Interstellar isotope ratios from mm-wave molecular absorption spectra

We have measured galactic λ3mm absorption spectra of HCO+, HCN, HNC, and CS toward compact extragalactic continuum sources in order to derive the isotopic abundance ratios 12C/13C, 14N/15N, 16O/18O, and 32S/34S in local diffuse clouds. For carbon, our data confirm recent results for the local ISM: we find 12C/13C = 59 ± 2. For nitrogen, we find 14N/15N = 237(−21,+27) consistent with the Solar value of 270, but substantially smaller than the values inferred from HCN emission in dense clouds. For sulfur we find 32S/34S ≈ 19 ± 8 consistent with the Solar value of 23. We also find one striking individual anomaly: toward 3C111 (B0415+379), H12CN/H13CN = 170±50 in one kinematic component. We attribute this to the fractionation of 13C into 13CO which may be so great that HCN is starved for 13C.