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-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]$\lambda63 \mu$m,
[OIII]$\lambda88 \mu$m, [OI]$\lambda145 \mu$m, and [CII]$\lambda158 \mu$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]$\lambda145 \mu$m and may have density
$>$10$^{10}$ m$^{-3}$, but the observed $\lambda145\mu$m/$\lambda63\mu$m and
$\lambda63\mu$m/$\lambda158\mu$m line ratios in the densest gas are well
outside model values. The HII region is well-traced by [CII], with the
$\lambda158\mu$m/$\lambda145\mu$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 $\sim3-8$% 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.
