Massive Quiescent Cores in Orion: VI. The Internal Structures and a
  Candidate of Transiting Core in NGC 2024 Filament
  We present a multi-wavelength observational study of the NGC 2024 filament
using infrared to sub-millimeter continuum and the NH$_3$ $(1,1)$ and $(2,2)$
inversion transitions centered on FIR-3, the most massive core therein. FIR-3
is found to have no significant infrared point sources in the Spitzer/IRAC
bands. But the NH$_3$ kinetic temperature map shows a peak value at the core
center with $T_{\rm k}=25$ K which is significantly higher than the surrounding
level ($T_{\rm k}=15-19$ K). Such internal heating signature without an
infrared source suggests an ongoing core collapse possibly at a transition
stage from first hydrostatic core (FHSC) to protostar. The eight dense cores in
the filament have dust temperatures between 17.5 and 22 K. They are much cooler
than the hot ridge ($T_{\rm d}=55$ K) around the central heating star IRS-2b.
Comparison with a dust heating model suggests that the filament should have a
distance of $3-5$ pc from IRS-2b. This value is much larger than the spatial
extent of the hot ridge, suggesting that the filament is spatially separated
from the hot region along the line of sight.
