IRAS sources beyond the solar circle. VII. The 12C/13C ratio in the far outer Galaxy
I. Physikalisches Institut, Zülpicher Strasse 77, D 50937 Köln, Germany
2 Istituto di Radioastronomia, CNR, Via Gobetti 101, I 40129 Bologna, Italy
Send offprint request to: J.G.A. Wouterloot
Accepted: 24 February 1996
We have investigated the 12C/13C abundance ratio in the far-outer Galaxy. We have used the IRAM 30-m telescope to obtain the 12CO and 13CO(1-0) and (2-1) distributions towards five IRAS sources at about 16-17 kpc from the galactic center. C18O(1-0) and (2-1) were observed towards the 13CO peak positions in those clouds. The source with the strongest C18O emission, WB 89-437, was subsequently observed in 13C18O(1-0) and (2-1) and in H2CS(3-2) and (6-5). To be able to compare our results with published data, we observed the same transitions towards the inner Galaxy source W 33, and towards W 3OH. The ratio of the 13CO and C18O column densities is about 14, slightly larger than what was found in local GMCs. This ratio is dominated by excitation and beam filling effects, and is therefore not indicative of the abundance ratios. The ratio C18O(1-0)/13C18O(1-0) directly yields the 12C/13C abundance ratio however, for which towards WB 89-437 we find a 3σ lower limit of 201±15, which means that the 12C/13C gradient found in the inner Galaxy continues further out. Our results for W 33 and W 3OH are consistent with earlier observations and give abundance ratios of 43.0±4.3 and 85±15, respectively. These J=1-0 measurements are however in contrast to results obtained from the corresponding J=2-1 transitions: we obtain abundance ratios of 104±60 (WB 89-437), 31±2 (W 33), and 24±2 (W 3OH). These differences may be due to the emission of the two transitions originating in different parts of the cloud with different excitation conditions. The 12CO emission towards WB 89-437 shows strong outflow emission, and that of WB 89-380 is dominated by self-absorption. The sizes of the 13CO clumps are 1-2 pc and they have peak positions located within 10″(0.5 pc) from the IRAS position. Their (virial) masses are typically several 1000 .
Key words: ISM: abundances / ISM: molecules / Galaxy: abundances / radio lines: ISM
© European Southern Observatory (ESO), 1996