Ultraviolet Survey of CO and H₂in Diffuse Molecular Clouds: The Reflection of Two Photochemistry Regimes in Abundance Relationships

Abstract

We carried out a comprehensive far-UV survey of 12 CO and H 2 column densities along diffuse molecular Galactic sight lines. This sample includes new measurements of CO from HST spectra along 62 sight lines and new measurements of H 2 from FUSE data along 58 sight lines. In addition, high-resolution optical data were obtained at the McDonald and European Southern Observatories, yielding new abundances for CH, CH + , and CN along 42 sight lines to aid in interpreting the CO results. These new sight lines were selected according to detectable amounts of CO in their spectra and provide information on both lower density ( 100 cm À3) and higher density diffuse clouds. A plot of log N (CO) versus log N (H 2 ) shows that two power-law relationships are needed for a good fit of the entire sample, with a break located at log N (CO; cm À2 ) ¼ 14:1 and log N (H 2 ) ¼ 20:4, corresponding to a change in production route for CO in higher density gas. Similar logarithmic plots among all five diatomic molecules reveal additional examples of dual slopes in the cases of CO versus CH (break at log N ¼ 14:1, 13.0), CH + versus H 2 (13.1, 20.3), and CH + versus CO (13.2, 14.1). We employ both analytical and numerical chemical schemes in order to derive details of the molecular environments. In the denser gas, where C 2 and CN molecules also reside, reactions involving C + and OH are the dominant factor leading to CO formation via equilibrium chemistry. In the low-density gas, where equilibrium chemistry studies have failed to reproduce the abundance of CH + , our numerical analysis shows that nonequilibrium chemistry must be employed for correctly predicting the abundances of both CH + and CO.

Dust, ice and gas in time (DIGIT):HerschelandSpitzerspectro-imaging of SMM3 and SMM4 in Serpens

“…In comparison with analogous studies, our estimates are different compared to the ratio of ∼2 × 10 −4 found for ∼50 K gas in Lacy et al (1994) but fall within the limits of log(CO/H 2 ) between -7.58 and -4.68 as measured by Sheffer et al (2008) for diffuse gas.…”

Carbon fractionation in photo-dissociation regions

“…In Fig. C.3 we show our model results for N(CH) versus N(H 2 ) together with the observational data (Rachford et al 2002;Magnani et al 2005Magnani et al , 2006Sheffer et al 2008;Qin et al 2010;Gerin et al 2010;Suutarinen et al 2011).…”

“…The column densities in our model are consistent with values for TMC-1 given by Suutarinen et al (2011) as well as with diffuse cloud observations presented by Gerin et al (2010) which give N(CH) ≈ 1−26 × 10 13 cm −2 for total H 2 columns between 10 21 −10 22 cm −2 . Sheffer et al (2008) present column densities along 42 diffuse molecular Galactic sight lines. They find total columns between 10 12 and 10 14 cm −2 and a very strong correlation between column densities of CH and H 2 .…”

AHerschel[C ii] Galactic plane survey

“…For the diffuse low column density molecular clouds (no or very little CO) in the solar neighborhood, the H 2 layer containing C + is observed to have thermal pressures of order a few × 10 3 K cm −3 (cf. Sheffer et al 2008;Goldsmith 2013). Densities and temperatures, and therefore thermal pressures, have been determined from observations of absorption lines in the optical and UV for nearby low column density clouds.…”

“…Densities and temperatures, and therefore thermal pressures, have been determined from observations of absorption lines in the optical and UV for nearby low column density clouds. For example, Sheffer et al (2008) analyzed UV absorption lines of H 2 in diffuse molecular clouds with N(H 2 ) column densities up to ∼10 21 cm −2 (or A V ∼ 1 mag.) and derived T kin ranging from 50 to 120 K, with an average of 76 K in the molecular portion of the cloud.…”