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To derive carbon isotopic ratios from optically thin tracers in the centralregions of the starburst galaxies M82 and NGC253. We present high sensitivityobservations of CCH and two of its 13C isotopologues, C13CH and 13CCH, as wellas the optically thin emission from C18O and 13C18O. We assume the columndensity ratio between isotopologues is representative of the 12C13C isotopicratio. From CCH, lower limits to the 12C/13C isotopic ratio of 138 in M82, and81 in NGC253, are derived. Lower limits to the 12C/13C ratios from COisotopologues support these. 13C18O is tentatively detected in NGC253, which isthe first reported detection in the extragalactic ISM. Based on these limits,we infer ratios of 16O/18O>350 and >300 in M82 and NGC253, respectively, and32S/34S>16 in NGC253. and the H2 column density determination through theoptically thin tracers 13CO and C18O. The derived CCH fractional abundancestoward these galaxies of <~1.1\times10^-8 are in good agreement with those ofmolecular clouds in the Galactic disk. Our lower limits to the 12C/13C ratiofrom CCH are a factor of 2-3 larger than previous limits. The results arediscussed in the context of molecular and nucleo-chemical evolution. The large12C/13C isotopic ratio of the molecular ISM in these starburst galaxies suggestthat the gas has been recently accreted toward their nuclear regions.

A large 12C/13C isotopic ratio in M82 and NGC253