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Astron. Astrophys. 334, 935-942 (1998)

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2. Observations

We observed [FORMULA] with the Swedish ESO Submillimeter Telescope (SEST) in December 14-19, 1994. Formally, the equatorial coordinates of [FORMULA] we used were [FORMULA] = [FORMULA] and [FORMULA] = [FORMULA]. These coordinates are proper motion corrected to the epoque of our observations. The radial velocity of the star is not very well determined: early works show a spread by a factor of nearly two and Hoffleit & Jaschek (1982) assigned the comment variable? to their value of [FORMULA] km s-1 in the Bright Star Catalogue. A more recent discussion is found in Vidal-Madjar et al. (1994), who place the CO absorption features at the heliocentric velocity [FORMULA] km s-1. This corresponds to [FORMULA] km s-1 on the LSR velocity scale, which is commonly exploited in galactic millimeter work.

In the 3 millimeter band, we used a cooled Schottky mixer as frontend, whereas at the CO (2-1) frequency ([FORMULA] mm) an SIS receiver was exploited. As backend we used a 2000 channel acousto-optical spectrometer (AOS) with 43 kHz wide channels, thus covering a total bandwidth of 86 MHz. In terms of radial velocity, the resolution at the various spectral line frequencies ranges between 0.1 and 0.3 km s-1 ([FORMULA], see: Table 1). With the exception of the SiO (v =0) line, we observed two transitions simultaneously, splitting the AOS into two halves, so that the spectral coverage ranges from about 50 to 300 km s-1 (see: Fig. 1).


[TABLE]

Table 1. Molecular line observations of [FORMULA] at the 15 m SEST


[FIGURE] Fig. 1. The observed millimeter wave spectra at four molecular line transitions of the [FORMULA] system. The stellar (photospheric) radial velocity of [FORMULA] is indicated by the vertical dotted line. CO (2-1) (top) was observed simultaneously with either CS or the highly excited SiO transition (v =2) by splitting the AOS into two halves. This resulted in less spectral coverage than for the low lying SiO transition (v =0, at bottom). The spectral resolution has been degraded somewhat by binning the velocity channels to about 1 km s-1 ([FORMULA] km s-1, [FORMULA] km s-1, [FORMULA] km s-1, [FORMULA] km s-1). The resulting values of the rms noise are shown in each panel

Depending on the frequency of observation, the beam of the 15 m SEST subtends a sky angle of about one third of an arcminute to one arcmin (Table 1). The mode of observation selected was wide dual beam switching, placing either the star or a reference position about 12 [FORMULA] away into the telescope beam (11 [FORMULA] 37'' beam throw in positive and negative azimuth respectively). The pointing stability of the telescope was regularly checked by observing the SiO maser source R Dor, only some [FORMULA] distant from [FORMULA], and was found to be excellent during these observations (within [FORMULA] rms). The relative alignment of the 1 and 3 millimeter beams is known to be within 3''.

Calibrations were done internally using a chopper wheel load resulting in the [FORMULA] scale (Ulich & Haas 1976). The atmospheric conditions were generally stable during our run. However, in local summer, the La Silla zenith optical depth at 230 GHz was quite high, about 0.5 (Table 1). From observations of Ori A we deduce that the day to day reproducibility of the signal was within an uncertainty of about 10 - 20%.

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© European Southern Observatory (ESO) 1998

Online publication: June 2, 1998

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