Rethinking a Mysterious Molecular Cloud

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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(H₂) maps of each cloud and are combined with ICO data to determine the CO-to-H₂ ratio, XCO. In addition, the probability distribution functions (PDFs) and cumulative mass fractions of the clouds are compared. For G216-2.5, < N(H₂) >=7.8× 10²⁰ cm⁻² and < XCO > =2.2× 10²⁰ (K km s⁻¹)⁻¹; for the Rosette, < N(H₂) > =1.8× 10²¹ cm⁻² and < XCO > =2.8× 10²⁰ (K km s⁻¹)⁻¹. The PDFs of both clouds are log-normal for extinctions below 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 of gas above the so-called extinction threshold for star formation, Aᵥ = 7.3 mag. The G216-2.5 cloud has 874 M of dense gas above this threshold.