NEW DETECTIONS OF ISOTOPIC MOLECULAR ABSORPTION-LINES - A LOW [C-12-C-13] RATIO IN NEARBY GAS

Molecular absorption line observations towards the background source S gr B2 'M' are presented. Previous observations have shown that there a re similar to 9 foreground clouds of moderate density along this line of sight, which produce absorption lines that are well spaced in veloc ity. In two of these clouds, first detections have now been made of th e rare isotopomers (CS)-C-13, (HNC)-C-13, (HCN)-N-15 and (HCO+)-O-18. For a feature at 1sr velocities of -4 to +18 km s(-1), the isotopic ra tio [C-12:C-13] has been estimated, from the relative intensities of ( CS)-C-12 and (CS)-C-13 J=1-0 lines, and also by comparing the strength of the (CS)-C-13 line with that of (CS)-S-34 J=1-0 observed previousl y. A convergent solution for the two methods is found if (CS)-C-12 is optically thick but the isotopomer lines are optically thin. In this c ase [C-12:C-13] is 24 +/- 11, which is surprisingly low if the gas lie s near the Sun, as indicated by its velocity. However, it has been sug gested that parts of this feature may in fact arise in hot gas close t o the Sgr B2 cloud, where a low isotope ratio is expected. If this reg ion of the line is excluded, the [C-12:C-13] ratio for the remaining 1 sr velocities of +11 to +18 km s(-1) is only slightly changed, with a value of 22 +/- 13. This is the true carbon isotope ratio in some near by gas, if effects such as peculiar velocities and isotopic fractionat ion are unimportant. The value found here is well below the local aver age of similar to 60-70 in the solar neighbourhood, which suggests tha t some of the nearby absorbing gas has been recently isotopically enri ched by stellar ejecta. This moderate density absorbing gas is then mo re likely to be material left over after star-formation, rather than a pre-star-forming cloud.