Gemini, SOFIA, and ATCA reveal very young, massive protostars in the collapsing molecular cloud BYF 73

We present multi-wavelength data on the globally infalling molecular cloud/protostellar cluster BYF 73. These include new far-infrared (FIR) spectral line and continuum data from the Stratospheric Observatory for Infrared Astronomy's (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/Infrared Array Camera (IRAC), and Herschel/Photodetecting Array Camera and Spectrometer (PACS) and Spectral and Photometric Imaging Receiver (SPIRE). The FIFI-LS spectroscopy in [O i], [O iii], [O i], and [C ii] highlights different gas environments in and between the dense molecular cloud and H ii region. The photo dissociation region (PDR) between the cloud and H ii region is best traced by [O i] and may have density >10¹⁰ m^-3, but the observed and λ63 μm/λ158 μm line ratios in the densest gas are well outside model values. The H ii region is well-traced by [C ii], with the λ158 μm/λ145 μm line ratio, indicating a density of 10^8.5 m^-3 and a relatively weak ionizing radiation field, 1.5 ≲ log(G/G ₀) ≲ 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 ∼240 M⊙, more massive than all the others combined by up to tenfold, with no obvious gas outflow, negligible cooling line emission, and ∼3%-8% of its 4.7 ×10³ L⊙ luminosity originating from the release of gravitational potential energy. MIR 2's dynamical age may be as little as 7000 years. 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.