Search

Context: A full understanding of the planet and moon formation process requires observations that probe the circumplanetary environment of accreting giant planets. The mid-infrared ELT imager and spectrograph (METIS) will provide a unique capability to detect warm-gas emission lines from circumplanetary disks. Aims: We aim to demonstrate the capability of the METIS instrument on the Extremely Large Telescope (ELT) to detect circumplanetary disks (CPDs) with fundamental v=1-0 transitions of $^{12}$CO from 4.5-5 $\mu$m. Methods: We consider the case of the well-studied HD 100546 pre-transitional disk to inform our disk modeling approach. We use the radiation-thermochemical disk modeling code ProDiMo to produce synthetic spectral channel maps. The observational simulator SimMETIS is employed to produce realistic data products with the integral field spectroscopic (IFU) mode. Results: The detectability of the CPD depends strongly on the level of external irradiation and the physical extent of the disk, favoring massive (~10 M$_{\rm J}$) planets and spatially extended disks with radii approaching the planetary Hill radius. The majority of $^{12}$CO line emission originates from the outer disk surface, and thus the CO line profiles are centrally peaked. The planetary luminosity does not contribute significantly to exciting disk gas line emission. If CPDs are dust-depleted, the $^{12}$CO line emission is enhanced as external radiation can penetrate deeper into the line emitting region. Conclusions: UV-bright star systems with pre-transitional disks are ideal candidates to search for CO-emitting CPDs with ELT/METIS. METIS will be able to detect a variety of circumplanetary disks via their fundamental $^{12}$CO ro-vibrational line emission in only 60 s of total detector integration time., Comment: 13 Pages, 11 Figures, accepted for publication in A&A

The subsurface oceans of icy satellites are among the most compelling among the potentially habitable environments in our Solar System. The question of whether a liquid subsurface layer can be maintained over geological timescales depends on its chemical composition. The composition of icy satellites is linked to that of the circumplanetary disk (CPD) in which they form. The CPD accretes material from the surrounding circumstellar disk in the vicinity of the planet, however, the degree of chemical inheritance is unclear. We aim to investigate the composition of ices in chemically reset or inherited circumplanetary disks to inform interior modeling and the interpretation of in situ measurements of icy solar system satellites, with an emphasis on the Galilean moon system. We used a radiation-thermochemical code to produce circumplanetary disk models and extract the ice composition from time-dependent chemistry, incorporating gas-phase and grain-surface reactions. The initial sublimation of ices during accretion may result in a CO2-rich ice composition. Sublimated ammonia ice is destroyed by background radiation while drifting towards the CPD midplane. Liberated nitrogen becomes locked in N2 due to efficient self-shielding, leaving ices depleted of ammonia. A significant ammonia ice component remains only when ices are inherited from the circumstellar disk. The observed composition of the Galilean moons is consistent with the sublimation of ices during accretion onto the CPD. In this scenario, the Galilean moon ices are nitrogen-poor and CO2 on Callisto is endogenous and primordial. The ice composition is significantly altered after an initial reset of accreted circumstellar ice. The chemical history of the Galilean moons stands in contrast to the Saturnian system, where the composition of the moons corresponds more closely with the directly inherited circumstellar disk material., 15 pages, 11 figures

Context. Understanding the conditions in which stars and stellar clusters form is of great importance. In particular, the role that stellar feedback may have is still hampered by large uncertainties. Aims. We aim to investigate the role played by ionising radiation and protostellar outflows during the formation and evolution of a stellar cluster. To self-consistently take into account gas accretion, we start with clumps of tens of parsecs in size. Methods. Using an adaptive mesh refinement code, we ran magneto-hydrodynamical numerical simulations aimed at describing the collapse of massive clumps with either no stellar feedback or taking into account ionising radiation and/or protostellar jets. Results. Stellar feedback substantially modifies the protostellar cluster properties in several ways. We confirm that protostellar outflows reduce the star formation rate by a factor of a few, although the outflows do not stop accretion and, likely enough, do not modify the final cluster mass. On the other hand, once sufficiently massive stars have formed, ionising radiation efficiently expels the remaining gas and reduces the final cluster mass by a factor of several. We found that while HII radiation and jets barely change the distribution of high density gas, the latter increases the dense gas velocity dispersion again by a factor of several in a few places. As we are starting from a relatively large scale, we found that the clusters whose mass and size are, respectively, of the order of a few 1000 M⊙ and a fraction of parsec, present a significant level of rotation. Moreover, we found that the sink particles that mimic the stars themselves tend to have rotation axes aligned with the cluster’s large-scale rotation. Finally, computing the classical Q parameter used to quantify stellar cluster structure, we infer that when jets are included in the calculation, the Q values are typical of observations, while when protostellar jets are not included, the Q values tend to be significantly lower. This is due to the presence of sub-clustering that is considerably reduced by the jets. Conclusions. Both large-scale gas accretion and stellar feedback, namely HII regions and protostellar jets, appear to significantly influence the formation and evolution of stellar clusters.

The large icy moons of Jupiter formed in a circumplanetary disk (CPD). CPDs are fed by infalling circumstellar gas and dust which may be shock-heated upon accretion or sublimated while passing through an optically thin gap. Accreted material is then either incorporated into moons, falls into the planet, or is lost beyond the disk edge on relatively short timescales. If ices are sublimated during accretion onto the CPD we know there must be sufficient time for them to recondense or moons such as Ganymede or Callisto could not form. The chemical timescale to form sufficiently icy solids places a novel constraint on the dynamical behaviour and properties of CPDs. We use the radiation thermochemical code ProDiMo to analyze how the radial ice abundance evolves in CPDs. We consider different initial chemical conditions of the disk to explore the consequences of infalling material being inherited from the circumstellar disk or being reset to atomic conditions by shock-heating. We contrast the timescales of ice formation with those of viscous evolution and radial dust drift. Water ice can form very efficiently in the CPD from initially atomic conditions, as a significant fraction is efficiently re-deposited on dust grains within < 1 yr. Radial grain drift timescales are in general longer than those of ice formation on grains. Icy grains of size $a < 3$ mm retain their icy mantles while crossing an optically thin circumstellar disk gap at 5 au for $L_* < 10 $ L$_{\odot}$. Three-body reactions play an important role in water formation in the dense midplane condition of CPDs. The CPD midplane must be depleted in dust relative to the circumstellar disk by a factor 10-50 to produce solids with the ice to rock ratio of the icy Galilean satellites. The CPD snowline is not erased by radial grain drift, which is consistent with the compositional gradient of the Galilean satellites being primordial., 20 pages, 17 figures

The nucleosynthetic isotope dichotomy between carbonaceous (CC) and non-carbonaceous (NC) meteorites has been interpreted as evidence for spatial separation and the coexistence of two distinct planet-forming reservoirs for several million years in the solar protoplanetary disk. The rapid formation of Jupiter's core within one million years after the formation of calcium-aluminium-rich inclusions (CAIs) has been suggested as a potential mechanism for spatial and temporal separation. In this scenario, Jupiter's core would open a gap in the disk and trap inward-drifting dust grains in the pressure bump at the outer edge of the gap, separating the inner and outer disk materials from each other. We performed simulations of dust particles in a protoplanetary disk with a gap opened by an early-formed Jupiter core, including dust growth and fragmentation as well as dust transport, using the dust evolution software DustPy. Our numerical experiments indicate that particles trapped in the outer edge of the gap rapidly fragment and are transported through the gap, contaminating the inner disk with outer disk material on a timescale that is inconsistent with the meteoritic record. This suggests that other processes must have initiated or at least contributed to the isotopic separation between the inner and outer Solar System., Comment: Accepted for publication in Astronomy & Astrophysics Letters

Numerical simulations of dwarf galaxies have so far failed to reproduce the observed metallicity-luminosity relation, down to the regime of ultra-faint dwarfs (UFDs). We address this issue by exploring how the first generations of metal-free stars (Pop III) could help increase the mean metallicity ([Fe/H]) of those small and faint galaxies. We ran zoom-in chemo-dynamical simulations of 19 halos extracted from a Λ Cold Dark Matter (CDM) cosmological box and followed their evolution down to redshift z = 0. Models were validated not only on the basis of galaxy global properties, but also on the detailed investigation of the stellar abundance ratios ([α/Fe]). We identified the necessary conditions for the formation of the first stars in mini-halos and derived constraints on the metal ejection schemes. The impact of Pop III stars on the final metallicity of UFDs was evaluated by considering different stellar mass ranges for their initial mass function (IMF), the influence of pair-instability supernovae (PISNe), and their energetic feedback, as well as the metallicity threshold that marks the transition from the first massive stars to the formation of low-mass long-lived stars. The inclusion of Pop III stars with masses below 140 M⊙, and a standard IMF slope of −1.3 does increase the global metallicity of UFDs, although these are insufficient to resolve the tension with observations. The PISNe with progenitor masses above 140 M⊙ do allow the metal content of UFDs to further increase. However, as PISNe are very rare and sometimes absent in the faintest UFDs, they have a limited impact on the global faint end of the metallicity-luminosity relation. Despite a limited number of spectroscopically confirmed members in UFDs, which make the stellar metallicity distribution of some UFDs uncertain, our analysis reveals that this is essentially the metal-rich tail that is missing in the models. The remaining challenges are thus both observational and numerical: (i) to extend high-resolution spectroscopy data samples and confirm the mean metallicity of the faintest UFDs; and (ii) to explain the presence of chemically enriched stars in galaxies with very short star formation histories.

This is an Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited., Context. Outer main belt asteroid (223) Rosa is a possible flyby target of opportunity for the European Space Agency (ESA) JUpiter ICy moons Explorer (JUICE) mission when it passes the asteroid belt on the way to Jupiter. The very low albedo and the featureless red spectra indicate a P-type asteroid in the Tholen taxonomy, though the yet known bulk density does not appear to match this classification. Aims. The aim of this work is to derive new estimates for the mass and bulk density of (223) Rosa. Methods. We derived the mass of Rosa by analyzing the gravitational deflection of small "test" asteroids that had a close encounter with Rosa in the past. To find such events suitable for the mass determination, we performed an encounter search with about 900 000 asteroids over the time span 1980–2030. Results. Three encounters were identified from which two independent mass estimates for Rosa were derived: M = (5.32 ± 2.17) × 1017 kg and M = (3.15 ± 1.14) × 1017 kg, respectively. The weighted mean is M = (3.62 ± 1.25) × 1017 kg. This yields to a bulk density of ρ = 1.2 ± 0.5 g cm−3, when adopting an effective diameter of D = 83 ± 8 km. This bulk density estimate is consistent with typical densities for Tholen taxonomy P-type asteroids. © M. Kretlow 2022., The occultation observations made by Roger Venable (USA), provided by the International Occultation Timing Association (IOTA) and by Herald et al. (2019) are acknowledged. Also appreciated are the services and data provided by the Asteroids Dynamic Site (AstDys), JPL Horizon and the Minor Planet Center (MPC). This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium)., With funding from the Spanish government through the Severo Ochoa Centre of Excellence accreditation SEV-2017-0709.

Context. Numerical integration ephemerides are widely used in research and engineering for their high precision. However, subject to their finite available time spans, their use is limited in theoretical research, such as the studies of rotation and evolution. Previously, we successfully experimented on the analytical representation of the mean longitude of Titan of the Jet Propulsion Laboratory (JPL) ephemeris, as a function of combinations of proper frequencies, and related the results with what is given in the synthetic ephemerides obtained by the Théorie Analytique des Satellites de Saturne (TASS). Aims. In this study, the analytical representations of the other osculating elements of the JPL Titan ephemeris are accomplished in order to construct the new synthetic representations, which have the advantages of both systems: long-lasting stability, the system details of TASS, and the high precision of JPL. Methods. A frequency analysis process was used to obtain the proper frequencies, amplitudes, and phases of the two ephemerides in the short term and the semi-long terms. For the proper frequency of the ascending node of Titan, which has a very long period, it is challenging to acquire the exact value, and the formula of TASS was used. The amplitude and phase of long terms were further calculated by a least-squares procedure. Results. Thanks to the accomplishments of the new synthetic representations of the JPL ephemeris, we report the complete combinations of the osculating elements of Titan. These combinations contain important dynamical information such as the proper frequencies. They will be useful in the theoretical research.

9 pags., 10 figs., 1 tab., Context. Explaining the presence of complex organic molecules (COMs) in interstellar environments requires a thorough understanding of the physics and chemistry occurring in the interplay between the gas phase and interstellar surfaces. Experiments and computer simulations are pivotal in building a comprehensive catalogue of processes of relevance for the build up of organic molecules in those environments. Aims. We combine experiments with tailored computer simulations to study the desorption dynamics of acetaldehyde CH3CHO - an important organic precursor in cold interstellar environments - on amorphous solid water for the first time. Our goals with this paper are twofold. Firstly, we want to contextualise the role of this molecule in the evolution of organic molecules in space. Secondly, we want to suggest a joint scheme to produce quantitative information on desorption magnitudes based on the combination of computations and experiments. This scheme can be adopted to refine measurements of other molecules. Methods. We determined desorption energies and pre-exponential factors of desorption theoretically using molecular dynamics simulations that combine semi-empirical and density functional calculations. We also performed temperature-programmed desorption experiments with acetaldehyde on top of non-porous amorphous solid water. The combination of theoretical and experimental results allows us to derive reliable quantities, which are required for understanding the desorption dynamics of interstellar COMs (iCOMs) atop interstellar ices. Results. The average theoretical and experimental desorption energies found for CH3CHO desorbing from non-porous amorphous solid water (np-ASW) surfaces are 3624K and 3774 K, respectively. The pre-exponential factor determined theoretically is νtheo = 2.4 × 1012 s-1 while from the experiments it was possible to constrain this magnitude to 1012 }1 s-1. Conclusions. The comparison of the desorption energies of CH3CHO with other COMs, such as CH3NH2 or CH3NO, shows that CH3CHO is more volatile. Therefore, we suggest that, in consideration of the average binding energy, CH3CHO should undergo preferential desorption during the ice-sublimation phase in hot cores enriching the gas-phase in this particular component. In addition, the overall low binding energy suggests a possible early return to the gas phase of pre-stellar cores due to non-thermal effects (i.e. reactive desorption or cosmic-ray-induced desorption). This could explain the prevalence of CH3CHO in the gas phase of pre-stellar cores. Dedicated laboratory and theoretical efforts are required to confirm this last point., Computer time was granted by the state of BadenWürttemberg through bwHPC and the German Research Foundation (DFG) through grant no. INST 40/467-1FUGG is greatly acknowledged. G.M thanks the Alexander von Humboldt Foundation for a post-doctoral research grant. We thank the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) for supporting this work by funding EXC 2075 – 390740016 under Germany’s Excellence Strategy. We acknowledge the support by the Stuttgart Center for Simulation Science (SimTech). V.J.H., R.J.P. and B.M. thank the funding by the Ministerio de Ciencia e Innovacion of Spain under grant PID2020-113084GB-I00, and the CSIC i-LINK+ project LINKA20353.

Context. Our thermal model is adapted and extended in this study. Specifically the aspect of handling indirect radiation, the solar limb darkening effect, and depth profiles are addressed. Aims. Our goal is to improve the existing thermal model to handle terrain scattering and re-radiation in an adaptive way. In addition, we aim to change previously fixed and manually chosen discretization of the solar limb darkening effect and depth profile to be adaptive and applicable for various planets and purposes. Methods. The temperature was modeled based on digital terrain models (DTMs) using data of the Mercury Laser Altimeter (MLA). New implementations to handle terrain scattering and re-radiation were introduced using level-of-detail techniques. The solar disk was discretized into a variable number of rings and the depth profile was introduced as an exponential function for which the number of nodes and the maximum depth can be chosen. Results. We present results for the ideal window size and degree of level-of-detail for thermal studies of the Hermean north pole. Further we show that the previous discretization of the solar limb darkening effect proved insufficient for Mercury, and we updated the implementation accordingly. Similarly we improved the implementation for the depth profile. For the first time, we derived depth-to-ice, as well as average and maximum temperature maps based on thermal modeling of the complete north polar MLA DTM.

Faraday tomography of radio polarimetric data below 200 MHz from LOFAR are providing us with a new perspective on the diffuse and magnetized interstellar medium (ISM). Of particular interest is the discovery of Faraday-rotated synchrotron polarization associated with neutral gas, as traced by atomic hydrogen (HI) and dust. Here we present the first in-depth numerical study of these LOFAR results. We produce and analyze comprehensive synthetic observations of low-frequency synchrotron polarization from MHD simulations of colliding super shells in the multiphase ISM. We define five distinct gas phases over more than four orders of magnitude in gas temperature and density, ranging from hot, and warm, fully ionized gas to cold neutral medium. We focus on the contribution of each gas phase to synthetic observations of both rotation measure and synchrotron polarized intensity below 200 MHz. We also investigate the link between the latter and synthetic observations of optically thin HI gas. We find that, not only the fully ionized gas but also the warm partially ionized and neutral phases strongly contribute to the total rotation measure and polarized intensity. However, the contribution of each phase to the observables strongly depends on the choice of integration axis and the orientation of the mean magnetic field with respect to the shell-collision axis. Strong correlation between HI synthetic data and synchrotron polarized intensity, reminiscent of LOFAR results, is obtained with lines of sight perpendicular to the mean magnetic field direction. Our study suggests that multiphase modelling of MHD processes is needed in order to interpret observations of the radio sky at low frequency. This work is a first step toward understanding the complexity of low-frequency synchrotron emission that will be soon revolutionized by large-scale surveys with LOFAR and the SKA., Comment: 17 pages, 20 figures, accepted by A&A, in press

Nearby pulsar wind nebulae exhibit complex morphological features: jets, torus, arcs and knots. These structures are well captured and understood in the scope of global magnetohydrodynamic models. However, the origin of knots in the inner radius of the Crab Nebula remains elusive. In this work, we investigate the dynamics of the shock front and downstream flow with a special emphasis on the reconnecting equatorial current sheet. We examine whether giant plasmoids produced in the reconnection process could be good candidates for the knots. To this end, we perform large semi-global three-dimensional particle-in-cell simulations in a spherical geometry. The hierarchical merging plasmoid model is used to extrapolate numerical results to pulsar wind nebula scales. The shocked material collapses into the midplane, forming and feeding a large-scale, but thin, ring-like current layer. The sheet breaks up into a dynamical chain of merging plasmoids reminiscent of three-dimensional reconnection. Plasmoids grow to a macroscopic size. The final number of plasmoids predicted is solely governed by the inverse of the dimensionless reconnection rate. The formation of giant plasmoids is a robust feature of pulsar wind termination shocks. They provide a natural explanation for the inner-ring knots in the Crab Nebula, provided that the nebula is highly magnetized., Comment: 8 pages, 7 figures, Accepted for publication in Astronomy & Astrophysics, revision includes A&A language editing

Some contracting or expanding stars are thought to host a large-scale magnetic field in their radiative interior. By interacting with the contraction-induced flows, such fields may significantly alter the rotational history of the star. They thus constitute a promising way to address the problem of angular momentum transport during the rapid phases of stellar evolution. In this work, we aim at studying the interplay between flows and magnetic fields in a contracting radiative zone. We propose a scenario that may account for the post-main sequence evolution of solar-like stars, in which a quasi-solid rotation can be maintained by a large-scale magnetic field during a contraction timescale. Then, an axisymmetric instability would destroy this large-scale structure and enables the differential rotation to set in. Such a contraction driven instability could also be at the origin of the observed dichotomy between strongly and weakly magnetic intermediate-mass stars., 31 pages, 29 figures, 3 tables, accepted for publication in Astronomy & Astrophysics

Context. Solar Orbiter and Parker Solar Probe jointly observed the solar wind for the first time in June 2020, capturing data from very different solar wind streams: calm, Alfvénic wind and also highly dynamic large-scale structures. Context. Our aim is to understand the origin and characteristics of the highly dynamic solar wind observed by the two probes, particularly in the vicinity of the heliospheric current sheet (HCS). Methods. We analyzed the plasma data obtained by Parker Solar Probe and Solar Orbiter in situ during the month of June 2020. We used the Alfvén-wave turbulence magnetohydrodynamic solar wind model WindPredict-AW and we performed two 3D simulations based on ADAPT solar magnetograms for this period. Results. We show that the dynamic regions measured by both spacecraft are pervaded by flux ropes close to the HCS. These flux ropes are also present in the simulations, forming at the tip of helmet streamers, that is, at the base of the heliospheric current sheet. The formation mechanism involves a pressure-driven instability followed by a fast tearing reconnection process. We further characterize the 3D spatial structure of helmet streamer born flux ropes, which appears in the simulations to be related to the network of quasi-separatrices.

Our ability to predict the structure and evolution of stars is in part limited by complex, 3D hydrodynamic processes such as convective boundary mixing. Hydrodynamic simulations help us understand the dynamics of stellar convection and convective boundaries. However, the codes used to compute such simulations are usually tested on extremely simple problems and the reliability and reproducibility of their predictions for turbulent flows is unclear. We define a test problem involving turbulent convection in a plane-parallel box, which leads to mass entrainment from, and internal-wave generation in, a stably stratified layer. We compare the outputs from the codes FLASH, MUSIC, PPMSTAR, PROMPI, and SLH, which have been widely employed to study hydrodynamic problems in stellar interiors. The convection is dominated by the largest scales that fit into the simulation box. All time-averaged profiles of velocity components, fluctuation amplitudes, and fluxes of enthalpy and kinetic energy are within $\lesssim 3\sigma$ of the mean of all simulations on a given grid ($128^3$ and $256^3$ grid cells), where $\sigma$ describes the statistical variation due to the flow's time dependence. They also agree well with a $512^3$ reference run. The $128^3$ and $256^3$ simulations agree within $9\%$ and $4\%$, respectively, on the total mass entrained into the convective layer. The entrainment rate appears to be set by the amount of energy that can be converted to work in our setup and details of the small-scale flows in the boundary layer seem to be largely irrelevant. Our results lend credence to hydrodynamic simulations of flows in stellar interiors. We provide in electronic form all outputs of our simulations as well as all information needed to reproduce or extend our study., Comment: 20 pages, 19 figures, new Fig. B.2, new clarifications, electronic materials updated, accepted for publication in A&A

Spatially resolved continuum observations of planet-forming disks show prominent ring and gap structures in their dust distribution. However, the picture from gas observations is much less clear and constraints on the radial gas density structure (i.e. gas gaps) remain rare and uncertain. We want to investigate the importance of thermo-chemical processes for the interpretation of high-spatial-resolution gas observations of planet-forming disks and their impact on derived gas properties. We apply the radiation thermo-chemical disk code ProDiMo (PROtoplanetary DIsk MOdel) to model self-consistently the dust and gas disk of HD 163296, using the DSHARP gas and dust observations. With this model we investigate the impact of dust gaps and gas gaps, considering chemistry and heating/cooling processes, on the observables and the derived gas properties. We find distinct peaks in the radial line intensity profiles of the CO line data of HD 163296 at the location of the dust gaps. Our model indicates that those peaks are not only a consequence of a gas temperature increase within the gaps but are mainly caused by the absorption of line emission from the back side of the disk by the dust rings. For two of the three prominent dust gaps in HD 163296, we find that thermo-chemical effects are negligible for deriving density gradients via measurements of the rotation velocity. However, for the gap with the highest dust depletion, the temperature gradient can be dominant and needs to be considered to derive accurate gas density profiles. Self-consistent gas and dust thermo-chemical modelling in combination with high-quality observations of multiple molecules are necessary to accurately derive gas gap depths and shapes. This is crucial to determine the origin of gaps and rings in planet-forming disks and to improve the mass estimates of forming planets if they are the cause of the gap., 12 pages, 9 figures, accepted for publication in A&A

Context. A large portion of stars is found to be part of binary or higher-order multiple systems. The ubiquity of planets found around single stars raises the question of whether and how planets in binary systems form. Protoplanetary disks are the birthplaces of planets, and characterizing them is crucial in order to understand the planet formation process. Aims. Our goal is to characterize the morphology of the GG Tau A disk, one of the largest and most massive circumbinary disks. We also aim to trace evidence for binary-disk interactions. Methods. We obtained observations in polarized scattered light of GG Tau A using the SPHERE/IRDIS instrument in the H-band filter. We analyzed the observed disk morphology and substructures. We ran 2D hydrodynamical models to simulate the evolution of the circumbinary ring over the lifetime of the disk. Results. The disk and also the cavity and the inner region are highly structured, with several shadowed regions, spiral structures, and streamer-like filaments. Some of these are detected here for the first time. The streamer-like filaments appear to connect the outer ring with the northern arc. Their azimuthal spacing suggests that they may be generated through periodic perturbations by the binary, which tear off material from the inner edge of the outer disk once during each orbit. By comparing observations to hydrodynamical simulations, we find that the main features, in particular, the gap size, but also the spiral and streamer filaments, can be qualitatively explained by the gravitational interactions of a binary with a semimajor axis of ~35 au on an orbit coplanar with the circumbinary ring., Astronomy & Astrophysics, 639, ISSN:0004-6361, ISSN:1432-0746

Molecular hydrogen (H2) is the main constituent of the gas in the planet-forming disks that surround many PMS stars. H2 can be incorporated in the atmosphere of the giant planets. HD has been detected in a few disks and can be considered the most reliable tracer of H2. We wish to form H2 and HD efficiently for the varied conditions encountered in protoplanetary disks: the densities vary from 1E4 to 1E16 cm^-3; the dust temperatures range from 5 to 1500 K, the gas temperatures go from 5 to a few 1000 Kelvin, and the ultraviolet field can be 1E7 stronger than the standard interstellar field. We implemented a comprehensive model of H2 and HD formation on cold and warm grain surfaces and via hydrogenated PAHs in the physico-chemical code ProDiMo. The H2 and HD formation can proceed via the Langmuir-Hinshelwood and Eley-Ridel mechanisms for physisorbed or chemisorbed H (D) atoms. H2 and HD also form by H (D) abstraction from hydrogenated neutral and ionised PAHs and via gas phase reactions. H2 and HD are formed efficiently on dust grain surfaces from 10 to 700 K. All the deuterium is converted into HD in UV shielded regions as soon as H2 is formed by gas-phase D abstraction reactions. The detailed model compares well with standard analytical prescriptions for H2 (HD) formation. At low temperatures, H2 is formed from the encounter of two physisorbed atoms. HD molecules form on the grain surfaces and in the gas-phase. At temperatures greater than 20 K, the meeting between a weakly bound H- (or D-) atom or a gas-phase H (D) atom and a chemisorbed atom is the most efficient H2 formation route. H2 formation through hydrogenated PAHs alone is efficient above 80 K. The contribution of hydrogenated PAHs to the overall H2 and HD formation is relatively low if chemisorption on silicate is taken into account and if a small hydrogen abstraction cross-section is used., Comment: 26 pages, accepted to A&A

We investigated the disc-halo connection in massive (M⋆ > 5 × 1010 M⊙) disc galaxies from the cosmological hydrodynamical simulations EAGLE and IllustrisTNG, and compared this connection with that inferred from the study of H I rotation curves in nearby massive spirals from the Spitzer Photometry and Accurate Rotation Curves dataset. We find that discrepancies between the simulated and observed discs arise both on global and local scales. Globally, the simulated discs inhabit halos that are a factor ∼4 (in EAGLE) and ∼2 (in IllustrisTNG) more massive than those derived from the rotation curve analysis of the observed dataset. We also used synthetic rotation curves of the simulated discs to demonstrate that the recovery of the halo masses from rotation curves are not systematically biased. We find that the simulations predict systems dominated by dark matter with stellar-to-total enclosed mass ratios that are a factor of 1.5−2 smaller than real galaxies at all radii. This is an alternative manifestation of the ‘failed feedback problem,’ since it indicates that simulated halos hosting massive discs have been too inefficient at converting their baryons into stars, possibly owing to an overly efficient stellar and/or AGN feedback implementation.

To reach its optimal performance, Fizeau interferometry requires that we work to resolve instrumental biases through calibration. One common technique used in high contrast imaging is angular differential imaging, which calibrates the point spread function and flux leakage using a rotation in the focal plane. Our aim is to experimentally demonstrate and validate the efficacy of an angular differential kernel-phase approach, a new method for self-calibrating interferometric observables that operates similarly to angular differential imaging, while retaining their statistical properties. We used linear algebra to construct new observables that evolve outside of the subspace spanned by static biases. On-sky observations of a binary star with the SCExAO instrument at the Subaru telescope were used to demonstrate the practicality of this technique. We used a classical approach on the same data to compare the effectiveness of this method. The proposed method shows smaller and more Gaussian residuals compared to classical calibration methods, while retaining compatibility with the statistical tools available. We also provide a measurement of the stability of the SCExAO instrument that is relevant to the application of the technique. Angular differential kernel phases provide a reliable method for calibrating biased observables. Although the sensitivity at small separations is reduced for small field rotations, the calibration is effectively improved and the number of subjective choices is reduced., Comment: To be published in Astronomy & Astrophysics, 11 pages, 7 figures

Context. Relativistic jets are ubiquitous in astrophysics. High Mass Microquasars (HMMQs) are useful labs to study such jets, as they are relatively close and evolve over observable time scales. The ambient medium into which the jet propagates is, however, far from homogeneous. Corresponding simulation studies to date consider various forms of a wind-shaped ambient medium but typically neglect radiative cooling and relativistic effects. Aims. We investigate the dynamical and structural effects of radiative losses and system parameters on relativistic jets in HMMQs, from the jet launch to its propagation over several tens of orbital separations. Methods. We use 3D relativistic hydrodynamical simulations including parameterized radiative cooling derived from relativistic thermal plasma distribution to carry out parameter studies around two fiducial cases inspired by Cygnus X-1 and Cygnus X-3. Results. Radiative losses are found to be more relevant in Cygnus X-3 than Cygnus X-1. Varying jet power, jet temperature, or the wind of the donor star tends to have a larger impact at early times, when the jet forms and instabilities initially develop, than at later times when the jet has reached a turbulent state. Conclusions. Radiative losses may be dynamically and structurally relevant at least in Cygnus X-3 case, and thus should be examined in more detail., Comment: 28 pages, 25 figures, 8 tables. Accepted for publication in A&A

Stellar orbits at the Galactic Center provide a very clean probe of the gravitational potential of the supermassive black hole. They can be studied with unique precision, beyond the confusion limit of a single telescope, with the near-infrared interferometer GRAVITY. Imaging is essential to search the field for faint, unknown stars on short orbits which potentially could constrain the black hole spin. Furthermore, it provides the starting point for astrometric fitting to derive highly accurate stellar positions. Here, we present $\mathrm{G^R}$, a new imaging tool specifically designed for Galactic Center observations with GRAVITY. The algorithm is based on a Bayesian interpretation of the imaging problem, formulated in the framework of information field theory and building upon existing works in radio-interferometric imaging. Its application to GRAVITY observations from 2021 yields the deepest images to date of the Galactic Center on scales of a few milliarcseconds. The images reveal the complicated source structure within the central $100\,\mathrm{mas}$ around Sgr A*, where we detected the stars S29 and S55 and confirm S62 on its trajectory, slowly approaching Sgr A*. Furthermore, we were able to detect S38, S42, S60, and S63 in a series of exposures for which we offset the fiber from Sgr A*. We provide an update on the orbits of all aforementioned stars. In addition to these known sources, the images also reveal a faint star moving to the west at a high angular velocity. We cannot find any coincidence with any known source and, thus, we refer to the new star as S300. From the flux ratio with S29, we estimate its K-band magnitude as $m_\mathrm{K}\left(\mathrm{S300}\right)\simeq 19.0 - 19.3$. Images obtained with CLEAN confirm the detection., 24 pages, 14 figures

Ferrero, I., et al. (DES Collaboration), The calibration and validation of scientific analysis in simulations is a fundamental tool to ensure unbiased and robust results in observational cosmology. In particular, mock galaxy catalogs are a crucial resource to achieve these goals in the measurement of baryon acoustic oscillation (BAO) in the clustering of galaxies. Here we present a set of 1952 galaxy mock catalogs designed to mimic the Dark Energy Survey Year 3 BAO sample over its full photometric redshift range 0:6 < zphoto < 1:1. The mocks are based upon 488 ICE-COLA fast N-body simulations of full-sky light cones and were created by populating halos with galaxies, using a hybrid halo occupation distribution halo abundance matching model. This model has ten free parameters, which were determined, for the first time, using an automatic likelihood minimization procedure.We also introduced a novel technique to assign photometric redshift for simulated galaxies, following a two-dimensional probability distribution with VIMOS Public Extragalactic Redshift Survey data. The calibration was designed to match the observed abundance of galaxies as a function of photometric redshift, the distribution of photometric redshift errors, and the clustering amplitude on scales smaller than those used for BAO measurements. An exhaustive analysis was done to ensure that the mocks reproduce the input properties. Finally, mocks were tested by comparing the angular correlation function w(θ), angular power spectrum ζp(rΓ) and projected clustering p(r?) to theoretical predictions and data. The impact of volume replication in the estimate of the covariance is also investigated. The success in accurately reproducing the photometric redshift uncertainties and the galaxy clustering as a function of redshift render this mock creation pipeline as a benchmark for future analyses of photometric galaxy surveys.

Intense X-ray and ultraviolet stellar irradiation can heat and inflate the atmospheres of closely orbiting exoplanets, driving mass outflows that may be significant enough to evaporate a sizable fraction of the planet atmosphere over the system lifetime. The recent surge in the number of known exoplanets, together with the imminent deployment of new ground and space-based facilities for exoplanet discovery and characterization, requires a prompt and efficient assessment of the most promising targets for intensive spectroscopic follow-ups. To this purpose, we developed ATES (ATmospheric EScape); a new hydrodynamics code that is specifically designed to compute the temperature, density, velocity and ionization fraction profiles of highly irradiated planetary atmospheres, along with the current, steady-state mass loss rate. ATES solves the one-dimensional Euler, mass and energy conservation equations in radial coordinates through a finite-volume scheme. The hydrodynamics module is paired with a photoionization equilibrium solver that includes cooling via bremsstrahlung, recombination and collisional excitation/ionization for the case of a primordial atmosphere entirely composed of atomic hydrogen and helium, whilst also accounting for advection of the different ion species. Compared against the results of 14 moderately-to-highly irradiated planets simulated with The PLUTO-CLOUDY Interface (TPCI), ATES yields remarkably good agreement at a significantly smaller fraction of the computational time. A convergence study shows that ATES recovers stable, steady-state hydrodynamic solutions for systems with $\log(-\phi_p) \lesssim 12.9 + 0.17\log F_{\rm XUV}$. Incidentally, atmospheres of systems above this threshold are generally thought to be undergoing Jeans escape. The code, which also features a user-friendly graphic interface, is available publicly as an online repository., Comment: 17 pages, 11 figures. Revised version. Accepted by A&A. Minor changes only; this new version includes a convergence study. Comments are welcome

Discoveries of extended rotation curves have suggested the presence of dark matter in spiral galaxy haloes. It has led to many studies that estimated the galaxy total mass, mostly by using the Navarro Frenk and White (NFW) density profile. We aim at verifying how the choice of the dark-matter profile may affect the predicted values of extrapolated total masses. We have considered the recent Milky Way (MW) rotation curve, firstly because of its unprecedented accuracy, and secondly because the Galactic disk is amongst the least affected by past major mergers having fully reshaped the initial disk. We find that the use of NFW profile (or its generalized form, gNFW) for calculating the dark-matter contribution to the MW rotation curve generates apparently inconsistent results, e.g., an increase of the baryonic mass leads to increase of the dark matter mass. Furthermore we find that NFW and gNFW profile narrow the total mass range, leading to a possible methodological bias particularly against small MW masses. By using the Einasto profile that is more appropriate to represent cold dark matter haloes, we finally find that the Milky Way slightly decreasing rotation curve favors total mass that can be as small as 2.6 $\times 10^{11}$ $M_{\odot}$, disregarding any other dynamical tracers further out in the MW. It is inconsistent with values larger than 18 $\times 10^{11}$ $M_{\odot}$ for any kind of CDM dark-matter halo profiles, under the assumption that stars and gas do not influence the predicted dark matter distribution in the MW. This methodological paper encourages the use of the Einasto profile for characterizing rotation curves with the aim of evaluating their total masses., 10 pages, 6 Figures, 5 Tables including Table A-1 with re-evaluated error bars on the Milky Way rotation curve. After proof editing and to be published in the Astronomy & Astrophysics journal

Radio galaxies play an important role in the seeding of cosmic rays and magnetic fields in galaxy clusters. Here, we simulate the evolution of relativistic electrons injected into the intracluster medium by radio galaxies. Using passive tracer particles added to magnetohydrodynamical adaptive-mesh simulations, we calculate the evolution of the spectrum of relativistic electrons taking into account energy losses and re-acceleration mechanisms associated with the dynamics of the intracluster medium. Re-acceleration can occur at shocks via diffusive shock acceleration, and in turbulent flows via second-order Fermi re-acceleration. This study confirms that relativistic electrons from radio galaxies can efficiently fill the intracluster medium over scales of several $100 \rm ~Myr$, and that they create a stable reservoir of fossil electrons that remains available for further re-acceleration by shock waves and turbulent gas motions. Our results also show that late evolution of radio lobes and remnant radio galaxies is significantly affected by the dynamics of the surrounding intracluster medium. Here the diffusive re-acceleration couples the evolution of relativistic particles to the gas perturbations. In the near future, deep radio observations, especially at low frequencies, can probe such mechanisms in galaxy clusters., 22 pages, 20 figures, A & A, in press

Recently a type of neural networks called Generative Adversarial Networks (GANs) has been proposed as a solution for fast generation of simulation-like datasets, in an attempt to bypass heavy computations and expensive cosmological simulations to run in terms of time and computing power. In the present work, we build and train a GAN to look further into the strengths and limitations of such an approach. We then propose a novel method in which we make use of a trained GAN to construct a simple autoencoder (AE) as a first step towards building a predictive model. Both the GAN and AE are trained on images issued from two types of N-body simulations, namely 2D and 3D simulations. We find that the GAN successfully generates new images that are statistically consistent with the images it was trained on. We then show that the AE manages to efficiently extract information from simulation images, satisfyingly inferring the latent encoding of the GAN to generate an image with similar large scale structures., submitted for publication in A&A, 15 pages

This is the first of a series of two papers that deepen our understanding of the transversal structure and the properties of recollimation shocks of axisymmetric, relativistic, superfast magnetosonic, overpressured jets. They extend previous work that characterized these properties in connection with the dominant type of energy (internal, kinetic, or magnetic) in the jet to models with helical magnetic fields with larger magnetic pitch angles and force-free magnetic fields. In this paper, the magnetohydrodynamical models were computed following an approach that allows studying the structure of steady, axisymmetric, relativistic (magnetized) flows using one-dimensional time-dependent simulations. In these approaches, the relevance of the magnetic tension and of the Lorentz force in shaping the internal structure of jets (transversal structure, radial oscillations, and internal shocks) is discussed. The radial Lorentz force controls the jet internal transversal equilibrium. Hence, highly magnetized non-force-free jets exhibit a thin spine of high internal energy around the axis. The properties of the recollimation shocks and sideways expansions and compressions of the jet result from the total pressure mismatch at the jet surface, which among other factors depends on the magnetic tension and the magnetosonic Mach number of the flow. Hot jets with low Mach number tend to have strong oblique shocks and wide radial oscillations. Highly magnetized jets with large toroidal fields tend to have weaker shocks and radial oscillations of smaller amplitude. In the second paper, we present synthetic synchrotron radio images of the magnetohydrodynamical models that are produced at a post-processing phase, focusing on the observational properties of the jets, namely the top-down emission asymmetries, spine brightening, the relative intensity of the knots, and polarized emission. © ESO 2021., JMM and MP acknowledge financial support from the Spanish Ministerio de Economia y Competitividad (grant AYA2016-77237-C33-P), the Spanish Ministerio de Ciencia (PID2019-107427GB-C33), and from the local Autonomous Government (Generalitat Valenciana, grant PROMETEO/2019/071). JMM acknowledges further financial support from the Spanish Ministerio de Economia y Competitividad (grant PGC2018-095984-B-I00). MP acknowledges further financial support from the Spanish Ministerio de Ciencia through grant PID2019-105510GB-C31. AF and JLG acknowledge financial support from the Spanish Ministerio de Economia y Competitividad (grants AYA2016-80889-P, PID2019-108995GB-C21), the Consejeria de Economia, Conocimiento, Empresas y Universidad of the Junta de Andalucia (grant P18-FR-1769), the Consejo Superior de Investigaciones Cientificas (grant 2019AEP112), and the State Agency for Research of the Spanish MCIU through the Center of Excellence Severo Ochoa award for the Instituto de Astrofisica de Andalucia (SEV-2017-0709). This research made use of Python (http://www.python.org), Numpy (van der Walt et al. 2011), Pandas (McKinney 2010), and Matplotlib (Hunter 2007). We also made use of Astropy (http://www.astropy.org), a community-developed core Python package for Astronomy (Astropy Collaboration 2013, 2018).

Aims. We present a detailed study of the magnetic evolution of AR 12673 using a magnetofrictional modelling approach. Methods. The fully data-driven and time-dependent model was driven with maps of the photospheric electric field, inverted from vector magnetogram observations obtained from the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO). Our analysis was aided by studying the evolution of metrics such as the free magnetic energy and the current-carrying helicity budget of the domain, maps of the squashing factor and twist, and plots of the current density. These allowed us to better understand the dynamic nature of the magnetic topology. Results. Our simulation captured the time-dependent nature of the active region and the erupting flux rope associated with the X-class flares on 6 September 2017, including the largest of solar cycle 24. Additionally, our results suggest a possible threshold for eruptions in the ratio of current-carrying helicity to relative helicity. Conclusion. The flux rope was found to be a combination of two structures that partially combine during the eruption process. Our time-dependent data-driven magnetofrictional model is shown to be capable of generating magnetic fields consistent with extreme ultraviolet (EUV) observations.

The phase-space structure of primordial dark matter halos is revisited using cosmological simulations with three sine waves and Cold Dark Matter (CDM) initial conditions. The simulations are performed with the tessellation based Vlasov solver ColDICE and a Particle-Mesh (PM) $N$-body code. The analyses include projected density, phase-space diagrams, radial density and pseudo-phase space density. Particular attention is paid to force and mass resolution. Because the phase-space sheet complexity, estimated in terms of total volume and simplices count, increases very quickly, ColDICE can follow only the early violent relaxation phase of halo formation. During the latter, agreement between ColDICE and PM simulations having one particle per cell or more is excellent and halos have a power-law density profile, $\rho(r) \propto r^{-\alpha}$, $\alpha \in [1.5,1.8]$. This slope, measured prior to any merger, is slightly larger than in the literature. The phase-space diagrams evidence complex but coherent patterns with clear signatures of self-similarity in the sine wave simulations, while the CDM halos are somewhat scribbly. After additional mass resolution tests, the PM simulations are used to follow the next stages of evolution. The power-law progressively breaks down with a convergence of the density profile to the well known "NFW"-like universal attractor, irrespectively of initial conditions, that is even in the three-sine wave simulations. This demonstrates again that mergers do not represent a necessary condition for convergence to the dynamical attractor. Not surprisingly, the measured pseudo phase-space density is a power-law $Q(r) \propto r^{-\alpha_Q}$, with $\alpha_{\rm Q}$ close to the prediction of secondary spherical infall model, $\alpha_{\rm Q} \simeq 1.875$. However this property is also verified during the early relaxation phase, which is non trivial., Comment: 37 pages, 20 figures, revised version with one typo corrected, accepted in Astronomy & Astrophysics

Context. The all-sky survey from the Planck space telescope has revealed that thermal emission from Galactic dust is polarized on scales ranging from the whole sky down to the inner regions of molecular clouds. Polarized dust emission can therefore be used as a probe for magnetic fields on different scales. In particular, the analysis of the relative orientation between the density structures and the magnetic field projected on the plane of the sky can provide information on the role of magnetic fields in shaping the structure of molecular clouds where star formation takes place. Aims. The orientation of the magnetic field with respect to the density structures has been investigated using different methods. The goal of this paper is to explicitly compare two of these: the Rolling Hough Transform (RHT) and the gradient technique (GRAD). Methods. We generated synthetic surface brightness maps at 353 GHz (850 μm) via magnetohydrodynamic simulations. We applied RHT and GRAD to two morphologically different regions identified in our maps. Region 1 is dominated by a dense and thick filamentary structure with some branches, while Region 2 includes a thinner filament with denser knots immersed in a more tenuous medium. Both methods derive the relative orientation between the magnetic field and the density structures, to which we applied two statistics, the histogram of relative orientation and the projected Rayleigh statistic, to quantify the variations of the relative orientation as a function of column density. Results. Both methods find areas with significant signal, and these areas are substantially different. In terms of relative orientations, in all our considered cases the predominant orientation of the density structures is perpendicular to the direction of the magnetic field. When the methods are applied to the same selected areas the results are consistent with each other in Region 2 but show some noticeable differences in Region 1. In Region 1, RHT globally finds the relative orientation becoming more perpendicular for increasing column density, while GRAD, applied at the same resolution as RHT, gives the opposite trend. These disparities are caused by the intrinsic differences in the methods and in the structures that they select. Conclusions. Our results indicate that the interpretation of the relative orientation between the magnetic field and density structures should take into account the specificity of the methods used to determine such orientation. The combined use of complementary techniques such as RHT and GRAD provides more complete information, which can be advantageously used to better understand the physical mechanisms operating in magnetized molecular clouds.

Aims: To trace the radial and vertical spatial distribution of H2CS, a key species of the S-bearing chemistry, in protoplanetary disks. To analyse the observed distributions in light of the H2CS binding energy, in order to discuss the role of thermal desorption in enriching the gas disk component. Methods: In the context of the ALMA chemical survey of Disk-Outflow sources in the Taurus star forming region (ALMA-DOT), we observed five Class I or early Class II sources with the o-H2CS(7_1,6-6_1,5) line on a 40 au scale. We estimated the binding energy (BEs) of H2CS using quantum mechanical calculations, for the first time, for an extended, periodic, crystalline ice. Results: We imaged H2CS in two rotating molecular rings in the HL Tau and IRAS04302+2247 disks. The outer radii are about 140 au (HL Tau), and 115 au (IRAS 04302+2247). The edge-on geometry of IRAS 04302+2247 reveals that H2CS emission peaks, at radii of 60-115 au, at z = +- 50 au from the equatorial plane. The column densities are about 10^14 cm^-2. For HL Tau, we derive, for the first time, the [H2CS]/[H] abundance in a protoplanetary disk (about 10^-14). The BEs of H2CS computed for extended crystalline ice and amorphous ices is 4258 K and 3000-4600 K, respectively, implying a thermal evaporation where dust temperature is larger than 50-80 K. Conclusions: H2CS traces the so-called warm molecular layer, a region previously sampled using CS, and H2CO. Thioformaldehyde peaks closer to the protostar than H2CO and CS, plausibly due to the relatively high-excitation level of observed 7_1,6-6_1,5 line (60 K). The H2CS BEs implies that thermal desorption dominates in thin, au-sized, inner and/or upper disk layers, indicating that the observed H2CS emitting up to radii larger than 100 au is likely injected in the gas due to non-thermal processes., A&A, in press

Aims. We present a detailed examination of the magnetic evolution of AR 12473 using time-dependent, data-driven magnetofrictional modelling. Methods. We used maps of the photospheric electric field inverted from vector magnetogram observations, obtained by the Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory (SDO), to drive our fully time-dependent, data-driven magnetofrictional model. Our modelled field was directly compared to extreme ultraviolet observations from the Atmospheric Imaging Assembly, also onboard SDO. Metrics were also computed to provide a quantitative analysis of the evolution of the magnetic field. Results. The flux rope associated with the eruption on 28 December 2015 from AR 12473 was reproduced by the simulation and found to have erupted due to a torus instability.

Initially far out-of-equilibrium self-gravitating systems form, through a collisionless relaxation dynamics, quasi-stationary states (QSS). These may arise from a bottom-up aggregation of structures or in a top-down frame; their quasi-equilibrium properties are well described by the Jeans equation and are not universal, i.e. they depend on initial conditions. To understand the origin of such dependence, we present results of numerical experiments of initially cold and spherical systems characterized by various choices of the spectrum of initial density fluctuations. The amplitude of such fluctuations determines whether the system relaxes in a top-down or a bottom-up manner. We find that statistical properties of the resulting QSS mainly depend upon the amount of energy exchanged during the formation process. In particular, in the violent top-down collapses the energy exchange is large and the QSS show an inner core with an almost flat density profile and a quasi Maxwell-Boltzmann (isotropic) velocity distribution, while their outer regions display a density profile $\rho(r) \propto r^{-\alpha}$ ($\alpha >0$) with radially elongated orbits. We analytically show that $\alpha=4$ in agreement with numerical experiments. In the less violent bottom-up dynamics, the energy exchange is much smaller, the orbits are less elongated and $0< \alpha(r) \le 4$, with a a density profile well fitted by the Navarro-Frenk-White behavior. Such a dynamical evolution is shown by both non-uniform spherical isolated systems and by halos extracted from cosmological simulations. We consider the relation of these results with the core-cusp problem concluding that this is naturally solved if galaxies form through a monolithic collapse., Comment: 15 pages, 24 figures. Accepted for publication in Astronomy and Astrophysics. The revised verion matches the published version

14 pags., 15 figs., 2 tabs., Context. The diffusion of volatile species on amorphous solid water ice affects the chemistry on dust grains in the interstellar medium as well as the trapping of gases enriching planetary atmospheres or present in cometary material. Aims. The aim of the work is to provide diffusion coefficients of CH4 on amorphous solid water (ASW) and to understand how they are affected by the ASW structure. Methods. Ice mixtures of H2O and CH4 were grown in different conditions and the sublimation of CH4 was monitored via infrared spectroscopy or via the mass loss of a cryogenic quartz crystal microbalance. Diffusion coefficients were obtained from the experimental data assuming the systems obey Fick's law of diffusion. Monte Carlo simulations were used to model the different amorphous solid water ice structures investigated and were used to reproduce and interpret the experimental results. Results. Diffusion coefficients of methane on amorphous solid water have been measured to be between 10-12 and 10-13 cm2 s-1 for temperatures ranging between 42 K and 60 K. We show that diffusion can differ by one order of magnitude depending on the morphology of amorphous solid water. The porosity within water ice and the network created by pore coalescence enhance the diffusion of species within the pores. The diffusion rates derived experimentally cannot be used in our Monte Carlo simulations to reproduce the measurements. Conclusions. We conclude that Fick's laws can be used to describe diffusion at the macroscopic scale, while Monte Carlo simulations describe the microscopic scale where trapping of species in the ices (and their movement) is considered., Funds from the Spanish MINECO/FEDER FIS2016-77726- C3-1-P and C3-3-P projects are acknowledged.

Context. The amount of data collected by spectrometers from radio to ultraviolet (UV) wavelengths opens a new era where the statistical and chemical information contained in the observations can be used concomitantly to investigate the thermodynamical state and the evolution of the interstellar medium (ISM). Aims. In this paper, we study the statistical properties of the HI-to-H2 transition observed in absorption in the local diffuse and multiphase ISM. Our goal is to identify the physical processes that control the probability of occurrence of any line of sight and the origins of the variations of the integrated molecular fraction from one line of sight to another. Methods. The turbulent diffuse ISM is modeled using the RAMSES code, which includes detailed treatments of the magnetohydrodynamics, the thermal evolution of the gas, and the chemistry of H2. The impacts of the UV radiation field, the mean density, the turbulent forcing, the integral scale, the magnetic field, and the gravity on the molecular content of the gas are explored through a parametric study that covers a wide range of physical conditions. The statistics of the HI-to-H2 transition are interpreted through analytical prescriptions and compared with the observations using a modified and robust version of the Kolmogorov-Smirnov test. Results. The analysis of the observed background sources shows that the lengths of the lines of sight follow a flat distribution in logarithmic scale from ~100 pc to ~3 kpc. Without taking into account any variation of the parameters along a line of sight or from one line of sight to another, the results of one simulation, convolved with the distribution of distances of the observational sample, are able to simultaneously explain the position, the width, the dispersion, and most of the statistical properties of the HI-to-H2 transition observed in the local ISM. The tightest agreement is obtained for a neutral diffuse gas modeled over ~200 pc, with a mean density n̅H̅ = 1−2 cm−3, illuminated by the standard interstellar UV radiation field, and stirred up by a large-scale compressive turbulent forcing. Within this configuration, the 2D probability histogram of the column densities of H and H2, poetically called the kingfisher diagram, is remarkably stable and is almost unaltered by gravity, the strength of the turbulent forcing, the resolution of the simulation, or the strength of the magnetic field Bx, as long as Bx < 4 μG. The weak effect of the resolution and our analytical prescription suggest that the column densities of HI are likely built up in large-scale warm neutral medium and cold neutral medium (CNM) structures correlated in density over ~20 pc and ~10 pc, respectively, while those of H2 are built up in CNM structures between ~3 and ~10 pc. Conclusions. Combining the chemical and statistical information contained in the observations of HI and H2 sheds new light on the study of the diffuse matter. Applying this new tool to several atomic and molecular species is a promising perspective to understanding the effects of turbulence, magnetic field, thermal instability, and gravity on the formation and evolution of molecular clouds.

Context.We assess statistical inversion of asteroid rotation periods, pole orientations, shapes, and phase curve parameters from photometric lightcurve observations, here sparse data from the ESAGaiaspace mission (Data Release 2) or dense and sparse data from ground-based observing programs.Aims.Assuming general convex shapes, we develop inverse methods for characterizing the Bayesian a posteriori probability density of the parameters (unknowns). We consider both random and systematic uncertainties (errors) in the observations, and assign weights to the observations with the help of Bayesian a priori probability densities.Methods.For general convex shapes comprising large numbers of parameters, we developed a Markov-chain Monte Carlo sampler (MCMC) with a novel proposal probability density function based on the simulation of virtual observations giving rise to virtual least-squares solutions. We utilized these least-squares solutions to construct a proposal probability density for MCMC sampling. For inverse methods involving triaxial ellipsoids, we update the uncertainty model for the observations.Results.We demonstrate the utilization of the inverse methods for three asteroids withGaiaphotometry from Data Release 2: (21) Lutetia, (26) Proserpina, and (585) Bilkis. First, we validated the convex inverse methods using the combined ground-based andGaiadata for Lutetia, arriving at rotation and shape models in agreement with those derived with the help of Rosetta space mission data. Second, we applied the convex inverse methods to Proserpina and Bilkis, illustrating the potential of theGaiaphotometry for setting constraints on asteroid light scattering as a function of the phase angle (the Sun-object-observer angle). Third, with the help of triaxial ellipsoid inversion as applied toGaiaphotometry only, we provide additional proof that the absoluteGaiaphotometry alone can yield meaningful photometric slope parameters. Fourth, for (585) Bilkis, we report, with 1-σuncertainties, a refined rotation period of (8.5750559 ± 0.0000026) h, pole longitude of 320.6° ± 1.2°, pole latitude of − 25.6° ± 1.7°, and the first shape model and its uncertainties from convex inversion.Conclusions.We conclude that the inverse methods provide realistic uncertainty estimators for the lightcurve inversion problem and that theGaiaphotometry can provide an asteroid taxonomy based on the phase curves.

Predictions of solar cycle 24 obtained from advection-dominated and diffusion-dominated kinematic dynamo models are different if the Babcock-Leighton mechanism is the only source of the poloidal field. Yeates et al. (2008) argue that the discrepancy arises due to different memories of the solar dynamo for advection- and diffusion-dominated solar convection zones. We aim to investigate the differences in solar cycle memory obtained from advection-dominated and diffusion-dominated kinematic solar dynamo models. Specifically, we explore whether the inclusion of Parker's mean-field $\alpha$ effect, in addition to the Babcock-Leighton mechanism, has any impact on the memory of the solar cycle. We used a kinematic flux transport solar dynamo model where poloidal field generation takes place due to both the Babcock-Leighton mechanism and the mean-field $\alpha$ effect. We additionally considered stochastic fluctuations in this model and explored cycle-to-cycle correlations between the polar field at the minima and toroidal field at cycle maxima. Solar dynamo memory is always limited to only one cycle in diffusion-dominated dynamo regimes while in advection-dominated regimes the memory is distributed over a few solar cycles. However, the addition of a mean-field alpha effect reduces the memory of the solar dynamo to within one cycle in the advection-dominated dynamo regime when there are no fluctuations in the mean-field $\alpha$ effect. When fluctuations are introduced in the mean-field poloidal source a more complex scenario is evident, with very weak but significant correlations emerging across a few cycles. Our results imply that inclusion of a mean-field alpha effect in the framework of a flux transport Babcock-Leighton dynamo model leads to additional complexities that may impact memory and predictability of predictive dynamo models of the solar cycle., Comment: Accepted in Astronomy and Astrophysics, 12 pages, 5 figures

Context. Radio-based techniques allow for a meteor detection of 24 h. Electromagnetic waves are scattered by the electrons produced by the ablated species colliding with the incoming air. As the electrons dissipate in the trail, the received signal decays. The interpretation of these measurements entails complex physical modelling of the flow. Aims. In this work, we present a procedure to compute extensive meteor trails in the rarefied segment of the trajectory. This procedure is a general and standalone methodology, which provides meteor physical parameters at given trajectory conditions, without the need to rely on phenomenological lumped models. Methods. We started from fully kinetic simulations of the evaporated gas that describe the nonequilibrium in the flow and the ionisation collisions experienced by metals in their encounter with air molecules. These simulations were employed as initial conditions for performing detailed chemical and multicomponent diffusion calculations of the extended trail, in order to study the processes which lead to the extinction of the plasma. In particular, we focused on the evolution of the trail generated by a 1 mm meteoroid flying at 32 km s−1, above 80 km. We retrieved the ambipolar diffusion coefficient and the electron line density and compared the outcome of our computations with classical results and observational fittings. Finally, the electron field was employed to estimate the resulting reflected signal, using classical radio-echo theory for underdense meteors. Results. A global and constant diffusion coefficient is sufficient to reproduce numerical profiles. A good agreement is found when we compare the extracted diffusion coefficients with theory and observations.

The present study aims at studying the flow and field produced by a stellar radiative zone which is initially made to rotate differentially in the presence of a large-scale poloidal magnetic field threading the whole domain. We focus both on the axisymmetric configurations produced by the initial winding-up of the magnetic field lines and on the possible instabilities of those configurations. We aim in particular at assessing the role of the stratification at stabilising the system. We perform 2D and 3D global Boussinesq numerical simulations started from an initial radial or cylindrical differential rotation and a large-scale poloidal magnetic field. Under the conditions of a large rotation frequency compared to the Alfv\'en frequency, a magnetic configuration strongly dominated by its toroidal component is built. The parameters of the simulations are chosen to respect the ordering of time scales of a typical stellar radiative zone. In this framework, the axisymmetric evolution is studied by varying the relative effects of the thermal diffusion, the Br\"unt-V\"ais\"al\"a frequency, the rotation and the initial poloidal field strength. We find that the axisymmetric state only depends on $t_{es}/t_{Ap}$, the ratio between the Eddington-Sweet circulation time scale and the Alfv\'en time scale. A scale analysis of the Boussinesq equations allows us to recover this result. In the cylindrical case, a magneto-rotational instability develops when the thermal diffusivity is sufficiently high to enable the favored wavenumbers to be insensitive to the effects of the stable stratification. In the radial case, the magneto-rotational instability is driven by the latitudinal shear created by the back-reaction of the Lorentz force on the flow. Increasing the level of stratification then leaves the growth rate of the instability mainly unaffected while its horizontal length scale grows., Comment: 20 pages, 13 figures, accepted for publication in A&A

Many extrasolar systems possessing planets in mean-motion resonance or resonant chain have been discovered to date. The transit method coupled with transit timing variation analysis provides an insight into the physical and orbital parameters of the systems, but suffers from observational limitations. When a (near-)resonant planetary system resides in the dynamical neighbourhood of a stable periodic orbit, its long-term stability, and thus survival, can be guaranteed. We use the intrinsic property of the periodic orbits, namely their linear horizontal and vertical stability, to validate or further constrain the orbital elements of detected two-planet systems. We computed the families of periodic orbits in the general three-body problem for several two-planet Kepler and K2 systems. The dynamical neighbourhood of the systems is unveiled with maps of dynamical stability. Additional validations or constraints on the orbital elements of K2-21, K2-24, Kepler-9, and (non-coplanar) Kepler-108 near-resonant systems were achieved. While a mean-motion resonance locking protects the long-term evolution of the systems K2-21 and K2-24, such a resonant evolution is not possible for the Kepler-9 system, whose stability is maintained through an apsidal anti-alignment. For the Kepler-108 system, we find that the stability of its mutually inclined planets could be justified either solely by a mean-motion resonance, or in tandem with an inclination-type resonance. Going forward, dynamical analyses based on periodic orbits could yield better constrained orbital elements of near-resonant extrasolar systems when performed in parallel to the fitting of the observational data., Accepted for publication in A&A

Clouds are ubiquitous in exoplanet atmospheres and represent a challenge for the model interpretation of their spectra. Complex cloud models are too numerically costly for generating a large number of spectra, while more efficient models may be too strongly simplified. We aim to constrain the atmospheric properties of the directly imaged planet HR 8799e with a free retrieval approach. We use our radiative transfer code petitRADTRANS for generating spectra, which we couple to the PyMultiNest tool. We added the effect of multiple scattering which is important for treating clouds. Two cloud model parameterizations are tested: the first incorporates the mixing and settling of condensates, the second simply parameterizes the functional form of the opacity. In mock retrievals, using an inadequate cloud model may result in atmospheres that are more isothermal and less cloudy than the input. Applying our framework on observations of HR 8799e made with the GPI, SPHERE and GRAVITY, we find a cloudy atmosphere governed by disequilibrium chemistry, confirming previous analyses. We retrieve that ${\rm C/O}=0.60_{-0.08}^{+0.07}$. Other models have not yet produced a well constrained C/O value for this planet. The retrieved C/O values of both cloud models are consistent, while leading to different atmospheric structures: cloudy, or more isothermal and less cloudy. Fitting the observations with the self-consistent Exo-REM model leads to comparable results, while not constraining C/O. With data from the most sensitive instruments, retrieval analyses of directly imaged planets are possible. The inferred C/O ratio of HR 8799e is independent of the cloud model and thus appears to be a robust. This C/O is consistent with stellar, which could indicate that the HR 8799e formed outside the CO$_2$ or CO iceline. As it is the innermost planet of the system, this constraint could apply to all HR 8799 planets., Comment: Accepted for publication in A&A, 28 pages, 13 figures, updated author list

Context. Extreme super-solar abundances of lithium and beryllium have been reported in recent years for classical novae based on absorption lines in ultraviolet and optical spectra during the optically thick stages, but these findings have not been compared with spectrum syntheses of the ejecta. Aims. We present a grid of nova ejecta models calculated with PHOENIX aimed at simulating the reported Li I and Be II features with super-solar abundances. Methods. We computed a sequence of models, finely exploring the parameter space of effective temperature, ejecta expansion velocity, and Li and Be overabundances. Results. Regardless of temperature and expansion velocity, the synthetic spectra for large Li and Be overabundances strongly disagree with those presented in recent literature. Assuming a wide range of Be overabundances (factors of 100 to 10 000 relative to solar), we predict a much stronger spectroscopic feature at Be II 3130 Å than those so far observed. A similar overabundance for Li would instead result in a barely observable change in the emitted flux at Li I 6709 Å. The observed extended absorption feature at 3131 Å reported in V838 Her and other novae appears even in zero-Be models with only solar abundances (which for novae are underestimates). Conclusions. The computed spectra do not support the lithium and beryllium abundances, and caution is warranted in the interpretation of the phenomenology.

Context. A 1D description of stellar dynamics is at the basis of stellar evolution modeling. Designed to investigate open problems in stellar evolution, the MUltidimensional Stellar Implicit Code expands a realistic 1D profile of a star’s internal structure to examine the interior dynamics of a specific star through either 2D or 3D hydrodynamic simulations. Aims. Extending our recent studies of 2D stellar convection to 3D stellar convection, we aim to compare 3D hydrodynamic simulations to identically set-up 2D simulations, for a realistic pre-main sequence star. Methods. We compare statistical quantities related to convective flows including: average velocity, vorticity, local enstrophy, and penetration depth beneath a convection zone. These statistics were produced during stationary, steady-state compressible convection in the star’s convection zone. Results. Our simulations confirm the common result that 2D simulations of stellar convection have a higher magnitude of velocity on average than 3D simulations. Boundary conditions and the extent of the spherical shell can affect the magnitude and variability of convective velocities. The difference between 2D and 3D velocities is dependent on these background points; in our simulations this can have an effect as large as the difference resulting from the dimensionality of the simulation. Nevertheless, radial velocities near the convective boundary are comparable in our 2D and 3D simulations. The average local enstrophy of the flow is lower for 2D simulations than for 3D simulations, indicating a different shape and structuring of 3D stellar convection. We performed a statistical analysis of the depth of convective penetration below the convection zone using the model proposed in our recent study (Pratt et al. 2017, A&A, 604, A125). That statistical model was developed based on 2D simulations, which allowed us to examine longer times and higher radial resolution than are possible in 3D. Here, we analyze the convective penetration in 3D simulations, and compare the results to identically set-up 2D simulations. In 3D simulations, the penetration depth is as large as the penetration depth calculated from 2D simulations.

The origin of the reservoirs of water on Earth is debated. The Earth's crust may contain at least three times more water than the oceans. This crust water is found in the form of phyllosilicates, whose origin probably differs from that of the oceans. We test the possibility to form phyllosilicates in protoplanetary disks, which can be the building blocks of terrestrial planets. We developed an exploratory rate-based warm surface chemistry model where water from the gas-phase can chemisorb on dust grain surfaces and subsequently diffuse into the silicate cores. We apply the phyllosilicate formation model to a zero-dimensional chemical model and to a 2D protoplanetary disk model (ProDiMo). The disk model includes in addition to the cold and warm surface chemistry continuum and line radiative transfer, photoprocesses (photodissociation, photoionization, and photodesorption), gas-phase cold and warm chemistry including three-body reactions, and detailed thermal balance. Despite the high energy barrier for water chemisorption on silicate grain surfaces and for diffusion into the core, the chemisorption sites at the surfaces can be occupied by a hydroxyl bond (-OH) at all gas and dust temperatures from 80 to 700 K for a gas density of 2E4 cm^-3. The chemisorption sites in the silicate cores are occupied at temperatures between 250 and 700 K. At higher temperatures thermal desorption of chemisorbed water occurs. The occupation efficiency is only limited by the maximum water uptake of the silicate. The timescales for complete hydration are at most 1E5 years for 1 mm radius grains at a gas density of 1E8 cm^-3. Phyllosilicates can be formed on dust grains at the dust coagulation stage in protoplanetary disks within 1 Myr. It is however not clear whether the amount of phyllosilicate formed by warm surface chemistry is sufficient compared to that found in Solar System objects., Comment: accepted to A&A

Mostly multiband photometric transit observations have been used so far to retrieve broadband transmission spectra of transiting exoplanets in order to study their atmosphere. An alternative method has been proposed and has only been used once to recover transmission spectra using chromatic Rossiter-McLaughlin observations. Stellar activity has been shown to potentially imitate narrow and broadband features in the transmission spectra retrieved from multiband photometric observations; however, there has been no study regarding the influence of stellar activity on the retrieved transmission spectra through chromatic Rossiter-McLaughlin. In this study with the modified SOAP3.0 tool, we consider different types of stellar activity features (spots and plages), and we generated a large number of realistic chromatic Rossiter-McLaughlin curves for different types of planets and stars. We were then able to retrieve their transmission spectra to evaluate the impact of stellar activity on them. We find that chromatic Rossiter-McLaughlin observations are also not immune to stellar activity, which can mimic broadband features, such as Rayleigh scattering slope, in their retrieved transmission spectra. We also find that the influence is independent of the planet radius, orbital orientations, orbital period, and stellar rotation rate. However, more general simulations demonstrate that the probability of mimicking strong broadband features is lower than 25% and that can be mitigated by combining several Rossiter-McLaughlin observations obtained during several transits. © ESO 2020., M.O. acknowledges the support of the Deutsche Forschungs-gemeinschft (DFG) priority program SPP 1992 “Exploring the Diversity of Extrasolar Planets (RE 1664/17-1)”. S.B. and M.O. also acknowledge the support of the FCT/DAAD bilateral grant 2019 (DAAD ID: 57453096). M.O and N.S. were also supported by Fundação para a Ciência e a Tecnologia (FCT, Portugal) /MCTES through national funds by FEDER through COMPETE2020 by these grants: UID/FIS/04434/2019 & POCI-01-0145-FEDER-028953 and PTDC/FIS-AST/32113/2017 & PTDC/FIS-AST/28953/2017 & POCI-01-0145-FEDER-032113. We would like to thank the anonymous referee for insightful suggestions, which added significantly to the clarity of this paper.

Context. As proven by measurements at Uranus and Neptune, the magnetic dipole axis and planetary spin axis can be off by a large angle exceeding 45°. The magnetosphere of such an (exo-)planet is highly variable over a one-day period and it does potentially exhibit a complex magnetic tail structure. The dynamics and shape of rotating magnetospheres do obviously depend on the planet’s characteristics but also, and very substantially, on the orientation of the planetary spin axis with respect to the impinging, generally highly supersonic, stellar wind. Aims. On its orbit around the Sun, the orientation of Uranus’ spin axis with respect to the solar wind changes from quasi-perpendicular (solstice) to quasi-parallel (equinox). In this paper, we simulate the magnetosphere of a fictitious Uranus-like planet plunged in a supersonic plasma (the stellar wind) at equinox. A simulation with zero wind velocity is also presented in order to help disentangle the effects of the rotation from the effects of the supersonic wind in the structuring of the planetary magnetic tail. Methods. The ideal magnetohydrodynamic (MHD) equations in conservative form are integrated on a structured spherical grid using the Message-Passing Interface-Adaptive Mesh Refinement Versatile Advection Code (MPI-AMRVAC). In order to limit diffusivity at grid level, we used background and residual decomposition of the magnetic field. The magnetic field is thus made of the sum of a prescribed time-dependent background field B0(t) and a residual field B1(t) computed by the code. In our simulations, B0(t) is essentially made of a rigidly rotating potential dipole field. Results. The first simulation shows that, while plunged in a non-magnetised plasma, a magnetic dipole rotating about an axis oriented at 90° with respect to itself does naturally accelerate the plasma away from the dipole around the rotation axis. The acceleration occurs over a spatial scale of the order of the Alfvénic co-rotation scale r*. During the acceleration, the dipole lines become stretched and twisted. The observed asymptotic fluid velocities are of the order of the phase speed of the fast MHD mode. In two simulations where the surrounding non-magnetised plasma was chosen to move at supersonic speed perpendicularly to the rotation axis (a situation that is reminiscent of Uranus in the solar wind at equinox), the lines of each hemisphere are symmetrically twisted and stretched as before. However, they are also bent by the supersonic flow, thus forming a magnetic tail of interlaced field lines of opposite polarity. Similarly to the case with no wind, the interlaced field lines and the attached plasma are accelerated by the rotation and also by the transfer of kinetic energy flux from the surrounding supersonic flow. The tailwards fluid velocity increases asymptotically towards the externally imposed flow velocity, or wind. In one more simulation, a transverse magnetic field, to both the spin axis and flow direction, was added to the impinging flow so that magnetic reconnection could occur between the dipole anchored field lines and the impinging field lines. No major difference with respect to the no-magnetised flow case is observed, except that the tailwards acceleration occurs in two steps and is slightly more efficient. In order to emphasise the effect of rotation, we only address the case of a fast-rotating planet where the co-rotation scale r* is of the order of the planetary counter-flow magnetopause stand-off distance rm. For Uranus, r*≫ rm and the effects of rotation are only visible at large tailwards distances r ≫ rm.

We present a machine-learning photometric redshift analysis of the Kilo-Degree Survey Data Release 3, using two neural-network based techniques: ANNz2 and MLPQNA. Despite limited coverage of spectroscopic training sets, these ML codes provide photo-zs of quality comparable to, if not better than, those from the BPZ code, at least up to zphot, A&A, in press. Data available from the KiDS website http://kids.strw.leidenuniv.nl/DR3/ml-photoz.php#annz2

Accepted: 2018-04-06, 資料番号: SA1180057000

The X-ray luminosities of T Tauri stars are about two to four orders of magnitude higher than the luminosity of the contemporary Sun. As these stars are born in clusters, their disks are not only irradiated by their parent star but also by an X-ray background field produced by the cluster members. We aim to quantify the impact of X-ray background fields produced by young embedded clusters on the chemical structure of disks. Further, we want to investigate the importance of the dust for X-ray radiative transfer in disks. We present a new X-ray radiative transfer module for the radiation thermo-chemical disk code ProDiMo, which includes X-ray scattering and absorption by both the gas and dust component. For the X-ray radiative transfer, we consider irradiation by the star and by X-ray background fields. To study the impact of X-rays on the chemical structure of disks we use the well-established disk ionization tracers N2H+ and HCO+. For evolved dust populations, X-ray opacities are mostly dominated by the gas; only for photon energies $E\gtrsim5-10\,$keV, dust opacities become relevant. Consequently, the local disk X-ray radiation field is only affected in dense regions close to the disk midplane. X-ray background fields can dominate the local X-ray disk ionization rate for disk radii $r\gtrsim20\,$au. However, the N2H+ and HCO+ column densities are only significantly affected in case of low cosmic-ray ionization rates, or if the background flux is at least a factor of ten higher than the flux level expected for clusters typical for the solar vicinity. Observable signatures of X-ray background fields in low-mass star-formation regions, like Taurus, are only expected for cluster members experiencing a strong X-ray background field. For the majority of the cluster members, the X-ray background field has only little impact on the disk chemical structure., 16 pages, 12 figures, accepted for publication in A&A