High-Density Gas in the Core of the Sagittarius B2 Molecular Cloud

We have observed several transitions of HC3N in the ground and vibrationally excited states in order to determine the density and kinetic temperature in the core of the Sagittarius B2 molecular cloud. Statistical equilibrium analysis of the J = 12 → 11 and 4 → 3 data suggests that warm and dense gas with a kinetic temperature of ∼20-40 K and a mean H₂ density of ∼10⁵ cm^-3 present in the inner region of the cloud ∼10 pc in size. The HC₃N column density does not correlate well with the C¹⁸O column density, suggesting strong variations of the HC₃N fractional abundance in the region by a factor of ∼3 on a 1' scale. The HC₃N fractional abundance derived at the center of the cloud is ∼3 × 10^-10, comparable to that derived for the ridge component of the Orion KL nebula. Detection of the J = 39 → 38 transition of HC₃N suggests H₂ densities greater than ∼10⁷ cm^-3 and mean kinetic temperatures greater than ∼60 K and 80 K in 21" FWHM beams centered at the positions of the Sgr B2(M) and (N) sources. Because this transition is most likely collisionally excited, the higher intensity and excitation temperature derived at the position of the Sgr B2(N) continuum source indicate higher density in the northern core compared to the middle core. Vibrationally excited HC₃N gives a ∼260 K estimate for the mean radiation field temperature for both cores in a 45" FWHM beam.