5 research outputs found
Dependence of the Star Formation Efficiency on the Parameters of Molecular Cloud Formation Simulations
We investigate the response of the star formation efficiency (SFE) to the
main parameters of simulations of molecular cloud formation by the collision of
warm diffuse medium (WNM) cylindrical streams, neglecting stellar feedback and
magnetic fields. The parameters we vary are the Mach number of the inflow
velocity of the streams, Msinf, the rms Mach number of the initial background
turbulence in the WNM, and the total mass contained in the colliding gas
streams, Minf. Because the SFE is a function of time, we define two estimators
for it, the "absolute" SFE, measured at t = 25 Myr into the simulation's
evolution (sfeabs), and the "relative" SFE, measured 5 Myr after the onset of
star formation in each simulation (sferel). The latter is close to the "star
formation rate per free-fall time" for gas at n = 100 cm^-3. We find that both
estimators decrease with increasing Minf, although by no more than a factor of
2 as Msinf increases from 1.25 to 3.5. Increasing levels of background
turbulence similarly reduce the SFE, because the turbulence disrupts the
coherence of the colliding streams, fragmenting the cloud, and producing
small-scale clumps scattered through the numerical box, which have low SFEs.
Finally, the SFE is very sensitive to the mass of the inflows, with sferel
decreasing from ~0.4 to ~0.04 as the the virial parameter in the colliding
streams increases from ~0.15 to ~1.5. This trend is in partial agreement with
the prediction by Krumholz & McKee (2005), since the latter lies within the
same range as the observed efficiencies, but with a significantly shallower
slope. We conclude that the observed variability of the SFE is a highly
sensitive function of the parameters of the cloud formation process, and may be
the cause of significant scatter in observational determinations.Comment: 19 pages, submitted to MNRA
High- and Low-Mass Star Forming Regions from Hierarchical Gravitational Fragmentation. High local Star Formation Rates with Low Global Efficiencies
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. In this simulation, the velocity dispersions at
all scales are caused primarily by infall motions rather than by random
turbulence. We study the properties (density, total gas+stars mass, stellar
mass, velocity dispersion, and star formation rate) of the cloud hosting the
first local, isolated "star formation" event in the simulation and compare them
with those of the cloud formed by a later central, global collapse event. We
suggest that the small-scale, isolated collapse may be representative of low-
to intermediate-mass star-forming regions, while the large-scale, massive one
may be representative of massive star forming regions. We also find that the
statistical distributions of physical properties of the dense cores in the
region of massive collapse compare very well with those from a recent survey of
the massive star forming region in the Cygnus X molecular cloud. The star
formation efficiency per free-fall time (SFE_ff) of the high-mass SF clump is
low, ~0.04. This occurs because the clump is accreting mass at a high rate, not
because its specific SFR (SSFR) is low. This implies that a low value of the
SFE_ff does not necessarily imply a low SSFR, but may rather indicate a large
gas accretion rate. We suggest that a globally low SSFR at the GMC level can be
attained even if local star forming sites have much larger values of the SSFR
if star formation is a spatially intermittent process, so that most of the mass
in a GMC is not participating of the SF process at any given time.Comment: Accepted by ApJ. Revised version, according to exchanges with
referee. Original results unchanged. Extensive new discussion on the low
global efficiency vs. high local efficiency of star formation. Abstract
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