Abstract
Abstract One of the most important and well-established empirical results in astronomy is the Kennicutt–Schmidt relation between the density of interstellar gas and the rate at which that gas forms stars. A tight correlation between these quantities has long been measured at galactic scales. More recently, using surveys of YSOs, a KS relationship has been found within molecular clouds relating the surface density of star formation to the surface density of gas; however, the scaling of these laws varies significantly from cloud to cloud. In this Letter, we use a recently developed, high-accuracy catalog of young stellar objects from Spitzer combined with high-dynamic-range gas column density maps of 12 nearby (<1.5 kpc) molecular clouds from Herschel to re-examine the KS relation within individual molecular clouds. We find a tight, linear correlation between clouds’ star formation rate per unit area and their gas surface density normalized by the gas freefall time. The measured intracloud KS relation, which relates star formation rate to the volume density, extends over more than two orders of magnitude within each cloud and is nearly identical in each of the 12 clouds, implying a constant star formation efficiency per freefall time ϵ ff ≈ 0.026. The finding of a universal correlation within individual molecular clouds, including clouds that contain no massive stars or massive stellar feedback, favors models in which star formation is regulated by local processes such as turbulence or stellar feedback such as protostellar outflows, and disfavors models in which star formation is regulated only by galaxy properties or supernova feedback on galactic scales.
Highlights
In galactic disks, there is a well-established correlation between the gas mass and star formation rate per unit area, when both quantities are measured in kpc-scale or larger patches (e.g., Kennicutt 1998; Bigiel et al 2008; Leroy et al 2013); this correlation is known as the Kennicutt-Schmidt (KS) relation (Schmidt 1959)
The analysis in Pokhrel et al (2020) is different from the one we perform here, in that Pokhrel et al (2020) examine the gas surface density around each protostar, whereas here we are investigating the properties of regions defined by the clouds column density; the latter has the advantage that it allows us to investigate the dependence of the intracloud KS relation on cloud volume density
We focus on the Perseus cloud as an example, and repeat our analysis using a column density map derived from extinction together with the c2d protostellar catalog; these are representative of the data quality available in earlier studies
Summary
There is a well-established correlation between the gas mass and star formation rate per unit area, when both quantities are measured in kpc-scale or larger patches (e.g., Kennicutt 1998; Bigiel et al 2008; Leroy et al 2013); this correlation is known as the Kennicutt-Schmidt (KS) relation (Schmidt 1959). The correlation, worsens as one measures progressively smaller regions, and there is little correlation between the carbon monoxide and ionizing or far-infrared luminosities – standard proxies for gas mass and star formation rate, respectively – of individual molecular clouds or filaments 100 pc in size (Mooney & Solomon 1988; Schruba et al 2010; Onodera et al 2010; Kruijssen & Longmore 2014; Ochsendorf et al 2017; Zhang et al 2019) This apparent lack of a correlation can be the result of a true spread in the star formation rate per unit mass among clouds (Lee et al 2016), or the failure of the proxies for mass and star formation rate. The uncertainties for individual clouds might be substantial, artificially creating scatter in the KS relation at smaller scales (Feldmann & Gnedin 2011; Kruijssen & Longmore 2014; Kreckel et al 2018)
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