Astron. Astrophys. 320, 972-992 (1997)

2. Observations

Molecular-line observations were carried out between 5-11 May 1990 with the 15-m Swedish-ESO Submillimeter Telescope (SEST) at La Silla, Chile. The SEST and its instrumentation have been described in detail by Booth et al. (1989). The receivers employed cooled Schottky barrier diode mixers; a high-resolution Acousto-Optical Spectrometer was used as a backend, with a total bandwidth of about 100 MHz and a resolution of 43 kHz, corresponding to velocity resolutions of 0.15  at 89 GHz, 0.12  at 115 GHz, and 0.06  at 230 GHz. The full width at half maximum (FWHM) beam width is at 89 GHz, at 115 GHz, and at 230 GHz, respectively.

The observations were made employing frequency-switching with switch interval depending on the line frequency: this interval is larger than the extent of the emission to avoid overlap, but still small enough that the emission feature is present both in the signal and reference cycles. The data were calibrated using the standard chopper wheel technique as described by Ulich & Haas (1976). Typical rms noise values are 0.5 K for ¹² CO(1-0), 0.7 K for ¹² CO(2-1), 0.2 K for ¹³ CO(1-0) and O(2-1), 0.6 K for ¹³ CO(2-1), 0.1 K for O(1-0), (1-0), and CO (1-0). Pointing was checked every 3 hours on the SiO() maser sources W Hya and Vx Sgr, and was found to be accurate to within . The temperature-scale was regularly checked on M17SW ( =33.3 K) and the local peak position in each of the two objects; the intensity calibration is accurate to within 10%.

The observed line intensities are expressed as main beam brightness temperature ( = ), with between 0.70 and 0.79.

The ¹² CO(1-0) and ¹³ CO(1-0) transitions were used mainly to determine the large scale distribution of the molecular gas, and as indicators of kinetic temperatures and column densities. These data are complemented with ¹² CO(2-1), ¹³ CO(2-1), O(1-0) and O(2-1) observations along strips through the ionization front in order to study variations in physical parameters, such as density, kinetic temperature, and isotopic ratios across the photon-dominated region (PDR). (1-0) and CO (1-0) emission were observed at selected positions to find the regions with higher density. For G353.1+0.6 observed positions are offsets (in arcseconds) in right ascension () and declination () from a zero point (1950.0). For G353.2+0.9 the zero point is (1950.0). In both cases the zero point coincides with the position of the ionization front.

Observations of ¹² CO(1-0) and ¹³ CO(1-0) were performed on a grid with a spacing of , and along two strips at (for G353.1+0.6) or (for G353.2+0.9) and cutting across the ionized gas. The other molecules and transitions were observed only along the two strips, with steps in declination of or integer multiples thereof.

The ¹² CO(1-0) spectra required some care before folding, because the emission is quite extended and there are a number of features (in the signal and reference cycles) at various velocities. Based on the difference in velocity between these features and the bulk of the emission, they do not seem to be related to the regions of interest, but can affect the spectrum of the main emission after folding. The ¹² CO(1-0) spectra with a frequency offset of 20 MHz have the red side of the main line affected by a negative feature in the reference (`dip') related to an emission feature at 40  . In these cases, before folding we have used gaussian fits to remove the large dip near the main line; the same procedure was followed for small peaks elsewhere in the spectrum which could produce dips in the main line after folding.

Finally, (1-0) and (2-1) spectra of the various CO-isotopes have been resampled to a velocity resolution of 0.12  and 0.06  , respectively, while the and CO spectra have been resampled to a velocity resolution of 0.15  .

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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