CO (J = 1-0) LINE EMISSION FROM GIANT MOLECULAR CLOUDS

We present numerical models of the (CO)-C-12 (J = 1-0) line emission from Giant Molecular Clouds (GMCs). The models calculate simultaneously the gas chemistry, thermal balance, and line transfer in a GMC of column density congruent-to 10(22) cm-2 (A(V) congruent-to 10) which is illuminated by an ultraviolet radiation field and subject to cosmic-ray heating and ionization. Line profiles are calculated for both microturbulent and macroturbulent cloud models. In the microturbulent case the line arises from gas with a supersonic velocity distribution with dispersion velocity on the order of the observed line width. In the macroturbulent case the line arises from a distribution of moving clumps, each with intrinsic line width which is on the order of the thermal line width and much smaller than the velocity dispersion of clumps. The typical clump-clump speed is of order of the observed line width. A theoretical plot of the CO luminosity versus cloud mass is constructed, and the variation in luminosity with incident radiation field and metallicity is investigated. Our main conclusions are the following: (1) The profile of the observed CO (J = 1-0) line cannot be interpreted as arising from a microturbulent gas velocity distribution. Microturbulent models produce emission with a wide range in profile shapes (i.e., self-absorbed profiles) and peak brightness temperatures in contrast to the observations. (2) Macroturbulent models can produce centrally peaked, smooth profiles, but only when many clumps are contained within the telescope beam. (3) Photoelectric heating dominates in the region where the CO (J = 1-0) line arises. Cosmic-ray heating alone is insufficient. In the absence of photoelectric heating, a cosmic-ray ionization rate of zeta = 2.4 x 10(-16) would be required to produce the observed CO (J = 1-0) line emission. This is approximately 15 times greater than the inferred rate from astrochemistry. (4) The integrated CO (J = 1-0) fine luminosity emitted from a GMC is only weakly dependent on the incident ultraviolet radiation field for fluxes up to approximately 10(3) times greater than the local Galactic field. At higher incident fluxes, GMCs with A(V) less than or similar to 10 are appreciably photodissociated resulting in a drop in CO luminosity. The critical field for dissociation is proportional to e1.6AV, and clouds with A(V) > 10 can remain molecular while illuminated by much higher fields. (5) If the hydrogen column density through a GMC is a constant (N congruent-to 10(22) cm-2), the CO luminosity is insensitive to small variations in metallicity Z up to a reduction in Z by a factor of 5, but thereafter decreases approximately linearly with Z. Alternatively, for a constant dust extinction of A(V) almost-equal-to 10 (and constant cloud mass), we find that the CO luminosity decreases by factors of 2.3 and 3.3 for reductions in Z by factors of 5 and 10, respectively.