No abstract

Observations indicate that massive stars form in regions of very high surface density, ~1 g cm^-2. Clusters containing massive stars and globular clusters have a comparable column density. The total pressure in clouds of such a column density is P/k~10^8-10^9 K cm^-3, far greater than that in the diffuse ISM or the average in GMCs. Observations show that massive star-forming regions are supersonically turbulent, and we show that the molecular cores out of which individual massive stars form are as well. The protostellar accretion rate in such a core is approximately equal to the instantaneous mass of the star divided by the free-fall time of the gas that is accreting onto the star (Stahler, Shu, & Taam 1980). The star-formation time in this Turbulent Core model for massive star formation is several mean free-fall timesscales of the core, but is about equal to that of the region in which the core is embedded. The typical time for a massive star to form is about 10^5 yr and the accretion rate is high enough to overcome radiation pressure due to the luminosity of the star. For the typical case we consider, in which the cores out of which the stars form have a density structure varying as r^{-1.5}, the protostellar accretion rate grows linearly with time. We calculate the evolution of the radius of a protostar and determine the accretion luminosity. At the high accretion rates that are typical in regions of massive star formation, protostars join the main sequence at about 20 solar masses. We apply these results to predict the properties of protostars thought to be powering several observed hot molecular cores, including the Orion hot core and W3(H2O). In the Appendixes, we discuss the pressure in molecular clouds and we argue that ``logatropic'' models for molecular clouds are incompatible with observation.Comment: ApJ accepted; 28 pages, some clarification of the text, results unchange

It has been known for more than 30 years that star formation in giant molecular clouds (GMCs) is slow, in the sense that only ~1% of the gas forms stars every free-fall time. This result is entirely independent of any particular model of molecular cloud lifetime or evolution. Here we survey observational data on higher density objects in the interstellar medium, including infrared dark clouds and dense molecular clumps, to determine if these objects form stars slowly like GMCs, or rapidly, converting a significant fraction of their mass into stars in one free-fall time. We find no evidence for a transition from slow to rapid star formation in structures covering three orders of magnitude in density. This has important implications for models of star formation, since competing models make differing predictions for the characteristic density at which star formation should transition from slow to rapid. The data are inconsistent with models that predict that star clusters form rapidly and in free-fall collapse. Magnetic- and turbulence-regulated star formation models can reproduce the observations qualitatively, and the turbulence-regulated star formation model of Krumholz & McKee quantitatively reproduces the infrared-HCN luminosity correlation recently reported by Gao & Solomon. Slow star formation also implies that the process of star cluster formation cannot be one of global collapse, but must instead proceed over many free-fall times. This suggests that turbulence in star-forming clumps must be driven, and that the competitive accretion mechanism does not operate in typical cluster-forming molecular clumps.Comment: Accepted for publication in ApJ. 14 pages, 5 figures, emulateapj format. This version has a more thorough error analysis and an expanded discussion. The basic conclusions are unchange

We investigate the formation and evolution of giant molecular clouds (GMCs) in a Milky-Way-like disk galaxy with a flat rotation curve. We perform a series of 3D adaptive mesh refinement (AMR) numerical simulations that follow both the global evolution on scales of ∼ 20 kpc and resolve down to scales 10 pc with a multiphase atomic interstellar medium (ISM). In this first study, we omit star formation and feedback, and focus on the processes of gravitational instability and cloud collisions and interactions. We define clouds as regions with n H ≥ 100 cm −3 and track the evolution of individual clouds as they orbit through the galaxy from their birth to their eventual destruction via merger or via destructive collision with another cloud. After ∼ 140 Myr a large fraction of the gas in the disk has fragmented into clouds with masses ∼ 10 6 M ⊙ and a mass spectrum similar to that of Galactic GMCs. The disk settles into a quasi steady state in which gravitational scattering of clouds keeps the disk near the threshold of global gravitational instability. The cloud collision time is found to be a small fraction, ∼ 1/5, of the orbital time, and this is an efficient mechanism to inject turbulence into the clouds. This helps to keep clouds only moderately gravitationally bound, with virial parameters of order unity. Many other observed GMC properties, such as mass surface density, angular momentum, velocity dispersion, and vertical distribution, can be accounted for in this simple model with no stellar feedback.

Massive stars (with mass m * > ∼ 8 M ⊙ ) are fundamental to the evolution of galaxies, because they produce heavy elements, inject energy into the interstellar medium, and possibly regulate the star formation rate. The individual star formation time, t * f , determines the accretion rate of the star; the value of the former quantity is currently uncertain by many orders of magnitude 1,2,3,4,5,6 , leading to other astrophysical questions. For example, the variation of t * f with stellar mass dictates whether massive stars can form simultaneously with low-mass stars in clusters. Here we show that t * f is determined by conditions in the star's natal cloud, and is typically ∼ 10 5 yr. The corresponding mass accretion rate depends on the pressure within the cloud-which we relate to the gas surface density-and on both the instantaneous and final stellar masses. Characteristic accretion rates are sufficient to overcome radiation pressure from ∼ 100 M ⊙ protostars, while simultaneously driving intense bipolar gas outflows. The weak dependence of t * f on the final mass of the star allows high-and low-mass star formation to occur nearly simultaneously in clusters.