Probing the possibility of a 12C/13C abundance gradient from observations of interstellar CH+

The author has obtained the interstellar 12C/13C isotope ratio in the neighborhood of the Sun, from very high signal-to-noise optical observations of CH+ toward the stars zeta Oph, zeta Per, P Cyg, 20 Tau, and 23 Tau. They have found that the 12CH+/13CH+ abundance ratios toward four different lines of sight are uniform within 12%. They provide evidence in favor of a locally-homogeneous ISM and have calculated a weighted mean 12C/13C ratio of 43 +/- 4. The similarity among the 12CH+/13CH+ ratios determined in four interstellar clouds possessing different physical conditions, precludes the possibility that fractionation or selective photodissociation of CH+ are taking place in these regions. Thus the observed 12CH+/13CH+ isotope ratios are representative of the 12C/13C ratios in the gas. Since the CH+ molecule seems to be the most sensitive probe of 12C/13C in the diffuse interstellar medium, they are obtaining CH+ observations at CTIO and at Lick Observatory toward more distant stars located up to 2.5 kpc from the Sun to probe the existence of a Galactic 12C/13C abundance gradient. Observations of the stars HD 183143, HD24432, and HD157038 are presented