Radio imaging of the NGC2024 FIR5/6 region: A hypercompact HII region candidate in Orion

The NGC2024 FIR5/6 region was observed in the 6.9mm continuum with an angular resolution of about 1.5arcsec. The 6.9mm continuum map shows four compact sources, FIR5w, 5e, 6c, and 6n, as well as an extended structure of the ionization front associated with the optical nebulosity. FIR6c has a source size of about 0.4arcsec or 150AU. The spectral energy distribution (SED) of FIR6c is peculiar: rising steeply around 6.9mm and flat around 1mm. The possibility of a hypercompact H II region is explored. If the millimeter flux of FIR6c comes from hot ionized gas heated by a single object at the center, the central object may be a B1 star of about 5800solar luminosities and about 13solar masses. The 6.9mm continuum of FIR6n may be a mixture of free-free emission and dust continuum emission. Archival data show that both FIR6n and 6c exhibit water maser activity, suggesting the existence of shocked gas around them. The 6.9mm continuum emission from FIR5w has a size of about 1.8arcsec or 760AU. The SEDs suggest that the 6.9mm emission of FIR5w and 5e comes from dust, and the masses of the dense molecular gas are about 0.6 and 0.5solar masses, respectively.