Astron. Astrophys. 361, 1079-1094 (2000)

2. Observations

2.1. Source list

For this study we observed those dense cores showing particularly strong CS emission ( K) in the surveys of Zinchenko et al. (1995, 1998) and Juvela (1996). Several strong SiO () sources detected by Harju et al. (1998) are also included in our sample. Sources observed at the SEST and at Onsala are presented in Table 1, Table 2. Sources also observed at Effelsberg or at the HHT are marked in both tables.

Table 1. Source list for SEST observations.
Notes:
a) also observed in Effelsberg,
b) also observed with HHT,
c) also observed in Onsala.

Table 2. Source list for Onsala observations.
Notes:
a) also observed in Effelsberg,
b) also observed with HHT,
c) also observed with SEST.

We designate most sources according to their galactic coordinates. Exceptions are Orion KL and Sgr A. For Sgr A we use the position observed by Jackson et al. (1984) for comparison (known as the M-0.13-0.08 cloud, see Lindqvist et al. 1995). Common identifications with some well known objects are given in the last column.

2.2. Observational procedures

The most important parameters of our SEST-15m, OSO-20m, Effelsberg 100-m and HHT measurements are summarized in Table 3, Table 4. Further details are given below for each instrument.

Table 3. Observing parameters ( is the spectral resolution).
Notes:
a) The frequencies and spectral resolutions for the observed transitions are presented in Table 4.
b) The system temperatures are given on a scale.
c) Beam sizes and main beam efficiencies are obtained by interpolating the data from the SEST manual (for SEST) and those provided by L.E.B. Johansson (for OSO) at nearby frequencies.

Table 4. Frequencies and spectral resolutions for the observed transitions. For other observing parameters see Table 3

2.2.1. SEST observations

The observations were performed with SIS receivers in a single-sideband (SSB) mode using dual beam switching with a beam throw of ´. At 220 GHz we used 2 acousto-optical spectrometers in parallel: (1) a 2000 channel high-resolution spectrometer (HRS) with 86 MHz bandwidth, 43 kHz channel separation and 80 kHz resolution and (2) a 1440 channel low-resolution spectrometer (LR1) with a 1000 MHz total bandwidth, 0.7 MHz channel separation and 1.4 MHz spectral resolution. The LR1 band was centered on the HNCO transition. However, it covered some other HNCO transitions too (see Table 4) as well as C¹⁸O (2-1), SO () and other lines (Fig. 1 shows a typical spectrum).

Fig. 1. A SEST low resolution spectrum. For identified features, molecular species and transitions are given

The 110 and 154 GHz observations were performed simultaneously; the spectra were recorded by the HRS which band was split into two equal parts. The 220 GHz HRS spectra were smoothed to 170 kHz resolution and the 110 and 154 mm spectra were smoothed to 86 kHz resolution. Pointing was checked periodically by observations of nearby SiO masers; the pointing accuracy was ".

The standard chopper-wheel technique was used for the calibration. We express the results in units of main beam brightness temperature () assuming the main beam efficiencies () as given in Table 3. The temperature scale was checked by observations of Orion KL.

In most sources only one position was observed, corresponding typically to the peak of the CS emission. In addition, G 270.26+0.83 and G 301.12-0.20 were mapped with a spacing of 10".

2.2.2. Onsala observations

At Onsala, the 110 GHz observing procedure was very similar to that at the SEST. The observations were also performed in a dual beam switching mode with a beam throw of . The front-end was a SIS receiver tuned to SSB operation. As backend we used 2 filter spectrometers in parallel: a 256 channel filterbank with 250 kHz resolution and a 512 channel filterbank with 1 MHz resolution. The calibration procedure was the same as at the SEST. The pointing accuracy checked by observations of nearby SiO masers was ". The strongest HNCO source from the Onsala sample, W51M, was mapped with 40" spacing.

2.2.3. Effelsberg observations

The 22 GHz observations in Effelsberg were performed with a K-band maser amplifier using position switching. The offset positions were displaced by 10´-15´ symmetrically in azimuth. Pointing was checked periodically by observations of nearby continuum sources; the pointing accuracy was ". The integration time per position was a few hours.

The main beam temperature scale was checked by observations of nearby continuum calibration sources, NGC 7027 and W3(OH); for Sgr A we used Sgr B2. The fluxes for the first two sources were taken from Ott et al. (1994). The Sgr B2 flux at 1.3 cm was taken from Martín-Pintado et al. (1990).

2.2.4. HHT observations

To observe the HNCO J = 21-20 lines at 461 GHz we have used the Heinrich Hertz Telescope (HHT) on Mt. Graham (Baars & Martin 1996) during Feb. 1999 with a beamwidth of 18". Spectra were taken employing an SIS receiver with backends consisting of two acousto optical spectrometers with 2048 channels each, channel spacing 480 and 120 kHz, frequency resolution 930 and 230 kHz, and total bandwidths of 1 GHz and 250 MHz, respectively. Receiver temperatures were 150 K, system temperatures were 1000 K on a scale. The receiver was sensitive to both sidebands. Any imbalance in the gains of the lower and upper sideband would thus lead to calibration errors. To account for this, we have observed the CO J = 4-3 line of Orion-KL with the same receiver tuning setup and obtain 70 K, in good agreement with Schulz et al. (1995).

HNCO (351.63346 GHz) and (329.66454 GHz) line emission was observed with a dual channel SIS receiver in early April 1999 at the HHT. The beamwidth was 22", receiver temperatures were 135 K; system temperatures were 700 K on a scale. The receivers were also sensitive to both sidebands. We have used published spectra from Orion-KL and IRC+10216 as calibrators (Groesbeck et al. 1994 , Schilke et al. 1997).

All results displayed are given in units of main beam brightness temperature (). This is related to via = (/) (cf. Downes 1989). The main beam efficiency, , was 0.38 at 461 GHz and 0.5 at 330 and 352 GHz as obtained by measurements of Saturn. The forward hemisphere efficiency, , is 0.75 at 461 GHz and 0.9 at 330 and 352 GHz (D. Muders, priv. comm.). The HHT is with an rms surface deviation of 20µm (i.e. /30 at 461 GHz) quite accurate. Thus emission from the sidelobes should not be a problem.

Pointing was obtained toward Jupiter (continuum pointing) and toward Orion-KL and R Cas (line pointing) with maximum deviations of order 5". Observations were carried out in a position switching mode with the off-position 1000" offset from the source position.

2.3. Data reduction and analysis

We have reduced the data and produced maps using the GAG (Groupe d'Astrophysique de Grenoble ) software package. The measured spectra were fitted by one or more gaussian components.

© European Southern Observatory (ESO) 2000

Online publication: October 10, 2000
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