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.