MODELING OF THE CONTINUUM AND MOLECULAR LINE EMISSION FROM THE SAGITTARIUS-B2 MOLECULAR CLOUD

We have modeled the continuum and molecular line emission from the Sagittarius B2 molecular cloud in order to determine the conditions in the core and the envelope of the cloud. The continuum models suggest that the total luminosity of the middle source Sgr B2(M), (1-2) × 107 L⊙, is an order of magnitude higher than that of the northern source Sgr B2(N). The peaking of the millimeter continuum emission at the position of the northern source is mostly a result of the higher column density at this position. The observed change in the middle-to-north peak flux ratio at submillimeter wavelengths is primarily a result of the luminosity difference. The continuum models confirm the very high H2 column density through the envelope of the cloud, ≳ 1024 cm-2 in a 60″ beam centered 1t the position of the Sgr B2(M) continuum source, and the total mass of the cloud, ≳5 × 106 M⊙. The microturbulent models of the molecular line emission predict the correct spatial intensity distribution of the J = 1 → 0 transitions of C18O and 13CO. They have difficulties, however, with reproducing the observed intensities of the higher transitions of these molecules. This may indicate that the envelope has a clumpy structure. Because turbulent dissipation is a major heating source for the gas in the envelope of the cloud, the kinetic temperature is very uniform over a broad range of distances from the cloud center. Sgr B2 differs significantly from typical disk giant molecular clouds (GMCs), in that it has higher mass and luminosity of the continuum sources, much greater H2 column density and mean volume density, together with different fractional abundances of many interstellar molecules. It seems also quite different from other Galactic center GMCs, which can be regarded as examples of an intermediate case between Sgr B2 and the disk clouds.