Building on our previous hydrodynamic study of the angular momenta of cloud cores formed during gravitational collapse of star-forming molecular gas in Kuznetsova et al., we now examine core ...properties assuming ideal magnetohydrodynamics (MHD). Using the same sink-patch implementation for the Athena MHD code, we characterize the statistical properties of cores, including the mass accretion rates, specific angular momenta, and alignments between the magnetic field and the spin axis of the core on the 0.1 pc scale. Our simulations, which reproduce the observed relation between magnetic field strength and gas density, show that magnetic fields can help collimate low-density flows and help seed the locations of filamentary structures. Consistent with our previous purely hydrodynamic simulations, stars (sinks) form within the heterogeneous environments of filaments, such that accretion onto cores is highly episodic leading to short-term variability but no long-term monotonic growth of the specific angular momenta. With statistical characterization of protostellar cores properties and behaviors, we aim to provide a starting point for building more realistic and self-consistent disk formation models, helping to address whether magnetic fields can prevent the development of (large) circumstellar disks in the ideal MHD limit.
ABSTRACT Our three-dimensional hydrodynamical simulations of starbursts examine the formation of superbubbles over a range of driving luminosities and mass loadings that determine superbubble growth ...and wind velocity. From this we determine the relationship between the velocity of a galactic wind (GW) and the power of the starburst. We find a threshold for the formation of a wind, above which the speed of the wind is not affected by grid resolution or the temperature floor of our radiative cooling. We investigate the effect that two different temperature floors in our radiative cooling prescription have on wind kinematics and content. We find that cooling to 10 K instead of to 104 K increases the mass fraction of cold neutral and hot X-ray gas in the GW, while halving that in warm H . Our simulations show that the mass of cold gas transported into the lower halo does not depend on the starburst strength. Optically bright filaments form at the edge of merging superbubbles, or where a cold dense cloud has been disrupted by the wind. Filaments formed by merging superbubbles will persist and grow to > 400 pc in length if anchored to a star forming complex. Filaments embedded in the hot GW contain warm and cold gas that moves 300−1200 km s−1 slower than the surrounding wind, with the coldest gas hardly moving with respect to the Galaxy. Warm and cold matter in the GW show asymmetric absorption profiles consistent with observations, with a thin tail up to the wind velocity.
Rapid star formation and global gravitational collapse Hartmann, Lee; Ballesteros-Paredes, Javier; Heitsch, Fabian
Monthly notices of the Royal Astronomical Society,
February 2012, Letnik:
420, Številka:
2
Journal Article
Recenzirano
Odprti dostop
Most young stars in nearby molecular clouds have estimated ages of 1-2 Myr, suggesting that star formation is rapid. However, small numbers of stars in these regions with inferred ages of >rsim5-10 ...Myr have been cited to argue that star formation is instead a slow, quasi-static process. When considering these alternative pictures it is important to recognize that the age spread in a given star-forming cloud is necessarily an upper limit to the time-scales of local collapse, as not all spatially distinct regions will start contracting at precisely the same instant. Moreover, star-forming clouds may dynamically evolve on time-scales of a few Myr; in particular, global gravitational contraction will tend to yield increasing star formation rates with time due to generally increasing local gas densities. We show that two different numerical simulations of dynamic, flow-driven molecular cloud formation and evolution (1) predict age spreads for the main stellar population roughly consistent with observations and (2) raise the possibility of forming small numbers of stars early in cloud evolution, before global contraction concentrates the gas and the bulk of the stellar population is produced. In general, the existence of a small number of older stars among a generally much younger population is consistent with the picture of dynamic star formation and may even provide clues to the time evolution of star-forming clouds.
Numerical simulations of star formation have found that a power-law mass function can develop at high masses. In a previous paper, we employed isothermal simulations that created large numbers of ...sinks over a large range in masses to show that the power-law exponent of the mass function, , asymptotically and accurately approaches Γ = −1. Simple analytic models show that such a power law can develop if the mass accretion rate , as in Bondi-Hoyle accretion; however, the sink mass accretion rates in the simulations show significant departures from this relation. In this paper, we show that the expected accretion rate dependence is more closely realized provided the gravitating mass is taken to be the sum of the sink mass and the mass in the near environment. This reconciles the observed mass functions with the accretion rate dependencies, and demonstrates that power-law upper mass functions are essentially the result of gravitational focusing, a mechanism present in, for example, the competitive accretion model.
Two aspects of filamentary molecular cloud evolution are addressed: (1) exploring analytically the role of the environment for the evolution of filaments demonstrates that considering them in ...isolation (i.e., just addressing the fragmentation stability) will result in unphysical conclusions about the filament's properties. Accretion can also explain the observed decorrelation between FWHM and peak column density. (2) Free-fall accretion onto finite filaments can lead to the characteristic "fans" of infrared-dark clouds around star-forming regions. The fans may form due to tidal forces mostly arising at the ends of the filaments, consistent with numerical models and earlier analytical studies.
The presence of short-lived radioisotopes (SLRs) in Solar system meteorites has been interpreted as evidence that the Solar system was exposed to a supernova shortly before or during its formation. ...Yet results from hydrodynamical models of SLR injection into the proto-solar cloud or disc suggest that gas-phase mixing may not be efficient enough to reproduce the observed abundances. As an alternative, we explore the injection of SLRs via dust grains as a way to overcome the mixing barrier. We numerically model the interaction of a supernova remnant containing SLR-rich dust grains with a nearby molecular cloud. The dust grains are subject to drag forces and both thermal and non-thermal sputtering. We confirm that the expanding gas shell stalls upon impact with the dense cloud and that gas-phase SLR injection occurs slowly due to hydrodynamical instabilities at the cloud surface. In contrast, dust grains of sufficient size ( ≳ 1 μm) decouple from the gas and penetrate into the cloud within 0.1 Myr. Once inside the cloud, the dust grains are destroyed by sputtering, releasing SLRs and rapidly enriching the dense (potentially star-forming) regions. Our results suggest that SLR transport on dust grains is a viable mechanism to explain SLR enrichment.
It has been recently shown that molecular clouds do not exhibit a unique shape for the column density probability distribution function (N-PDF). Instead, clouds without star formation seem to possess ...a lognormal distribution, while clouds with active star formation develop a power-law tail at high column densities. The lognormal behaviour of the N-PDF has been interpreted in terms of turbulent motions dominating the dynamics of the clouds, while the power-law behaviour occurs when the cloud is dominated by gravity. In the present contribution, we use thermally bi-stable numerical simulations of cloud formation and evolution to show that, indeed, these two regimes can be understood in terms of the formation and evolution of molecular clouds: a very narrow lognormal regime appears when the cloud is being assembled. However, as the global gravitational contraction occurs, the initial density fluctuations are enhanced, resulting, first, in a wider lognormal N-PDF, and later, in a power-law N-PDF. We thus suggest that the observed N-PDF of molecular clouds are a manifestation of their global gravitationally contracting state. We also show that, contrary to recent suggestions, the exact value of the power-law slope is not unique, as it depends on the projection in which the cloud is being observed.
Abstract
We present numerical simulations of reorienting oblique shocks that form in the collision layer between magnetized colliding flows. Reorientation aligns post-shock filaments normal to the ...background magnetic field. We find that reorientation begins with pressure gradients between the collision region and the ambient medium. This drives a lateral expansion of post-shock gas, which reorients the growing filament from the outside-in (i.e. from the flow/ambient boundary, towards the colliding flows axis). The final structures of our simulations resemble polarization observations of filaments in Taurus and Serpens South, as well as the integral-shaped filament in Orion A. Given the ubiquity of colliding flows in the interstellar medium, shock reorientation may be relevant to the formation of filaments normal to magnetic fields.
We study numerically the formation of molecular clouds in large-scale colliding flows including self-gravity. The models emphasize the competition between the effects of gravity on global and local ...scales in an isolated cloud. Global gravity builds up large-scale filaments, while local gravity, triggered by a combination of strong thermal and dynamical instabilities, causes cores to form. The dynamical instabilities give rise to a local focusing of the colliding flows, facilitating the rapid formation of massive protostellar cores of a few hundred M unk. The forming clouds do not reach an equilibrium state, although the motions within the clouds appear to be comparable to virial. The self-similar core mass distributions derived from models with and without self-gravity indicate that the core mass distribution is set very early on during the cloud formation process, predominantly by a combination of thermal and dynamical instabilities rather than by self-gravity.
The Smith Cloud (SC) is a gaseous high-velocity cloud (HVC) in an advanced state of accretion, only 2.9 kpc below the Galactic plane and due to impact the disk in asymptotically =27 Myr. It is unique ...among HVCs in having a known distance (12.4 + or - 1.3 kpc) and a well-constrained 3D velocity (296 km s super(-1)), but its origin has long remained a mystery. Here we present the first absorption-line measurements of its metallicity, using Hubble Space Telescope/ COS UV spectra of three active galactic nuclei lying behind the Cloud together with Green Bank Telescope 21 cm spectra of the same directions. Using Voigt-profile fitting of the S II lambdalambda1250, 1253, 1259 triplet together with ionization corrections derived from photoionization modeling, we derive the sulfur abundance in each direction; a weighted average of the three measurements gives S/H = -0.28 + or - 0.14, or (ProQuest: Formulae and/or non-USASCII text omitted) solar metallicity. The finding that the SC is metal-enriched lends support to scenarios where it represents recycled Galactic material, rather than the remnant of a dwarf galaxy or accreting intergalactic gas. The metallicity and trajectory of the Cloud are both indicative of an origin in the outer disk. However, its large mass and prograde kinematics remain to be fully explained. If the cloud has accreted cooling gas from the corona during its fountain trajectory, as predicted in recent theoretical work, its current mass would be higher than its launch mass, alleviating the mass concern.
PDF
