Cosmic rays and the masses of giant molecular clouds

Abstract

The use of cosmic gamma rays in elucidating the spatial density of cosmic rays in the Galaxy depends crucially on knowledge of the distribution of the target gas. There has been argument as to the latter quantity and a new analysis is timely. In the present work we make a critical analysis of the various methods of determining the masses of local giant molecular clouds with particular emphasis being placed on the use of far-infrared data in examining the Taurus and Orion GMCS. Consistent results are found for the conversion factor, X, in coverting the 12CO signal ( integral T(12CO)d nu ) to the column density of molecular gas (in 1020 H molecules cm-2): X approximately=1.5. A notable result is that the far-infrared method applied to the nearest and coldest GMC (Taurus) reveals no evidence for the presence of significant quantities of very cold dust (and associated gas). There is thus support for the validity of the authors use of this method in the Inner Galaxy and their previous work in this field (Broadbent et al. 1989); in that work they found a slightly lower value of X ( approximately=1.0) in the Inner Galaxy. The reasons for other workers having found higher values of X in the Inner Galaxy are examined and it is concluded that problems of cloud definition and confusion in this crowded region are responsible. Returning to the question of the cosmic-ray distribution, the confirmation of their previous gas estimates indicates that their claim (e.g. in Bhat et al. 1986) that there is a significant dependence of cosmic-ray intensity on Galactocentric distance is well founded. The significance of the result is that the majority of the observed cosmic rays at Earth are of Galactic origin.