Rethinking a Mysterious Molecular Cloud

Abstract

I present high-resolution column density maps of two molecular clouds (MCs) having strikingly different star formation rates. To better understand the unusual, massive G216-2.5, an MC with no massive star formation, the distribution of its molecular gas is compared to that of the Rosette MC. Far-infrared data from Herschel are used to derive N(H₂) maps of each cloud and are combined with {{I}_CO} data to determine the CO-to-H₂ ratio, {{X}_CO}. In addition, the probability distribution functions (PDFs) and cumulative mass fractions of the clouds are compared. For G216-2.5, < N({{H}₂})> =7.8× {{10}²⁰} cm^-2 and < {{X}_CO}> =2.2× {{10}²⁰} cm^-2 (K km s^-1)^-1 for the Rosette, < N({{H}₂})> =1.8× {{10}²¹} cm^-2 and < {{X}_CO}> =2.8× {{10}²⁰} cm^-2 (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, {{A}_V}=7.3 mag. The G216-2.5 cloud has 874 {{M}_⊙ } of dense gas above this threshold.