The High-Mass Protostellar Population of a Massive Infrared Dark Cloud
  We conduct a census of the high-mass protostellar population of the
$\sim70,000\:M_\odot$ Infrared Dark Cloud (IRDC) G028.37+00.07, identifying 35
sources based on their $70\:\mu$m emission, as reported in the {\it Herschel}
Hi-GAL catalog of Molinari et al. (2016). We perform aperture photometry to
construct spectral energy distributions (SEDs), which are then fit with the
massive protostar models of Zhang & Tan (2018). We find that the sources span a
range of isotropic luminosities from $\sim$20 to 4,500$\:L_\odot$. The most
luminous sources are predicted to have current protostellar masses of
$m_{*}\sim10\:M_\odot$ forming from cores of mass $M_{c}\sim40$ to
$400\:M_\odot$. The least luminous sources in our sample are predicted to be
protostars with masses as low as $\sim 0.5\:M_\odot$ forming from cores with
$M_{c}\sim10\:M_\odot$, which are the minimum values explored in the
protostellar model grid. The detected protostellar population has a total
estimated protostellar mass of $M_{*}\sim 100\:M_\odot$. Allowing for
completeness corrections, which are constrained by comparison with an ALMA
study in part of the cloud, we estimate a star formation efficiency per
free-fall time of $\sim3\%$ in the IRDC. Finally, analyzing the spatial
distribution of the sources, we find relatively low degrees of central
concentration of the protostars. The protostars, including the most massive
ones, do not appear to be especially centrally concentrated in the protocluster
as defined by the IRDC boundary.
