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

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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, [FORMULA], 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 [FORMULA] and [FORMULA] for NRAO150 and 3C111, respectively, while ours (4/3 [FORMULA] N(C3H2-(o))are [FORMULA] and [FORMULA].

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

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 AV = 1-10 mag. The first column is a digest of species seen in optical and UV absorption toward [FORMULA] Oph, mostly from the tables of Van Dishoeck & Black (1986); this line of sight has AV = 1 mag and the HCO+ column density is taken from the study of HCO+ emission by Liszt & Lucas (1994), for a density of [FORMULA]. 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 [FORMULA] or just over 10 magnitudes of visual extinction. The last column shows the results of Model G of the [FORMULA] 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[FORMULA] N()/N([FORMULA]).
This work: N(C3H2) [FORMULA] N(ortho-C3H2); CH is our unpublished work and N(OH) [FORMULA] N(HCO+) (Liszt & Lucas, 1996)
TMC-1: results from Ohishi et al. (1992)
[FORMULA] Oph: N(C+) is from Cardelli et al. (1993)
BD-G: results from model [FORMULA] 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[FORMULA] and C3H/C2H [FORMULA] 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 [FORMULA] km s-1 toward W49, where [FORMULA] (some three times larger than any single feature seen here), our recent work (Lucas & Liszt, 2000) showed N(HCO+) [FORMULA] 60 N(H13CO) [FORMULA] or N(C3H2)/N(HCO+) [FORMULA] 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.

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Online publication: June 20, 2000