The dense molecular ridge in NGC 2024

Abstract

NGC 2024 is a star-forming region in Orion B which contains a dense molecular ridge and a number of dense cores seen by their dust emission (Mezger et al., 1992). This area is investigated using observations of four formaldehyde transitions, and the J = 3 - 2 transition of HCO⁺ and H¹³CO⁺. Maps of the ridge are produced and used to provide temperatures and densities, with a focus on the dense core positions. The resulting temperatures are very warm (45-85 K), and derived H₂CO number densities are found to be ~ 2×10⁶ cm^-3. Using the derived properties in combination with sub-mm data from Johnstone et al. (2006), core masses of ~2 M_&#8857 are found within a 15" beamsize, and H₂CO fractional abundances are calculated to resemble previous values found in similar star-forming regions. HCO⁺ and H¹³CO⁺ observations are used to obtain temperatures and also optical depths. The virial theorem is applied to FIR cores 3, 4, 5 and 6 suggests that all four cores are unstable against gravitational collapse.