SIMBAD references

Molecular absorption line observations towards the background source Sgr B2 `M' are presented. Previous observations have shown that there are ∼9 foreground clouds of moderate density along this line of sight, which produce absorption lines that are well spaced in velocity. In two of these clouds, first detections have now been made of the rare isotopomers ¹³CS, HN¹³C, HC¹⁵N and HC¹⁸O⁺. For a feature at lsr velocities of -4 to +18km/s, the isotopic ratio [¹²C:¹³C] has been estimated, from the relative intensities of ¹²CS and ¹³CS J=1-0 lines, and also by comparing the strength of the ¹³CS line with that of C³⁴S J=1-0 observed previously. A convergent solution for the two methods is found if ¹²CS is optically thick but the isotopomer lines are optically thin. In this case [¹²C:¹³C] is 24±11, which is surprisingly low if the gas lies near the Sun, as indicated by its velocity. However, it has been suggested that parts of this feature may in fact arise in hot gas close to the Sgr B2 cloud, where a low isotope ratio is expected. If this region of the line is excluded, the [¹²C:¹³C] ratio for the remaining lsr velocities of +11 to +18km/s is only slightly changed, with a value of 22±13. This is the true carbon isotope ratio in some nearby gas, if effects such as peculiar velocities and isotopic fractionation are unimportant. The value found here is well below the local average of ∼60-70 in the solar neighbourhood, which suggests that some of the nearby absorbing gas has been recently isotopically enriched by stellar ejecta. This moderate density absorbing gas is then more likely to be material left over after star-formation, rather than a pre-star-forming cloud.