1998A&A...329..443H

¹²C/¹³C line intensity ratios have been derived from several carbon-bearing molecules to confine the range of carbon and oxygen isotope abundance ratios toward the nuclear regions of two infrared bright galaxies. The most stringent limits are obtained from CN mm-wave emission lines. Supplementary measurements toward the Galactic center region indicate that overall I(¹²CN)/I(¹³CN) line intensity ratios are giving lower limits to the corresponding ¹²C/¹³C abundance ratio. Toward M 82 and IC 342, we find ¹²C/¹³C >40 and >30. Therefore the smaller ¹²C/¹³C ratio of our own Galactic center region (25) may not be typical for central regions of galaxies which are more luminous in the infrared. The ¹²C/¹³C limits and data from various isotopic species of CO also infer ¹⁶O/¹⁸O abundance ratios of >90 and >125 for M 82 and IC 342, respectively. From the I(HC¹⁴N)/I(HC¹⁵N) line intensity ratio, ¹⁴N/¹⁵N>100 is derived for M 82.