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

2. Observations

2.1. C₂H

The bulk of the work reported here is a survey of 87 GHz N=0-1 C₂H absorption meant to complement our earlier, approximately flux-limited survey of HCO⁺ (Lucas & Liszt, 1996). The 87 GHz spectrum of C₂H (Tucker et al., 1974; Gottlieb et al., 1983a) has 6 hyperfine components of relative LTE strengths 43:417:208:208:83:43 in the spectral region from 87.28416 to 87.44651 GHz. The strongest line at 87.316924 GHz was taken as the zero-velocity rest frequency for our work. We typically observed the strongest four components as shown in Fig. 1.

Fig. 1. C2H absorption spectra for two sources, showing four hyperfine components. The velocity scale is relative to the frequency of the strongest component; in the upper figure, the velocity scale differs between the right and left frames.

Fig. 2. Digest of detected C2H 87.3GHz absorption profiles seen at the Plateau de Bure Interferometer for the strongest hyperfine component of the N=0-1 transition. The channel spacing is 78kHz and the resolution is 140 kHz (0.48 km s-1)

Table 1 shows the list of background sources observed, their galactic coordinates and the rms error in the line/continuum ratio, which is also the rms error in optical depth in the optically thin limit. In Table A1 of the Appendix we show the results of gaussian fitting, done simultaneously to whichever subset of the hyperfine structure was actually observed. The optical depth quoted at line center is for the strongest component (but results from a fit to multiple components assumed to appear in the LTE ratio) and the integrated optical depth is the sum over all six hyperfine components, which we derive by a simple scaling to account for that (smaller) fraction of the line which was not actually observed. In the limit of no collisional excitation above the black body background (all excitation temperatures = 2.73 K), the optical depth integrals are related to the total column density via N(C₂H) = 1.70 N, where we have taken the permanent dipole moment as 0.8 Debye. Comparison with the analogous expressions for C₃H and C₄H in Sects. 2.3 and 2.4 will show why our limits on those species are poor: the correction for the partition function is much larger.

Table 1. Background Source and profile rms

The data were taken at various times between mid-1993 and mid-1997. In all cases the spectral channel separation was 78.1 kHz (0.268 km s^-1 at the strongest hyperfine component). The actual resolution of the data shown here is lower, however, 140 kHz (0.481 km s^-1).

2.2. Cyclic C₃H₂

We observed absorption from ortho cyclic-C₃H₂ at 85338.91GHz and absorption from para cyclic C₃H₂ at 82093.56 GHz, both of which arise from levels within about 2 K of the ground state (Vrtilek et al., 1987). The ratio of statistical weights is ortho:para = 3:1. The spectra were taken over the period 1994-1997 with the usual 140 kHz- wide channels sampled at 78.1 kHz intervals, leading to a velocity resolutions of 0.285 and 0.274 km s^-1 for the para and ortho lines. For B2200+420, we also took a 1997 spectrum with two times narrower channels, as shown in Fig. 3. The 7 sources observed are noted in Table 1, where the rms noise in baseline line/continuum ratio for the ortho-species is given in the last column.

Fig. 3. C3H2-(o) absorption spectra for four sources. The profile for B1730-130 has been multiplied by a factor two. Toward B0415+379 at upper right, the profile for C3H2-(p) is shown shaded.

The dipole moment of C₃H₂ is 3.27 Debye (Lovas et al., 1992). The column density and integrated optical depth (for the transitions observed here) are related by N(C₃H₂-(o)) , or N(C₃H₂-(p)) , in the limit of no collisional excitation above the cosmic blackbody background.

The products of gaussian fitting of the C₃H₂-(o) profiles are given in Table A2 of the Appendix.

2.3. Linear C₃H

We observed the J=7/2-9/2 transitions of linear C₃H near 97995.45 GHz, the two hyperfine components separated by 0.75 MHz (Gottlieb et al., 1986). For a dipole moment of 3.1 Debye, using the partition function of Thaddeus et al. (1985a), we find N(C₃H) = 44.7 N in the limit of no excitation above the cosmic microwave background. For the line of sight toward B0415+379, we find N(C₃H) 0.065 N(HCO⁺) at the level, while for B0355+508, B0528+134, B1730-130, and B2251+158, we have coincidentally similar but much poorer limits N(C₃H) 4 N(HCO⁺).

2.4. C₄H

We observed one of the paired spin doublets of the J=9-10 transition of C₄H at 95188.94 GHz (Gottlieb et al., 1983b). For a dipole moment of 0.9 Debye, and in the limit of no collisional excitation, it follows that N(C₄H) = 1545 N where the integral is taken over one spin doublet. This leads to rather poor limits, the best of which is N(C₄H) 666 N(HCO⁺) toward B0415+379 . For B0355+508, B0528+134, B1730-130, and B2251+158, we have only that N(C₄H) 1500-2200 N(HCO⁺). It is straightforward to show that much better limits on the abundance of C₄H would be available for the 10 GHz transitions from lower-lying transitions, in cases of weak collisional excitation.

© European Southern Observatory (ESO) 2000

Online publication: June 20, 2000
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