Carbon Isotope Fractionation in CO - the 13C / 12C Ratio and the Nature of the Diffuse Interstellar Cloud Toward Zeta-Ophiuchi

Details Carbon Isotope Fractionation in CO - the 13C / 12C Ratio and the Nature of the Diffuse Interstellar Cloud Toward Zeta-Ophiuchi Carbon Isotope Fractionation in CO - the 13C / 12C Ratio and the Nature of the Diffuse Interstellar Cloud Toward Zeta-Ophiuchi

Mar 1981

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1981apj...244..855c&link_type=abstract

Astrophysical Journal, Vol.244, P. 855, 1981

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54

Scientific paper

Observations have been performed toward ζ Oph at the frequencies of the J= 1-0 transitions of 12CO and 13CO. A conservative upper limit of 1/50 is obtained for the abundance ratio 13CO/12CO. in contrast, a ratio greater than 1/20 would be expected due to the effects of chemical fractionation if the temperature of the gas cloud were as low (≍20 K) as indicated in certain previous studies. We derive a lower limit of 50 K for the temperature of the gas that contains CO, and thus by inference for the temperature of the primary dense region toward ζ Oph. Other critical data-the populations of the fine-structure states of carbon atoms, the populations of the rotational states of CO, HD, and H2, and the strength of the radio-recombination line of carbon- are then reexamined. A single set of physical conditions (density ≍200 cm-3 and temperature ≍65 K) is shown to be compatible with the observational constraints for the bulk of the primary component of the interstellar gas toward ζ Oph. Previous studies have yielded densities that are an order-of-magnitude higher.
Our 13CO/12CO ratio, together with the optical and ultraviolet data which indicate a temperature less than 85 K, gives an upper limit of about 1/75 for the 13C/12C ratio. These data are thus only barely compatible with an enhancement of 13C/12C over the terrestrial ratio.

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