Further 12C/13C Ratios from Formaldehyde - a Variation with Distance from the Galactic Center

Details Further 12C/13C Ratios from Formaldehyde - a Variation with Distance from the Galactic Center Further 12C/13C Ratios from Formaldehyde - a Variation with Distance from the Galactic Center

May 1982

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1982a%26a...109..344h&link_type=abstract

Astronomy and Astrophysics, Vol.109, P. 344, 1982

Astronomy and Astrophysics

Astrophysics

63

Scientific paper

New measurements of the 110 - 111 line of H212C16O and H213C16O with the Effelsberg 100-m telescope are reported. The measured (H212C16O/H213 C16O) ratios, corrected for hyperfine splitting in H2133CO, are: 76±7 toward W3, 52±4 toward NGC 2024, a composite value of ˜60 toward M 17 and 55±3 toward G49.4 -0.3. Toward Cas A, we find 63±5 for the -47km s-1 component, 58±3 for the -39km s-1 component and ≳90 for the 0 k-1s component (all errors are one standard deviation). Our estimated corrections for photon trapping raise the ratios significantly: for W3, we get 91±16, for NGC2024, 68±11, and for G 49.4-0.3, 82±12. For sources near the Sun an average value of 80±7 close to the solar system ratio, is obtained. In addition there is some evidence for real cloud to cloud variation in the (12C/13C) ratio. We list the best estimates for the (H2CO/H213CO) ratios (including all corrections) for sources in the galactic disk. A linear least squares fit to these corrected ratios gives (5.5±0.1) DGC +24.2±0.8. DGC is the distance from galactic center in kpc; the quoted error does not include any systematic effects. The correlation coefficient for this fit is 0.57. Clumping in the molecular clouds will tend to raise the (12C/13C) ratios. This effect will probably be greater for sources in the inner part of the galaxy and thus would decrease our value for the (12C/13C) gradient.

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