The Distribution of Mass Surface Densities in a High-mass Protocluster

September 2016 • 2016ApJ...829L..19L

Authors • Lim, Wanggi • Tan, Jonathan C. • Kainulainen, Jouni • Ma, Bo • Butler, Michael J.

Abstract • We study the probability distribution function (PDF) of mass surface densities, Σ, of infrared dark cloud (IRDC) G028.37+00.07 and its surrounding giant molecular cloud. This PDF constrains the physical processes, such as turbulence, magnetic fields, and self-gravity, that are expected to be controlling cloud structure and star formation activity. The chosen IRDC is of particular interest since it has almost 100,000 solar masses within a radius of 8 pc, making it one of the most massive, dense molecular structures known and is thus a potential site for the formation of a “super star cluster.” We study Σ in two ways. First, we use a combination of NIR and MIR extinction maps that are able to probe the bulk of the cloud structure up to Σ ∼ 1 g cm^-2(A _V ≃ 200 mag). Second, we study the FIR and submillimeter dust continuum emission from the cloud utilizing Herschel-PACS and SPIRE images and paying careful attention to the effects of foreground and background contamination. We find that the PDFs from both methods, applied over a ∼20‧(30 pc)-sized region that contains ≃1.5 × 10⁵ M _⊙ and enclosing a minimum closed contour with Σ ≃ 0.013 g cm^-2 (A _V ≃ 3 mag), shows a lognormal shape with the peak measured at Σ ≃ 0.021 g cm^-2 (A _V ≃ 4.7 mag). There is tentative evidence for the presence of a high-Σ power-law tail that contains from ∼3% to 8% of the mass of the cloud material. We discuss the implications of these results for the physical processes occurring in this cloud.

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Wanggi Lim

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