Inefficient star formation in high Mach number environments I. The turbulent support analytical model
arxiv(2024)
摘要
The star formation rate (SFR), the number of stars formed per unit of time,
is a fundamental quantity in the evolution of the Universe. While turbulence is
believed to play a crucial role in setting the SFR, the exact mechanism remains
unclear. Turbulence promotes star formation by compressing the gas, but also
slows it down by stabilizing the gas against gravity. Most widely-used
analytical models rely on questionable assumptions, including: i) integrating
over the density PDF, a one-point statistical description that ignores spatial
correlation, ii) selecting self-gravitating gas based on a density threshold
that often ignores turbulent dispersion, iii) assuming the freefall time as
the timescale for estimating SFR without considering the need to rejuvenate the
density PDF, iv) assuming the density PDF to be lognormal. Improving upon the
only existing model that incorporates the spatial correlation of the density
field, we present a new analytical model. We calculate the time needed to
rejuvenate density fluctuations of a given density and spatial scale, revealing
that it is generally much longer than the freefall time, rendering the latter
inappropriate for use. We make specific predictions regarding the role of the
Mach number, M, and the driving scale of turbulence divided by the mean
Jeans length. At low to moderate Mach numbers, turbulence does not reduce and
may even slightly promote star formation by broadening the PDF. However, at
higher Mach numbers, most density fluctuations are stabilized by turbulent
dispersion, leading to a steep drop in the SFR as the Mach number increases.
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