Radio Imaging of the NGC 2024 FIR 5/6 Region: a Hypercompact H II Region Candidate in Orion

TL;DR

This study uses high-resolution radio imaging to identify a potential hypercompact H II region in Orion's NGC 2024 FIR 5/6 area, revealing complex structures and maser activity indicative of early star formation stages.

Contribution

First detailed radio imaging of FIR 5/6 region revealing a candidate hypercompact H II region and associated maser activity, advancing understanding of early massive star formation.

Findings

FIR 6c may be a hypercompact H II region with a B1 star.

FIR 6n shows mixed free-free and dust emission.

FIR 5w and 5e are dominated by dust emission with low gas masses.

Abstract

The NGC 2024 FIR 5/6 region was observed in the 6.9 mm continuum with an angular resolution of about 1.5 arcsec. The 6.9 mm continuum map shows four compact sources, FIR 5w, 5e, 6c, and 6n, as well as an extended structure of the ionization front associated with the optical nebulosity. FIR 6c has a source size of about 0.4 arcsec or 150 AU. The spectral energy distribution (SED) of FIR 6c is peculiar: rising steeply around 6.9 mm and flat around 1 mm. The possibility of a hypercompact H II region is explored. If the millimeter flux of FIR 6c comes from hot ionized gas heated by a single object at the center, the central object may be a B1 star of about 5800 solar luminosities and about 13 solar masses. The 6.9 mm continuum of FIR 6n may be a mixture of free-free emission and dust continuum emission. Archival data show that both FIR 6n and 6c exhibit water maser activity, suggesting the existence of shocked gas around them. The 6.9 mm continuum emission from FIR 5w has a size of about 1.8 arcsec or 760 AU. The SEDs suggest that the 6.9 mm emission of FIR 5w and 5e comes from dust, and the masses of the dense molecular gas are about 0.6 and 0.5 solar masses, respectively.