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Astron. Astrophys. 324, 221-236 (1997)
1. Introduction
Because of the large cosmic abundance of oxygen, this element plays a major role in the chemistry of interstellar molecular clouds. Much theoretical work has been devoted to the synthesis of oxygen-bearing molecules. At the same time, around 30 molecules containing oxygen, representing nearly half of the molecules so far detected in space, have now been observed in interstellar and circumstellar environments. Many of these oxygen-bearing species are only trace compounds with very low abundances but some of them are among the most complex organic molecules observed, revealing the surprisingly richness of interstellar chemistry. The most abundant and famous oxygen-bearing molecule is CO which is observed in a large variety of physical conditions and used as a tracer of molecular gas in galaxies. Chemical models of interstellar clouds, beginning with the first ones dealing with steady-state abundance calculations of the simplest molecules (Herbst & Klemperer 1973, Viala & Walmsley 1976) predict that molecular oxygen O2 has an abundance comparable to that of CO deep inside molecular clouds and plays an important role for controlling the abundance of other molecules. This has been confirmed by further chemical models, at least by those assuming steady-state equilibrium.
For these reasons, observation of molecular oxygen and measurement
of its abundance could permit to test the interstellar chemistry and
to give some insight on the total amount of oxygen in the gas phase of
molecular clouds. Despite the presumed importance of O2 in
the interstellar clouds, this molecule has not yet been observed. The
presence of a large quantity of molecular oxygen in the terrestrial
atmosphere is a major obstacle to the detection of interstellar
O2 by ground-based telescopes. Its observation requires the
use of receivers embarked on balloons or satellites and four
experiments are in course of development to detect rotational lines
within the ground vibrational level. Our laboratory is involved in two
of these experiments for which we are developing two submillimeter
heterodyne supraconducting (SIS) receivers. The first one will be
borne by the stratospheric balloon-borne observatory PRONAOS-SMH
developed under the responsability of the CNES to search for the
(N, J): (3, 2) ![[FORMULA]](img3.gif)
(1, 1) line of O2 at 368 GHz (Beaudin et al.
1994). The second one will equip the swedish-french balloon PIROG 8 to
search for the (3, 2) (1, 2) line at
425 GHz. Two other projects are: the US satellite SWAS which will
observe the (3, 3) (1, 2) line at 487
GHz and the swedish satellite ODIN, developed with the collaboration
of Canada, France and Finland devoted to a search for both the
119 GHz and 487 GHz lines.
The main purpose of this paper is to make theoretical predictions of the emissivities of these O2 lines and of some others. To do this, radiative and collisional transfers between the rotational levels of O2 have been included in a model which calculates chemical and thermal balance in molecular clouds. Modelling the chemistry and rotational excitation of molecular oxygen previously done by Black & Smith (1984). New data about chemical and collisional processes led us to adress this problem again and to make a more extensive study of the influence of several physical parameters on the abundance and rotational excitation of O2. The model is presented in Sect. 2 including the determination of the spectroscopic parameters of O2 in its ground vibrational level which are needed to compute its rotational population. Several parameters could affect the abundance and the emissivity of O2 such as total visual extinction, temperature, density, ultraviolet radiation field or C/O elemental abundance ratio. The influence of these parameters are discussed in Sect. 3. The effects of radiative cooling due to O2 have been accurately estimated and are presented in the same section. Sect. 4 summarizes our main conclusions.
© European Southern Observatory (ESO) 1997
Online publication: May 26, 1998
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