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The streaming instability is a mechanism whereby pebble-sized particles in protoplanetary discs spontaneously come together in dense filaments, which collapse gravitationally to form planetesimals upon reaching the Roche density. The extent of the filaments along the orbital direction is nevertheless poorly characterised, due to a focus in the literature on small simulation domains where the behaviour of the streaming instability on large scales cannot be determined. We present here computer simulations of the streaming instability in boxes with side lengths up to 6.4 scale heights in the plane. This is 32 times larger than typically considered simulation domains and nearly a factor 1,000 times the volume. We show that the azimuthal extent of filaments in the non-linear state of the streaming instability is limited to approximately one gas scale height. The streaming instability will therefore not transform the pebble density field into axisymmetric rings; rather the non-linear state of the streaming instability appears as a complex structure of loosely connected filaments. Including the self-gravity of the pebbles, our simulations form up to 4,000 planetesimals. This allows us to probe the high-mass end of the initial mass function of planetesimals with much higher statistical confidence than previously. We find that this end is well-described by a steep exponential tapering. Since the resolution of our simulations is moderate -- a necessary trade-off given the large domains -- the mass distribution is incomplete at the low-mass end. When putting comparatively less weight on the numbers at low masses, at intermediate masses we nevertheless reproduce the power-law shape of the distribution established in previous studies., Comment: 15 pages, 14 figures, 2 tables; accepted for publication in A&A

Context. While Class II stars have already accreted most of their mass, the continued inflow of fresh material via Bondi-Hoyle accretion acts as an additional mass reservoir for their circumstellar disks. This may explain the observed accretion rates of pre-main- sequence (PMS) stars, as well as observational inconsistencies in the mass and angular momentum balance of their disks. Aims. Using a new simulation that reproduces the stellar IMF, we want to quantify the role of Bondi-Hoyle accretion in the formation of Class II disks, as well address the prospect of its observational detection with the James Webb Space Telescope (JWST). Methods. We study the mass and angular momentum of the accreting gas using passively-advected tracer particles in the simulation, and we carry out radiative transfer calculations of near-infrared scattering to generate synthetic JWST observations of Bondi-Hoyle trails of PMS stars. Results. Gas accreting on Class II PMS stars approximately 1 Myr after their formation has enough mass and angular momentum to strongly affect the evolution of the preexisting disks. The accreted angular momentum is large enough to also explain the observed size of Class II disks. The orientation of the angular momentum vector can differ significantly from that of the previously accreted gas, which may result in a significant disk warping or misalignment. We also predict that JWST observations of Class II stars will be able to detect Bondi-Hoyle trails with a 90% success rate with only 2 min exposure time, if stars with accretion rates \dot{M} > 5e-10 Msol/yr and luminosity of L > 0.5 Lsol are selected., Comment: 12 pages, 16 figures, submitted to A&A

We present Paicos, a new object-oriented Python package for analyzing simulations performed with Arepo. Paicos strives to reduce the learning curve for students and researchers getting started with Arepo simulations. As such, Paicos includes many examples in the form of Python scripts and Jupyter notebooks as well as an online documentation describing the installation procedure and recommended first steps. Paicos' main features are automatic handling of cosmological and physical units, computation of derived variables, 2D visualization (slices and projections), 1D and 2D histograms, and easy saving and loading of derived data including units and all the relevant metadata., Comment: Published in JOSS, code at https://github.com/tberlok/paicos

Star formation is a multi-scale problem, and only global simulations that account for the connection from the molecular cloud scale gas flow to the accreting protostar can reflect the observed complexity of protostellar systems. Star-forming regions are characterised by supersonic turbulence and as a result, it is not possible to simultaneously design models that account for the larger environment and in detail reproduce observed stellar systems. Instead, the stellar inventories can be matched statistically, and best matches found that approximate specific observations. Observationally, a combination of single-dish telescopes and interferometers are now able to resolve the nearest protostellar objects on all scales from the protostellar core to the inner 10 AU. We present a new non-parametric methodology which uses high-resolution simulations and post-processing methods to match simulations and observations using deep learning. Our goal is to perform a down-selection from large data sets of synthetic images to a ranked list of best-matching candidates with respect to the observation. This is particularly useful for binary and multiple stellar systems that form in turbulent environments. The objective is to accelerate the rate at which we can do such comparisons, remove biases from hand-picking matches, and contribute to identifying the underlying physical processes that drive the creation and evolution of observed protostellar systems., Comment: 13 pages, 8 figures, submitted to MNRAS

Protostellar binaries harbour complex environment morphologies. Observations represent a snapshot in time, and projection and optical depth effects impair our ability to interpret them. Careful comparison with high-resolution models that include the larger star-forming region can help isolate the driving physical processes and give observations context in the time domain. We carry out zoom-in simulations with AU-scale resolution, and for the first time ever we follow the evolution until a circumbinary disk is formed. We investigate the gas dynamics around the young stars and extract disk sizes. Using radiative transfer, we obtain evolutionary tracers of the binary systems. We find that the centrifugal radius in prestellar cores is a poor estimator of the resulting disk size due to angular momentum transport at all scales. For binaries, the disk sizes are regulated periodically by the binary orbit, having larger radii close to the apastron. The bolometric temperature differs systematically between edge-on and face-on views and shows a high frequency time dependence correlated with the binary orbit and a low frequency time dependence with larger episodic accretion events. These oscillations can bring the system appearance to change rapidly from class 0 to class I and for short time periods even bring it to class II. The highly complex structure in early stages, as well as the binary orbit itself, affects the classical interpretation of protostellar classes and direct translation to evolutionary stages has to be done with caution and include other evolutionary indicators such as the extent of envelope material., Comment: 15 pages, 13 figures, submitted to MNRAS

Characterizing prestellar cores in star-forming regions is an important step towards the validation of theoretical models of star formation. Thanks to their sub-arcsecond resolution, ALMA observations can potentially provide samples of prestellar cores up to distances of a few kpc, where regions of massive star formation can be targeted. However, the extraction of real cores from dust-continuum observations of turbulent star-forming clouds is affected by complex projection effects. In this work, we study the problem of core extraction both in the idealized case of column-density maps and in the more realistic case of synthetic 1.3\,mm ALMA observations. The analysis is carried out on 12 regions of high column density from our 250 pc simulation. We find that derived core masses are highly unreliable, with only {\em a weak correlation between the masses of cores selected in the synthetic ALMA maps and those of the corresponding three-dimensional cores}. The fraction of real three-dimensional cores detected in the synthetic maps increases monotonically with mass and remains always below 50\%. Above $\sim 1\,M_{\odot}$, the core mass function derived from the column-density maps is steeper than that of the three-dimensional cores, while the core mass function from the synthetic ALMA maps has a slope closer to that of the real three-dimensional cores. Because of the mass uncertainties, proper guidance from realistic simulations is essential if ALMA observations of protoclusters at kpc distances are to be used to test star-formation models., Comment: 20 pages, 21 figures, MNRAS, in press

Meteorites display an isotopic composition dichotomy between non-carbonaceous (NC) and carbonaceous (CC) groups, indicating that planetesimal formation in the solar protoplanetary disk occurred in two distinct reservoirs. The prevailing view is that a rapidly formed Jupiter acted as a barrier between these reservoirs. We show a fundamental inconsistency in this model: if Jupiter is an efficient blocker of drifting pebbles, then the interior NC reservoir is depleted by radial drift within a few hundred thousand years. If Jupiter lets material pass it, then the two reservoirs will be mixed. Instead, we demonstrate that the arrival of the CC pebbles in the inner disk is delayed for several million years by the viscous expansion of the protoplanetary disk. Our results support that Jupiter formed in the outer disk (>10 AU) and allowed a considerable amount of CC material to pass it and become accreted by the terrestrial planets., Comment: Authors' version, 13 pages, 4 figures. Published in Science Advances in the issue of April 22, 2022

The dynamical state of massive clumps is key to our understanding of the formation of massive stars. In this work, we study the kinematic properties of massive clumps using synthetic observations. We have previously compiled a very large catalog of synthetic dust-continuum compact sources from our 250 pc, SN-driven, star formation simulation. Here, we compute synthetic $\rm N_{2}H^{+}$ line profiles for a subsample of those sources and compare their properties with the observations and with those of the corresponding three-dimensional (3D) clumps in the simulation. We find that the velocity dispersion of the sources estimated from the $\rm N_{2}H^{+}$ line is a good estimate of that of the 3D clumps, although its correlation with the source size is weaker than the velocity-size correlation of the 3D clumps. The relation between the mass of the 3D clumps, $M_{\rm main}$, and that of the corresponding synthetic sources, $M_{\rm SED}$, has a large scatter and a slope of 0.5, $M_{\rm main} \propto M_{\rm SED}^{0.5}$, due to uncertainties arising from the observational band-merging procedure and from projection effects along the line of sight. As a result, the virial parameters of the 3D clumps are not correlated with the clump masses, even if a negative correlation is found for the compact sources, and the virial parameter of the most massive sources may significantly underestimate that of the associated clumps., Comment: 19 pages, 12 figures, 1 table. Accepted for publication in MNRAS

Systematic surveys of massive clumps have been carried out to study the conditions leading to the formation of massive stars. These clumps are typically at large distances and unresolved, so their physical properties cannot be reliably derived from the observations alone. Numerical simulations are needed to interpret the observations. To this end, we generate synthetic Herschel observations using our large-scale star-formation simulation, where massive stars explode as supernovae driving the interstellar-medium turbulence. From the synthetic observations, we compile a catalog of compact sources following the exact same procedure as for the Hi-GAL compact source catalog. We show that the sources from the simulation have observational properties with statistical distributions consistent with the observations. By relating the compact sources from the synthetic observations to their three-dimensional counterparts in the simulation, we find that the synthetic observations overestimate the clump masses by about an order of magnitude on average due to line-of-sight projection, and projection effects are likely to be even worse for Hi-GAL Inner Galaxy sources. We also find that a large fraction of sources classified as protostellar are likely to be starless, and propose a new method to partially discriminate between true and false protostellar sources., Comment: 17 pages, 15 figures, submitted to MNRAS

While the importance of supernova feedback in galaxies is well established, its role on the scale of molecular clouds is still debated. In this work, we focus on the impact of supernovae on individual clouds, using a high-resolution magneto-hydrodynamic simulation of a region of 250 pc where we resolve the formation of individual massive stars. The supernova feedback is implemented with real supernovae that are the natural evolution of the resolved massive stars, so their position and timing are self-consistent. We select a large sample of molecular clouds from the simulation to investigate the supernova energy injection and the resulting properties of molecular clouds. We find that molecular clouds have a lifetime of a few dynamical times, less then half of them contract to the point of becoming gravitationally bound, and the dispersal time of bound clouds, of order one dynamical time, is a factor of two shorter than that of unbound clouds. We stress the importance of internal supernovae, that is massive stars that explode inside their parent cloud, in setting the cloud dispersal time, and their huge overdensity compared to models where the supernovae are randomly distributed. We also quantify the energy injection efficiency of supernovae as a function of supernova distance to the clouds. We conclude that intermittent driving by supernovae can maintain molecular-cloud turbulence and may be the main process of cloud dispersal. The role of supernovae in the evolution of molecular clouds cannot be fully accounted for without a self-consistent implementation of their feedback., Comment: 33 pages, 23 figures, submitted to ApJ

We address the problem of the origin of massive stars, namely the origin, path and timescale of the mass flows that create them. Based on extensive numerical simulations, we propose a scenario where massive stars are assembled by large-scale, converging, inertial flows that naturally occur in supersonic turbulence. We refer to this scenario of massive-star formation as the "Inertial-Inflow Model". This model stems directly from the idea that the mass distribution of stars is primarily the result of turbulent fragmentation. Under this hypothesis, the statistical properties of the turbulence determine the formation timescale and mass of prestellar cores, posing definite constraints on the formation mechanism of massive stars. We quantify such constraints by the analysis of a simulation of supernova-driven turbulence in a 250-pc region of the interstellar medium, describing the formation of hundreds of massive stars over a time of approximately 30 Myr. Due to the large size of our statistical sample, we can say with full confidence that massive stars in general do not form from the collapse of massive cores, nor from competitive accretion, as both models are incompatible with the numerical results. We also compute synthetic continuum observables in Herschel and ALMA bands. We find that, depending on the distance of the observed regions, estimates of core mass based on commonly-used methods may exceed the actual core masses by up to two orders of magnitude, and that there is essentially no correlation between estimated and real core masses., Comment: 35 pages, 30 figures, ApJ in press

Jupiter and Saturn formed early, before the gas disk dispersed. The presence of gap-opening planets affects the dynamics of the gas and embedded solids and halts the inward drift of grains above a certain size. A drift barrier can explain the absence of calcium aluminium rich inclusions (CAIs) in chondrites originating from parent bodies that accreted in the inner solar system. Employing an interdisciplinary approach, we use a $\mu$-X-Ray-fluorescence scanner to search for large CAIs and a scanning electron microscope to search for small CAIs in the ordinary chondrite NWA 5697. We carry out long-term, two-dimensional simulations including gas, dust, and planets to characterize the transport of grains within the viscous $\alpha$-disk framework exploring the scenarios of a stand-alone Jupiter, Jupiter and Saturn \textit{in situ}, or Jupiter and Saturn in a 3:2 resonance. In each case, we find a critical grain size above which drift is halted as a function of the physical conditions in the disk. From the laboratory search we find four CAIs with a largest size of $\approx$200$\,\mu$m. \Combining models and data, we provide an estimate for the upper limit of the $\alpha$-viscosity and the surface density at the location of Jupiter, using reasonable assumptions about the stellar accretion rate during inward transport of CAIs, and assuming angular momentum transport to happen exclusively through viscous effects. Moreover, we find that the compound gap structure in the presence of Saturn in a 3:2 resonance favors inward transport of grains larger than CAIs currently detected in ordinary chondrites., Comment: 16 pages, 10 figures, updated to match published version in Astrophysical Journal

We present Lampray: a multi-group long characteristics ray tracing method for adaptive mesh radiation hydrodynamics in the Ramses code. It avoids diffusion, captures shadows, and treats colliding beams correctly, and therefore complements existing moment-based ray tracing in Ramses. Lampray includes different options for interpolation between ray and cell domain, and use either integral, Fourier, or an implicit method for hydrogen ionization to solve the radiative transfer. The opacity can either be tabulated or computed through a coupling to the general non-equilibrium astro-chemistry framework Krome. We use an H-He-C-O network with 36 species and 240 reactions to track the photo-chemistry in the interstellar medium across 6 and 10 orders of magnitude in temperature and density. Self-shielding prescriptions for H$_2$ and CO are used together with a new model for the diffuse interstellar UV-field. We also track the dust temperature, formation of H$_2$ on grains, and H$_2$O and CO ices in detail. Lampray is tested against standard benchmarks for molecular cloud and star formation simulations, including the formation of a Str\"omgren sphere, the expansion of an ionization front, the photo-evaporation of a dense clump, and the H-He-C-O chemistry in a static photo-dissociation front. Efficient parallelisation is achieved with a separate domain decomposition for rays where points along a ray reside in the same memory space, and data movement from cell- to ray-domain is done with a direct hash-table lookup algorithm. Point sources are treated without splitting rays, and therefore the method currently only scales to a few point sources, while diffuse radiation has excellent scaling., Comment: 31 pages, 38 figures

In this paper the non-linear effect of massive neutrinos on cosmological structures is studied in a conceptually new way. We have solved the non-linear continuity and Euler equations for the neutrinos on a grid in real space in $N$-body simulations, and closed the Boltzmann hierarchy at the non-linear Euler equation using the stress and pressure perturbations from linear theory. By comparing with state-of-the art cosmological neutrino simulations, we are able to simulate the non-linear neutrino power spectrum very accurately. This translates into a negligible error in the matter power spectrum, and so our $\scriptstyle{\rm CO}N{\rm CEPT}$ code is ideally suited for extracting the neutrino mass from future high precision non-linear observational probes such as Euclid., Comment: Accepted for publication in JCAP. 29 pages, 10 figures

Stars and their corresponding protoplanetary disks form in diverse environments. To account for these natural variations, we investigate the formation process around nine solar mass stars with a maximum resolution of 2 AU in a Giant Molecular Cloud of (40 pc)$^3$ in volume by using the adaptive mesh refinement code \ramses. The magnetohydrodynamic simulations reveal that the accretion process is heterogeneous in time, in space, and among protostars of otherwise similar mass. During the first roughly 100 kyr of a protostar evolving to about a solar mass, the accretion rates peak around $10^{-5}$ to $10^{-4}$ M$_{\odot}$ yr$^{-1}$ shortly after its birth, declining with time after that. The different environments also affect the spatial accretion, and infall of material to the star-disk system is mostly through filaments and sheets. Furthermore, the formation and evolution of disks varies significantly from star to star. We interpret the variety in disk formation as a consequence of the differences in the combined effects of magnetic fields and turbulence that may cause differences in the efficiency of magnetic braking, as well as differences in the strength and distribution of specific angular momentum., Comment: Contribution to conference proceedings of "Star Formation in Different Environments" (SFDE) in Quy-Nhon, Vietnam 2016. 4 pages, 3 figures

Hertzsprung-Russell diagrams of star forming regions show a large luminosity spread. This is incompatible with well-defined isochrones based on classic non-accreting protostellar evo- lution models. Protostars do not evolve in isolation of their environment, but grow through accretion of gas. In addition, while an age can be defined for a star forming region, the ages of individual stars in the region will vary. We show how the combined effect of a protostellar age spread, a consequence of sustained star formation in the molecular cloud, and time-varying protostellar accretion for individual protostars can explain the observed luminosity spread. We use a global MHD simulation including a sub-scale sink particle model of a star forming region to follow the accretion process of each star. The accretion profiles are used to compute stellar evolution models for each star, incorporating a model of how the accretion energy is distributed to the disk, radiated away at the accretion shock, or incorporated into the outer layers of the protostar. Using a modelled cluster age of 5 Myr we naturally reproduce the lumi- nosity spread and find good agreement with observations of the Collinder 69 cluster, and the Orion Nebular Cluster. It is shown how stars in binary and multiple systems can be externally forced creating recurrent episodic accretion events. We find that in a realistic global molecular cloud model massive stars build up mass over relatively long time-scales. This leads to an important conceptual change compared to the classic picture of non-accreting stellar evolution segmented in to low-mass Hayashi tracks and high-mass Henyey tracks., Comment: 20 pages, 16 figures, Updated to match published article in MNRAS

Using zoom-simulations carried out with the adaptive mesh-refinement code RAMSES with a dynamic range of up to $2^{27} \approx 1.34 \times 10^8$ we investigate the accretion profiles around six stars embedded in different environments inside a (40 pc)$^3$ giant molecular cloud, the role of mass infall and disc instabilities on the accretion profile, and thus on the luminosity of the forming protostar. Our results show that the environment in which the protostar is embedded determines the overall accretion profile of the protostar. Infall on to the circumstellar disc may trigger gravitational disc instabilities in the disc at distances of around ~10 to ~50 au leading to rapid transport of angular momentum and strong accretion bursts. These bursts typically last for about ~10 to a ~100 yr, consistent with typical orbital times at the location of the instability, and enhance the luminosity of the protostar. Calculations with the stellar evolution code mesa show that the accretion bursts induce significant changes in the protostellar proper- ties, such as the stellar temperature and radius. We apply the obtained protostellar properties to produce synthetic observables with RADMC3D and predict that accretion bursts lead to ob- servable enhancements around 20 to 200 $\mu$m in the spectral energy distribution of Class 0 type young stellar objects., Comment: 17 pages, 14 figures, accepted by MNRAS

We address the turbulent fragmentation scenario for the origin of the stellar initial mass function (IMF), using a large set of numerical simulations of randomly driven supersonic MHD turbulence. The turbulent fragmentation model successfully predicts the main features of the observed stellar IMF assuming an isothermal equation of state without any stellar feedback. As a test of the model, we focus on the case of a magnetized isothermal gas, neglecting stellar feedback, while pursuing a large dynamic range in both space and timescales covering the full spectrum of stellar masses from brown dwarfs to massive stars. Our simulations represent a generic 4 pc region within a typical Galactic molecular cloud, with a mass of 3000 Msun and an rms velocity 10 times the isothermal sound speed and 5 times the average Alfven velocity, in agreement with observations. We achieve a maximum resolution of 50 au and a maximum duration of star formation of 4.0 Myr, forming up to a thousand sink particles whose mass distribution closely matches the observed stellar IMF. A large set of medium-size simulations is used to test the sink particle algorithm, while larger simulations are used to test the numerical convergence of the IMF and the dependence of the IMF turnover on physical parameters predicted by the turbulent fragmentation model. We find a clear trend toward numerical convergence and strong support for the model predictions, including the initial time evolution of the IMF. We conclude that the physics of isothermal MHD turbulence is sufficient to explain the origin of the IMF., Comment: 25 pages, 21 figures, Accepted by ApJ

We compute the star formation rate (SFR) in molecular clouds (MCs) that originate {\it ab initio} in a new, higher-resolution simulation of supernova-driven turbulence. Because of the large number of well-resolved clouds with self-consistent boundary and initial conditions, we obtain a large range of cloud physical parameters with realistic statistical distributions, an unprecedented sample of star-forming regions to test SFR models and to interpret observational surveys. We confirm the dependence of the SFR per free-fall time, $SFR_{\rm ff}$, on the virial parameter, $\alpha_{\rm vir}$, found in previous simulations, and compare a revised version of our turbulent fragmentation model with the numerical results. The dependences on Mach number, ${\cal M}$, gas to magnetic pressure ratio, $\beta$, and compressive to solenoidal power ratio, $\chi$ at fixed $\alpha_{\rm vir}$ are not well constrained, because of random scatter due to time and cloud-to-cloud variations in $SFR_{\rm ff}$. We find that $SFR_{\rm ff}$ in MCs can take any value in the range $0 \le SFR_{\rm ff} \lesssim 0.2$, and its probability distribution peaks at a value $SFR_{\rm ff}\approx 0.025$, consistent with observations. The values of $SFR_{\rm ff}$ and the scatter in the $SFR_{\rm ff}$--$\alpha_{\rm vir}$ relation are consistent with recent measurements in nearby MCs and in clouds near the Galactic center. Although not explicitly modeled by the theory, the scatter is consistent with the physical assumptions of our revised model and may also result in part from a lack of statistical equilibrium of the turbulence, due to the transient nature of MCs., Comment: ApJ, in press

The current study was developed to provide a database of relatively simple numerical simulations of protostellar collapse, as a template library for observations of cores and very young protostars, and for researchers who wish to test their chemical modeling under dynamic astrophysical conditions. It was also designed to identify statistical trends that may appear when running many models of the formation of low-mass stars by varying the initial conditions. A large set of 143 calculations of the gravitational collapse of an isolated sphere of gas with uniform temperature and a Bonnor-Ebert like density profile was undertaken using a 1D fully implicit Lagrangian radiation hydrodynamics code. The parameter space covered initial masses from 0.2 to 8 Msun, temperatures of 5-30 K and radii between 3000 and 30,000 AU. A spread in the thermal evolutionary tracks of the runs was found, due to differing initial conditions and optical depths. Within less than an order of magnitude, all first and second Larson cores had masses and radii independent of the initial conditions. The time elapsed between the formation of the first and second cores was found to strongly depend on the first core mass accretion rate, and no first core in our grid of models lived for longer than 2000 years, before the onset of the second collapse. The end product of a protostellar cloud collapse, the second Larson core, is, at birth, a canonical object with a mass and radius of about 3 Mjup and 8 Rjup, independent of its initial conditions. The evolution sequence which brings the gas to stellar densities can however proceed in a variety of scenarios, on different timescales, along different isentropes, but each story line can largely be predicted by the initial conditions. All the data from the simulations are publicly available at this address: http://starformation.hpc.ku.dk/grid-of-protostars., Comment: 24 pages, 14 figures, accepted for publication in A&A

We present a comparison of molecular clouds (MCs) from a simulation of supernova-driven interstellar medium (ISM) turbulence with real MCs from the Outer Galaxy Survey. The radiative transfer calculations to compute synthetic CO spectra are carried out assuming the CO relative abundance depends only on gas density, according to four different models. Synthetic MCs are selected above a threshold brightness temperature value, $T_{\rm B,min}=1.4$ K, of the $J=1-0$ $^{12}$CO line, generating 16 synthetic catalogs (four different spatial resolutions and four CO abundance models), each containing up to several thousands MCs. The comparison with the observations focuses on the mass and size distributions and on the velocity-size and mass-size Larson relations. The mass and size distributions are found to be consistent with the observations, with no significant variations with spatial resolution or chemical model, except in the case of the unrealistic model with constant CO abundance. The velocity-size relation is slightly too steep for some of the models, while the mass-size relation is a bit too shallow for all models only at a spatial resolution $dx\approx 1$ pc. The normalizations of the Larson relations show a clear dependence on spatial resolution, for both the synthetic and the real MCs. The comparison of the velocity-size normalization suggests that the SN rate in the Perseus arm is approximately 70\% or less of the rate adopted in the simulation. Overall, the realistic properties of the synthetic clouds confirm that supernova-driven turbulence can explain the origin and dynamics of MCs., Comment: accepted for publication in ApJ

Context: Understanding how protostars accrete their mass is a central question of star formation. One aspect of this is trying to understand whether the time evolution of accretion rates in deeply embedded objects is best characterised by a smooth decline from early to late stages or by intermittent bursts of high accretion. Aims: We create synthetic observations of deeply embedded protostars in a large numerical simulation of a molecular cloud, which are compared directly to real observations. The goal is to compare episodic accretion events in the simulation to observations and to test the methodology used for analysing the observations. Methods: Simple freeze-out and sublimation chemistry is added to the simulation, and synthetic C$^{18}$O line cubes are created for a large number of simulated protostars. The spatial extent of C$^{18}$O is measured for the simulated protostars and compared directly to a sample of 16 deeply embedded protostars observed with the Submillimeter Array. If CO is distributed over a larger area than predicted based on the protostellar luminosity, it may indicate that the luminosity has been higher in the past and that CO is still in the process of refreezing. Results: Approximately 1% of the protostars in the simulation show extended C$^{18}$O emission, as opposed to approximately 50% in the observations, indicating that the magnitude and frequency of episodic accretion events in the simulation is too low relative to observations. The protostellar accretion rates in the simulation are primarily modulated by infall from the larger scales of the molecular cloud, and do not include any disk physics. The discrepancy between simulation and observations is taken as support for the necessity of disks, even in deeply embedded objects, to produce episodic accretion events of sufficient frequency and amplitude., Comment: Accepted for publication in A&A, 11 pages, 8 figures; v2 contains minor updates to the language

(abridged) Context: Both observations and simulations of embedded protostars have progressed rapidly in recent years. Bringing them together is an important step in advancing our knowledge about the earliest phases of star formation. Aims: To compare synthetic continuum images and SEDs, calculated from large-scale numerical simulations, to observational studies, thereby aiding in both the interpretation of the observations and in testing the fidelity of the simulations. Methods: The radiative transfer code RADMC-3D is used to create synthetic continuum images and SEDs of protostellar systems in a large numerical simulation of a molecular cloud. More than 13000 unique radiative transfer models are produced of a variety of different protostellar systems. Results: Over the course of 0.76 Myr the simulation forms more than 500 protostars, primarily within two sub-clusters. Synthetic SEDs are used to the calculate evolutionary tracers Tbol and Lsmm/Lbol. It is shown that, while the observed distributions of the tracers are well matched by the simulation, they generally do a poor job of tracking the protostellar ages. Disks form early in the simulation, with 40 % of the Class 0 protostars being encircled by one. The flux emission from the simulated disks is found to be, on average, a factor of 6 too low relative to real observations. The distribution of protostellar luminosities spans more than three order of magnitudes, similar to the observed distribution. Cores and protostars are found to be closely associated with one another, with the distance distribution between them being in excellent agreement with observations. Conclusions: The analysis and statistical comparison of synthetic observations to real ones is established as a powerful tool in the interpretation of observational results., Comment: Accepted for publication in A&A, 20 pages, 16 figures. Corrected typos

The compressibility of molecular cloud (MC) turbulence plays a crucial role in star formation models, because it controls the amplitude and distribution of density fluctuations. The relation between the compressive ratio (the ratio of powers in compressive and solenoidal motions) and the statistics of turbulence has been previously studied systematically only in idealized simulations with random external forces. In this work, we analyze a simulation of large-scale turbulence (250 pc) driven by supernova (SN) explosions that has been shown to yield realistic MC properties. We demonstrate that SN driving results in MC turbulence with a broad lognormal distribution of the compressive ratio, with a mean value $\approx 0.3$, lower than the equilibrium value of $\approx 0.5$ found in the inertial range of isothermal simulations with random solenoidal driving. We also find that the compressibility of the turbulence is not noticeably affected by gravity, nor are the mean cloud radial (expansion or contraction) and solid-body rotation velocities. Furthermore, the clouds follow a general relation between the rms density and the rms Mach number similar to that of supersonic isothermal turbulence, though with a large scatter, and their average gas density PDF is described well by a lognormal distribution, with the addition of a high-density power-law tail when self-gravity is included., Comment: Accepted by ApJ

We have carefully studied how local measurements of the Hubble constant, $H_0$, can be influenced by a variety of different parameters related to survey geometry, depth, and size, as well as observer position in space. Our study is based on N-body simulations of structure in the standard $\Lambda$CDM model and our conclusion is that the expected variance in measurements of $H_0$ is far too small to explain the current discrepancy between the low value of $H_0$ inferred from measurements of the cosmic microwave background (CMB) by the Planck collaboration and the value measured directly in the local universe by use of Type Ia supernovae. This conclusion is very robust and does not change with different assumptions about survey geometry and effective sky coverage or observer position in space., Comment: 15 pages, 9 figures

We investigate the role of mass infall in the formation and evolution of protostars. To avoid ad hoc initial and boundary conditions, we consider the infall resulting self-consistently from modeling the formation of stellar clusters in turbulent molecular clouds. We show that infall rates in turbulent clouds are comparable to accretion rates inferred from protostellar luminosities or measured in pre-main-sequence stars. They should not be neglected in modeling the luminosity of protostars and the evolution of disks, even after the embedded protostellar phase. We find large variations of infall rates from protostar to protostar, and large fluctuations during the evolution of individuals protostars. In most cases, the infall rate is initially of order 10$^{-5}$\msun\ yr$^{-1}$, and may either decay rapidly in the formation of low-mass stars, or remain relatively large when more massive stars are formed. The simulation reproduces well the observed characteristic values and scatter of protostellar luminosities and matches the observed protostellar luminosity function. The luminosity problem is therefore solved once realistic protostellar infall histories are accounted for, with no need for extreme accretion episodes. These results are based on a simulation of randomly-driven magneto-hydrodynamic turbulence on a scale of 4pc, including self-gravity, adaptive-mesh refinement to a resolution of 50AU, and accreting sink particles. The simulation yields a low star formation rate, consistent with the observations, and a mass distribution of sink particles consistent with the observed stellar initial mass function during the whole duration of the simulation, forming nearly 1,300 sink particles over 3.2 Myr., Comment: 21 pages, 16 figures, accepted for publication in ApJ

Context: The R CrA cloud hosts a handful of Class 0/I low-mass young stellar objects. The chemistry and physics at scales $>500$ AU in this cloud are dominated by the irradiation from the nearby Herbig Be star R CrA. The luminous large-scale emission makes it necessary to use high-resolution spectral imaging to study the chemistry and dynamics of the inner envelopes and discs of the protostars. Aims: We aim to better understand the structure of the inner regions of these protostars and, in particular, the interplay between the chemistry and the presence of discs. Methods: Using Atacama Large Millimeter/submillimeter Array (ALMA) high-resolution spectral imaging interferometry observations, we study the molecular line and dust continuum emission at submillimetre wavelengths. Results: We detect dust continuum emission from four circumstellar discs around Class 0/I objects within the R CrA cloud. Towards IRS7B we detect C$^{17}$O emission showing a rotation curve consistent with a Keplerian disc with a well-defined edge that gives a good estimate for the disc radius at 50 AU. We derive the central object mass to $2.3M_{\odot}$ and the disc mass to $0.024M_{\odot}$. The observations are also consistent with a model of material infalling under conservation of angular momentum; however, this model provides a worse fit to the data. We also report a likely detection of faint CH$_3$OH emission towards this point source, as well as more luminous CH$_3$OH emission in an outflow orthogonal to the major axis of the C$^{17}$O emission. Conclusions: The faint CH$_3$OH emission seen towards IRS7B can be explained by a flat density profile of the inner envelope caused by the disc with a radius $\lesssim50$ AU. We propose that the regions of the envelopes where complex organic molecules are present in Class 0/I young stellar objects can become quenched as the disc grows., Comment: Accepted for publication in A&A; 11 pages, 9 figures

We present a novel method for implementing massive neutrinos in N-body simulations. Instead of sampling the neutrino velocity distribution by individual point particles we take neutrino free-streaming into account by treating it as an effective redshift dependent sound speed in a perfect isothermal fluid, and assume a relation between the sound speed and velocity dispersion of the neutrinos. Although the method fails to accurately model the true neutrino power spectrum, it is able to calculate the total matter power spectrum to the same accuracy as more complex hybrid neutrino methods, except on very small scales. We also present an easy way to update the publicly available Gadget-2 version with this neutrino approximation., Comment: 13 pages, 7 figures

We present an analysis of peculiar velocities and their effect on supernova cosmology. In particular, we study (a) the corrections due to our own motion, (b) the effects of correlations in peculiar velocities induced by large-scale structure, and (c) uncertainties arising from a possible local under- or over-density. For all of these effects we present a case study of their impact on the cosmology derived by the Sloan Digital Sky Survey-II Supernova Survey (SDSS-II SN Survey). Correcting supernova redshifts for the CMB dipole slightly over-corrects nearby supernovae that share some of our local motion. We show that while neglecting the CMB dipole would cause a shift in the derived equation of state of Delta w ~ 0.04 (at fixed matter density) the additional local-motion correction is currently negligible (Delta w<0.01). We use a covariance-matrix approach to statistically account for correlated peculiar velocities. This down-weights nearby supernovae and effectively acts as a graduated version of the usual sharp low-redshift cut. Neglecting coherent velocities in the current sample causes a systematic shift of ~2% in the preferred value of w and will therefore have to be considered carefully when future surveys aim for percent-level accuracy. Finally, we perform n-body simulations to estimate the likely magnitude of any local density fluctuation (monopole) and estimate the impact as a function of the low-redshift cutoff. We see that for this aspect the low-z cutoff of z=0.02 is well-justified theoretically, but that living in a putative local density fluctuation leaves an indelible imprint on the magnitude-redshift relation., Comment: 16 pages, 12 figures, accepted by ApJ

Even in a universe that is homogeneous on large scales, local density fluctuations can imprint a systematic signature on the cosmological inferences we make from distant sources. One example is the effect of a local under-density on supernova cosmology. Also known as a Hubble-bubble, it has been suggested that a large enough under-density could account for the supernova magnitude- redshift relation without the need for dark energy or acceleration. Although the size and depth of under-density required for such an extreme result is extremely unlikely to be a random fluctuation in an on-average homogeneous universe, even a small under-density can leave residual effects on our cosmological inferences. In this paper we show that there remain systematic shifts in our cosmological parameter measure- ments, even after excluding local supernovae that are likely to be within any small Hubble-bubble. We study theoretically the low-redshift cutoff typically imposed by supernova cosmology analyses, and show that a low-redshift cut of z0 \sim 0.02 may be too low based on the observed inhomogeneity in our local universe. Neglecting to impose any low-redshift cutoff can have a significant effect on the cosmological pa- rameters derived from supernova data. A slight local under-density, just 30% under-dense with scale 70h^{-1} Mpc, causes an error in the inferred cosmological constant density {\Omega}{\Lambda} of \sim 4%. Imposing a low-redshift cutoff reduces this systematic error but does not remove it entirely. A residual systematic shift of 0.99% remains in the inferred value {\Omega}{\Lambda} even when neglecting all data within the currently pre- ferred low-redshift cutoff of 0.02. Given current measurement uncertainties this shift is not negligible, and will need to be accounted for when future measurements yield higher precision., Comment: 12 pages, 11 figures. Accepted for publication in The Astrophysical Journal