VUV SURVEY OF $^{12}$CO/$^{13}$CO IN THE SOLAR NEIGHBORHOOD WITH HST

Abstract

VUV measurements of $A-X$ (4$^{th}$ positive) absorption bands of CO can yield precise column densities along interstellar sight lines. We analyzed 21 sight lines to deduce both $^{12}$CO and $^{13}$CO column densities in diffuse molecular clouds within the solar neighborhood (toward stars approx. 100 to 1000 pc away). The data were obtained from the {\it Hubble Space Telescope} archive, and include 18 sight lines observed with STIS ($\lambda$/$\Delta$$\lambda$ = 46,000---160,000) and 3 that were observed with GHRS ($\lambda$/$\Delta$$\lambda$ = 19,000---85,000). Three more stars for which we previously published ratios of $^{12}$CO/$^{13}$CO were added to sample. The average value of the isotopic carbon ratio in the solar neighborhood is known to be 70 $\pm$ 7, as determined from radio observations of rotational transitions in emission from high-density molecular clouds. This ratio is the ``parent'' of the CO isotopomeric ratios, which are known to be affected by two opposing fractionation processes, selective photodissociation and isotopic charge exchange with C$^+$. Here we find that 40% of the $^{12}$CO/$^{13}$CO values are within 2 $\sigma$ of the carbon average, while most sight lines show that CO is fractionated to higher and lower values by a factor of $\leq$ 2. } }