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

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3. Systematics

3.1. C2H

The individual C2H spectra are displayed in Figs. 1 and 2. There were no detectable anomalies in the ratios of the hyperfine lines and the line parameters in Table A1 derive from a simultaneous fitting of several hyperfine components under the assumption of LTE line ratios.

In our earlier work (Lucas & Liszt, 1997) the systematic variation of C2H and HCO+ was not readily apparent, owing to the small number of lines of sight, and it appeared that C2H and HCO+ were largely decoupled. Fig. 4 at bottom shows perhaps the most important results of this work: that C2H is as widespread as HCO+, that the C2H/HCO+ ratio actually is fairly high (in the mean) at low column density where values as high as 35 are seen at N(HCO+)[FORMULA], and that the column densities of C2H and HCO+ are strongly coupled.

[FIGURE] Fig. 4. Top: variation of C2H with C3H2: the regression line has a power law slope of [FORMULA]. Bottom: N(C2H) [FORMULA] N(HCO+) on linear and (inset) log scales.

For the C2H/HCO+ abundance ratio we calculate an unweighted mean over the individual components of 14.5[FORMULA]6.7, implying N(C2H)/N([FORMULA]) [FORMULA] assuming N(HCO+)/N([FORMULA]) [FORMULA] (Lucas & Liszt, 1996; Liszt & Lucas, 1996, 2000).

3.2. C3H2

Spectra of C3H2-(o) toward 4 sources are shown in Fig. 3 and Table A2 lists the decomposition products of all detected lines. The detection toward B0528+134 is rather marginal. The column densities of C3H2 and C2H are strongly and nearly linearly related, as shown in Fig. 4 at top. Given the long-known ubiquity of C3H2, the strong correlation between C3H2 and C2H reinforces the notion that C2H is ubiquitous as well. The mean abundance ratio weighted by the variance is [FORMULA]C2H/C3H2- o[FORMULA] = 27.7[FORMULA]8, and for the abundance we have N(C3H2)/N([FORMULA]) = (4/3) N(C3H2-(o))/N([FORMULA].

3.2.1. The ortho/para ratio in C3H2

We detected para-C3H2 toward B0355, B0415, and B1730 finding para/ortho ratios of 0.29[FORMULA]0.060, 0.56[FORMULA]0.083, and 0.243[FORMULA]0.060 respectively. The weighted mean of these is (coincidentally) 0.33. The line of para-C3H2 toward B0415 is shown in Fig. 3.

3.3. C4H and linear C3H

For the line of sight toward B0415+379, we find N(C3H) [FORMULA] at the [FORMULA] level, or N(C3H) [FORMULA] 0.065 N(HCO+). For B0355+508, B0528+134, B1730-130, and B2251+158, we have coincidentally similar but much poorer limits N(C3H) [FORMULA] 4 N(HCO+).

As noted in Sect. 2, the mm-wave lines of C4H are too high up the rotation ladder to derive good limits in the absence of substantial excitation. Our result that N(C4H) [FORMULA] 1000-2000 N(HCO+) is of little interest. C4H is better studied at [FORMULA]3cm than [FORMULA]3mm.

3.4. Linewidths and kinematics

A plot of the linewidths of C2H and HCO+ shows a dominant linear relationship with some significant outliers. The outliers can all be attributed to weak or blended lines and we conclude that there is no statistically significant difference between the derived linewidths of these species.

Fig. 5 shows a comparison of C2H (histogram) and HCO+ (shaded) profiles in three sources (see the figure caption for an explanation of scaling). The tendency for the C2H/HCO+ ratio to be high in some weak lines is evident in the -11 km s-1 gas toward B0212+735 and perhaps between the two components around -10 km s-1 toward B0355+508, although the HCO+ profile in this direction has somewhat higher resolution. Toward B1730-130 the HCO+ may be narrower, but the baseline level in the C2H profile seems somewhat uncertain. Fig. 6 shows a comparison of the C2H toward B0415+379 with OH, H13CO+ (because H12CO+ is somewhat saturated) and C3H2. The best match for the C2H is actually with C3H2.

[FIGURE] Fig. 5. Comparison of C2H (lines) and HCO+ (shaded bars) spectra scaled to the area of the C2H profile over various velocity ranges: at top, for v [FORMULA] -1 km s-1; otherwise, over the whole profile.

[FIGURE] Fig. 6. Comparison of C2H and other spectra (shaded) scaled to the area of the C2H profile, toward B0415+379.

Clearly, there are no systematic differences between the kinematics of C2H and those of the other species.

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© European Southern Observatory (ESO) 2000

Online publication: June 20, 2000