The destruction of Giant Molecular Clouds is a key component in galaxy evolution. We theoretically model the destruction of GMCs by HII regions, which evaporate ionized gas and eject neutral gas during their expansion. HII regions follow one of three tracks, depending on the EUV luminosity, $S$, of the ionizing OB association: the expansion can stall inside the cloud; it can break out, forming a blister (champagne) flow; or, for $S>S_{\rm com}$, it can result in the formation of a cometary cloud. We present results for the accumulated mass loss, $M_{\rm loss}(t)$, and the final mass loss, $M_{{\rm loss},f}$, by evaporation and ejection for a range of cloud masses ($10^4 S_{\rm com}$ associations destroy at least 70% of the initial cloud. We find a critical cloud mass $M_{\rm survive}$ above which clouds never become cometary and lose $<$ 70% of their mass via a single association. Low mass clouds mostly lose mass via ejection of neutral gas.

The young supernova remnant Cassiopeia A is currently interacting with the circumstellar medium expelled by its progenitor star. We analyzed the proper motion of dense circumstellar clumps, known as quasi-stationary flocculi (QSFs), embedded within the remnant using archived images from the Hubble Space Telescope (HST). Our study focused on approximately 40 QSFs that are bright in optical bands. We determined the proper motions within each QSF by cross-correlating the HST images taken 15 yr apart, and presented the results as vector maps that show both the proper-motion magnitudes and directions. For most QSFs, the proper-motion vectors are well aligned, but some show significant variations in expansion direction within the QSF. We derived the mean proper motion of each QSF and investigated its properties. The mean proper-motion vectors of the QSFs predominantly point radially away from the explosion center of Cas A, though not perfectly aligned with a strict radial direction, with corresponding velocities typically in the range of a few hundred kilometers per second. We found no clear correlation between the proper motion and the projected distance from the explosion center, nor between the proper motion and the line-of-sight velocity of the QSFs. This result indicates that the apparent expansion of QSFs is primarily driven by interactions with the supernova blast wave, rather than by their initial ejection. We present an exploratory model of the three-dimensional distribution of QSFs.

We analyze measurements of the thermal Sunyaev-Zeldovich (tSZ) effect arising in the circumgalactic medium (CGM) of L* galaxies, reported by J. N. Bregman et al. (B+22) and S. Das et al. (D+23). In our analysis, we use the Y. Faerman et al. CGM models, a new power-law model (PLM), and the TNG100 simulation. For a given M-vir, our PLM has four parameters: the fraction, f(hCGM), of the halo baryon mass in hot CGM gas, the ratio, phi(T), of the actual gas temperature at the virial radius to the virial temperature, and the power-law indices, a(P,th) and a(n) for the thermal electron pressure and the hydrogen nucleon density. The B+22 Compton-y profile implies steep electron pressure slopes (a(P,th) similar or equal to 2). For isothermal conditions, the temperature is at least 1.1 x 10(6) K, with a hot CGM gas mass of up to 3.5 x 10(11)M(circle dot) for a virial mass of 2.75 x 10(12)M(circle dot). However, if isothermal, the gas must be expanding out of the halos. An isentropic equation of state is favored for which hydrostatic equilibrium is possible. The B+22 and D+23 results are consistent with each other and with recent (0.5-2 keV) CGM X-ray observations of Milky Way mass systems. For M-vir similar or equal to 3 x 10(12)M(circle dot), the scaled Compton pressure integrals, E(z)-2/3Y500/Mvir,125/3, lie in the narrow range, 2.5 x 10(-4)-5.0 x 10(-4) kpc(2), for all three sets of observations. TNG100 underpredicts the tSZ parameters by factors similar to 0.5 dex for the L* galaxies, suggesting that the feedback strengths and CGM gas losses are overestimated in the simulated halos at these mass scales

Strong lensing offers a precious opportunity for studying the formation and early evolution of super star clusters that are rare in our cosmic backyard. The Sunburst Arc, a lensed Cosmic Noon galaxy, hosts a young super star cluster with escaping Lyman continuum radiation. Analyzing archival HST images and emission line data from VLT/MUSE and X-shooter, we construct a physical model for the cluster and its surrounding photoionized nebula. We confirm that the cluster is $\lesssim4\,$Myr old, is extremely massive $M_\star \sim 10^7\,M_\odot$ and yet has a central component as compact as several parsecs, and we find a gas-phase metallicity $Z=(0.22\pm0.03)\,Z_\odot$. The cluster is surrounded by $\gtrsim 10^5\,M_\odot$ of dense clouds that have been pressurized to $P\sim 10^9\,{\rm K}\,{\rm cm}^{-3}$ by perhaps stellar radiation at within ten parsecs. These should have large neutral columns $N_{\rm HI} > 10^{22.5}\,{\rm cm}^{-2}$ to survive rapid ejection by radiation pressure. The clouds are likely dusty as they show gas-phase depletion of silicon, and may be conducive to secondary star formation if $N_{\rm HI} > 10^{24}\,{\rm cm}^{-2}$ or if they sink further toward the cluster center. Detecting strong ${\rm N III]}\lambda\lambda$1750,1752, we infer heavy nitrogen enrichment $\log({\rm N/O})=-0.21^{+0.10}_{-0.11}$. This requires efficiently retaining $\gtrsim 500\,M_\odot$ of nitrogen in the high-pressure clouds from massive stars heavier than $60\,M_\odot$ up to 4 Myr. We suggest a physical origin of the high-pressure clouds from partial or complete condensation of slow massive star ejecta, which may have important implication for the puzzle of multiple stellar populations in globular clusters.

Beginning with cosmological initial conditions at z=100, we simulate the effects of magnetic fields on the formation of Population III stars and compare our results with the predictions of Paper I. We use Gadget-2 to follow the evolution of the system while the field is weak. We introduce a new method for treating kinematic fields by tracking the evolution of the deformation tensor. The growth rate in this stage of the simulation is lower than expected for diffuse astrophysical plasmas, which have a very low resistivity (high magnetic Prandtl number); we attribute this to the large numerical resistivity in simulations, corresponding to a magnetic Prandtl number of order unity. When the magnetic field begins to be dynamically significant in the core of the minihalo at z=27, we map it onto a uniform grid and follow the evolution in an adaptive mesh refinement, MHD simulation in Orion2. The nonlinear evolution of the field in the Orion2 simulation violates flux-freezing and is consistent with the theory proposed by Xu Lazarian. The fields approach equipartition with kinetic energy at densities 10^10 - 10^12 cm^-3. When the same calculation is carried out in Orion2 with no magnetic fields, several protostars form, ranging in mass from 1 to 30 M_sol with magnetic fields, only a single 30 M_sol protostar forms by the end of the simulation. Magnetic fields thus suppress the formation of low-mass Pop III stars, yielding a top-heavy Pop III IMF and contributing to the absence of observed Pop III stars.

ABSTRACT Optical and infrared polarization mapping and recent Planck observations of the filametary cloud L1495 in Taurus show that the large-scale magnetic field is approximately perpendicular to the long axis of the cloud. We use the HAWC + polarimeter on SOFIA to probe the complex magnetic field in the B211 part of the cloud. Our results reveal a dispersion of polarization angles of 36°, about five times that measured on a larger scale by Planck. Applying the Davis–Chandrasekhar–Fermi (DCF) method with velocity information obtained from Institut de Radioastronomie Millimétrique 30 m C18O(1-0) observations, we find two distinct sub-regions with magnetic field strengths differing by more than a factor 3. The quieter sub-region is magnetically critical and sub-Alfv$\acute{\rm e}$nic; the field is comparable to the average field measured in molecular clumps based on Zeeman observations. The more chaotic, super-Alfv$\acute{\rm e}$nic sub-region shows at least three velocity components, indicating interaction among multiple substructures. Its field is much less than the average Zeeman field in molecular clumps, suggesting that the DCF value of the field there may be an underestimate. Numerical simulation of filamentary cloud formation shows that filamentary substructures can strongly perturb the magnetic field. DCF and true field values in the simulation are compared. Pre-stellar cores are observed in B211 and are seen in our simulation. The appendices give a derivation of the standard DCF method that allows for a dispersion in polarization angles that is not small, present an alternate derivation of the structure function version of the DCF method, and treat fragmentation of filaments.

The formation of stars and stellar clusters remains a grand challenge problem in astrophysics that has important implications for the evolution of the interstellar medium as well as shaping the evolution of galaxies.The computational challenges are formidable and involve a coupling of highly non-linear physical processes such as hydrodynamics, self-gravity, magnetic fields, radiation transfer, supersonic turbulence, ionization, protostellar outflows, stellar winds and chemistry that have both disparate timescales as well as operate over many decades of physical length scale.These processes can regulate the feedback from nascent protostars onto the surrounding turbulent gas clouds that are the embryos of new star formation, and as a result, the feedback itself can influence the gaseous reservoir feeding newly formed protostars which in turn influence the star formation process.

A significant fraction of binary neutron star mergers occur in star-forming galaxies where the UV-optical and soft X-ray afterglow emission from the relativistic jet may be absorbed by dust and re-emitted at longer wavelengths. We show that, for mergers occurring in gas-rich environment (n_H > 0.5 cm^-3 at a few to tens of pc) and when the viewing angle is less than about 30 degrees, the emission from heated dust should be detectable by James Webb Space Telescope (JWST), with a detection rate of the order once per year. The spatial separation between the dust emission and the merger site is a few to 10 milli-arcsecs (for a source distance of 150 Mpc), which may be astrometrically resolved by JWST for sufficiently high signal-noise-ratio detections. Measuring the superluminal apparent speed of the flux centroid directly gives the orbital inclination of the merger, which can be combined with gravitational wave data to measure the Hubble constant. For a line of sight within the jet opening angle, the dust echoes are much brighter and may contaminate the search for kilonova candidates from short gamma-ray bursts, such as the case of GRB 130603B.

The largest ever simulation of astrophysical turbulence substantially improves our understanding of how energy injection on large interstellar scales governs how stars form on small scales.

We construct a new analytic phenomenological model for the extended circumgalactic material (CGM) of $L^*$ galaxies. Our model reproduces the OVII/OVIII absorption observations of the Milky Way (MW) and the OVI measurements reported by the COS-Halos and eCGM surveys. The warm/hot gas is in hydrostatic equilibrium in a MW gravitational potential, and we adopt a barotropic equation of state, resulting in a temperature variation as a function of radius. A pressure component with an adiabatic index of $\gamma=4/3$ is included to approximate the effects of a magnetic field and cosmic rays. We introduce a metallicity gradient motivated by the enrichment of the inner CGM by the Galaxy. We then present our fiducial model for the corona, tuned to reproduce the observed OVI-OVIII column densities, and with a total mass of $M_{\rm gas} \approx 5.5 \times 10^{10}~{\rm M_{\odot}}$ inside $r_{\rm cgm} \approx 280$ kpc. The gas densities in the CGM are low ($n_{\rm H} = 10^{-5} - 3 \times 10^{-4}~{\rm cm^{-3}}$) and its collisional ionization state is modified by the metagalactic radiation field (MGRF). We show that for OVI-bearing warm/hot gas with typical observed column densities $N_{\rm OVI} \sim 3 \times 10^{14}~{\rm cm^{-2}}$ at large ($\gtrsim 100$ kpc) impact parameters from the central galaxies, the ratio of the cooling to dynamical times, $t_{\rm cool}/t_{\rm dyn}$, has a model-independent upper limit of $\lesssim 4$. In our model, $t_{\rm cool}/t_{\rm dyn}$ at large radii is $\sim 2-3$. We present predictions for a wide range of future observations of the warm/hot CGM, from UV/X-ray absorption and emission spectroscopy, to dispersion measure (DM) and Sunyaev-Zeldovich CMB measurements. We provide the model outputs in machine-readable data files, for easy comparison and analysis.

Gravitationally bound clusters that survive gas removal represent an unusual mode of star formation in the Milky Way and similar spiral galaxies. While forming, they can be distinguished observationally from unbound star formation by their high densities, virialized velocity structures, and star formation histories that accelerate towards the present, but extend multiple free-fall times into the past. In this paper, we examine several proposed scenarios for how such structures might form and evolve, and carry out a Bayesian analysis to test these models against observed distributions of protostellar age, counts of young stellar objects relative to gas, and the overall star formation rate of the Milky Way. We show that models in which the acceleration of star formation is due either to a large-scale collapse or a time-dependent increase in star formation efficiency are unable to satisfy the combined set of observational constraints. In contrast, models in which clusters form in a 'conveyor belt' mode where gas accretion and star formation occur simultaneously, but the star formation rate per free-fall time is low, can match the observations.

While magnetic fields are important in contemporary star formation, their role in primordial star formation is unknown. Magnetic fields of order 10^-16 G are produced by the Biermann battery due to the curved shocks and turbulence associated with the infall of gas into the dark matter minihalos that are the sites of formation of the first stars. These fields are rapidly amplified by a small-scale dynamo until they saturate at or near equipartition with the turbulence in the central region of the gas. Analytic results are given for the outcome of the dynamo, including the effect of compression in the collapsing gas. The mass-to-flux ratio in this gas is 2-3 times the critical value, comparable to that in contemporary star formation. Predictions of the outcomes of simulations using smooth particle hydrodynamics (SPH) and grid-based adaptive mesh refinement (AMR) are given. Because the numerical viscosity and resistivity for the standard resolution of 64 cells per Jeans length are several orders of magnitude greater than the physical values, dynamically significant magnetic fields affect a much smaller fraction of the mass in simulations than in reality. An appendix gives an analytic treatment of free-fall collapse, including that in a constant density background. Another appendix presents a new method of estimating the numerical viscosity; results are given for both SPH and grid-based codes.

EDITORIAL article Front. Astron. Space Sci., 16 April 2020Sec. Stellar and Solar Physics Volume 7 - 2020 | https://doi.org/10.3389/fspas.2020.00013

Star clusters stand at the intersection of much of modern astrophysics: the ISM, gravitational dynamics, stellar evolution, and cosmology. Here, we review observations and theoretical models for the formation, evolution, and eventual disruption of star clusters. Current literature suggests a picture of this life cycle including the following several phases: Clusters form in hierarchically structured, accreting molecular clouds that convert gas into stars at a low rate per dynamical time until feedback disperses the gas. The densest parts of the hierarchy resist gas removal long enough to reach high star-formation efficiency, becoming dynamically relaxed and well mixed. These remain bound after gas removal. In the first similar to 100 Myr after gas removal, clusters disperse moderately fast, through a combination of mass loss and tidal shocks by dense molecular structures in the star-forming environment. After similar to 100 Myr, clusters lose mass via two-body relaxation and shocks by giant molecular clouds, processes that preferentially affect low-mass clusters and cause a turnover in the cluster mass function to appear on similar to 1-10-Gyr timescales. Even after dispersal, some clusters remain coherent and thus detectable in chemical or action space for multiple galactic orbits. In the next decade, a new generation of space- and adaptive optics-assisted ground-based telescopes will enable us to test and refine this picture.

We present dark-matter minihalo models for the Ultra-Compact High Velocity HI Clouds (UCHVCs) recently discovered in the 21 cm ALFALFA survey. We assume gravitational confinement of similar to 10(4) K HI gas by flat-cored dark-matter subhalos within the Local Group. For the UCHVCs we calculate the photoionization-limited hydrostatic gas profiles for any distance-dependent total observed HI mass and predict the associated (projected) HI half-mass radii. The observed 21 cm fluxes and half-mass angular radii then constrain the source distances or DM halo parameters. As a consistency check we model the gas-rich dwarf galaxy Leo T, for which the distance is known (420 kpc) and there is a well-resolved HI column density profile. We derive an upper limit for the pressure of any enveloping hot IGM gas at the distance of Leo T. Our analysis supports the scenario that some of the UCHVCs may constitute a population of 21-cm-selected but optically-faint dwarf galaxies in the Local Volume.

Star formation in a filamentary infrared dark cloud ( IRDC) is simulated over the dynamic range of 4.2 pc to 28 au for a period of 3.5 x 10(5) yr, including magnetic fields and both radiative and outflow feedback from the protostars. At the end of the simulation, the star formation efficiency is 4.3 per cent and the star formation rate per free-fall time is epsilon(ff) similar or equal to 0.04, within the range of observed values. The total stellar mass increases as similar to t(2), whereas the number of protostars increases as similar to t(1.5). We find that the density profile around most of the simulated protostars is similar to rho alpha r(-1.5). At the end of the simulation, the protostellar mass function approaches the Chabrier stellar initial mass function. We infer that the time to form a star of median mass 0.2M(circle dot) is about 1.4 x 10(5) yr from the median mass accretion rate. We find good agreement among the protostellar luminosities observed in the large sample of Dunham et al., our simulation and a theoretical estimate, and we conclude that the classical protostellar luminosity problem is resolved. The multiplicity of the stellar systems in the simulation agrees, to within a factor of 2, with observations of Class I young stellar objects; most of the simulated multiple systems are unbound. Bipolar protostellar outflows are launched using a subgrid model, and extend up to 1 pc from their host star. The mass-velocity relation of the simulated outflows is consistent with both observation and theory.

For dark matter (DM) particles with masses in the 0.6 - 6 m_p range, we set stringent constraints on the interaction cross-sections for scattering with ordinary baryonic matter. These constraints follow from the recognition that such particles can be captured by - and thermalized within - the Earth, leading to a substantial accumulation and concentration of DM that interact with baryons. Here, we discuss the probability that DM intercepted by the Earth will be captured, the number of DM particles thereby accumulated over Earth's lifetime, the fraction of such particles retained in the face of evaporation, and the density distribution of such particles within the Earth. In the latter context, we note that a previous treatment of the density distribution of DM, presented by Gould and Raffelt and applied subsequently to DM in the Sun, is inconsistent with considerations of hydrostatic equilibrium. Our analysis provides an estimate of the DM particle density at Earth's surface, which may exceed 1.E+14 cm-3 for the mass range under consideration. Based upon our determination of the DM density at Earth's surface, we derive constraints on the scattering cross-sections. These constraints are placed by four considerations: (1) the lifetime of the relativistic proton beam at the Large Hadron collider (LHC); (2) the orbital decay of spacecraft in low Earth orbit (LEO); (3) the vaporization rate of cryogenic liquids in well-insulated storage dewars; and (4) the thermal conductivity of Earth's crust. As an example application of our results, we show that for the scattering cross-sections that were invoked recently in Barkana's original explanation for the anomalously deep 21 cm absorption reported by EDGES, DM particle masses in the 0.6 - 4 m_p range are ruled out.

Many spiral galaxies have extended outer H i disks and display low levels of star formation, inferred from the far-ultraviolet emission detected by the Galaxy Evolution Explorer ( GALEX ), well beyond the optical radius. Here, we investigate the supernova (SN) rate in the outskirts of galaxies, using the largest and most homogeneous set of nearby supernovae (SNe) from the Lick Observatory Supernova Search (LOSS). While SN rates have been measured with respect to various galaxy properties, such as stellar mass and metallicity, their relative frequency in the outskirts versus the inner regions has not yet been studied. Understanding the SN rate as a function of intragalactic environment has many ramifications, including the interpretation of LIGO observations, the formation of massive stars, and the puzzlingly high velocity dispersion of the outer H i disk. Using data from the LOSS survey, we find that the rate beyond the optical radius of spiral galaxies is 2.5 ± 0.5 SNe per millennium, while dwarf galaxies host 4.0 ± 2.2 SNe per millennium. The rates of core-collapse SNe (that may collapse to form the massive black holes detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO)/Virgo) in the outer disks of spirals is 1.5 ± 0.15 SNe per millennium and in dwarf galaxies is 2.6 ± 1.5 SNe per millennium. Core-collapse SNe in spiral outskirts contribute 7600 ± 1700 SNe Gpc −3 yr −1 to the volumetric rate, and dwarf galaxies have a rate of 31,000 ± 18,000 SNe Gpc −3 yr −1 . The relative ratio of core-collapse to SNe Ia is comparable in the inner and outer parts of spirals, and in dwarf galaxies.

Star-forming molecular clouds are observed to be both highly magnetized and turbulent. Consequently, the formation of protostellar discs is largely dependent on the complex interaction between gravity, magnetic fields, and turbulence. Studies of non-turbulent protostellar disc formation with realistic magnetic fields have shown that these fields are efficient in removing angular momentum from the forming discs, preventing their formation. However, once turbulence is included, discs can form in even highly magnetized clouds, although the precise mechanism remains uncertain. Here, we present several high-resolution simulations of turbulent, realistically magnetized, high-mass molecular clouds with both aligned and random turbulence to study the role that turbulence, misalignment, and magnetic fields have on the formation of protostellar discs. We find that when the turbulence is artificially aligned so that the angular momentum is parallel to the initial uniform field, no rotationally supported discs are formed, regardless of the initial turbulent energy. We conclude that turbulence and the associated misalignment between the angular momentum and the magnetic field are crucial in the formation of protostellar discs in the presence of realistic magnetic fields.