Astron. Astrophys. 329, 443-450 (1998)

1. Introduction

Isotope ratios derived from CNO elements have significantly contributed to our understanding of the nuclear processing in stars and the 'chemical' evolution of galaxies, since these elements are abundant and have stable 'primary' and 'secondary' nuclei. From a theoretical point of view, ¹²C/¹³C is the least controversial CNO isotope ratio: ¹²C is a 'primary' product of helium-burning, ¹³C is mainly a 'secondary' product of hydrogen-burning with ¹²C as the seed nucleus. Some primary ¹³C may also be synthesized during the third dredge up in stars of intermediate mass ('hot bottom burning', e.g. Renzini & Voli 1981).

There is evidence for high ¹²C/¹³C ratios in the central regions of active star-forming galaxies with high luminosities in the far infrared (for a summary, see Henkel & Mauersberger 1993). First hints were obtained from distant mergers (being ultraluminous in the infrared) which were showing integrated I (¹²CO)/I (CO) J =1-0 line intensity ratios 20 (Aalto et al. 1991; Combes et al. 1991; Casoli et al. 1992a,b). An interpretation in terms of an extended halo of weak ¹²CO and negligible ¹³CO emission is not supported by a comparison of filled-aperture with interferometric CO data (P.M. Solomon, priv. comm.). The generally accepted explanation involves inflow of disk gas with high ¹²C/¹³C ratios into the central region, possibly combined with a ¹²C enhancement caused by nucleosynthesis in massive stars. Most direct evidence for high ¹²C/¹³C ratios was obtained from recent studies of the central regions of the nearby starburst galaxies NGC 253 and NGC 4945; this is based on ¹²C/¹³C line intensity ratios from a variety of molecular species (Henkel et al. 1993, 1994). The estimated ¹²C/¹³C abundance ratios, 40 - 50, are larger than the value for the central region of the Milky Way, 25 (e.g. Wilson & Rood 1994).

Fig. 1. Observed spectra of molecules and their 13C -bearing isotopic species toward M 82 and IC 342. The spectra are smoothed to a channel spacing of 4 MHz ( 12 @

Since two well studied extragalactic sources represent too small a sample for a comparison with the Milky Way, we have extended this list, including M 82 (NGC 3034) and IC 342. These contain powerful far infrared sources in their central regions and show an impressive amount of strong molecular lines (e.g. Henkel et al. 1986, 1991). From our experience with NGC 253, the best limits to the ¹²C/¹³C abundance ratio are obtained from the I (¹²CN)/I (¹³CN) line intensity ratios. We observed CN not only toward M 82 and IC 342 but also toward the Galactic center region (see Table 1), where the interstellar ¹²C/¹³C ratio is known and where isotope ratios deduced from CN can thus be tested.

© European Southern Observatory (ESO) 1998

Online publication: December 8, 1997
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