Free Access
Astron. Astrophys. Suppl. Ser.
Volume 119, Number 3, November I 1996
Page(s) 439 - 457
Published online 15 November 1996
IRAS sources beyond the solar circle. VII. The 12C/13C ratio in the far outer Galaxy DOI: 10.1051/aas:1996258

Astronomy and Astrophysics Supplement Series, Vol. 119, November I 1996, 439-457

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

J.G.A. Wouterloot and J. Brand
Send offprint requests to: J.G.A. Wouterloot

I. Physikalisches Institut, Zülpicher Strasse 77, D 50937 Köln, Germany
Istituto di Radioastronomia, CNR, Via Gobetti 101, I 40129 Bologna, Italy

Received October 20, 1995; accepted February 24, 1996


We have investigated the C/C abundance ratio in the far-outer Galaxy. We have used the IRAM 30-m telescope to obtain the CO and CO(1-0) and (2-1) distributions towards five IRAS sources at about 16-17 kpc from the galactic center. CO(1-0) and (2-1) were observed towards the CO peak positions in those clouds. The source with the strongest CO emission, WB 89-437, was subsequently observed in CO(1-0) and (2-1) and in HCS(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 CO and CO 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 CO(1-0)/CO(1-0) directly yields the C/C abundance ratio however, for which towards WB 89-437 we find a 3 lower limit of 20115, which means that the C/C 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 43.04.3 and 8515, 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 10460 (WB 89-437), 312 (W 33), and 242 (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 CO emission towards WB 89-437 shows strong outflow emission, and that of WB 89-380 is dominated by self-absorption. The sizes of the CO 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

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