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Astron. Astrophys. 364, L1-L5 (2000)

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2. Observations and data reduction

Details on the UVES spectrograph and its performances may be found in D'Odorico et al. (2000) as well as in the UVES User Manual (D'Odorico & Kaper, 2000). The data we used consists of a spectrum obtained on October 16th 1999 with the dichroic # 1 and the standard setting centered at 3460 Å in the blue arm and 5800 Å in the red arm. The exposure time was of 1200 s for both arms. The detector in the blue arm is an EEV CCD, while in the red it is a mosaic of one EEV (identical to that used in the blue arm) and one MIT CCD. All CCDs are composed of [FORMULA] square pixels of 15 [FORMULA] side. The slit width was [FORMULA] in the blue and [FORMULA] in the red. The data was reduced using the ECHELLE context of MIDAS and each CCD was treated independently; reduction included background subtraction, cosmic ray filtering, flat fielding, extraction, wavelength calibration and order merging. Since no arc spectra for wavelength calibration with this setting are available for the date of observation, we used calibration spectra acquired on different days. From our previous experience with UVES (Bonifacio et al. 2000), we expect the wavelength scale to be reproducible to within a shift of a few tenths of a pixel; since we are not interested in accurate radial velocities such a shift is of no consequence for our analysis. The resolution, as measured from the Th lines of the calibration lamp is [FORMULA] for the whole blue arm spectrum. For the red arm spectrum we could not find an arc taken with a slit of [FORMULA] and used one taken with a slit of [FORMULA] instead, whose measured resolution is [FORMULA], we therefore expect the resolution of the red spectra to be slightly lower, i.e. about 60000, as predicted by the UVES manual (D'Odorico & Kaper 2000). To give an idea of the dynamic range offered by the instrument we mention that at the peak of the He II 3203.104 Å line the spectrum detects 7831 [FORMULA], while only 88 [FORMULA] in the adjacent continuum. An estimate of the S/N ratio in this range, assuming Poisson noise, gives S/N [FORMULA] 88 in the He II line and S/N [FORMULA] 9 in the continuum. We also made use of a short exposure spectrum taken on October 10th, with the same setting and a slit of [FORMULA] in the blue arm and [FORMULA] in the red arm, the exposure times were 120 s and 60 s respectively. This spectrum was reduced in the same way as the long exposure and was used only to check the profiles of very strong lines.

The measured wavelength of the sharp, unblended Fe II lines is shifted by [FORMULA] km s-1 with respect to the laboratory (air) wavelength. After correction for the earth motion ([FORMULA]) the heliocentric radial velocity derived from the Fe II lines is [FORMULA], consistent with the heliocentric radial velocity reported in previous works ([FORMULA] km s-1, Thackeray (1977)). Hereafter, we consider the system of the FeII lines as the rest frame of RR Tel and the observed wavelengths refer to a frame that is at rest with respect to the Fe II lines.

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© European Southern Observatory (ESO) 2000

Online publication: December 15, 2000
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