Star formation thresholds: real and illusory. (arXiv:1902.00934v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Khullar_S/0/1/0/all/0/1">Shivan Khullar</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Krumholz_M/0/1/0/all/0/1">Mark R. Krumholz</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Federrath_C/0/1/0/all/0/1">Christoph Federrath</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Cunningham_A/0/1/0/all/0/1">Andrew J. Cunningham</a>
Based on both theory and observations, a number of authors have proposed that
there is a threshold for star formation. This threshold is a characteristic
density or extinction level below which gas forms stars inefficiently, and
above which star formation becomes efficient. In this paper we use detailed
simulations of turbulent, magnetised star-forming clouds, including stellar
radiation and outflow feedback, to investigate whether thresholds for star
formation exist and whether it is possible to recover them using current
observational techniques. We show that the simulations do possess a threshold,
indicated by a sharp increase in star formation rate per free-fall time
$epsilon_{rm ff}$ at a number density $n_{rm th}sim 10^{6.5}$ cm$^{-3}$.
Using mock observations of the simulations at realistic resolutions, we show
that plots of projected $epsilon_{rm ff}$ can detect the presence of a
threshold, but that the resolutions typical of current dust emission or
absorption surveys are insufficient to determine its value. In contrast,
proposed alternative diagnostics based on a change in the slope of the gas
surface density versus star formation rate surface density (Kennicutt-Schmidt
relation) or on the correlation between young stellar object counts and gas
mass as a function of density are ineffective at detecting thresholds even when
they are present. The signatures in these diagnostics sometimes taken as
indicative of a threshold in observations, which we generally reproduce in our
mock observations, do not prove to correspond to real physical features in the
3D gas distribution.
Based on both theory and observations, a number of authors have proposed that
there is a threshold for star formation. This threshold is a characteristic
density or extinction level below which gas forms stars inefficiently, and
above which star formation becomes efficient. In this paper we use detailed
simulations of turbulent, magnetised star-forming clouds, including stellar
radiation and outflow feedback, to investigate whether thresholds for star
formation exist and whether it is possible to recover them using current
observational techniques. We show that the simulations do possess a threshold,
indicated by a sharp increase in star formation rate per free-fall time
$epsilon_{rm ff}$ at a number density $n_{rm th}sim 10^{6.5}$ cm$^{-3}$.
Using mock observations of the simulations at realistic resolutions, we show
that plots of projected $epsilon_{rm ff}$ can detect the presence of a
threshold, but that the resolutions typical of current dust emission or
absorption surveys are insufficient to determine its value. In contrast,
proposed alternative diagnostics based on a change in the slope of the gas
surface density versus star formation rate surface density (Kennicutt-Schmidt
relation) or on the correlation between young stellar object counts and gas
mass as a function of density are ineffective at detecting thresholds even when
they are present. The signatures in these diagnostics sometimes taken as
indicative of a threshold in observations, which we generally reproduce in our
mock observations, do not prove to correspond to real physical features in the
3D gas distribution.
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