Massive Quiescent Cores in Orion: VI. The Internal Structures and a Candidate of Transiting Core in NGC 2024 Filament

TL;DR

This study investigates the internal structures of dense cores in the NGC 2024 filament, revealing signs of core collapse and a potential transiting core stage, through multi-wavelength observations and temperature analysis.

Contribution

It provides new insights into the internal temperature structures and possible evolutionary stages of cores in the NGC 2024 filament using combined infrared and sub-millimeter data.

Findings

FIR-3 shows internal heating without infrared sources, indicating core collapse.

Dense cores are cooler than the surrounding hot ridge, suggesting spatial separation.

The filament is located 3-5 pc from the hot ridge, based on dust heating models.

Abstract

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.