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Issue A&A
Volume 375, Number 2, August IV 2001
Page(s) L23 - L26
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:20010939



A&A 375, L23-L26 (2001)
DOI: 10.1051/0004-6361:20010939

13C I in high-mass star-forming clouds

A. R. Tieftrunk1, K. Jacobs1, C. L. Martin2, O. Siebertz1, A. A. Stark2, J. Stutzki1, C. K. Walker3 and G. A. Wright4

1  KOSMA, I. Physikalisches Institut der Universität Köln, Zülpicher Str. 77, 50937 Köln, Germany
2  Smithsonian Astrophysical Observatory, 60 Garden Street, Cambridge MA 02138, USA
3  Steward Observatory, University of Arizona, Tucson AZ 85721, USA
4  Bell Laboratories, 791 Holmdel-Keyport Rd., Holmdel NJ 07733, USA

(Received 11 May 2001 / Accepted 2 June 2001 )

Abstract
We report measurements of the 12C/13C abundance ratio in the three galactic regions G 333.0-0.4, NGC 6334 A and G 351.6-1.3 from observations of the 12CI $^3P_2\rightarrow $ 3P1 transition and the hyperfine components of the corresponding 13CI transition near 809 GHz. These transitions were observed simultaneously with the CO 7-6 line emission at 806 GHz with the AST/RO telescope located at the South Pole. From a simultaneous fit to the 12CI $^3P_2\rightarrow $ 3P1 transition and the HF components of the corresponding 13CI transition and an independent estimate of an upper limit to the optical depth of the 12CI emission we determine intrinsic 12CI/13CI column density ratios of $23\pm 1$ for G 333.0-0.4, $56\pm 14$ for NGC 6334 A and $69\pm 12$ for G 351.6-1.3. As the regions observed are photon dominated, we argue that the apparent enhancement in the abundance of 13C towards G 333.0-0.4 may be due to strong isotope-selective photodissociation of $^{13}{\rm CO}$, outweighing the effects of chemical isotopic fractionation as suggested by models of PDRs. Towards NGC 6334 A and G 351.6-1.3 these effects appear to be balanced, similar to the situation for the Orion Bar region observed by Keene et al. (1998).


Key words: ISM: abundances -- atoms -- clouds -- H II regions -- submillimeter

Offprint request: A. R. Tieftrunk, atieftru@ph1.uni-koeln.de

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