Astron. Astrophys. 358, 1069-1076 (2000)

4. Comparative chemistry of simple carbon chains and hydrocarbons

4.1. Comparison with some prior results for C₃H₂

C₃H₂ was discovered in the interstellar medium along lines of sight lacking truly dark clouds (Matthews & Irvine, 1985), for instance toward Cas A. Cox et al. (1988) observed C₃H₂ in absorption from diffuse gas, including two lines of sight in common with this work: NRAO150 (=B0355+508), and 3C111 (=B0415+379). The ambient hydrogen number densities they derived from a comparison of 18 and 21 GHz ortho-and para-absorption line strengths, , which were used in their excitation calculations, seem ruled out by the general weakness of mm-emission from species like HCO⁺ and HCN (Lucas & Liszt, 1996; Liszt, 1997). Nonetheless, their column densities are in excellent agreement with ours. Their values are and for NRAO150 and 3C111, respectively, while ours (4/3 N(C₃H₂-(o))are and .

Therefore, differences in quoted relative abundances must arise from normalization, i.e. the means used to estimate N(). For instance, toward 3C111, which we discussed at some length in Lucas & Liszt (1998) (N(HCO⁺) ), Cox et al. (1988) used N() , implying a very high value for N(HCO⁺)/() . The extinction in this direction is probably not less than 2 magnitudes, which is consistent with a relative abundance of HCO⁺ below about .

4.2. Relative abundances in diffuse, translucent, and dense clouds

Table 2 is a compilation of relative abundances covering diffuse and translucent clouds in the range A_V = 1-10 mag. The first column is a digest of species seen in optical and UV absorption toward Oph, mostly from the tables of Van Dishoeck & Black (1986); this line of sight has A_V = 1 mag and the HCO⁺ column density is taken from the study of HCO⁺ emission by Liszt & Lucas (1994), for a density of . The column headed "This work" is meant to be representative of the present results. The column marked "TMC-1" contains entries from the compilation of Ohishi et al. (1992), which were normalized using or just over 10 magnitudes of visual extinction. The last column shows the results of Model G of the Oph line of sight by Van Dishoeck & Black (1986), the most exhaustive study of quiescent, moderate-density, diffuse cloud chemistry.

Table 2. Relative abundances 10 N()/N().
Notes:
This work: N(C₃H₂) N(ortho-C₃H₂); CH is our unpublished work and N(OH) N(HCO⁺) (Liszt & Lucas, 1996)
TMC-1: results from Ohishi et al. (1992)
Oph: N(C⁺) is from Cardelli et al. (1993)
BD-G: results from model Oph G of Van Dishoeck & Black (1986)

Clearly, the abundances of many species - OH, HCO⁺ and C₂H - vary little from diffuse to dark gas: paradoxically, the seemingly ubiquitous species C₃H₂ is not one of them. Instead, it, like C₃H, grows substantially more abundant in denser gas seen at higher extinction in dark clouds. By contrast, the abundance of CH probably declines by a small factor in denser material. An identical pattern, C₃/C and C₃H/C₂H is repeated in diffuse gas and TMC-1. The abundances of species determined in emission are typically subject to uncertainties of factors of a few toward TMC-1 while the abundances seen in this work inherently vary by like amounts.

4.3. The relative abundance of the linear and cyclic forms of C₃H₂

Cernicharo et al. (1999) recently detected linear C₃H₂ (l-C₃H₂) in absorption from the so-called spiral-arm clouds seen in absorption toward giant HII regions in the galactic plane (W49, W51, Sgr B2): we have also studied some of these sources, albeit for a very limited purpose; with reference mostly to SiO (Lucas & Liszt, 2000). Cernicharo et al. (1999) find that the o-C₃H₂/l-C₃H₂ ratio in the spiral-arm clouds, roughly 5, is an order of magnitude larger than what is found in dark clouds like TMC1 (where it is of order 50-100). The spiral-arm clouds have somewhat thicker gas columns than those seen in absorption toward our extragalactic sources, by factors of a few or more. For instance, at km s^-1 toward W49, where (some three times larger than any single feature seen here), our recent work (Lucas & Liszt, 2000) showed N(HCO⁺) 60 N(H¹³CO) or N(C₃H₂)/N(HCO⁺) 0.18, about a factor 4 higher than the mean for the clouds studied here. Apparently, the relative abundance of cyclic-C₃H₂ increases faster than that of linear-C₃H₂ during the transition from diffuse to darker material.

© European Southern Observatory (ESO) 2000

Online publication: June 20, 2000
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