INTERSTELLAR CN AND CH⁺IN DIFFUSE MOLECULAR CLOUDS:¹²C/¹³C RATIOS AND CN EXCITATION

Abstract

We present very high signal-to-noise ratio absorption-line observations of CN and CH + along 13 lines of sight through diffuse molecular clouds. The data are examined to extract precise isotopologic ratios of 12 CN/ 13 CN and 12 CH + / 13 CH + in order to assess predictions of diffuse cloud chemistry. Our results on 12 CH + / 13 CH + confirm that this ratio does not deviate from the ambient 12 C/ 13 C ratio in local interstellar clouds, as expected if the formation of CH + involves nonthermal processes. We find that 12 CN/ 13 CN, however, can be significantly fractionated away from the ambient value. The dispersion in our sample of 12 CN/ 13 CN ratios is similar to that found in recent surveys of 12 CO/ 13 CO. For sight lines where both ratios have been determined, the 12 CN/ 13 CN ratios are generally fractionated in the opposite sense compared to 12 CO/ 13 CO. Chemical fractionation in CO results from competition between selective photodissociation and isotopic charge exchange. An inverse relationship between 12 CN/ 13 CN and 12 CO/ 13 CO follows from the coexistence of CN and CO in diffuse cloud cores. However, an isotopic charge exchange reaction with CN may mitigate the enhancements in 12 CN/ 13 CN for lines of sight with low 12 CO/ 13 CO ratios. For two sight lines with high values of 12 CO/ 13 CO, our results indicate that about 50% of the carbon is locked up in CO, which is consistent with the notion that these sight lines probe molecular cloud envelopes where the transition from C + to CO is expected to occur. An analysis of CN rotational excitation yields a weighted mean value for T 01 ( 12 CN) of 2.754 ± 0.002 K, which implies an excess over the temperature of the cosmic microwave background of only 29 ± 3 mK. This modest excess eliminates the need for a local excitation mechanism beyond electron and neutral collisions. The rotational excitation temperatures in 13 CN show no excess over the temperature of the CMB.

Diffuse interstellar bands in Upper Scorpius: probing variations in the DIB spectrum due to changing environmental conditions

“…CH traces mainly cold (UV shielded) cloud material in the line-of-sight which is confirmed by the improved regression coefficient with respect to E (B−V) for ζ-type sightlines (r = 0.83) compared to its σ-type equivalent (r = 0.53). In line with previous results we observe no significant velocity difference between the CH and CH + lines in the Upp Sco region, thus finding no evidence for the production of CH + in shocks (Ritchey et al 2011).…”

Carbon fractionation in photo-dissociation regions

“…Casassus et al (2005) report an average ratio of 78 ± 2 from measurements along 9 lines of sight. Recent absorption-line observations by Ritchey et al (2011) along 13 lines of sight through diffuse molecular clouds confirm a FR(CH + ) close to the ambient ER. They report total column densities of CH + of a few 10 13 cm −2 .…”

THE C 14 N/C 15 N RATIO IN DIFFUSE MOLECULAR CLOUDS

“…The observations examined here were previously discussed by Ritchey et al (2011), who studied 12 CN/ 13 CN and 12 CH + / 13 CH + ratios along 13 lines of sight through diffuse molecular clouds.…”

“…The relevant data, acquired using the Ultraviolet and Visual Echelle Spectrograph (UVES) of the Very Large Telescope (VLT), were obtained from the European Southern Observatory (ESO) Science Archive Facility and were reduced with the UVES pipeline software (as described in more detail in Ritchey et al 2011). The spectra have very high signal-to-noise ratios (S/N ∼ 1200-2000 per resolution element near 3875Å) and a resolution of approximately 3.5 km s −1 (R = 85, 000).…”