Ground-based Observations Of Interstellar CN And CH+ In Diffuse Molecular Clouds: 12C/13C Ratios And CN Excitation

Details Ground-based Observations Of Interstellar CN And CH+ In Diffuse Molecular Clouds: 12C/13C Ratios And CN Excitation Ground-based Observations Of Interstellar CN And CH+ In Diffuse Molecular Clouds: 12C/13C Ratios And CN Excitation

May 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009aas...21442503f&link_type=abstract

American Astronomical Society, AAS Meeting #214, #425.03; Bulletin of the American Astronomical Society, Vol. 41, p.694

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We present high-resolution observations of optical transitions in CN and CH+ for a number of Galactic diffuse clouds. We extract the 12CN/13CN and 12CH+/13CH+ ratios along each direction in order to assess predictions of diffuse cloud chemistry. We find a weighted mean 12CH+/13CH+ ratio of 74.4 ± 7.6. This result is consistent with the average 12C/13C ratio of 70 ± 7 for local interstellar clouds and confirms theoretical expectations that 12CH+/13CH+ represents the ambient carbon isotopic ratio. Thus, we find no evidence for variation in 12C/13C within 1 kpc of the Sun. The isotopologic ratios in CN and CO, however, show significant fractionation away from the ambient value due to the opposing effects of photodissociation and charge exchange reactions. Our 12CN/13CN measurements are combined with 12CO/13CO ratios from the literature to study the effects of chemical fractionation in diffuse molecular clouds. We find evidence for an inverse relationship between 12CN/13CN and 12CO/13CO, resulting from the physical association of CN and CO in these cloud cores. The isotopologic ratios suggest that about 20 percent of C is locked up in CO in typical cores. Finally, we examine rotational excitation temperatures in 12CN and 13CN. Our weighted mean value of T01(12CN) = 2.754 ± 0.002 K implies an excess over the cosmic microwave background (CMB) of only 29 ± 3 mK, considerably smaller than some recent surveys suggested. This modest excess likely arises from collisional excitation by electrons occurring locally in some clouds, with electron densities of 0.1 to 0.5 cm-3. Given the dispersion of 134 mK in our individual T01 measurements, the excess may not be physical. There is some indication of a greater excess in T12(12CN), but the dispersion in these measurements is also greater (259 mK). The rotational excitation temperature observed in 13CN shows no excess over the CMB.

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