IRAS sources beyond the solar circle. VII. The 12C/13C ratio in the far outer Galaxy

¹ 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

Received: 20 October 1995
Accepted: 24 February 1996

Abstract

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 .