The dense molecular ridge in NGC 2024

Abstract

NGC 2024 is a star-forming region in Orion B that contains a dense molecular ridge. A number of dense cores are seen by their dust emission. This area is investigated using observations of four formaldehyde transitions - namely, 303 → 202, 322 → 2 21, 505 → 404, and 523 → 422. Maps of the ridge are used to provide temperatures and densities, with a focus on the dense-core positions. The formaldehyde transition ratios 303 → 202/322 → 2 21 and 505 → 404/523 → 422 offer measurements of the kinetic temperature, while the ratio 303 → 202/505 → 404 allows the evaluation of number densities. The resulting temperatures are very warm (45-85 K), with no observed variation between the core centers and their envelopes. H2CO number densities of 2 × 106 cm -3 at far-infrared (FIR) core positions and H2CO column densities in the range of 6 × 1014 to 1 × 1015 cm-2 are derived. Using the derived properties in combination with previous submillimeter continuum data, core masses of 2 M are found within a 15″ beam size. H2CO fractional abundances are calculated and resemble previous values found in similar star-forming regions - between 2 × 10-9 and 5 × 10-9. The virial theorem applied to FIR cores 3, 4, 5, and 6 suggests that all four cores are unstable against gravitational collapse. © 2008. The American Astronomical Society. All rights reserved.