arXiv subjects

Enrique Vázquez-Semadeni

Publications and source records attributed to Enrique Vázquez-Semadeni.

At least 19 recordsLinked to original sources

arXiv2026

The ALMA-QUARKS Survey: Discovery of Superfine Dusty Fibrils inside Massive Star-forming Clumps

We report the discovery of more than 320 superfine dusty filamentary structures (fibrils) inside 121 massive star forming clumps that are located in widely different Galactic environments (Galactocentric distances of ~1.2 - 12.9 kpc). These fibrils are identified from the 1.3-mm continuum emission in the ALMA-QUARKS survey, which has a linear resolution of ~900 AU for a source at ~3 kpc. The typical width of these fibrils identified in clumps is found to be ~0.01 pc, which is about ten times narrower than that of dusty filaments in nearby clouds identified by the Herschel Space Observatory. The mass (M) versus length (l) relation for these fibrils follows M ~ l^2, similar to that of Galactic filaments identified in space (e.g., Herschel) and ground-based single-dish (e.g., APEX) surveys. However, these fibrils in the ALMA-QUARKS survey are significantly denser (N_{H_2} = 10^{23} - 10^{24} cm^{-2}) than the filaments found in previous Herschel surveys (N_{H_2} = 10^{20} - 10^{23} cm^{-2}). This work contributes a large sample of newly found superfine fibrils in massive clumps, following the identification of large 0.1 pc-wide filaments and associated internal velocity coherent molecular fibers in nearby molecular clouds, further emphasizing the crucial role played by filamentary structures in star formation at various physical scales.

astro-ph.GA↗

arXiv2026

Effect of gravity-driven longitudinal flows in filaments on angular momentum transport to embedded cores

Different models of filament formation predict distinct patterns of angular momentum redistribution toward embedded cores, set by the underlying velocity-field structure, which can set the initial conditions for a preferential orientation between protostellar outflows and filaments. However, the absence of a dominant alignment in observations keeps this connection open to debate. We investigate whether gravity-driven longitudinal flows along filaments can redistribute angular momentum (AM) toward collapse centers. To this end, we analyze the distributions of 3D and 2D-projected angles between sink angular momentum vectors and host filament orientations in an SPH simulation of giant molecular cloud and filament formation. We also characterize the filament velocity field by measuring the angles between SPH particle velocity vectors and filament axes, and the degree of convergent flow toward filament density peaks. No preferred alignment between the sinks' AM and the filament direction is found at early evolutionary stages, neither in 3D nor in 2D. Later, however, a predominantly perpendicular configuration emerges in 3D. Tracking individual sinks indicates that this alignment is not primordial but develops as gravity strengthens. In individual filaments, the onset of perpendicular alignment coincides with the development of convergent longitudinal flows. Finally, we estimate the minimum excess fraction of intrinsically perpendicular 3D orientations needed for a projected sample to retain a detectable perpendicular alignment in 2D as a function of sample size, and the minimum sample size for the trend to be detectable in projection, which is at least $\sim 35$ measurements, making conclusions from limited 2D samples potentially misleading.

astro-ph.GA↗

arXiv2026

The efficiency per free-fall time as a ratio of the Star Formation Rate to the gas-infall rate in collapsing cores: dependence on the core definition, accretion, and radial structure

A parameter used to characterise star formation activity in MCs is the efficiency per free-fall time, $ε_{\rm ff}$, although commonly referred to as an efficiency, it is formally the ratio between the star formation rate (SFR) and the gas-infall rate. Here we numerically study the collapse of cores and define $ε_{\rm ff}\equiv\langle\dot{M}_\star\rangle/(M_{\rm core}/τ_{\rm ff})$, where $\langle\dot{M}_\star\rangle$ is the average SFR, $M_{\rm core}$ is the gas mass within the core (as the gas cells above a density threshold), and $τ_{\rm ff}$ is the free-fall time of the core gas. We perform simplified numerical experiments of the gravitational collapse of an isolated core, varying the initial mean number density ($n_0=100$ and $1000~\rm cm^{-3}$) and adopting closed/open BCs to (dis)allow fresh gas accretion into the domain. The simulations start with a slight central Gaussian overdensity that evolved into a power-law profile, $n\propto r^{-p}$ with $p\to2$. As the collapse proceeds, a sink particle forms in the center of the core. We find that both the BCs and the adopted core definition modify the measured core properties and, consequently, the inferred $ε_{\rm ff}$. Low-density models have less mass available, and their accretion histories are therefore much more sensitive to the choice of BCs, while high-density runs, with their larger mass reservoirs, maintain similar accretion histories regardless of the BCs. In all models, after sink formation, $ε_{\rm ff}$ rises and then remains relatively stable while accretion continues to replenish the core's mass, but increases once the gas reservoir is exhausted. Somewhat counterintuitively, $ε_{\rm ff}$ is higher in the low-mass cores, since the larger gas infall rates onto the high-mass cores compensate for their higher SFR. We conclude that the inferred $ε_{\rm ff}$ depends sensitively on both the adopted core definition and external mass supply

astro-ph.SR↗

arXiv2026

Evidence for the gravity-driven and magnetically-regularized gas flows feeding the massive protostellar cluster in Cepheus A

The hierarchical interplay among gravity, magnetic fields, and turbulence in forming massive protostellar clusters remains elusive. We present high-resolution ($\sim$14 arcsec $\simeq$ 0.05 pc) 850 $μ$m dust polarization and C$^{18}$O line observations of Cepheus A using JCMT SCUBA-2/POL-2 and HARP. Our analysis reveals aligned gravitational (G), magnetic (B), and velocity fields (K), with an energy hierarchy of $E_{\mathrm{G}}$ > $E_{\mathrm{B}}$ > $E_{\mathrm{K}}$. Gravity, as the primary driver, induces gas flows and drags in B-field lines. Magnetic tension, as a secondary force, regulates turbulence, enabling ordered flows with an accretion rate of $\sim$ 2.1 $\pm$ 0.4 $\times$ 10$^{-4}$ M$_\odot$ yr$^{-1}$. This challenges the conventional view of B-fields resisting collapse in the clump/hub scale, instead showing cooperation with gravity. The $\sim$0.6 pc clump-scale B-field (with mean PA $\sim$ 45°) aligns coherently with fields at cloud ($\sim$5 pc), core ($\sim$0.05 pc), and disk ($\sim$2000 AU) scales, offering new insights into the role of magnetic fields in multiscale star formation dynamics.

astro-ph.GA↗

arXiv2025

What is the contribution of gravitational infall on the mass assembly of star-forming clouds? A case study in a numerical simulation of the interstellar medium

Star formation in galaxies is a complex phenomenon occurring on a very wide range of scales, and molecular clouds are at the heart of this process. The formation of these structures and the subsequent collapse of the gas within them to form new stars remain unresolved scientific questions. In particular, the role and importance of gravity at between the disk scale height and prestellar cores (100 to 0.01 pc) are still topics of debate. In this work, we conduct a case study examining the mass assembly and evolution of a giant molecular cloud complex in a numerical stratified-box simulation of the interstellar medium with photo-ionizing and supernova driving and resolving down to scales $\gtrsim 1$ pc and densities up to $10^3$ cm$^{-3}$. By introducing tracer particles to precisely track the forces acting on the gas during its evolution towards and within the clouds, we are able to quantify how much of the mass inflow is driven by the self-gravity of the gas and the gravity from the stellar disk. We find that up to 20% of the gas is gravity-driven at a scale of 100 pc, contributing 10% of the inflow from the warm to the cold phase and 20% from the cold phase to the individual molecular clouds, reaching up to 45% inside the molecular gas, at densities $\gtrsim 400$ cm$^{-3}$. However, at the 100 pc scale, the contribution of gravity-driven gas on the linewidth is negligible. We conclude that the bulk of the gas motions assembling the clouds in our simulation are caused by the supernova-driven supersonic turbulence, which results in locally convergent flows, with a small contribution from the stellar gravitational potential.

astro-ph.GA↗

arXiv2025

Gravity or turbulence? VII. The Schmidt-Kennicutt law, the star formation efficiency, and the mass density of clusters from gravitational collapse rather than turbulent support

We explore the Schmidt-Kennicutt (SK) relations and the star formation efficiency per free-fall time ($ε_{\rm ff}$), mirroring observational studies, in numerical simulations of filamentary molecular clouds undergoing gravitational contraction. We find that (a)~collapsing clouds accurately replicate the observed SK relations for galactic clouds and (b)~$ε_{\rm ff}$ is small and constant in space and in time, with values similar to those found in local clouds. We propose that this constancy arises from the similar radial scaling of the free-fall time ($τ_{\rm ff}$) and the internal mass in density structures with spherically-averaged density profiles near $r^{-2}$. We additionally show that (c)~the star formation rate (SFR) increases rapidly in time; (d)~the low values of $ε_{\rm ff}$ result from evaluating $τ_{\rm ff}$ and the characteristic star-formation time scale over different time intervals, combined with the increasing SFR, and (e)~the fact that star clusters are significantly denser than the gas clumps from which they form is a natural consequence of the rapidly increasing SFR, the continuous replenishment of the star-forming gas by the accretion flow, and the near $r^{-2}$ density profile induced by the collapse. Finally, we argue that interpreting $ε_{\rm ff}$ as an efficiency is problematic since it is not bounded by unity, and because the gas mass in clouds evolves. Instead, we propose that viewing $ε_{\rm ff}$ as the ratio of the actual SFR to the gas free-fall rate. In summary, our results show that the SK relation, the low values of $ε_{\rm ff}$, and the mass density of stellar clusters arise naturally from gravitational contraction.

astro-ph.GA↗

arXiv2025

The Turbulent Support (TS) and Global Hierarchical Collapse (GHC) models for molecular clouds compared. Differences, convergence, and myths

We provide a detailed comparison between the ``turbulent support'' (TS) and ``global hierarchical collapse'' (GHC) models for molecular clouds and star formation, their respective interpretations of the observational data, the features they share, and suggested tests and observations to discern between them. Also, we clarify common misconceptions in recent literature about the global and hierarchical nature of the GHC scenario, and briefly discuss the evolution of some aspects of both models toward convergence. TS assumes that star-forming molecular clouds and their substructures are either in approximate virial equilibrium between gravity and turbulence or overvirial, so that the cloud is either confined or expanding, and its substructures (clumps, filaments and cores) are produced by turbulent compressions. In this scheme, the star formation rate (SFR) is time-independent and determined by the turbulent and gravitational parameters of the clouds, in particular the virial parameter $\av$. Conversely, GHC assumes that most star-forming molecular clouds and their substructures are part of a continuous gravitationally-driven flow, each accreting from their parent structure. Therefore, GHC is an intrinsically {\it evolutionary} model for the clouds and their star formation rate, determined by the evolution of the collapse flow. It interprets nonthermal motions as a mixture of infall and turbulent components, with the relative importance of the former increasing as the objects become denser and/or more massive, and thus $\av$ is an {\it evolving variable} of the clouds. Tests that may provide clues to distinguishing between TS and GHC must take into account that the innermost parts of globally gravitationally bound structures may not locally appear bound, and thus the binding may have to be searched for at the largest scale of the structure.

astro-ph.GA↗

arXiv2025

A neutral hydrogen absorption study of cold gas in the outskirts of the Magellanic Clouds using the GASKAP-HI survey

Cold neutral hydrogen (HI) is a crucial precursor for molecular gas formation and can be studied via HI absorption. This study investigates HI absorption in low column density regions of the Small and Large Magellanic Clouds (SMC and LMC) using the Galactic-ASKAP HI (GASKAP-HI) survey, conducted by the Australian Square Kilometer Array Pathfinder (ASKAP). We select 10 SMC directions in the outer regions and 18 LMC directions, with 4 in the outskirts and 14 within the main disk. Using the radiative transfer method, we decompose the emission and absorption spectra into individual cold neutral medium (CNM) and warm neutral medium (WNM) components. In the SMC, we find HI peak optical depths of 0.09-1.16, spin temperatures of ~20-50 K, and CNM fractions of 1-11%. In the LMC, optical depths range from 0.03 to 3.55, spin temperatures from ~10 to 100 K, and CNM fractions from 1% to 100%. The SMC's low CNM fractions likely result from its low metallicity and large line-of-sight depth. Additionally, the SMC's outskirts show lower CNM fractions than the main body, potentially due to increased CNM evaporation influenced by the hot Magellanic Corona. Shell motions dominate the kinematics of the majority of CNM clouds in this study and likely supply cold HI to the Magellanic Stream. In the LMC, high CNM fraction clouds are found near supergiant shells, where thermal instability induced by stellar feedback promotes WNM-to-CNM transition. Although no carbon monoxide (CO) has been detected, enhanced dust shielding in these areas helps maintain the cold HI.

astro-ph.GA↗

arXiv2025

The role of shocks and the velocity gradient in the relative orientation of the magnetic field and dense gas clouds

We investigate the alignment mechanism between the magnetic field and interstellar clouds formed by the collision of warm atomic gas. We find that the magnetic field, initially oriented parallel to the flow, is perturbed by a fast MHD shock, which amplifies magnetic field fluctuations parallel to the shock front. Behind the shock, the compressive downstream velocity field further amplifies the magnetic field component parallel to the shock front. This mechanism causes the magnetic field to become increasingly parallel to the dense layer, and the development of a shear flow around the latter. Furthermore, the bending-mode perturbations on the dense layer are amplified by the non-linear thin-shell instability (NTSI), stretching the density structures formed by the thermal instability, and rendering them parallel to the bent field lines. By extension, we suggest that a tidal stretching velocity gradient such as that produced in gas infalling into a self-gravitating structure must straighten the field lines along the accretion flow, orienting them perpendicular to the density structures. We also find that the upstream superalfvénic regime transitions to a transalfvénic regime between the shock and the condensation front, and then to a subalfvénic regime inside the condensations.

astro-ph.GA↗

arXiv2024

ATOMS: ALMA Three-millimeter Observations of Massive Star-forming regions $-$ XVII. High-mass star-formation through a large-scale collapse in IRAS 15394$-$5358

Hub-filament systems are considered as natural sites for high-mass star formation. Kinematic analysis of the surroundings of hub-filaments is essential to better understand high-mass star formation within such systems. In this work, we present a detailed study of the massive Galactic protocluster IRAS 15394$-$5358, using continuum and molecular line data from the ALMA Three-millimeter Observations of Massive Star-forming Regions (ATOMS) survey. The 3~mm dust continuum map reveals the fragmentation of the massive ($\rm M=843~M_{\odot}$) clump into six cores. The core C-1A is the largest (radius = 0.04~pc), the most massive ($\rm M=157~M_{\odot}$), and lies within the dense central region, along with two smaller cores ($\rm M=7~and~3~M_{\odot}$). The fragmentation process is consistent with the thermal Jeans fragmentation mechanism and virial analysis shows that all the cores have small virial parameter values ($\rm α_{vir}<<2$), suggesting that the cores are gravitationally bound. The mass vs. radius relation indicates that three cores can potentially form at least a single massive star. The integrated intensity map of $\rm H^{13}CO^{+}$ shows that the massive clump is associated with a hub-filament system, where the central hub is linked with four filaments. A sharp velocity gradient is observed towards the hub, suggesting a global collapse where the filaments are actively feeding the hub. We discuss the role of global collapse and the possible driving mechanisms for the massive star formation activity in the protocluster.

astro-ph.GA↗

arXiv2024

On the properties and implications of collapse-driven MHD turbulence

We numerically investigate the driving of MHD turbulence by gravitational contraction using simulations of an initially spherical, magnetically supercritical cloud core with initially transonic and trans-Alfvénic turbulence. We perform a Helmholtz decomposition of the velocity field, and investigate the evolution of its solenoidal and compressible parts, as well as of the velocity component along the gravitational acceleration vector, a proxy for the infall component of the velocity field. We find that: 1) In spite of being supercritical, the core first contracts to a sheet perpendicular to the mean field, and the sheet itself collapses. 2) The solenoidal component of the turbulence remains at roughly its initial level throughout the simulation, while the compressible component increases continuously. This implies that turbulence does {\it not} dissipate towards the center of the core. 3) The distribution of simulation cells in the $B$-$ρ$ plane occupies a wide triangular region at low densities, bounded below by the expected trend for fast MHD waves ($B \propto ρ$, applicable for high local Alfvénic Mach number $\Ma$) and above by the trend expected for slow waves ($B \sim$ constant, applicable for low local $\Ma$). At high densities, the distribution follows a single trend $B \propto ρ^{\gamef}$, with $1/2 < \gamef < 2/3$, as expected for gravitational compression. 4) The measured mass-to-magnetic flux ratio $λ$ increases with radius $r$, due to the different scalings of the mass and magnetic flux with $r$. At a fixed radius, $λ$ increases with time due to the accretion of material along field lines. 5) The solenoidal energy fraction is much smaller than the total turbulent component, indicating that the collapse drives the turbulence mainly compressibly, even in directions orthogonal to that of the collapse.

astro-ph.GA↗

arXiv2024

Multi-scale accretion in dense cloud cores and the delayed formation of massive stars

The formation mechanism of massive stars remains one of the main open problems in astrophysics, in particular the relationship between the mass of the most massive stars, and that of the cores in which they form. Numerical simulations of the formation and evolution of large molecular clouds, within which dense cores and stars form self-consistently, show in general that the cores' masses increase in time, and also that the most massive stars tend to appear later (by a few to several Myr) than lower-mass stars. Here we present an idealized model that incorporates accretion onto the cores as well as onto the stars, in which the core's mass growth is regulated by a ``gravitational choking'' mechanism that does not involve any form of support. This process is of purely gravitational origin, and causes some of the mass accreted onto the core to stagnate there, rather than being transferred to the central stars. Thus, the simultaneous mass growth of the core and of the stellar mass can be computed. In addition, we estimate the mass of the most massive allowed star before its photoionizing radiation is capable of overcoming the accretion flow onto the core. This model constitutes a proof-of-concept for the simultaneous growth of the gas reservoir and the stellar mass, the delay in the formation of massive stars observed in cloud-scale numerical simulations, the need for massive, dense cores in order to form massive stars, and the observed correlation between the mass of the most massive star and the mass of the cluster it resides in. Also, our model implies that by the time massive stars begin to form in a core, a number of low-mass stars are expected to have already formed.

astro-ph.GA↗

arXiv2023

The ALMA Survey of Star Formation and Evolution in Massive Protoclusters with Blue Profiles (ASSEMBLE): Core Growth, Cluster Contraction, and Primordial Mass Segregation

The ALMA Survey of Star Formation and Evolution in Massive Protoclusters with Blue Profiles (ASSEMBLE) aims to investigate the process of mass assembly and its connection to high-mass star formation theories in protoclusters in a dynamic view. We observed 11 massive (Mclump>1000 Msun), luminous (Lbol>10,000 Lsun), and blue-profile (infall signature) clumps by ALMA with resolution of 2200-5500 au at 350 GHz (870 um) in continuum and line emission. 248 dense cores were identified, including 106 cores showing protostellar signatures and 142 prestellar core candidates. Compared to early-stage infrared dark clouds (IRDCs) by ASHES, the core mass and surface density within the ASSEMBLE clumps exhibited significant increment, suggesting concurrent core accretion during the evolution of the clumps. The maximum mass of prestellar cores was found to be 2 times larger than that in IRDCs, indicating evolved protoclusters have the potential to harbor massive prestellar cores. The mass relation between clumps and their most massive core (MMCs) is observed in ASSEMBLE but not in IRDCs, which is suggested to be regulated by multiscale mass accretion. The mass correlation between the core clusters and their MMCs has a steeper slope compared to that observed in stellar clusters, which can be due to fragmentation of the MMC and stellar multiplicity. We observe a decrease in core separation and an increase in central concentration as protoclusters evolve. We confirm primordial mass segregation in the ASSEMBLE protoclusters, possibly resulting from gravitational concentration and/or gas accretion.

astro-ph.GA↗

arXiv2023

The kinetic and magnetic energy budget of hub-filament systems during the gravitational fragmentation of molecular clouds

We present a numerical study of the balance between the gravitational (Eg), kinetic (Ek), and magnetic (Em) energies of structures within a hub-filament system in a simulation of the formation and global hierarchical collapse (GHC) of a giant molecular cloud. For structures defined by various density thresholds, and at different evolutionary stages, we investigate the scaling of the virial parameter, $α$, with mass $M$, and of the Larson ratio, ${\cal{L}}\equivσ_v/R^{1/2}$, with column density $Σ$, where $σ_v$ is the 1D velocity dispersion, and $R$ is an effective radius. We also investigate these scalings for the corresponding magnetic parameters $α_m$ and ${\cal{L}}_m$. Finally, we compare our numerical results with an observational sample of massive clumps. We find that: 1) $α_m$ and ${\cal{L}}_m$ follow similar scalings as their kinetic counterparts, although the ratio Em/Ek decreases as |Eg| increases. 2) The largest objects, defined by the lowest thresholds, tend to appear gravitationally bound (and magnetically supercritical), while their internal substructures tend to appear unbound (and subcritical). This suggests that the latter are being compressed by the infall of their parent structures, and supports earlier suggestions that the measured mass-to-magnetic flux ratio $μ$ decreases inwards in a centrally-peaked cloud under ideal MHD. 3)~The scatter in the $α$-$M$ and ${\cal{L}}$-$Σ$ plots is reduced when Ek and Em are plotted directly against Eg, suggesting that the scatter is due to an ambiguity between mass and size. 4) The clumps in our GHC simulation follow the same trends as the observational sample of massive clumps in the $α$-$M$ and ${\cal{L}}$-$Σ$ diagrams. We conclude that the main controlling parameter of the energy budget in the structures is Eg, with the kinetic and magnetic energies being derived from it.

astro-ph.GA↗

arXiv2023

Simulated observations of star formation regions: infrared evolution of globally collapsing clouds

The direct comparison between hydrodynamical simulations and observations is needed to improve the physics included in the former and test biases in the latter. Post-processing radiative transfer and synthetic observations are now the standard way to do this. We report on the first application of the \texttt{SKIRT} radiative transfer code to simulations of a star-forming cloud. The synthetic observations are then analyzed following traditional observational workflows. We find that in the early stages of the simulation, stellar radiation is inefficient in heating dust to the temperatures observed in Galactic clouds, thus the addition of an interstellar radiation field is necessary. The spectral energy distribution of the cloud settles rather quickly after $\sim3$ Myr of evolution from the onset of star formation, but its morphology continues to evolve for $\sim8$ Myr due to the expansion of \textsc{Hii} regions and the respective creation of cavities, filaments, and ridges. Modeling synthetic \textit{Herschel} fluxes with 1- or 2-component modified black bodies underestimates total dust masses by a factor of $\sim2$. Spatially-resolved fitting recovers up to about $70\%$ of the intrinsic value. This ``missing mass'' is located in a very cold dust component with temperatures below $10$ K, which does not contribute appreciably to the far-infrared flux. This effect could bias real observations if such dust exists in large amounts. Finally, we tested observational calibrations of the SFR based on infrared fluxes and concluded that they are in agreement when compared to the intrinsic SFR of the simulation averaged over $\sim100$ Myr.

astro-ph.GA↗

arXiv2023

ATOMS: ALMA Three-millimeter Observations of Massive Star-forming regions -- XV. Steady Accretion from Global Collapse to Core Feeding in Massive Hub-filament System SDC335

We present ALMA Band-3/7 observations towards "the Heart" of a massive hub-filament system (HFS) SDC335, to investigate its fragmentation and accretion. At a resolution of $\sim0.03$ pc, 3 mm continuum emission resolves two massive dense cores MM1 and MM2, with $383(^{+234}_{-120})$ $M_\odot$ (10-24% mass of "the Heart") and $74(^{+47}_{-24})$ $M_\odot$, respectively. With a resolution down to 0.01 pc, 0.87 mm continuum emission shows MM1 further fragments into six condensations and multi-transition lines of H$_2$CS provide temperature estimation. The relation between separation and mass of condensations at a scale of 0.01 pc favors turbulent Jeans fragmentation where the turbulence seems to be scale-free rather than scale-dependent. We use the H$^{13}$CO$^+$ (1-0) emission line to resolve the complex gas motion inside "the Heart" in position-position-velocity space. We identify four major gas streams connected to large-scale filaments, inheriting the anti-clockwise spiral pattern. Along these streams, gas feeds the central massive core MM1. Assuming an inclination angle of $45(\pm15)^{\circ}$ and a H$^{13}$CO$^+$ abundance of $5(\pm3)\times10^{-11}$, the total mass infall rate is estimated to be $2.40(\pm0.78)\times10^{-3}$ $M_\odot$ yr$^{-1}$, numerically consistent with the accretion rates derived from the clump-scale spherical infall model and the core-scale outflows. The consistency suggests a continuous, near steady-state, and efficient accretion from global collapse, therefore ensuring core feeding. Our comprehensive study of SDC335 showcases the detailed gas kinematics in a prototypical massive infalling clump and calls for further systematic and statistical analyses in a large sample.

astro-ph.GA↗

arXiv2022

Synthetic Spectral Signatures from Isothermal Collapsing Gas and the Interpretation of Infall Profiles

We revisit the interpretation of blue-excess molecular lines from dense collapsing cores, considering recent numerical results that suggest prestellar core collapse occurs from the outside-in, and not inside-out. We thus create synthetic molecular-line observations of simulated collapsing, spherically-symmetric, density fluctuations of low initial amplitude, embedded in a uniform, globally gravitationally unstable background, without a turbulent component. The collapsing core develops a flattened, Bonnor-Ebert-like density profile, but with an outside-in radial velocity profile, where the peak infall speeds are at large radii, in lower density gas, with cloud-to-core accretion, and no hydrostatic outer envelope. Using optically thick HCO$^+$ J=1-0 and 3-2 rotational lines, we consider several "typical" beamwidths and use a simple line-fitting model to infer infall speeds from the synthetic profiles similarly to what is done in standard line modeling. We find that the inferred infall speeds are ~ 25-30% of the actual peak infall speed as the largest speeds are downweighted by the low density gas in which they occur, due to the outside-in nature of the actual radial collapse profile. Also, with the N$_2$H$^+$ $J_{F_1F}=1_{01}-0_{12}$ hyperfine line, we investigate the change in the asymmetry parameter, $δv\equiv(V_{thick}-V_{thin})/Δv_{thin}$, during the collapse, finding good agreement with observed values. Finally, the HCO$^+$ J=3-2 lines exhibit extreme $T_b$/$T_r$-ratios like those for evolved cores, for larger beams late in the collapse. Our results suggest that standard dynamical infall reproduces several observed features, but that low-mass core infall speeds are generally undervalued, often interpreted as being subsonic although the actual speeds are supersonic, due to incorrectly assuming an inside-out infall radial velocity profile with a static outer envelope.

astro-ph.GA↗

arXiv2022

A resolution criterion based on characteristic timescales for MHD simulations of molecular clouds

We investigate the effect of numerical magnetic diffusion in magnetohydrodynamic simulations of magnetically supported molecular clouds. To this end, we have performed numerical studies on adaptive mesh isothermal simulations of marginally sub-critical molecular clouds. We find that simulations with low and intermediate resolutions collapse, contrary to what is theoretically expected. However, the simulation with the highest numerical resolution oscillates around an equilibrium state without collapsing. In order to quantify the numerical diffusion of the magnetic field, we ran a second suit of current-sheet simulations in which the numerical magnetic diffusion coefficient can be directly measured, and computed the corresponding diffusion times at various numerical resolutions. On this basis, we propose a criterion for the resolution of magnetic fields in MHD simulations based on requiring that the diffusion time to be larger than the characteristic timescale of the physical process responsible for the dynamic evolution of the structure.

astro-ph.GA↗