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Astron. Astrophys. 355, 333-346 (2000)

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2. Observations

2.1. VLA observations of 1667MHz OH

Earlier we took OH spectra at the VLA but with an inadvertantly mis-set bandwidth code, which permitted only a measurement of the integrated absorption in the resultant spectra (Liszt and Lucas 1996). This dataset, and a subsequent OH spectrum acquired at Nançay (the latter normalized to VLA data provided by A. Marscher) enabled the discovery of a tight, linear OH-HCO+ correlation but provided little kinematic information otherwise. This situation was rectified in 1998 August when we used the VLA to observe 1667.359 MHz OH absorption toward the sources listed in Table 1 with the narrowest possible resolution 763Hz, or 0.137 km s-1. To do so required not using on-line smoothing.


Table 1. Background Sources and Line/Continuum Rms

We observed bandpass calibrators (3C286, 3C48) every few hours and spent 1-4 hours on each source. No phase calibrators were observed and the data are entirely self-calibrated. As in our earlier work, an AIPS task UVLSD formed the final spectra as line/continuum ratios, by continuous weighted averaging of the data, without recourse to external phase calibration. All the sources studied here are point-source VLA calibrators, except for 3C111 which has weak extended structure. Moore and Marscher (1995) show profiles at various positions over this source.

Toward B0212+735, the bluer half of the bandpass could not be used, owing to some instability which induced a strong curvature. Toward B0355+508, the signal/noise turned out somewhat lower than we hoped (although still better than our previous spectrum from Nançay) and perhaps with some low-level fluctuations in the baseline. The sources toward which we have acquired new OH observations are listed in Table 1 along with the rms noise in line/continuum at the spectral baseline. Profile integrals are given in Table 2 for the new and old data.


Table 2. OH Profile Integrals (km s-1)

2.2. Plateau de Bure Interferometer observations of HCO+

Compared to six years ago when we began this work, the Plateau de Bure Interferometer has five rather than three antennas, better receivers, and higher spectral resolution. We took new HCO+ spectra of the sources listed in Table 1 using correlators configured for 140 kHz resolution sampled at 78.1 kHz (0.47 and 0.26 km s-1, respectively) and 70 kHz resolution sampled at 39.1 kHz. As earlier, the baseline established for a broader band was interpolated to and subtracted from the line-bearing regions of the spectrum. In many cases, the HCO+ absorption extended beyond the range of the higher-resolution data. The broad underlying HCO+ absorption toward B0355+508 and B0415+379 was, in retrospect, present in our earlier data. The sources for which we have new HCO+ profiles are listed in Table 1. The rms error in the baseline line/continuum ratio pertains to the higher-resolution data. Profile and optical depth integrals for the new and old data are given in Table 3.


Table 3. HCO+ Profile and Optical Depth Integrals (km s-1)

2.3. Extant observations of atomic hydrogen emission and absorption, and CO absorption in the UV

The HI emission profiles displayed here are from the Leiden-Dwingeloo all sky survey of (Hartmann and Burton 1997). The absorption profiles are those of (Dickey et al. 1983) and (Garwood and Dickey 1989), courtesy of J. Dickey and B. Garwood. CO column densities measured in UV absorption are from an updated version (from Federman) of the summary in (Federman et al. 1994).

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© European Southern Observatory (ESO) 2000

Online publication: March 17, 2000