¹³C I in high-mass star-forming clouds

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

Corresponding author: A. R. Tieftrunk, atieftru@ph1.uni-koeln.de

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_1 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_1 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 for G 333.0-0.4, for NGC 6334 A and 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 , 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).