Astrochemistry of sub-millimeter sources in Orion^*

Studying the variations of molecular tracers with changing physical conditions

¹ National Research Council Canada, Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, B.C., V9E 2E7, Canada
² Department of Physics & Astronomy, University of Victoria, Victoria, BC, V8P 1A1, Canada
³ Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands

Corresponding author: D. Johnstone, doug.johnstone@nrc-cnrc.gc.ca

Received: 28 January 2003
Accepted: 29 August 2003

Abstract

Cornerstone molecules (CO, H₂CO, CH₃OH, HCN, HNC, CN, CS, SO) were observed toward seven sub-millimeter bright sources in the Orion molecular cloud in order to quantify the range of conditions for which individual molecular line tracers provide physical and chemical information. Five of the sources observed were protostellar, ranging in energetics from , while the other two sources were located at a shock front and within a photodissociation region (PDR).
Statistical equilibrium calculations were used to deduce from the measured line strengths the physical conditions within each source and the abundance of each molecule. In all cases except the shock and the PDR, the abundance of CO with respect to H₂ appears significantly below (factor of ten) the general molecular cloud value of 10^-4. Formaldehyde measurements were used to estimate a mean temperature and density for the gas in each source. Evidence was found for trends between the derived abundance of CO, H₂CO, CH₃OH, and CS and the energetics of the source, with hotter sources having higher abundances. Determining whether this is due to a linear progression of abundance with temperature or sharp jumps at particular temperatures will require more detailed modeling. The observed methanol transitions require high temperatures ( K), and thus energetic sources, within all but one of the observed protostellar sources. The same conclusion is obtained from observations of the CS 7–6 transition. Analysis of the HCN and HNC 4–3 transitions provides further support for high densities cm^-3 in all the protostellar sources.
The shape of the CO 3–2 line profile provides evidence for internal energetic events (outflows) in all but one of the protostellar sources, and shows an extreme kinematic signature in the shock region. In general, the CO line and its isotopes do not significantly contaminate the 850m broadband flux (less than 10%); however, in the shock region the CO lines alone account for more than two thirds of the measured sub-millimeter flux. In the energetic sources, the combined flux from all other measured molecular lines provides up to an additional few percent of line contamination.