Carbon monoxide emission from interstellar dust clouds

Observations of the J = 1-0 rotational lines of 12C16O and 13C16O in two dust clouds, the North America nebula dust cloud and L1630, are reported. The observations have higher velocity resolution (delta v = 0.25 km s-1) than had been available previously. In an earlier paper, unusually bright CO emission associated with the reflection nebulae NGC 2023 and NGC 2068 in L1630 is noted. Bright CO emission is looked for from the directions o individual H α-emission stars in L1630 and the North America dust cloud, but the CO emission at these places is not enhanced above that of the surrounding material. It is concluded that these stars are too cool to heat the gas appreciably. The shapes of the CO profiles are determined by the large-scale motions of the gas in dust clouds. The possibility that magnetic fields in dust clouds prevent rapid dissipation of turbulent motions in these clouds is discussed. The shapes of the CO profiles observed near NGC 2023 show that the gas motions there are turbulent, rather than radial. From the observations of the North America dust cloud, it is shown that not more than about 10 percent of the carbon present in the gas is in the form of CO. A model of CO excitation in a homogeneous, spherical cloud with a constant source function is described, and the effects of radiative trapping in the model are studied. Some numerical results are presented, that show how the CO brightness temperature is determined by the CO column density and the density of the gas. The problem of making realistic models of dust clouds is briefly discussed, and some suggestions are made about possible future observations that would be useful to determine whether the motions of the gas are predominantly turbulent or radial (expansion or collapse)