CO LINES TOWARD NGC-2024 AND OTHER STAR-FORMING REGIONS - A CLOSER LOOK AT THE WARM GAS COMPONENT

We present observations of (CO)-C-13 J = 6 --> 5 emission from a variety of Galactic star-forming regions. Sources spanning a luminosity range from 1 to 10(6) L. have been detected in this isotopic transition. The high line intensity (typically T(mb) = 20-100 K) confirms the presence of large column densities (N(H2) almost-equal-to 10(23) cm-2) of warm (T(kin) greater-than-or-equal-to 100 K) molecular gas in typical regions of massive star formation. The (CO)-C-13/(CO)-C-12 J = 6 --> 5 intensity ratio is typically much higher than the (CO)-C-13/(CO)-C-12 abundance ratio. Hence, even the mid-J transitions of the main CO isotope are very optically thick in most sources. The (CO)-C-13 6 --> 5 emission is found to arise from quiescent (DELTAv almost-equal-to 5 km s-1) gas. Strip maps of selected sources (M17, Cepheus A, W51) show that the (CO)-C-13 6 --> 5 emitting gas is confined to narrow zones (< 1 pc) close to the H II regions in these star-forming regions. Thus, the spatial distribution and the quiescence of the warm gas support the idea that it is being heated by the UV radiation field just outside the H II regions. Although the column density of the warm gas is much higher than predicted by present photodissociation region (PDR) models, photoheating seems to be the most probable dominating heating process in these regions. We further present a detailed multiline study of the molecular gas in a selected source: NGC 2024. Measurements of nine different CO lines enable us to derive precise physical parameters of the two major source components along the line of sight toward the mm continuum peak FIR 5. The component in front of the H II region is cold (almost-equal-to 23 K) and contains about one-fifth of the total molecular column density (5 x 10(22) cm-2). The bulk of the column density (2 x 10(23) cm-2) is located behind the H II region in a warm (67 K) gas component. For a standard H-2/CO ratio, the low intensity of low-J isotopic CO lines excludes the presence of large amounts (N(H2) greater-than-or-equal-to 10(24) cm-2) of cold (T(kin) < 25 K) molecular gas. Maps recorded in four CO transitions indicate that the warm gas is located in the immediate vicinity of the H II region and that its spatial distribution is very similar to the 1.3 mm dust continuum. The 1.3 mm continuum, therefore, is likely to originate from-probably warm- dust embedded within warm gas.