Proplyds in the Flame Nebula NGC 2024

Abstract

A recent survey of the inner $0.35\times 0.35$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 $\sim 0.2-0.5$Myr old, with an ALMA mm continuum disc detection rate of about $45$per cent. However this drops to only $\sim 15$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.

14 pages, Accepted for publication in MNRAS

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• The influence of the environment on the spin evolution of low-mass stars. I. External photoevaporation of circumstellar disks
• Warm millimetre dust in protoplanetary discs near massive stars
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• [OI] 6300Å emission as a probe of external photoevaporation of protoplanetary discs
• Determining dispersal mechanisms of protoplanetary disks using accretion and wind mass loss rates
• Deep diving off the `Cosmic Cliffs': previously hidden outflows in NGC 3324 revealed by JWST
• An APEX search for carbon emission from NGC 1977 proplyds