Rethinking a Mysterious Molecular Cloud

Abstract

I present high-resolution column density maps of two molecular clouds having strikingly different star formation rates. To better understand the unusual, massive G216-2.5, a molecular cloud with no massive star formation, the distribution of its molecular gas is compared to that of the Rosette Molecular Cloud. Far-infrared data from Herschel are used to derive $N(\mathrm{H}_2)$ maps of each cloud and are combined with $I_{\mathrm{CO}}$ data to determine the CO-to-H$_2$ ratio, $X_{\mathrm{CO}}$. In addition, the probability distribution functions (PDFs) and cumulative mass fractions of the clouds are compared. For G216-2.5, $< N(\mathrm{H}_2) >=7.8\times 10^{20} cm^{-2}$ and $< X_{\mathrm{CO}} > =2.2\times 10^{20} (K km s^{-1})^{-1}$; for the Rosette, $< N(\mathrm{H}_2) > =1.8\times 10^{21} cm^{-2}$ and $ < X_{\mathrm{CO}} > =2.8\times 10^{20} (K km s^{-1})^{-1}$. The PDFs of both clouds are log-normal for extinctions below $\sim 2$ mag and both show departures from log-normality at high extinctions. Although it is the less-massive cloud, the Rosette has a higher fraction of its mass in the form of dense gas and contains $1389 M_\odot$ of gas above the so-called extinction threshold for star formation, $A_V = 7.3$ mag. The G216-2.5 cloud has $874 M_\odot$ of dense gas above this threshold.