Determination of ¹²C/¹³C in the interstellar medium toward ζ ophiuchi and ξ persei

Over the past 7 years ¹²CH⁺/¹³CH⁺ observations performed by different groups of authors at various observatories toward the ζ Oph cloud have yielded interstellar ¹²C/¹³C isotope ratios varying at the factor of 2 level. In an effort to determine the cause of this discrepancy, we have (1) reanalyzed the Lick Observatory Reticon data presented in Hawkins, Jura, & Meyer, (2) obtained a new set of Lick Observatory observations toward ζ Oph using a CCD detector, and (3) carried out high-resolution, high signal-to-noise ratio observations toward ζ Oph and ξ Per using the KPNO coudé feed echelle spectrograph and a CCD detector. The reanalysis of the Lick Observatory Reticon data, performed with different software and using a different way of placing the continuum, has yielded ¹²CH⁺/¹³CH⁺ ratios that are completely consistent with the original analysis presented in the Hawkins, Jura, & Meyer paper. The Lick Observatory CCD data toward ζ Oph suffer from a large amount of telluric contamination, and the ¹²C/¹³C results are quite uncertain, ranging from 40 to 60, owing to the error in continuum placement. The KPNO CCD data are of highest quality in terms of resolution, signal-to-noise ratio, and absence of telluric lines. These KPNO data cover the CH⁺ 4232 and 3957 Å regions for ζ Oph and ξ Per. In the case of ζ Oph, the KPNO data also yielded a measurement of the ¹²CN/¹³CN ratio at 3874 Å. The KPNO results toward ζ Oph are ¹²CH⁺/¹³CH⁺ (0, 0) = 63 ± 8, ¹²CH⁺/¹³CH⁺(1, 0) = 67 ± 19, and ¹²CN/¹³CN = 100⁺⁸⁸_-33. Toward ξ Per, our KPNO measurements yield ¹²CH⁺/¹³CH⁺ = 49 ± 15 from the 4232 Å band, and a lower limit of 45 from the 3957 Å observations. Our KPNO CCD ¹²CH⁺/¹³CH⁺ and ¹²CN/¹³CN ζ Oph results presented here are consistent with recent determinations by Crane, Hegyi, & Lambert, Stahl & Wilson, and Kaiser & Wright. By contrast, the work by Hawkins, Jura, & Meyer and Crane & Hegyi deviates beyond the 1 σ errors from the above results. Toward ξ Per our KPNO CCD ¹²CH⁺/¹³CH⁺ results agree with the Hawkins & Jura measurements. The CH⁺ molecule provides the best probe of the interstellar ¹²C/¹³C ratio in the gas because it is not believed to suffer from isotope-selective effects such as chemical fractionation. A comparison of the solar system ¹²C/¹³C ratio of 89 to the ¹²CH⁺/¹³CH⁺ KPNO values of 63 and 49 which we measure in the interstellar medium toward ζ Oph and ξ Per, respectively, implies that the Galaxy has undergone a significant amount of chemical evolution since the formation of the solar system.