English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools annual

Global: Zendy Free Plan

Global: Zendy Tools monthly

Global: Zendy Plus monthly

We have investigated the 12 C/13 C abundance ratio in the far-outer Galaxy. We have used the IRAM 30-m telescope to obtain the 12 CO and 13 CO(1-0) and (2-1) distributions towards five IRAS sources at about 16-17 kpc from the galactic center. C18 O(1-0) and (2-1) were observed towards the 13 CO peak positions in those clouds. The source with the strongest C18 O emission, WB 89-437, was subsequently observed in 13 C18 O(1-0) and (2-1) and in H2 CS(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 13 CO and C18 O 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 C18 O(1-0)/13 C18 O(1-0) directly yields the 12 C/13 C abundance ratio however, for which towards WB 89-437 we find a 3σ lower limit of 201± 15, which means that the 12 C/13 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 43.0± 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 12 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 13 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 .

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