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Astron. Astrophys. 348, 584-593 (1999)

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2. Observations and data reduction

The observations were obtained at the 3.8 m UKIRT telescope on Mauna Kea, Hawaii over two nights in May/June 1997. The longslit capability of the CGS4 cooled grating spectrometer was used with the 150 mm focal length camera, and 75 l/mm grating, to measure H2 emission in the K-band in the outflow of L483, giving a wavelength coverage of 1.85 [FORMULA] to 2.52 [FORMULA]. The slit size is approximately 90[FORMULA]1.2 arcsec2, and the configuration gives a spectral resolution of 0.003 [FORMULA] at 2.2 [FORMULA]. The seeing was 1.8 ", obtained from the FWHM size of a standard star.

The slit was placed in three different positions (Fig. 1) through the outflow, chosen to cut the bright jet head in two directions and also cover the more extended emission between the jet head and the source. The slits were positioned using offsets from a nearby star. The positions from Fig. 1 are correct to within [FORMULA] 1 ". The details of the observations are given in Table 1. Towards the source, there is a nebulosity which has been imaged in the K band, and slit 2, which is along the jet axis, also passes through this bright nebulosity near the source.

[FIGURE] Fig. 1. Position of the CGS4 slits relative to the [FORMULA] emission. The image of [FORMULA] emission is formed from the difference between 2.12 [FORMULA] and 2.22 [FORMULA] images (Fuller et al. 1995). A star marks the IRAS source, and crosses mark the slit centres. Slit 1 runs almost perpendicular to the outflow axis, through the head of the jet. Slits 2 and 3 are parallel to each other, with slit 2 being 2 " north of slit 3.


[TABLE]

Table 1. CGS4 Observations


The observations were taken in object-sky pairs. The integration times gave an RMS noise in the spectra of [FORMULA] erg s-1 cm- 2[FORMULA]. The data were reduced using the CGS4 data reduction package. Wavelength calibration was undertaken using the OH sky lines from the raw spectra. The RMS uncertainty in the wavelength scale corresponds to [FORMULA] 45 km s-1. We didn't detect any velocity shift [FORMULA] 3 [FORMULA] 45 km s-1.

For each slit position, every object observation was flat-fielded and sky-subtracted, removing the dark current, and then added together to form a single group. The observations were taken in non-destructive read mode, so removing the need to subtract a bias level. Flux calibration was provided by dividing the reduced spectra by the spectra of a standard star. This also corrected for atmospheric absorption lines. Standard stars were observed at the start and end of each set of observations. The standard stars used were BS7288, with a K magnitude of 6.13 (Catalog of IR Observations), and BS6830, whose K magnitude was taken to be 6.1 (Koornneef 1983). The uncertainty is estimated to be 4 [FORMULA] in the line fluxes. However, given the seeing and the slit width, it is unlikely that the observations contain all of the flux from the standard star, as the star is not exactly centred on the slit. Therefore, the calibration method provides a lower limit on the stellar flux, and therefore an upper limit on the line intensities. Since the analysis is based on line ratios, this does not unduly influence the conclusions, except that the derived column densities are upper limits.

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© European Southern Observatory (ESO) 1999

Online publication: July 26, 1999
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