Proplyds in the Flame Nebula NGC 2024
  A recent survey of the inner $0.35\times0.35$pc of the NGC 2024 star forming
region revealed two distinct millimetre continuum disc populations that appear
to be spatially segregated by the boundary of a dense cloud. The eastern (and
more embedded) population is $\sim0.2-0.5$Myr old, with an ALMA mm continuum
disc detection rate of about $45\,$per cent. However this drops to only
$\sim15$per cent in the 1Myr western population. When presenting this result,
van Terwisga et al. (2020) suggested that the two main UV sources, IRS 1 (a
B0.5V star in the western region) and IRS 2b (an O8V star in the eastern
region, but embedded) have both been evaporating the discs in the depleted
western population.
  In this paper we report the firm discovery in archival HST data of 4 proplyds
and 4 further candidate proplyds in NGC 2024, confirming that external
photoevaporation of discs is occurring. However, the locations of these
proplyds changes the picture. Only three of them are in the depleted western
population and their evaporation is dominated by IRS 1, with no obvious impact
from IRS 2b. The other 5 proplyds are in the younger eastern region and being
evaporated by IRS 2b. We propose that both populations are subject to
significant external photoevaporation, which happens throughout the region
wherever discs are not sufficiently shielded by the interstellar medium. The
external photoevaporation and severe depletion of mm grains in the 0.2-0.5Myr
eastern part of NGC 2024 would be in competition even with very early planet
formation.
