HIGH- AND LOW-MASS STAR-FORMING REGIONS FROM HIERARCHICAL GRAVITATIONAL FRAGMENTATION. HIGH LOCAL STAR FORMATION RATES WITH LOW GLOBAL EFFICIENCIES

Journal Article · Sun Dec 20 04:00:00 EST 2009 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/707/2/1023· OSTI ID:21392603

Vazquez-Semadeni, Enrique; Gomez, Gilberto C; Ballesteros-Paredes, Javier ^[1]; Jappsen, A -Katharina ^[2]; Klessen, Ralf S ^[3]

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1. Centro de RadioastronomIa y AstrofIsica, Universidad Nacional Autonoma de Mexico, Apdo. Postal 3-72, Morelia, Michoacan 58089 (Mexico)
2. School of Physics and Astronomy, Cardiff University, Queens Buildings, The Parade, Cardiff CF24 3AA (United Kingdom)
3. Zentrum fuer Astronomie der Universitaet Heidelberg, Institut fuer Theoretische Astrophysik, 69120 Heidelberg (Germany)

We investigate the properties of 'star-forming regions' in a previously published numerical simulation of molecular cloud formation out of compressive motions in the warm neutral atomic interstellar medium, neglecting magnetic fields and stellar feedback. We study the properties (density, total gas + stars mass, stellar mass, velocity dispersion, and star formation rate (SFR)) of the cloud hosting the first local, isolated 'star formation' event and compare them with those of the cloud formed by the central, global collapse event. In this simulation, the velocity dispersions at all scales are caused primarily by infall motions rather than by random turbulence. We suggest that the small-scale isolated collapses may be representative of low- to intermediate-mass star-forming regions, with gas masses (M{sub gas}) of hundreds of solar masses, velocity dispersions sigma{sub v} approx 0.7 km s{sup -1}, and SFRs approx3 x 10{sup -5} M{sub sun} yr{sup -1}, while the large-scale, massive ones may be representative of massive star-forming regions, with M{sub gas} of thousands of solar masses, sigma{sub v}approx a few km s{sup -1}, and SFRs approx3 x 10{sup -4} M{sub sun} yr{sup -1}. We also compare the statistical distributions of the physical properties of the dense cores appearing in the central region of massive collapse with those from a recent survey of the massive star-forming region in the Cygnus X molecular cloud, finding that the observed and simulated distributions are in general very similar. However, we find that the star formation efficiency per free-fall time (SFE{sub ff}) of the high mass region, similar to that of OMC-1, is low, approx0.04. In the simulated cloud, this is not a consequence of a 'slow' SFR in a nearly hydrostatic cloud supported by turbulence, but rather of the region accreting mass at a high rate. Thus, we find that measuring a low SFE{sub ff} may be incorrectly interpreted as implying a lifetime much longer than the core's local free-fall time, and an SFR much slower than that given by the free-fall rate, if the accretion is not accounted for. We suggest that rather than requiring a low value of the SFE{sub ff} everywhere in the Galaxy, attaining a globally low specific SFR requires star formation to be a spatially intermittent process, so that most of the mass in a giant molecular cloud (GMC) is not participating in the SF process at any given time. Locally, the specific SFR of a star-forming region can be much larger than the global GMC's average.

• OSTI ID: 21392603
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 2 Vol. 707; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

71 CLASSICAL AND QUANTUM MECHANICS
GENERAL PHYSICS
79 ASTRONOMY AND ASTROPHYSICS
COMPUTERIZED SIMULATION
DISTRIBUTION
FEEDBACK
FRAGMENTATION
GALAXIES
LIFETIME
MAGNETIC FIELDS
MASS
RANDOMNESS
SIMULATION
STARS
TURBULENCE
VELOCITY