Search for molecular oxygen in dense interstellar clouds.

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.

Submillimeter heterodyne detection of interstellar carbon monoxide at 434 micrometers.

A submillimeter heterodyne radiometer, developed for astronomical applications, uses an optically pumped laser local oscillator and a quasi-optical Schottky diode mixer. The resultant telescope-mounted system, which has a noise temperature less than 4000 K (double sideband) and high frequency and spatial resolution, has been used to detect the J = 6 --> 5 rotational transition of carbon monoxide at 434 micrometers in the Orion molecular cloud. The measurements, when compared with previous millimeter-wave data, indicate that the broad carbon monoxide emission feature is produced by an optically thin gas whose temperature exceeds 180 K.