The Self-gravitating Gas Fraction and The Critical Density for Star Formation.
2018
We analytically calculate the
star formationefficiency and dense
self-gravitatinggas fraction in the presence of magneto-gravo-turbulence using the model of Burkhart (2018), which employs a piecewise lognormal and powerlaw density Probability Distribution Function (
PDF). We show that the
PDFtransition density from lognormal to powerlaw forms is a mathematically motivated critical density for
star formationand can be physically related to the density where the Jeans length is comparable to the sonic length, i.e. the post-shock critical density for collapse. When the
PDFtransition density is taken as the critical density, the instantaneous
star formationefficiency ($\epsilon_{\rm inst}$) and depletion time ($\tau_{\rm depl}$) can be calculated from the dense
self-gravitatinggas fraction represented as the fraction of gas in the
PDFpowerlaw tail. We minimize the number of free parameters in the analytic expressions for $\epsilon_{\rm inst}$ and $\tau_{\rm depl}$ by using the
PDFtransition density instead of a parameterized critical density for collapse and thus provide a more direct pathway for comparison with observations. We test the analytic predictions for the transition density and
self-gravitatinggas fraction against AREPO moving mesh gravoturbulent simulations and find good agreement. We predict that, when gravity dominates the density distribution in the star-
forming gas, the
star formationefficiency should be weakly anti-correlated with the sonic Mach number while the depletion time should increase with increasing sonic Mach number. The
star formationefficiency and depletion time depend primarily on the dense
self-gravitatinggas fraction, which in turn depends on the interplay of gravity, turbulence and stellar feedback. Our model prediction is in agreement with recent observations, such as the M51 PdBI Arcsecond
WhirlpoolSurvey (PAWS).
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