Gamma Rays, Cosmic Rays and Local Molecular Clouds.

Available from UMI in association with The British Library. Requires signed TDF. An estimate of the CO-to-N(H_2 ) conversion factor X has been obtained using gamma ray data from local molecular clouds at medium latitudes. Evidence is found for the production of low-energy cosmic rays in the cloud environs which complicates the derivation of X, but an interpretation of the results that is consistent with extinction and optically thin molecular emission data is possible and suggests X ~ (1 - 2) times 10 ^{20} mol cm^{ -2} (K kms^{-1})^ {-1}. Some variations in the cosmic ray intensity not associated with the clouds but in the same line-of-sight may be indicated by the gamma ray analysis. Differences between the value proposed by COS -B (X = 2.6 times 10^{20} in the Orion molecular cloud) and the present work are ascribed to their disregard for the production of cosmic rays in or around the clouds. Several models that might account for the cosmic ray excess are examined in detail. The most promising is that by Morfill (1982) in which electrons are convected into the cloud by Alfven waves generated by a few GeV protons. The optical and molecular emission data suggest that X is ~1 times 10^{20} in low CO luminosity parts of the cloud and remains < 2 times 10^ {20} in more luminous regions. At the highest luminosities there is evidence that X declines once again. It is suggested that such a dependence on cloud luminosity arises because X is temperature-dependent, being roughly proportional to T^{-1 }. This contention supports the conclusions of Bhat et al. (1986c) that X decreases to about 1 times 10^{20} in the inner Galaxy, since mean cloud temperatures are higher there than in the solar neighbourhood. The Massachusetts-Stony Brook virial analysis of inner Galaxy giant molecular clouds is reappraised, leading to X < 1 times 10^{20} after allowance has been made for saturation broadening of the CO emission line. The mass of molecular gas in the inner Galaxy is derived as ~4.5 times 10^8 M_odot , or about 60% of the HI mass over the same distance interval.