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To understand massive star formation requires study of its initial conditions. Two massive starless core candidates, C1-N & C1-S, have been detected in IRDC G028.37+00.07 in N2D+(3-2) with ALMA. From their line widths, either the cores are subvirial and are thus young structures on the verge of near free-fall collapse, or they are threaded by ~1 mGB-fields that help support them in near virial equilibrium and potentially have older ages. We modeled the deuteration rate of N2H+ to constrain collapse rates of the cores. First, to measure their current deuterium fraction, DN2H+frac = [N2D+]/[N2H+], we observed multiple transitions of N2H+ and N2D+ with CARMA, SMA, JCMT, NRO 45m and IRAM 30m, to complement the ALMA data. For both cores we derived DN2H+ frac ~ 0.3, several orders of magnitude above the cosmic [D]/[H] ratio. We then carried out chemodynamical modeling, exploring how collapse rate relative to free-fall, αff, affects the level of DN2H+ frac that is achieved from a given initial condition. To reach the observed DN2H+ frac, most models require slow collapse with αff~0.1, i.e., ~1/10th of free-fall. This makes it more likely that the cores have been able to reach a near virial equilibrium state and we predict that strong B-fields will eventually be detected. The methods developed here will be useful for measurement of the pre-stellar core mass function. [ABSTRACT FROM AUTHOR]

Almost two decades ago, the discovery of a handful of Infrared Dark Clouds (IRDCs) heralded a new avenue for exploring the elusive early phases of massive star formation (Egan et al. 1998). Since then tremendous progress has been made in our understanding of these clouds. It is now well established that IRDCs are ubiquitous and harbor a broad spectrum of star forming stages. Thus studies of star formation and IRDCs are intricately tied together. I summarize some of the recent progress in understanding the magnetic field structure and gas dynamics in IRDCs in the context of high-mass star formation formation. [ABSTRACT FROM AUTHOR]

I review (1) Physics of Star Formation & Open Questions; (2) Structure & Dynamics of Star-Forming Clouds & Young Clusters; (3) Star Formation Rates: Observations & Theoretical Implications. [ABSTRACT FROM AUTHOR]

Context. The convective envelopes of cool main-sequence stars harbour magnetic fields with a complex global and local structure. These fields affect the near-surface convection and the outer stellar atmospheres in many ways and are responsible for the observable magnetic activity of stars. Aims.Our aim is to understand the local structure in unipolar regions with moderate average magnetic flux density. These correspond to plage regions covering a substantial fraction of the surface of the Sun (and likely also the surface of other Sun-like stars) during periods of high magnetic activity. Methods.We analyse the results of 18 local-box magnetohydrodynamics simulations covering the upper layers of the convection zones and the photospheres of cool main-sequence stars of spectral types F to early M. The average vertical field in these simulations ranges from 20 to 500 G. Results.We find a substantial variation of the properties of the surface magnetoconvection between main-sequence stars of different spectral types. As a consequence of a reduced efficiency of the convective collapse of flux tubes, M dwarfs lack bright magnetic structures in unipolar regions of moderate field strength. The spatial correlation between velocity and the magnetic field as well as the lifetime of magnetic structures and their sizes relative to the granules vary significantly along the model sequence of stellar types. [ABSTRACT FROM AUTHOR]

Context. Theoretical arguments indicate that close-in terrestial exoplanets may have weak magnetic fields, especially in the case of planets more massive than Earth (super-Earths). Planetary magnetic fields, however, constitute one of the shielding layers that protect the planet against cosmic-ray particles. In particular, a weak magnetic field results in a high flux of Galactic cosmic rays that extends to the top of the planetary atmosphere. Aims. We wish to quantify the flux of Galactic cosmic rays to an exoplanetary atmosphere as a function of the particle energy and of the planetary magnetic moment. Methods. We numerically analyzed the propagation of Galactic cosmic-ray particles through planetary magnetospheres. We evaluated the efficiency of magnetospheric shielding as a function of the particle energy (in the range 16 MeV < E < 524 GeV) and as a function of the planetary magnetic field strength (in the range 0 Af® < M < 10 Af®). Combined with the flux outside the planetary magnetosphere, this gives the cosmic-ray energy spectrum at the top of the planetary atmosphere as a function of the planetary magnetic moment. Results. We find that the particle flux to the planetary atmosphere can be increased by more than three orders of magnitude in the absence of a protecting magnetic field. For a weakly magnetized planet ( Af = 0.05 Af® ), only particles with energies below 512 MeV are at least partially shielded. For a planet with a magnetic moment similar to that of Earth, this limit increases to to 32 GeV, whereas for a strongly magnetized planet (Af = 10.0 Af®), partial shielding extends up to 200 GeV, Over the parameter range we studied, strong shielding does not occur for weakly magnetized planets. For a planet with a magnetic moment similar to that of Earth, particles with energies below 512 MeV are strongly shielded, and for strongly magnetized planets, this limit increases to 10 GeV, Conclusions. We find that magnetic shielding strongly controls the number of cosmic-ray particles reaching the planetary atmosphere. The implications of this increased particle flux are discussed in a companion article. [ABSTRACT FROM AUTHOR]

Context. Detailed modelling of the high-energy emission from gamma-ray binaries has been propounded as a path to pulsar wind physics. Aims. Fulfilling this ambition requires a coherent model of the flow and its emission in the region where the pulsar wind interacts with the stellar wind of its companion. Methods. We have developed a code that follows the evolution and emission of electrons in the shocked pulsar wind based on inputs from a relativistic hydrodynamical simulation. The code is used to model the well-documented spectral energy distribution and orbital modulations from LS 5039. Results. The pulsar wind is fully confined by a bow shock and a back shock. The particles are distributed into a narrow Maxwellian, emitting mostly GeV photons, and a power law radiating very efficiently over a broad energy range from X-rays to TeV gamma rays. Most of the emission arises from the apex of the bow shock. Doppler boosting shapes the X-ray and very high energy (VHE) lightcurves, constraining the system inclination to i « 35°. There is tension between the hard VHE spectrum and the level of X-ray to MeV emission, which requires differing magnetic field intensities that are hard to achieve with constant magnetisation a and Lorentz factor rp of the pulsar wind. Our best compromise implies a « 1 and rp « 5 x 10з, so respectively higher and lower than the typical values in pulsar wind nebulae. Conclusions. The high value of a derived here, where the wind is confined close to the pulsar, supports the classical picture that has pulsar winds highly magnetised at launch. However, such magnetisations will require that further investigations are based on relativistic MHD simulations. [ABSTRACT FROM AUTHOR]

We present the first study of the relationship between the column density distribution of molecular clouds within nearby Galactic spiral arms and their evolutionary status as measured from their stellar content. We analyze a sample of 195 molecular clouds located at distances below 5.5 kpc, identified from the ATLASGAL 870 μm data. We define three evolutionary classes within this sample: starless clumps, star-forming clouds with associated young stellar objects, and clouds associated with Hii regions. We find that the N(H2) probability density functions (N-PDFs) of these three classes of objects are clearly different: the N-PDFs of starless clumps are narrowest and close to log-normal in shape, while star-forming clouds and Hii regions exhibit a power-law shape over a wide range of column densities and log-normal-like components only at low column densities. We use the N-PDFs to estimate the evolutionary time-scales of the three classes of objects based on a simple analytic model from literature. Finally, we show that the integral of the N-PDFs, the dense gas mass fraction, depends on the total mass of the regions as measured by ATLASGAL: more massive clouds contain greater relative amounts of dense gas across all evolutionary classes [ABSTRACT FROM AUTHOR]

Context. An equation of state (EoS) is a relation between thermodynamic state variables and it is essential for closing the set of equations describing a fluid system. Although an ideal EoS with a constant adiabatic index Γ is the preferred choice owing to its simplistic implementation, many astrophysical fluid simulations may benefit from a more sophisticated treatment that can account for diverse chemical processes. Aims. In the present work we first review the basic thermodynamic principles of a gas mixture in terms of its thermal and caloric EoS by including effects like ionization, dissociation, and temperature dependent degrees of freedom such as molecular vibrations and rotations. The formulation is revisited in the context of plasmas that are either in equilibrium conditions (local thermodynamicor collisional excitation-equilibria) or described by non-equilibrium chemistry coupled to optically thin radiative cooling. We then present a numerical implementation of thermally ideal gases obeying a more general caloric EoS with non-constant adiabatic index in Godunov-type numerical schemes. Methods. We discuss the necessary modifications to the Riemann solver and to the conversion between total energy and pressure (or vice versa) routinely invoked in Godunov-type schemes. We then present two different approaches for computing the EoS. The first employs root-finder methods and it is best suited for EoS in analytical form. The second is based on lookup tables and interpolation and results in a more computationally effcient approach, although care must be taken to ensure thermodynamic consistency. Results. A number of selected benchmarks demonstrate that the employment of a non-ideal EoS can lead to important differences in the solution when the temperature range is 500-104 K where dissociation and ionization occur. The implementation of selected EoS introduces additional computational costs although the employment of lookup table methods (when possible) can significantly reduce the overhead by a factor of ∼3-4. [ABSTRACT FROM AUTHOR]

Context. Angular momentum transport in accretion discs is often believed to be due to magnetohydrodynamic turbulence mediated by the magnetorotational instability (MRI). Despite an abundant literature on the MRI, the parameters governing the saturation amplitude of the turbulence are poorly understood and the existence of an asymptotic behaviour in the Ohmic diffusion regime has not been clearly established. Aims. We investigate the properties of the turbulent state in the small magnetic Prandtl number limit. Since this is extremely computationally expensive, we also study the relevance and range of applicability of the most common subgrid scale models for this problem. Methods. Unstratified shearing box simulations are performed both in the compressible and incompressible limits, with a resolution up to 800 cells per disc scale height. This is the highest resolution ever attained for a simulation of MRI turbulence. Different magnetic field geometry and a wide range of dimensionless dissipative coefficients are considered. We also systematically investigate the relevance of using large eddy simulations (LES) in place of direct numerical simulations. Results. In the presence of a mean magnetic field threading the domain, angular momentum transport converges to a finite value in the small Pm limit. When the mean vertical field amplitude is such that β (the ratio between the thermal and magnetic pressure) equals 103, we find α ∼ 3.2 × 10-2 when Pm approaches zero. In the case of a mean toroidal field for which α = 100, we find α ∼ 1.8 × 10-2 in the same limit. Implicit LES and the Chollet-Lesieur closure model both reproduce these results for the α parameter and the power spectra. A reduction in computational cost by a factor of at least 16 (and up to 256) is achieved when using such methods. Conclusions. MRI turbulence operates efficiently in the small Pm limit provided there is a mean magnetic field. Implicit LES offers a practical and efficient means of investigation of this regime but should be used with care, particularly in the case of a vertical field. The Chollet-Lesieur closure model is perfectly suited for simulations done with a spectral code. [ABSTRACT FROM AUTHOR]

We present a mass estimate of the Planck-discovered cluster PLCK G100.2-30.4, derived from a weak lensing analysis of deep Subaru griz images. We perform a careful selection of the background galaxies using the multi-band imaging data, and undertake the weak lensing analysis on the deep (1 h) r-band image. The shape measurement is based on the Kaiser-Squires-Broadhurst algorithm; we adopt the PSFex software to model the point spread function (PSF) across the field and correct for this in the shape measurement. The weak lensing analysis is validated through extensive image simulations. We compare the resulting weak lensing mass profile and total mass estimate to those obtained from our re-analysis of XMM-Newton observations, derived under the hypothesis of hydrostatic equilibrium. The total integrated mass profiles agree remarkably well, within 1σ across their common radial range. A mass M500 ∼ 7 × 1014 M☉ is derived for the cluster from our weak lensing analysis. Comparing this value to that obtained from our reanalysis of XMM-Newton data, we obtain a bias factor of (1-b) = 0.8 ± 0.1. This is compatible within 1σ with the value of (1-b) obtained in Planck 2015 from the calibration of the bias factor using newly available weak lensing reconstructed masses. [ABSTRACT FROM AUTHOR]

Context. Photodissociation regions (PDRs) contain a large portion of all of the interstellar matter in galaxies. Classical examples include the boundaries between ionized regions and molecular clouds in regions of massive star-formation, marking the point where all of the photons that are energetic enough to ionize hydrogen have been absorbed. Aims. To determine the physical properties of the PDRs associated with the star-forming regions IRAS 23133+6050 and S 106 and present them in the context of other Galactic PDRs associated with massive star-forming regions. Methods. We employ Herschel PACS and SPIRE spectroscopic observations to construct a full 55-650 μm spectrum of each object from which we measure the PDR cooling lines, other fine- structure lines, CO lines, and the total far-infrared flux. These measurements (and combinations thereof) are then compared to standard PDR models. Subsequently, detailed numerical PDR models are compared to these predictions, yielding additional insight into the dominant thermal processes in the PDRs and their structures. Results. We find that the PDRs of each object are very similar and can be characterized by a two-phase PDR model with a very dense, highly UV irradiated phase (n ∼ 106 cm-3, G0 ∼ 105) interspersed within a lower density, weaker radiation field phase (n ∼ 104 cm-3, G0 ∼ 104). We employed two different numerical models to investigate the data. We first used RADEX models to fit the peak of the 12CO ladder, which in conjunction with the properties derived, yielded a temperature of around 300 K. Subsequent numerical modeling with a full PDR model revealed that the dense phase has a filling factor of around 0.6 in both objects. The shape of the 12CO ladder was consistent with these components, with heating dominated by grain photoelectric heating. An extra excitation component for the heightest-J lines (J > 20) is required for S 106. [ABSTRACT FROM AUTHOR]

Context. The observations of the synchrotron emission from the supernova remnant (SNR) SN 1006 indicate a bilateral morphology, which is mainly characterized by two opposite bright limbs with knots and filaments along the boundary of the remnant. The morphology is not strictly circular with disturbances and bumps at the rim of the remnant. For instance, one big bump is located on the northeastern (NE) limb and several relatively small ones on the southwestern (SW) limb on the detected morphologies from the observations in the radio and X-rays. Aims. The generation of the asymmetric morphology of non-thermal X-rays for SN 1006 is investigated through three-dimensional (3D) magnetohydrodynamic simulations. Moreover, the density distribution of the ambient material can be investigated by comparing the resulting synchrotron morphology with the detected images of the hard X-ray emission. Methods. First, the remnant was simulated as a supernova explosion evolved in a turbulent plasma with a relatively small amplitude for the turbulence. In the model, several spherical cavities with lower densities compared with the background plasma were employed to reproduce the detected bumps at the bright limbs. Second, the e ect of the modification on the morphology of the remnant due to efficient cosmic ray acceleration was investigated by adopting a lower adiabatic index. Results. If we assume that the injection efficiency of electrons into the diffusive shock acceleration process at the shock follows the quasi-parallel scenario, the hard X-ray morphology of SN 1006 with bumps in the bright NE and SW regions can be generally reproduced by employing cavities in the background medium in the non-modification scenario. Furthermore, in the modification case with a lower adiabatic index of 1:2 for the remnant evolved in a uniform medium, bubbles can be produced with relatively small extensions on the SNR boundary that result from the hydrodynamical instabilities that in turn overtake the forward shock. Conclusions. The SNR propagating in the turbulent medium with a relatively small amplitude can reproduce the knotty and filamentary morphology of the remnant better than what evolved in the uniform environment; moreover, the big bump on the NE limb can be explained as the result of a lower-density region, which has a radius of about 2.5 pc and a density of about 0.4 times of the general ambient medium, swept by the shock front, and the other smaller ones on the SW limb can either be reproduced with smaller regions with lower densities swept by the shock or be explained as the protrusions in the scenario of the efficient cosmic ray acceleration when the instability fingers effectively overtake the forward shock. [ABSTRACT FROM AUTHOR]

Astrophysical shocks are often collisionless shocks, in which the changes in plasma flow and temperatures across the shock are established not through Coulomb interactions, but through electric and magnetic fields. An open question about collisionless shocks is whether electrons and ions each establish their own post-shock temperature (non-equilibration of temperatures), or whether they quickly equilibrate in the shock region. Here we provide a simple, thermodynamic, relation for the minimum electron-ion temperature ratios that should be expected as a function of Mach number. The basic assumption is that the enthalpy-flux of the electrons is conserved separately, but that all particle species should undergo the same density jump across the shock, in order for the plasma to remain charge neutral. The only form of additional electron heating that we allow for is adiabatic heating, caused by the compression of the electron gas. These assumptions result in an analytic treatment of expected electron-ion temperature ratio that agrees with observations of collisionless shocks: at low sonic Mach numbers, Ms ≲ 2, the electron-ion temperature ratio is close to unity, whereas for Mach numbers above Ms ≈ 60 the electron-ion temperature ratio asymptotically approaches a temperature ratio of Te=Ti = me=/(mi). In the intermediate Mach number range the electron-ion temperature ratio scales as Te/Ti ∝ M-2 s . In addition, we calculate the electron-ion temperature ratios under the assumption of adiabatic heating of the electrons only, which results in a higher electronion temperature ratio, but preserves the Te/Ti ∝ M-2 s scaling. We also show that for magnetised shocks the electron-ion temperature ratio approaches the asymptotic value e=Ti = me=/(mi) for lower magnetosonic Mach numbers (Mms), mainly because for a strongly magnetised shock the sonic Mach number is larger than the magnetosonic Mach number (Mms ≤ Ms). The predicted scaling of the electron-ion temperature ratio is in agreement with observational data for magnetosonic Mach numbers between 2 and 10, but for supernova remnants the relation requires that the inferred Mach numbers for the observations are overestimated, perhaps as a result of upstream heating in the cosmic-ray precursor. In addition to predicting a minimal electron-ion temperature ratio, we also heuristically incorporate ion-electron heat exchange at the shock, quantified with a dimensionless parameter ξ, which is the fraction of the enthalpy-flux difference between electrons and ions that is used for equilibrating the electron and ion temperatures. Comparing the model to existing observations in the solar system and supernova remnants suggests that the data are best described by ξ ≳ 5%, but also provides a hint that the Mach number of some supernova remnant shocks may have been overestimated; perhaps as a result of heating in the cosmic-ray precursor. [ABSTRACT FROM AUTHOR]

Massive primordial halos exposed to moderate UV backgrounds are the potential birthplaces of supermassive black holes. In these halos, an initially isothermal collapse will occur, leading to high accretion rates of ~0.1 M⊙ yr-1. During the collapse, the gas in the interior will turn into a molecular state, and will form accretion disk in order to conserve angular momentum. We consider here the structure of such an accretion disk and the role of viscous heating in the presence of high accretion rates for a central star of 10, 100, and 104 M⊙. Our results show that the temperature in the disk increases considerably due to viscous heating, leading to a transition from the molecular to the atomic cooling phase. We found that the atomic cooling regime may extend out to several 100 AU for a 104 M⊙ central star and that it provides substantial support to stabilize the disk. It therefore favors the formation of a massive central object. The comparison of clump migration and contraction time scales shows that stellar feedback from these clumps may occur during the later stages of the evolution. Overall, viscous heating provides an important pathway to obtain an atomic gas phase within the center of the halo, and helps in the formation of very massive objects. The massive object may collapse to form a massive black hole of about ≥104 M⊙. [ABSTRACT FROM AUTHOR]

Context. Close-in exoplanets interact with their host stars gravitationally as well as via their magnetized plasma outflows. The rich dynamics that arises may result in distinct observable features. Aims. Our objective is to study and classify the morphology of the different types of interaction that can take place between a giant close-in planet (a hot Jupiter) and its host star, based on the physical parameters that characterize the system. Methods. We perform 3D magnetohydrodynamic numerical simulations to model the star-planet interaction, incorporating a star, a hot Jupiter, and realistic stellar and planetary outflows. We explore a wide range of parameters and analyze the flow structures and magnetic topologies that develop. Results. Our study suggests the classification of star-planet interactions into four general types, based on the relative magnitudes of three characteristic length scales that quantify the effects of the planetary magnetic field, the planetary outflow, and the stellar gravitational field in the interaction region. We describe the dynamics of these interactions and the flow structures that they give rise to, which include bow shocks, cometary-type tails, and inspiraling accretion streams. We point out the distinguishing features of each of the classified cases and discuss some of their observationally relevant properties. Conclusions. The magnetized interactions of star-planet systems can be categorized, and their general morphologies predicted, based on a set of basic stellar, planetary, and orbital parameters. [ABSTRACT FROM AUTHOR]

Context. Magnetic fields play an important role in the formation and stabilization of spiral structures in galaxies, but the interaction between interstellar gas and magnetic fields has not yet been understood. In particular, the phenomenon of "magnetic arms" located between material arms is a mystery. Aims. The strength and structure of interstellar magnetic fields and their relation to spiral arms in gas and dust are investigated in the nearby and almost face-on spiral galaxy IC 342. Methods. The total and polarized radio continuum emission of IC 342 was observed with high spatial resolution in four wavelength bands with the Effelsberg and VLA telescopes. At λ6.2 cm the data from both telescopes were combined. I separated thermal and nonthermal (synchrotron) emission components with the help of the spectral index distribution and derived maps of the magnetic field strength, degree of magnetic field order, magnetic pitch angle, Faraday rotation measure, and Faraday depolarization. Results. IC 342 hosts a diffuse radio disk with an intensity that decreases exponentially with increasing radius. The frequency dependence of the scalelength of synchrotron emission indicates energy-dependent propagation of the cosmic-ray electrons, probably via the streaming instability. The equipartition strength of the total field in the main spiral arms is typically 15 µG, that of the ordered field about 5 µG. The total radio emission, observed with the VLA's high resolution, closely follows the dust emission in the infrared at 8 µm (Spitzer telescope) and 22 µm (WISE telescope). The polarized emission is not diffuse, but concentrated in spiral arms of various types: (1) a narrow arm of about 300 pc width, displaced inwards with respect to the eastern arm by about 200 pc, indicating magnetic fields compressed by a density wave; (2) a broad arm of 300-500 pc width around the northern arm with systematic variations in polarized emission, polarization angles, and Faraday rotation measures on a scale of about 2 kpc, indicative of a helically twisted flux tube generated by the Parker instability; (3) a rudimentary magnetic arm in an interarm region in the north-west; (4) several broad spiral arms in the outer galaxy, related to spiral arms in the total neutral gas; (5) short features in the outer south-western galaxy, probably distorted by tidal interaction. Faraday rotation of the polarization angles reveals an underlying regular field of only 0.5 µG strength with a large-scale axisymmetric spiral pattern, probably a signature of a mean-field α-Ω dynamo, and an about 10× stronger field that fluctuates on scales of a few 100 pc. The magnetic field around the bar in the central region of IC 342 resembles that of large barred galaxies; it has a regular spiral pattern with a large pitch angle, is directed outwards, and is opposite to the large-scale regular field in the disk. Polarized emission at λ20.1 cm is strongly affected by Faraday depolarization in the western and northern parts of the galaxy. Helical fields extending from disk to halo may account for this asymmetry. Conclusions. Interstellar magnetic fields interact with the gas and gas flows. Density-wave compression generates polarized radio emission at the inner edge of some spiral arms. Fast MHD density waves can generate coincident spiral arms in gas and magnetic fields in the outer parts of IC 342. Magnetic armsar e offset from the spiral pattern in gas and dust; their generation and development by mean-field dynamo action probably need a spiral pattern that is stable over a few galactic rotation periods, which is probably the case for the galaxy NGC 6946. The mean-field dynamo in IC 342 is slow and weak, probably disturbed by the bar, tidal interaction, or a transient spiral pattern. [ABSTRACT FROM AUTHOR]

Context. Superbubbles and supershells are the channels for transferring mass and energy from the Galactic disk to the halo. Magnetic fields are believed to play a vital role in their evolution. Aims. We study the radio continuum and polarized emission properties of theW4 superbubble to determine its magnetic field strength. Methods. New sensitive radio continuum observations were made atλ6 cm, λ11 cm, and λ21 cm. The total intensity measurements were used to derive the radio spectrum of the W4 superbubble. The linear polarization data were analysed to determine the magnetic field properties within the bubble shells. Results. The observations show a multi-shell structure of the W4 superbubble. A flat radio continuum spectrum that stems from optically thin thermal emission is derived from 1.4 GHz to 4.8 GHz. By fitting a passive Faraday screen model and considering the filling factor fine, we obtain the thermal electron density ne = 1.0/√fne (±5%) cm-3 and the strength of the line-of-sight component of the magnetic field B// = -5.0/√fne (±10%) µG (i.e. pointing away from us) within the western shell of the W4 superbubble. When the known tilted geometry of the W4 superbubble is considered, the total magnetic field Btot in its western shell is greater than 12 µG. The electron density and the magnetic field are lower and weaker in the high-latitude parts of the superbubble. The rotation measure is found to be positive in the eastern shell but negative in the western shell of the W4 superbubble, which is consistent with the case where the magnetic field in the Perseus arm is lifted up from the plane towards high latitudes. Conclusions. The magnetic field strength and the electron density we derived for the W4 superbubble are important parameters for evolution models of superbubbles breaking out of the Galactic plane. [ABSTRACT FROM AUTHOR]

Aims. Magnetic fields play an important role during the formation and evolution of stars. Of particular interest in stellar evolution is what effect they have on the transport angular momentum and mixing of chemical elements along the radial direction in radiative regions. Current theories suggest a dynamo loop as the mechanism responsible for maintaining the magnetic field in the radiative zone. This loop consists of differential rotation on one side and magnetohydrodynamic (MHD) instability the so-called Tayler instability on the other. However, how this might work quantitatively is still an unsettled question, largely because we do not yet understand all the properties of the instability in question. In this paper we explore some properties of the Tayler instability. Methods. We present 3D MHD simulations of purely toroidal and mixed poloidal-toroidal magnetic field configurations to study the behavior of the Tayler instability. For the first time the simultaneous action of rotation and magnetic diffusion are taken into account and the effects of a poloidal field on the dynamic evolution of unstable toroidal magnetic fields is included. Results. In the absence of diffusion, fast rotation (rotation rate, Ω||, compared to Alfvén frequency, ωA,Φ) is able to suppress the instability when the rotation and magnetic axes are aligned and when the radial field strength gradient p< 1.5 (where p = ∂lnB/∂lnϖ and ϖ is the cylindrical radius coordinate). When diffusion is included, this system turns unstable for diffusion dominated and marginally diffusive dominated regions. If the magnetic and rotation axes are perpendicular to each other, Ω, the stabilizing effect induced by the Coriolis force is scale dependent and decreases with increasing wavenumber. In toroidal fields with radial field gradients bigger than p> 1.5, rapid rotation does not suppress the instability but instead introduces a damping factor ωA/ 2Ω|| to the growth rate, in agreement with the analytic predictions. For the mixed poloidal-toroidal fields we find an unstable axisymmetric mode, not predicted analytically, right at the stability threshold for the non-axisymmetric modes; it has been argued that an axisymmetric mode is necessary for the closure of the Tayler-Spruit dynamo loop. [ABSTRACT FROM AUTHOR]

The present work studies the characteristics of an equatorial streamer visible above the east limb of the Sun on March 2008, during the most recent minimum of solar activity. We analysed the visible light coronagraphic images of SOHO/LASCO and the ultraviolet observations in the HI Lyα spectral line obtained by SOHO/UVCS, and exploited the Doppler dimming effect of the coronal Lyα line to derive the outflow velocity profile of the scattering neutral hydrogen atoms in the streamer region. Taking advantage of the synergy between visible light and ultraviolet observations, we were able to determine all the properties of the coronal structure. In particular, the actual extent of the streamer along the line of sight has been evaluated for the first time. In so doing, the solar wind outflow velocity turned out to be the only free parameter in the theoretical modelling of the Lyα intensity. We found nearly static conditions below 3.5 R☉ along the streamer axis, whereas the solar wind flows at velocities from 40 kms-1 to 140 km s-1 in the altitude range 2.5-5.0 R☉ along the southern boundary of the streamer. We also derived the intensity distribution of the F coronal component in the LASCO C2 field of view, by combining total and polarized brightness data. Finally, we investigated the dependence of the Lyα resonant scattering process on the kinetic temperature of the coronal neutral hydrogen atoms and found that the value of this temperature mostly affects the scattering process at low heliocentric distances, where the solar wind flows with low velocity. [ABSTRACT FROM AUTHOR]

Context. In binary star systems, the winds from the two components impact each other, leading to strong shocks and regions of enhanced density and temperature. Potentially habitable circumbinary planets must continually be exposed to these interaction regions. Aims. We study, for the first time, the interactions between winds from low-mass stars in a binary system to show the wind conditions seen by potentially habitable circumbinary planets. Methods. We use the advanced 3D numerical hydrodynamic code Nurgush to model the wind interactions of two identical winds from two solar mass stars with circular orbits and a binary separation of 0.5 AU. As input into this model, we use a 1D hydrodynamic simulation of the solar wind, run using the Versatile Advection Code. We derive the locations of stable and habitable orbits in this system to explore what wind conditions potentially habitable planets will be exposed to during their orbits. Results. Our wind interaction simulations result in the formation of two strong shock waves separated by a region of enhanced density and temperature. The wind-wind interaction region has a spiral shape due to Coriolis forces generated by the orbital motions of the two stars. The stable and habitable zone in this system extends from ∼1.4 AU to ∼2.4 AU. Habitable planets have to pass through strong shock waves several times per orbit and spend a significant amount of time embedded in the higher density matter between the shocks. The enhanced density in the wind-wind interaction region is likely to lead to a 20% decrease in the size of a planet's magnetosphere. Conclusions. Our results indicate that wind-wind interactions are likely to influence the magnetospheres and upper atmospheres of circumbinary planets and could have moderate implications for the development of habitable planetary environments. [ABSTRACT FROM AUTHOR]

Aims. We present an inversion method based on Bayesian analysis to constrain the interior structure of terrestrial exoplanets, in the form of chemical composition of the mantle and core size. Specifically, we identify what parts of the interior structure of terrestrial exoplanets can be determined from observations of mass, radius, and stellar elemental abundances. Methods. We perform a full probabilistic inverse analysis to formally account for observational and model uncertainties and obtain confidence regions of interior structure models. This enables us to characterize how model variability depends on data and associated uncertainties. Results. We test our method on terrestrial solar system planets and find that our model predictions are consistent with independent estimates. Furthermore, we apply our method to synthetic exoplanets up to 10 Earth masses and up to 1.7 Earth radii, and to exoplanet Kepler-36b. Importantly, the inversion strategy proposed here provides a framework for understanding the level of precision required to characterize the interior of exoplanets. Conclusions. Our main conclusions are (1) observations of mass and radius are sufficient to constrain core size; (2) stellar elemental abundances (Fe, Si, Mg) are principal constraints to reduce degeneracy in interior structure models and to constrain mantle composition; (3) the inherent degeneracy in determining interior structure from mass and radius observations does not only depend on measurement accuracies, but also on the actual size and density of the exoplanet. We argue that precise observations of stellar elemental abundances are central in order to place constraints on planetary bulk composition and to reduce model degeneracy. We provide a general methodology of analyzing interior structures of exoplanets that may help to understand how interior models are distributed among star systems. The methodology we propose is sufficiently general to allow its future extension to more complex internal structures including hydrogen- and water-rich exoplanets. [ABSTRACT FROM AUTHOR]

The apparent correlation between the specific star formation rate (sSFR) and total stellar mass (M*) of galaxies is a fundamental relationship indicating how they formed their stellar populations. To attempt to understand this relation, we hypothesize that the relation and its evolution is regulated by the increase in the stellar and gas mass surface density in galaxies with redshift, which is itself governed by the angular momentum of the accreted gas, the amount of available gas, and by self-regulation of star formation. With our model, we can reproduce the specific SFR-M* relations at z ∼ 1-2 by assuming gas fractions and gas mass surface densities similar to those observed for z = 1-2 galaxies. We further argue that it is the increasing angular momentum with cosmic time that causes a decrease in the surface density of accreted gas. The gas mass surface densities in galaxies are controlled by the centrifugal support (i.e., angular momentum), and the sSFR is predicted to increase as, sSFR(z) = (1 + z)3=tH0, as observed (where tH0 is the Hubble time and no free parameters are necessary). In addition, the simple evolution for the star-formation intensity we propose is in agreement with observations of Milky Way-like galaxies selected through abundance matching. At z ≳ 2, we argue that star formation is self-regulated by high pressures generated by the intense star formation itself. The star formation intensity must be high enough to either balance the hydrostatic pressure (a rather extreme assumption) or to generate high turbulent pressure in the molecular medium which maintains galaxies near the line of instability (i.e. Toomre Q ∼ 1). We provide simple prescriptions for understanding these self-regulation mechanisms based on solid relationships verified through extensive study. In all cases, the most important factor is the increase in stellar and gas mass surface density with redshift, which allows distant galaxies to maintain high levels of sSFR. Without a strong feedback from massive stars, such galaxies would likely reach very high sSFR levels, have high star formation efficiencies, and because strong feedback drives outflows, ultimately have an excess of stellar baryons. [ABSTRACT FROM AUTHOR]

Aims. In light of the recent detection of direct evidence for the formation of Kelvin-Helmholtz instabilities in the Orion nebula, we expand upon previous modelling efforts by numerically simulating the shear-flow driven gas and dust dynamics in locations where the HII region and the molecular cloud interact. We aim to directly confront the simulation results with the infrared observations. Methods. To numerically model the onset and full nonlinear development of the Kelvin-Helmholtz instability we take the setup proposed to interpret the observations, and adjust it to a full 3D hydrodynamical simulation that includes the dynamics of gas as well as dust. A dust grain distribution with sizes between 5-250 nm is used, exploiting the gas+dust module of the MPI-AMRVAC code, in which the dust species are represented by several pressureless dust fluids. The evolution of the model is followed well into the nonlinear phase. The output of these simulations is then used as input for the SKIRT dust radiative transfer code to obtain infrared images at several stages of the evolution, which can be compared to the observations. Results. We confirm that a 3D Kelvin-Helmholtz instability is able to develop in the proposed setup, and that the formation of the instability is not inhibited by the addition of dust. Kelvin-Helmholtz billows form at the end of the linear phase, and synthetic observations of the billows show striking similarities to the infrared observations. It is pointed out that the high density dust regions preferentially collect on the flanks of the billows. To get agreement with the observed Kelvin-Helmholtz ripples, the assumed geometry between the background radiation, the billows and the observer is seen to be of critical importance. [ABSTRACT FROM AUTHOR]

Context. Galactic and extragalactic relativistic jets are surrounded by rich environments that are full of moving objects, such as stars and dense medium inhomogeneities. These objects can enter into the jets and generate shocks and non-thermal emission. Aims. We characterize the emitting properties of the downstream region of a standing shock formed due to the interaction of a relativistic jet with an obstacle. We focus on the case of red giants interacting with an extragalactic jet. Methods. We perform relativistic axisymmetric hydrodynamical simulations of a relativistic jet meeting an obstacle of very large inertia. The results are interpreted in the framework of a red giant whose dense and slow wind interacts with the jet of an active galactic nucleus. Assuming that particles are accelerated in the standing shock generated in the jet as it impacts the red giant wind, we compute the non-thermal particle distribution, the Doppler boosting enhancement, and the non-thermal luminosity in gamma rays. Results. The available non-thermal energy from jet-obstacle interactions is potentially enhanced by a factor of ~100 when accounting for the whole surface of the shock induced by the obstacle, instead of just the obstacle section. The observer gamma-ray luminosity, including the effective obstacle size, the flow velocity and Doppler boosting effects, can be ~300 (γj/10)2 times higher than when the emitting flow is assumed at rest and only the obstacle section is considered, where γj is the jet Lorentz factor. For a whole population of red giants inside the jet of an active galactic nucleus, the predicted persistent gamma-ray luminosities may be potentially detectable for a jet pointing approximately to the observer. Conclusions. Obstacles interacting with relativistic outflows, for instance clouds and populations of stars for extragalactic jets, or stellar wind inhomogeneities in microquasar jets and in winds of pulsars in binaries, should be taken into account when investigating the origin of the non-thermal emission from these sources. [ABSTRACT FROM AUTHOR]

Context. Carbon chemistry plays a pivotal role in the interstellar medium (ISM) but even the synthesis of the simplest hydrocarbons and how they relate to polycyclic aromatic hydrocarbons (PAHs) and grains is not well understood. Aims. We study the spatial distribution and chemistry of small hydrocarbons in the Orion Bar photodissociation region (PDR), a prototypical environment in which to investigate molecular gas irradiated by strong UV fields. Methods. We used the IRAM 30 m telescope to carry out a millimetre line survey towards the Orion Bar edge, complemented with ~2' × 2' maps of the C2H and c-C3H2 emission. We analyse the excitation of the detected hydrocarbons and constrain the physical conditions of the emitting regions with non-LTE radiative transfer models. We compare the inferred column densities with updated gas-phase photochemical models including 13CCH and C13CH isotopomer fractionation. Results. Approximately 40% of the lines in the survey arise from hydrocarbons (C2H, C4H, c-C3H2, c-C3H, C13CH, 13CCH, l-C3H, and l-H2C3 in decreasing order of abundance). We detect new lines from l-C3H+ and improve its rotational spectroscopic constants. Anions or deuterated hydrocarbons are not detected, but we provide accurate upper limit abundances: [C2D]/[C2H] < 0.2%, [C2H-]/[C2H] < 0.007%, and [C4H-]/[C4H] < 0.05%. Conclusions. Our models can reasonably match the observed column densities of most hydrocarbons (within factors of <3). Since the observed spatial distribution of the C2H and c-C3H2 emission is similar but does not follow the PAH emission, we conclude that, in high UV-flux PDRs, photodestruction of PAHs is not a necessary requirement to explain the observed abundances of the smallest hydrocarbons. Instead, gas-phase endothermic reactions (or with barriers) between C+, radicals, and H2 enhance the formation of simple hydrocarbons. Observations and models suggest that the [C2H]/[c-C3H2] ratio (~32 at the PDR edge) decreases with the UV field attenuation. The observed low cyclic-to-linear C3H column density ratio (≤3) is consistent with a high electron abundance (xe) PDR environment. In fact, the poorly constrained xe gradient influences much of the hydrocarbon chemistry in the more UV-shielded gas. The inferred hot rotational temperatures for C4H and l-C3H+ also suggest that radiative IR pumping affects their excitation. We propose that reactions of C2H isotopologues with 13C+ and H atoms can explain the observed [C13CH]/[13CCH] = 1.4 ± 0.1 fractionation level. [ABSTRACT FROM AUTHOR]

Aims. The importance of radiation to the physical structure of protoplanetary disks cannot be understated. However, protoplanetary disks evolve with time, and so to understand disk evolution and by association, disk structure, one should solve the combined and time-dependent equations of radiation hydrodynamics. Methods. We implement a new implicit radiation solver in the AZEuS adaptive mesh refinement magnetohydrodynamics fluid code. Based on a hybrid approach that combines frequency-dependent ray-tracing for stellar irradiation with non-equilibrium flux limited diffiusion, we solve the equations of radiation hydrodynamics while preserving the directionality of the stellar irradiation. The implementation permits simulations in Cartesian, cylindrical, and spherical coordinates, on both uniform and adaptive grids. Results. We present several hydrostatic and hydrodynamic radiation tests which validate our implementation on uniform and adaptive grids as appropriate, including benchmarks specifically designed for protoplanetary disks. Our results demonstrate that the combination of a hybrid radiation algorithm with AZEuS is an effiective tool for radiation hydrodynamics studies, and produces results which are competitive with other astrophysical radiation hydrodynamics codes. [ABSTRACT FROM AUTHOR]

Context. Rossby wave instability (RWI) at dead zone boundaries may play an important role in planet formation. Viscous hydrodynamics results suggest RWI is excited only when the viscosity changes over a radial distance less than two density scale heights. However in the disks around Solar-mass T Tauri stars, it is not viscosity but magnetic forces that provide the accretion stress beyond about 10 AU, where surface densities are low enough so stellar X-rays and interstellar cosmic rays can penetrate. Aims. We explore the conditions for RWI in the smooth transition with increasing distance, from resistive and magnetically-dead to conducting and magnetically-active. Methods. We perform 3D unstratified MHD simulations with the Pencil code, using static resistivity profiles. Results. We find that in MHD, contrary to viscous models, the RWI is triggered even with a gradual change in resistivity extending from 10 to 40 AU (i.e., spanning 15 scale heights for aspect ratio 0.1). This is because magneto-rotational turbulence sets in abruptly when the resistivity reaches a threshold level. At higher resistivities the longest unstable wavelength is quenched, resulting in a sharp decline of the Maxwell stress towards the star. The sharp gradient in the magnetic forces leads to a localized density bump, that is in turn Rossby wave unstable. Conclusions. Even weak gradients in the resistivity can lead to sharp transitions in the Maxwell stress. As a result the RWI is more easily activated in the outer disk than previously thought. Rossby vortices at the outer dead zone boundary thus could underlie the dust asymmetries seen in the outer reaches of transition disks. [ABSTRACT FROM AUTHOR]

Aims. We modelled an imaged Kelvin-Helmholtz (KH) instability in coronal mass ejections (CME) in the lower corona by investigating conditions under which kink (m = 1) and m = -3 magnetohydrodynamic (MHD) modes in a uniformly twisted flux tube moving along its axis become unstable. Methods. We employed the dispersion relations of MHD modes derived from the linearised MHD equations. We assumed real wave numbers and complex angular wave frequencies, namely complex wave phase velocities. The dispersion relations were solved numerically at fixed input parameters (taken from observational data) and various mass flow velocities. Results. It is shown that the stability of the modes depends upon four parameters: the density contrast between the flux tube and its environment, the ratio of the background magnetic fields in the two media, the twist of the magnetic field lines inside the tube, and the value of the Alfvén Mach number (the ratio of the tube velocity to Alfvén speed inside the flux tube). For a twisted magnetic flux tube at a density contrast of 0.88, a background magnetic field ratio of 1.58, and a normalised magnetic field twist of 0.2, the critical speed for the kink (m = -3) mode (where m is the azimuthal mode number) is 678 km s-1, just as observed. The growth rate for this harmonic at KH wavelength of 18.5 Mm and ejecta width of 4.1 Mm is equal to 0.037 s-1, in agreement with observations. KH instability of the m = -3 mode may also explain why the KH vortices are seen only at one side of arising CME. Conclusions. The good agreement between observational and computational data shows that the imaged KH instability on CME can be explained in terms of an emerging KH instability of the m = -3 MHD mode in twisted magnetic flux tubes moving along its axis. [ABSTRACT FROM AUTHOR]

Aims. Understanding the formation of binary and multiple stellar systems largely comes down to studying the circumstances under which a condensing core fragments (or not) during the first stages of the collapse. However, both the probability of fragmentation and the number of fragments seem to be determined to a large degree by the initial conditions. In this work we explore this dependence by studying the fate of the linear perturbations of a homogeneous gas sphere, both analytically and numerically. Methods. In particular, we investigate the stability of the well-known homologous solution that describes the collapse of a uniform spherical cloud. One problem that arises in such treatments is the mathematical singularity in the perturbation equations, which corresponds to the location of the sonic point of the flow. This difficulty is surpassed here by explicitly introducing a weak shock next to the sonic point as a natural way of connecting the subsonic to the supersonic regimes. In parallel, we perform adaptive mesh refinement (AMR) numerical simulations of the linear stages of the collapse and compare the growth rates obtained by each method. Results. With this combination of analytical and numerical tools, we explore the behavior of both axisymmetric and non-axisymmetric perturbations. The numerical experiments provide the linear growth rates as a function of the core's initial virial parameter and as a function of the azimuthal wave number of the perturbation. The overlapping regime of the numerical experiments and the analytical predictions is the situation of a cold and large cloud, and in this regime the analytically calculated growth rates agree very well with the ones obtained from the simulations. Conclusions. The use of a weak shock as part of the perturbation allows us to find physically acceptable solutions to the equations for a continuous range of growth rates. The numerical simulations agree very well with the analytical prediction for the most unstable cores, while they impose a limit of a virial parameter of 0.1 for core fragmentation in the absence of rotation. [ABSTRACT FROM AUTHOR]

We examine the long-term evolution of the intra-hour variable quasar, J1819+3845, whose variations have been attributed to interstellar scintillation by extremely local turbulent plasma, located only 1-3 pc from Earth. The variations in this source ceased some time in the period between June 2006 and February 2007. The evolution of the source spectrum and the long-term light curve, and the persistent compactness of the source VLBI structure indicate that the cessation of rapid variability was associated with the passage of the scattering material out of the line of sight to the quasar. We present an extensive analysis of the linear polarization variations and their relation to total intensity variations. The proper motion of polarized features in the quasar jet is found to be subluminal. Systematic time delays between Stokes I, Q, and U, in combination with the structure of the source obtained from 8.4 GHz VLBI data confirm the estimate of the screen distance: 1-2 pc, making the screen one of the nearest objects to the solar system. We determine the physical properties of this scattering material. The electron density in the scattering region is extremely high with respect to the warm ionized ISM, with an estimated density of ne ~ 97 l01/3 0 L100-1/2 cm-3, where l0 is the outer scale of the turbulence in AU and L = 100 L100 AU is the depth of the scattering region. If this plasma is in pressure balance with the local magnetic field, one expects a ~2 rad m-2 rotation measure (RM) change associated with the passage of this material past the quasar. To that end, we examine the RMs of sources and the diffuse polarized emission in the surrounding region. We place a limit of 10 rad m-2 on the RM change based upon 21 cm polarization observations. The variability of sources near J1819+3845 is examined to deduce limits on the transverse extent of the screen, and we find that no other sources exhibit variations on comparable timescales and that the screen must therefore be either very small (of order 100 AU) or patchy. [ABSTRACT FROM AUTHOR]

Aims. Ionized carbon is the main gas-phase reservoir of carbon in the neutral diffuse interstellar medium (ISM) and its 158 µm fine structure transition [CII] is the most important cooling line of the diffuse ISM. We combine [CII] absorption and emission spectroscopy to gain an improved understanding of physical conditions in the different phases of the ISM. Methods. We present high-resolution [CII] spectra obtained with the Herschel/HIFI instrument towards bright dust continuum regions in the Galactic plane, probing simultaneously the diffuse gas along the line of sight and the background high-mass star forming regions. These data are complemented by single pointings in the 492 and 809 GHz fine structure lines of atomic carbon and by medium spectral resolution spectral maps of the fine structure lines of atomic oxygen at 63 and 145 µm with Herschel/PACS. Results. We show that the presence of foreground absorption may completely cancel the emission from the background source in medium spectral resolution PACS data and that high spectral resolution spectra are needed to interpret the [C II] and [OI] emission and the [C II]/FIR ratio. This phenomenon may explain part of the [C II]/FIR deficit seen in external luminous infrared galaxies where the bright emission from the nuclear regions may be partially canceled by absorption from diffuse gas in the foreground. The C+ and C excitation in the diffuse gas is consistent with a median pressure of ~5900 Kcm-3 for a mean kinetic temperature of 100 K. A few higher pressure regions are detected along the lines of sight, as emission features in both fine structure lines of atomic carbon. The knowledge of the gas density allows us to determine the filling factor of the absorbing gas along the selected lines of sight. The derived median value of the filling factor is 2.4%, in good agreement with the properties of the Galactic cold neutral medium. The mean excitation temperature is used to derive the average cooling due to C+ in the Galactic plane : 9.5 × 10-26 erg-1H-1. Along the observed lines of sight, the gas phase carbon abundance does not exhibit a strong gradient as a function of Galacto-centric radius and has a weighted average of C/H = 1.5 ± 0.4 × 10-4. [ABSTRACT FROM AUTHOR]

Context. The formation of massive (O-type) stars through the same accretion processes as low-mass stars is problematic, mainly because of the feedback massive stars provide to the environment, which halts the accretion. In order to constrain theoretical models of high-mass star formation, observational signatures of mass accretion in O-type forming stars are desirable. The high-mass star forming region NGC 7538 IRS1 (distance = 2.7 kpc) is an ideal target, because VLBI measurements of CH3OH masers recently identified a triple system of high-mass young stellar object (YSOs) in the region: IRS1a, IRS1b, and IRS1c. The first two YSOs seem to be surrounded by rotating disks. Aims. We want to characterize physical conditions and kinematics of circumstellar molecular gas around O-type young stars. Subarcsecond resolution observations of highly-excited lines from high-density tracers are useful, since these probe the hottest and densest gas, which presumably is close to O-type forming stars, i.e., in disks and the innermost portions of envelopes. Methods. Using the Karl Jansky Very Large Array (JVLA), we have mapped the hot and dense molecular gas in the hot core associated with NGC 7538 IRS1, with ~0".2 angular resolution, in seven metastable (J = K) inversion transitions of ammonia (NH3): (J, K) = (6, 6), (7, 7), (9, 9), (10, 10), (12, 12), (13, 13), and (14, 14). These lines arise from energy levels between ~400 K and ~1950 K above the ground state, and are observed in absorption against the HC-HII region associated with NGC 7538 IRS1. The CH3OH JK = 132-131 and CH3CN (2-1) lines were also included in our spectral setup, but only the former was detected. We also obtained sensitive continuum maps at frequencies between 25 and 35 GHz. Results. For each transition, we produced resolved images of total intensity and velocity field, as well as position-velocity diagrams. The intensity maps show that the NH3 absorption follows the continuum emission closely. With a 500 AU linear resolution, we resolve the elongated north-south NH3 structure into two compact components: the main core and a southernmost component. Previous observations of the radio continuum with a 0".08 (or 200 AU) resolution, resolved in turn the compact core in two (northern and southern) components. The velocity maps of the compact core show a clear velocity gradient in all lines, which is indicative of rotation. It is possible that the rotation is not in an accretion disk but in a (circumbinary) envelope, containing ~40 Mʘ (dynamical mass). The core hosts a binary system of massive YSOs, associated with the two (northern and southern) components of the radio continuum, which have a separation of about 500 AU and velocities around -59 km s-1 and -56.4 km s-1, respectively. The southernmost component, separated by 1000 AU and resolved in our NH3 maps (0".2 beamsize) from the core, is associated with a third massive YSO, with a velocity around -60 km s-1. These features correspond to the triple system of high-mass YSOs IRS1a, IRS1b, and IRS1c. In addition, we derive rotational temperatures, NH3 column densities, H2 gas densities, and gas masses from the NH3 data. Surprisingly, measurements of the hyperfine structure show total optical depths of 10-26 even for these highly-excited lines, among the largest found so far in hot NH3 in high-mass star forming regions. From ratios of optical depths as well as rotational temperature diagrams of the observed ortho and para transitions, we derive a rotational temperature ~280 K, NH3 column densities ~1.4-2.5 × 1019 cm-2, total H2 volume densities ~3.5-6.2 × 1010 cm-3, and a total gas mass in the range of 19-34 Mʘ (the latter two assume [NH3]/[H2] = 10-7), for the main core. For the southern component, we derive a temperature of 110 K, a molecular density of ~ 0.7-2 × 1010 cm-3 and a gas mass in the range of 4-12 M. Conclusions. We conclude that NGC 7538 IRS1 is the densest hot molecular core known to date, containing a rotating envelope which hosts a multiple system of high-mass YSOs, possibly surrounded by accretion disks. Future JVLA observations in the A-configuration are needed to resolve the binary system in the core and may allow the study of the gas kinematics in the accretion disks associated with individual binary members. [ABSTRACT FROM AUTHOR]

Aims. Two-dimensional magnetohydrodynamic (MHD) simulations are used to model the emission properties of TeV-bright shelltype supernova remnants (SNRs) and to explore their nature. Methods. In the leptonic scenario for the TeV emission, the ?-ray emission is produced via inverse Compton scattering of background soft photons by high-energy electrons accelerated by the shocks of the SNRs. In a previous paper, we showed that since the energy densities of the cosmic microwave background radiation and that of the IR/optical background photons are much higher than that of the photons produced by the same high-energy electrons via the synchrotron process, the observed correlation between X-ray and TeV brightness of SNR RX?J1713.7-3946 can be readily explained with the assumption that the energy density of relativistic electrons is proportional to that of the magnetic field. The TeV emissivity is therefore proportional to the magnetic field energy density and MHD?simulations can be used to model the TeV structure of such remnants directly. Two-dimensional MHD simulations for SNRs are then performed under the assumption that the ambient interstellar medium is turbulent with the magnetic field and density fluctuations, following a Kolmogorov-like power-law spectrum. Results. (1) As expected, these simulations confirm early 1D and 2D modelings of these sources, namely the hydrodynamical evolution of the shock waves and amplification of magnetic field by Rayleigh-Taylor convective flows and by shocks propagating in a turbulent medium; (2) we reproduce rather complex morphological structure for ?-rays, for example, the bright thin rim and significant asymmetry, suggesting intrinsic variations of the source morphology not related to the structure of the progenitor and environment; and (3) the observed radial profile of several remnants are well reproduced with an ambient medium density of 0.1-1 cm-3. An even lower ambient density leads to a sharper drop of the TeV brightness with radius than what is observed near the outer edge of these remnants. Conclusions. In a turbulent background medium, we can reproduce the observed characteristics of several shell-type TeV SNRs with reasonable parameters except for a higher ambient density than that inferred from X-ray observations. [ABSTRACT FROM AUTHOR]

Aims. The cosmic distance-duality relation, dL(z) /dA(z)(1 + z)² = ?, where dL(z) and dA(z) are, respectively, the luminosity and the angular diameter distances and where ? = 1 holds as long as the number of photons is conserved and gravity is described by a metric theory. Testing such hypotheses is, therefore, an important task for both cosmology and fundamental physics. Methods. We estimated the gas mass fraction of 91 galaxy clusters (GC) from current measurements of the GC total mass recently reported by the Atacama Cosmology Telescope collaboration. We use these data, along with type Ia supernovae observations of the Union 2.1 compilation, to test a possible deviation from the value ? = 1. Results. Although they agree with the standard hypothesis, we find that this combination of both data tends to favor values of ? less than 1, which might be associated with some physical processes that increase the number of photons and modifying the above relation to dL< (1 + z)²dA. [ABSTRACT FROM AUTHOR]

Context. Jets are dynamic, impulsive, well-collimated plasma events developing at many different scales and in different layers of the solar atmosphere. Aims. Jets are believed to be induced by magnetic reconnection, a process central to many astrophysical phenomena. Studying their dynamics can help us to better understand the processes acting in larger eruptive events (e.g., flares and coronal mass ejections) as well as mass, magnetic helicity, and energy transfer at all scales in the solar atmosphere. The relative simplicity of their magnetic geometry and topology, compared with larger solar active events, makes jets ideal candidates for studying the fundamental role of reconnection in energetic events. Methods. In this study, using our recently developed numerical solver ARMS, we present several parametric studies of a 3D numerical magneto-hydrodynamic model of solar-jet-like events. We studied the impact of the magnetic field inclination and photospheric field distribution on the generation and properties of two morphologically different types of solar jets, straight and helical, which can account for the observed so-called standard and blowout jets. Results. Our parametric studies validate our model of jets for different geometric properties of the magnetic configuration. We find that a helical jet is always triggered for the range of parameters we tested. This demonstrates that the 3D magnetic null-point configuration is a very robust structure for the energy storage and impulsive release characteristic of helical jets. In certain regimes determined by magnetic geometry, a straight jet precedes the onset of a helical jet. We show that the reconnection occurring during the straight-jet phase influences the triggering of the helical jet. Conclusions. Our results allow us to better understand the energization, triggering, and driving processes of straight and helical jets. Our model predicts the impulsiveness and energetics of jets in terms of the surrounding magnetic field configuration. Finally, we discuss the interpretation of the observationally defined standard and blowout jets in the context of our model, as well as the physical factors that determine which type of jet will occur. [ABSTRACT FROM AUTHOR]

The UC-HII region of MWC 349A is the prototype of an ionized wind driven by a massive star surrounded by a disk. Recent high angular resolution observations of the millimeter recombination lines have shown that the disk rotates with a Keplerian law in its outer parts. However, the kinematics of innermost regions in the UC-HII region of MWC 349A is still unknown, in particular, the radius where the wind is launched from the disk. We performed hydrogen recombination line observations with the Heterodyne Instrument for the Far Infrared (HIFI) onboard the Herschel Space Observatory to study the kinematics of its innermost regions by studying their spectral features. In addition to the two laser peaks, we report the first detection of two new components that are blueshifted with respect to the laser peaks for all the recombination lines with principal quantum number n ⩽ 21. These new spectral features originate from the region where the wind is ejected from the disk.We used our 3D non-LTE radiative transfer model for recombination lines (MORELI) to show that these features are consistent with the wind being ejected at a radius of ∼24 AU from the star, which supports magnetohydrodynamic wind models. [ABSTRACT FROM AUTHOR]

In galaxy clusters the thermal Sunyaev-Zel'dovich (SZ) effect from the hot intracluster medium (ICM) provides a direct, self-contained measure of the pressure integrated over crossing lines of sight, that is intrinsically independent of redshift and well suited for evolutionary studies. We show in detail how the size of the effect and its pattern on the sky plane are directly related to the entropy levels in the ICM, and how they characterize the cluster cores and outskirts independently. We find that at redshifts z ≲ 0.3 the signals to be expected in the cores considerably exceed those detected at 10' resolution with the Planck satellite. We propose that at 1' resolutions as implemented on recent ground instrumentation for mapping features in individual clusters, the average patterns of the SZ signals can provide a direct and effective way to find and count cool, low-entropy cores and hot, high-entropy outskirts out to z ∼ 2. Such counts will tell the timing and the mode of the processes that drive the evolution of the ICM from the distant to the local cluster population. [ABSTRACT FROM AUTHOR]

We describe the all-sky Planck catalogue of clusters and cluster candidates derived from Sunyaev-Zeldovich (SZ) effect detections using the first 15.5 months of Planck satellite observations. The catalogue contains 1227 entries, making it over six times the size of the Planck Early SZ (ESZ) sample and the largest SZ-selected catalogue to date. It contains 861 confirmed clusters, of which 178 have been confirmed as clusters, mostly through follow-up observations, and a further 683 are previously-known clusters. The remaining 366 have the status of cluster candidates, and we divide them into three classes according to the quality of evidence that they are likely to be true clusters. The Planck SZ catalogue is the deepest all-sky cluster catalogue, with redshifts up to about one, and spans the broadest cluster mass range from (0.1 to 1.6) × 1015 M☉. Confirmation of cluster candidates through comparison with existing surveys or cluster catalogues is extensively described, as is the statistical characterization of the catalogue in terms of completeness and statistical reliability. The outputs of the validation process are provided as additional information. This gives, in particular, an ensemble of 813 cluster redshifts, and for all these Planck clusters we also include a mass estimated from a newly-proposed SZ-mass proxy. A refined measure of the SZ Compton parameter for the clusters with X-ray counter-parts is provided, as is an X-ray flux for all the Planck clusters not previously detected in X-ray surveys. [ABSTRACT FROM AUTHOR]

Structure formation in the current Universe operates through the accretion of group-scale systems onto massive clusters. The detection and study of such accreting systems is crucial to understand the build-up of the most massive virialized structures we see today. We report the discovery with XMM-Newton of an irregular X-ray substructure in the outskirts of the massive galaxy cluster Abell 2142. The tip of the X-ray emission coincides with a concentration of galaxies. The bulk of the X-ray emission of this substructure appears to be lagging behind the galaxies and extends over a projected scale of at least 800 kpc. The temperature of the gas in this region is 1.4 keV, which is a factor of ~4 lower than the surrounding medium and is typical of the virialized plasma of a galaxy group with a mass of a few 1013 M☉. For this reason, we interpret this structure as a galaxy group in the process of being accreted onto the main dark-matter halo. The X-ray structure trailing behind the group is due to gas stripped from its original dark-matter halo as it moves through the intracluster medium (ICM). This is the longest X-ray trail reported to date. For an infall velocity of ~1200 km s-1 we estimate that the stripped gas has been surviving in the presence of the hot ICM for at least 600 Myr, which exceeds the Spitzer conduction timescale in the medium by a factor of ≳400. Such a strong suppression of conductivity is likely related to a tangled magnetic field with small coherence length and to plasma microinstabilities. The long survival time of the low-entropy intragroup medium suggests that the infalling material can eventually settle within the core of the main cluster. [ABSTRACT FROM AUTHOR]

Context. The [CII] 158 μm line is the brightest far-infrared cooling line in galaxies, representing 0.1 to 1% of their far-infrared continuum emission, and is therefore a potentially powerful tracer of star formation activity. The [CII] line traces different phases of the interstellar medium (ISM), including the diffuse ionized medium, warm and cold atomic clouds, clouds in transition from atomic to molecular, and dense and warm photon dominated regions (PDRs). Therefore without being able to separate the contributions to the [CII] emission, the relationship of this fine structure line emission to star formation has been unclear. Aims. We study the relationship between the [CII] emission and the star formation rate (SFR) in the Galactic plane and separate the relationship of different ISM phases to the SFR. We compare these relationships to those in external galaxies and local clouds, allowing examinations of these relationships over a wide range of physical scales. Methods. We compare the distribution of the [CII] emission, with its different contributing ISMphases, as a function of Galactocentric distance with the SFR derived from radio continuum observations. We also compare the SFR with the surface density distribution of atomic and molecular gas, including the CO-dark H2 component. Results. The [CII] and SFR are well correlated at Galactic scales with a relationship that is in general agreement with that found for external galaxies. By combining [CII] and SFR data points in the Galactic plane with those in external galaxies and nearby star forming regions, we find that a single scaling relationship between the [CII] luminosity and SFR applies over six orders of magnitude. The [CII] emission from different ISM phases are each correlated with the SFR, but only the combined emission shows a slope that is consistent with extragalactic observations. These ISM components have roughly comparable contributions to the Galactic [C II] luminosity: dense PDRs (30%), cold HI (25%), CO-dark H2 (25%), and ionized gas (20%). The SFR-gas surface density relationship shows a steeper slope compared to that observed in galaxies, but one that it is consistent with those seen in nearby clouds. The different slope is a result of the use of a constant CO-to-H2 conversion factor in the extragalactic studies, which in turn is related to the assumption of constant metallicity in galaxies. We find a linear correlation between the SFR surface density and that of the dense molecular gas. [ABSTRACT FROM AUTHOR]

Context. What exactly controls star formation in the Galaxy remains controversial. In particular, the role of feedback and magnetic field are still partially understood. Aims. We investigate the role played by supernovae feedback and magnetic field on the star formation and the structure of the Galactic disc. Methods. We perform numerical simulations of the turbulent, magnetized, self-gravitating, multi-phase, supernovae regulated ISM within a 1 kpc stratified box. We implemented various schemes for the supernovae. This goes from a random distribution at a fixed rate to distributions for which the supernovae are spatially and temporally correlated to the formation of stars. To study the influence of magnetic field on star formation, we perform both hydrodynamical and magneto-hydrodynamical simulations. Results. We find that supernovae feedback has a drastic influence on the galactic evolution. The supernovae distribution plays a very significant role. When the supernovae are not correlated with star formation events, they do not significantly modify the very high star formation rate obtained without feedback. When the supernovae follow the accretion, the star formation rate can be reduced by a factor up to 30. The magnetic field is also playing a significant role. It reduces the star formation rate by a factor up to 2-3 and reduces the number of collapse sites by a factor of about 2. Conclusions. The exact correlation between the supernovae and the dense gas appears to have significant consequences on the galactic disc evolution and the star formation. This implies that small scale studies are necessary to understand and quantify the feedback efficiency. The magnetic field does influence the star formation at galactic scales by reducing the star formation rate and the number of star formation sites. [ABSTRACT FROM AUTHOR]

Context. Tens of light hydrides and small molecules have now been detected over several hundreds sightlines sampling the diffuse interstellar medium (ISM) in both the solar neighbourhood and the inner Galactic disk. They provide unprecedented statistics on the first steps of chemistry in the diffuse gas. Aims. These new data confirm the limitations of the traditional chemical pathways driven by the UV photons and the cosmic rays (CR) and the need for additional energy sources, such as turbulent dissipation, to open highly endoenergetic formation routes. The goal of the present paper is to further investigate the link between specific species and the properties of the turbulent cascade in particular its space-time intermittency. Methods. We have analysed ten different atomic and molecular species in the framework of the updated model of turbulent dissipation regions (TDR). We study the influence on the abundances of these species of parameters specific to chemistry (density, UV field, and CR ionisation rate) and those linked to turbulence (the average turbulent dissipation rate, the dissipation timescale, and the ion-neutral velocity drift in the regions of dissipation). Results. The most sensitive tracers of turbulent dissipation are the abundances of CH+ and SH+, and the column densities of the J = 3, 4, 5 rotational levels of H2. The abundances of CO, HCO+, and the intensity of the 158 μm [CII] emission line are significantly enhanced by turbulent dissipation. The vast diversity of chemical pathways allows the independent determinations of free parameters never estimated before: an upper limit to the average turbulent dissipation rate, ε ≲ 10-23 erg cm-3 s-1 for nH = 20 cm-3, from the CH+ abundance; an upper limit to the ion-neutral velocity drift, vin ≲ 3.5 kms-1, from the SH+ to CH+ abundance ratio; and a range of dissipation timescales, 100 ≲ τv ≲ 1000 yr, from the CO to HCO+ abundance ratio. For the first time, we reproduce the large abundances of CO observed on diffuse lines of sight, and we show that CO may be abundant even in regions with UV-shieldings as low as 5 × 10-3 mag. The best range of parameters also reproduces the abundance ratios of OH, C2H, and H2O to HCO+ and are consistent with the known properties of the turbulent cascade in the Galactic diffuse ISM. Conclusions. Our results disclose an unexpected link between the dissipation of turbulence and the emergence of molecular richness in the diffuse ISM. Some species, such as CH+ or SH+, turn out to be unique tracers of the energy trail in the ISM. In spite of some degeneracy, the properties of the turbulent cascade, down to dissipation, can be captured through specific molecular abundances. [ABSTRACT FROM AUTHOR]

Context. Some runaway stars are known to display IR arc-like structures around them, resulting from their interaction with surrounding interstellar material. The properties of these features as well as the processes involved in their formation are still poorly understood. Aims. We aim to understand the physical mechanisms that shape the dust arc observed near the runaway O-star AEAur (HD34078). Methods. We obtained and analyzed a high spatial resolution (4.4") map of the 12CO(1-0) emission that is centered on HD34078, and that combines data from both the IRAM interferometer and 30 m single-dish antenna. Results. One third of the 30 m flux mainly originates from two small (no larger than 5" × 10" or 0.013 × 0.026 pc), and bright (1 and 3K peak temperatures) CO globulettes. The line of sight toward HD34078 intersects the outer part of one of the globulettes, which accounts for both the properties of diffuse UV light observed in the field and the numerous molecular absorption lines detected in HD34078's spectra, including those from highly excited H2. Their modeled distance from the star (0.2 pc) is compatible with the fact that they lie on the 3D paraboloid, which fits the arc detected in the 24 μm Spitzer image. Four other compact CO globulettes are detected in the mapped area, all lying close to the rim of this paraboloid. These globulettes have a high density and linewidth, and are strongly pressure-confined or transient. Conclusions. The presence of molecular globulettes at such a close distance from an O star is unexpected, and probably related to the high proper motion of HD34078. Indeed, the good spatial correlation between the CO globulettes and the IR arc suggests that they result from the interaction of the radiation and wind emitted by HD34078 with the ambient gas. However, the details of this interaction remain unclear. A wind mass-loss rate significantly larger than the value inferred from UV lines is favored by the large IR arc size, but does not easily explain the low velocity of the CO globulettes. The effect of radiation pressure on dust grains also meets several issues in explaining the observations. Further observational and theoretical work is needed to fully elucidate the processes shaping the gas and dust in bow shocks around runaway O stars. [ABSTRACT FROM AUTHOR]

Disks around T Tauri stars are known to lose mass, as best shown by the profiles of the forbidden emission lines of low-ionization species. At least two separate kinematic components have been identified, one characterized by velocity shifts of tens to hundreds of km s-1 (HVC) and one with a much lower velocity of a few km s-1 (LVC). The HVC are convincingly associated to the emission of jets, but the origin of the LVC is still unknown. In this paper we analyze the forbidden line spectrum of a sample of 44 mostly lowmass young stars in Lupus and σ Ori observed with the X-shooter ESO spectrometer. We detect forbidden line emission of Oi, Oii, S ii, Ni, and Nii, and characterize the line profiles as LVC, blueshifted HVC, and redshifted HVC. We focus our study on the LVC. We show that there is a good correlation between line luminosity and both Lstar and the accretion luminosity (or the mass accretion rate) over a large interval of values (Lstar ∼ 10-2-1 L☉ ; Lacc ∼ 10-5-10-1 L☉; Macc ∼ 10-11-10-7 M☉/yr). The lines show the presence of a slow wind (Vpeak < 20 km s-1) that is dense (nH > 108 cm-3), warm (T ∼ 5000-10 000 K), mostly neutral. We estimate the mass of the emitting gas and provide a value for the maximum volume it occupies. Both quantities increase steeply with the stellar mass, from ∼10-12 M☉ and ∼0.01 AU3 for Mstar ∼ 0:1 M☉, to ∼3 x 10-10 M☉ and ∼1 AU3 for Mstar ∼ 1 M☉, respectively. These results provide quite stringent constraints to wind models in low-mass young stars, that need to be explored further. [ABSTRACT FROM AUTHOR]