Gemini, SOFIA, and ATCA Reveal Very Young, Massive Protostars in the Collapsing Molecular Cloud BYF 73

Abstract

We present multi-wavelength data on the globally infalling molecular cloud/protostellar cluster BYF 73. These include new far-IR spectral line and continuum data from SOFIA's Far Infrared Field-Imaging Line Spectrometer (FIFI-LS), mid-infrared (MIR) observations with the Thermal-Region Camera Spectrograph (T-ReCS) on Gemini-South, and 3 mm continuum data from the Australia Telescope Compact Array (ATCA), plus archival data from Spitzer/IRAC, and Herschel/PACS and SPIRE. The FIFI-LS spectroscopy in [OI]$\lambda 63\mu$m, [OIII]$\lambda 88\mu$m, [OI]$\lambda 145\mu$m, and [CII]$\lambda 158\mu$m highlights different gas environments in and between the dense molecular cloud and HII region. The photo-dissociation region (PDR) between the cloud and HII region is best traced by [OI]$\lambda 145\mu$m and may have density $>$10${}^{10}$ m${}^{-3}$, but the observed $\lambda 145\mu$m/$\lambda 63\mu$m and $\lambda 63\mu$m/$\lambda 158\mu$m line ratios in the densest gas are well outside model values. The HII region is well-traced by [CII], with the $\lambda 158\mu$m/$\lambda 145\mu$m line ratio indicating a density of 10${}^{8.5}$ m${}^{-3}$ and a relatively weak ionizing radiation field, 1.5 $\lesssim$ log$(G/G_0)\lesssim$ 2. The T-ReCS data reveal eight protostellar objects in the cloud, of which six appear deeply embedded ($A_V$ $>$ 30${}^m$ or more) near the cloud's center. MIR 2 has the most massive core at $\sim$240 M${}_{\odot }$, more massive than all the others combined by up to tenfold, with no obvious gas outflow, negligible cooling line emission, and $\sim 3-8$% of its 4.7$\times$10${}^3$ L${}_{\odot }$ luminosity originating from the release of gravitational potential energy. MIR 2's dynamical age may be as little as 7000 yr. This fact, and the cloud's total embedded stellar mass being far less than its gas mass, confirm BYF 73's relatively early stage of evolution.

arXiv comment6 pages, 5 figures, published in Astrophysical Journal Letters

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