Protoplanetary disk masses in NGC 2024: Evidence for two populations
  Protoplanetary disks in dense, massive star-forming regions are strongly
affected by their environment. How this environmental impact changes over time
is an important constraint on disk evolution and external photoevaporation
models. We characterize the dust emission from 179 disks in the core of the
young (0.5 Myr) NGC 2024 cluster. By studying how the disk mass varies within
the cluster, and comparing these disks to those in other regions, we aim to
determine how external photoevaporation influences disk properties over time.
Using the Atacama Large Millimeter/submillimeter Array, a 2.9' x 2.9' mosaic
centered on NGC 2024 FIR 3 was observed at 225 GHz with a resolution of 0.25'',
or ~100 AU. The imaged region contains 179 disks identified at IR wavelengths,
seven new disk candidates, and several protostars. The overall detection rate
of disks is $32 \pm 4\%$. Few of the disks are resolved, with the exception of
a giant (R = 300 AU) transition disk. Serendipitously, we observe a millimeter
flare from an X-ray bright young stellar object (YSO), and resolve continuum
emission from a Class 0 YSO in the FIR 3 core. Two distinct disk populations
are present: a more massive one in the east, along the dense molecular ridge
hosting the FIR 1-5 YSOs, with a detection rate of $45 \pm 7\%$. In the western
population, towards IRS 1, only $15 \pm 4\%$ of disks are detected. NGC 2024
hosts two distinct disk populations. Disks along the dense molecular ridge are
young (0.2 - 0.5 Myr) and partly shielded from the far ultraviolet radiation of
IRS 2b; their masses are similar to isolated 1 - 3 Myr old SFRs. The western
population is older and at lower extinctions, and may be affected by external
photoevaporation from both IRS 1 and IRS 2b. However, it is possible these
disks had lower masses to begin with.
