Complex Molecules in Star-Forming Regions

Doktorsavhandling, 1998

Results from spectral-line observations of molecular gas in star-forming regions, performed with the Swedish-ESO Submillimetre Telescope (SEST), are presented. The purpose of this work has been to provide better estimates of the chemical composition as a function of the physical conditions in the sources. The main investigation in this Thesis is a spectral line survey covering the 218.3--263.5 GHz band. Three positions towards the Galactic Centre molecular cloud Sgr B2 were observed. Two of them, Sgr B2(N) and Sgr B2(M), are dense cores where signs of advanced stages of star-formation are present, while the third position, Sgr B2(NW), sample gas in the surrounding cloud. More than 2500 emission lines originating from 42 molecular species were detected. A modified rotation diagram method, which accounts for the line optical depths, was applied in order to determine the rotational temperatures and molecular column densities. For a number of the species, most notably SO and SO2, the size of the emission regions could be estimated by virtue of their well-constrained optical depths. Most species are considerably more excited in the Sgr B2(N) and Sgr B2(M) cores than in Sgr B2(NW), as demonstrated by the detection of, e.g., 114 transitions of C2H3CN in its .ny.11 and .ny.15 vibrational states in Sgr B2(N). The beam-averaged column densities indicate, with a few exceptions, relatively small abundance variations among the three observed positions. However, in those cases where the source size is known, the abundances are considerably higher in Sgr B2(N) and Sgr B2(M), which emphasises the impact of source structure on the abundance estimates. The cores of Sgr B2 have the chemical characteristics of both the Compact Ridge and Hot Core sources in the Orion Molecular Cloud. The high abundance of SO2 in Sgr B2(M) is similar to that in the Orion outflow. By combining data from Haystack Observatory, our SEST survey, and a lower-frequency survey of the same positions carried out at Nobeyama Radio Observatory a rare cyclic molecule, ethylene oxide (c-C2H4O), could be identified in Sgr B2(N) for the first time in the interstellar medium. To verify the identification, a search towards a number of southern molecular clouds was subsequently made. Ethylene oxide and its isomer acetaldehyde (CH3CHO) were detected in four additional sources, NGC 6334F, G327.3-0.6, G31.41+0.31, and G34.3+0.2, all of which are hot cloud cores similar to Sgr B2(N). The abundances of CH3CHO were found to exceed those of c-C2H4O by a factor of 2.6 to 8.5, and the abundances of both species are orders of magnitudes larger than what is predicted by present models of gas-phase chemistry.