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