title: Search for molecular oxygen in dense interstellar clouds.
abstract: We have carried out a search for the 234 GHz N = 2 --> 0, J = 1 --> 1 transition of 16O18O using the 13.7 m FCRAO radio telescope.  No emission was detected toward six giant molecular clouds.  Observations of the 220 GHz J = 2 --> 1 transition of C18O yield column densities for this species 1-3 x 10(16) cm-2; the resulting limits on the [O2]/[CO] ratio lie between <0.5 and <4.  According to various chemical models, the ratio of molecular oxygen to carbon monoxide is primarily sensitive to the age of a cloud and to its carbon to oxygen ratio.  For ages exceeding 3 x 10(6) yr and total carbon-to-oxygen ratio < 1, [O2]/[CO] can approach unity.  Our best limits can be interpreted as indicating that the observed clouds are not chemically "mature" or that [carbon]/[oxygen] > 1.  However, significant exploitation of molecular oxygen as a tracer of cloud structure and evolution will require more sensitive observations, which may be best carried out from above Earth's atmosphere.
authors.affiliation: Department of Physics and Astronomy, University of Massachusetts, Amherst, USA.
authors.forename: P F
authors.initials: PF
authors.lastname: Goldsmith
authors.forename: R L
authors.initials: RL
authors.lastname: Snell
authors.forename: N R
authors.initials: NR
authors.lastname: Erickson
authors.forename: R L
authors.initials: RL
authors.lastname: Dickman
authors.forename: F P
authors.initials: FP
authors.lastname: Schloerb
authors.forename: W M
authors.initials: WM
authors.lastname: Irvine
mesh_terms: D055580:Astronomical Phenomena; D001256:Astronomy; D002248:Carbon Monoxide / Q000032:analysis; D019443:Cosmic Dust / Q000032:analysis*; D005118:Extraterrestrial Environment; D008956:Models, Chemical; D010100:Oxygen / Q000032:analysis* / Q000737:chemistry; D013057:Spectrum Analysis
chemical_list: D019443:Cosmic Dust; D002248:Carbon Monoxide; D010100:Oxygen