A Three‐Position Spectral Line Survey of Sagittarius B2 between 218 and 263 GHz. II. Data Analysis

Abstract

We present an analysis of the data from our Swedish-ESO Submillimetre Telescope molecular line survey in the 1.3 mm band of the N, M, and NW positions in the Sgr B2 molecular cloud. The line emissions from 42 molecular species, and some of their isotopomers, were analyzed assuming a single temperature and a homogeneous source. In cases where a source size much smaller than the antenna beam (23'') could be estimated, optical depth effects were also accounted for. In this way rotation temperatures, molecular column densities, and in several cases also source sizes, were determined. Observed and modeled intensities are presented in rotation diagrams. A few complex molecular species, NH2CHO, CH3CHO, C2H3CN, C2H5CN, and CH3OCHO, mainly in the N source, exhibit anomalously strong intensities in their intrinsically weak b- and c-type lines. We argue that this effect can hardly be explained by high optical depths alone, and therefore propose that the possibility of radiative pumping via the low-lying vibrational states of these molecules should be investigated as an alternative explanation. The highest rotation temperatures, up to about 500 K, were found for large molecules toward Sgr B2(N), closely followed by Sgr B2(M). In Sgr B2(NW), which samples the cloud envelope, the rotation temperatures are 15-50 K. For molecules with too few observed transitions to allow determination of rotation temperatures, the column density was calculated assuming optically thin emission, and we adopted rotation temperatures of 50, 50, and 20 K in M, N, and NW, respectively. Column density ratios of isotopomers were determined. After a critical discussion of the resolution-dependent H2 column density toward the observed positions, abundances relative to H2 were calculated. We discuss the chemical differences between the three observed cloud positions and compare with the hot core, compact ridge, and outflow in Orion A. Hot core-type molecules like CH2NH, NH2CN, CH3CN, C2H3CN, and C2H5CN, as well as H2CS, are more abundant in Sgr B2(N) by factors of 3-8 as compared to the M position. Large oxygen-containing species like CH3OH, CH3CHO, CH3OCHO, CH3OCH3, and NH2CHO, of compact ridge-type, show similar or slightly enhanced abundances in N as compared to M. The C2H5OH abundances are similar in N, M, and also in NW. The SO2 and SO abundances in the M core (4 × 10-7 and 1 × 10-7, respectively) are 13 and 5 times higher than in N and are very enhanced (103 and 102 times) as compared to NW. Such high SO2 and SO abundances are also found in the prominent Orion A outflow source. In M the SO18O and S18O data suggest a 16O/18O ratio of 120. The HOCO+ ion is detected in all three positions and appears to be 3 times more abundant in the NW position. HCNH+ is seen only in NW.