Astron. Astrophys. 359, 991-997 (2000)

2. Observations and data reduction

2.1. Observations

The spectroscopic observations were obtained with the ELODIE spectrograph at the 1.93-m telescope at the Observatoire de Haute-Provence (Baranne et al. 1996). This instrument covers a spectral range of about 3000 Å from 3906 Å to 6811 Å and has a resolving power . For RR Lyrae, an exposure of 5 to 10 mn, which corresponds to 1% of the pulsation period (13 h 36 mn), results in a signal-to-noise ratio around 50. Therefore, this spectrograph is ideally suited for studying the effect of the pulsation on line profiles, especially for determining the turbulence level in the atmosphere. The spectra, used in this paper, have been recorded during several runs that each lasted two to three consecutive nights. These nights and the corresponding Blazhko phase are listed in Table 1. These Blazhko phases correspond to the mean value over the period of each run.

Table 1. The Blazhko phase and their corresponding observing nights.

The pulsation and Blazhko phases have been calculated from the ephemeris given by Chadid & Gillet (1997). The Blazhko phase corresponds to the end of the 4-year cycle around September 1994, while the four others Blazhko phases , , and refer to the same and following 4-year cycle.

2.2. Data reduction

In this paper, we have used the profile of a singly ionized metallic absorption line Fe II 4923.921 Å and the correlation profiles of Chadid & Gillet (1996a). The data reduction of the CCD images was done using the Munich Image Data Analysis System (MIDAS). In this study, all the spectra were treated in the same way. A detailed description of the observations and data reduction can be found in Chadid & Gillet (1996a).

Throughout this paper, the pulsation phase is , the Blazhko phase is and the average radial velocity over one pulsation period is .

2.3. Error estimation

During individual runs, the mechanical, thermal and optical characteristics of the spectrograph did not change. At the beginning of each night, the positioning of the echelle orders on the CCD was calibrated. Consequently, the "zero point" of the spectrograph is never larger than 5 m/s (Naef 1999 private communication). The wavelength calibration was done with a thorium lamp. A thorium arc was taken at the beginning and at the end of each night, except June 24 and 25, 1996 where a thorium arc was also done during the middle of the night. The room that contains the spectrograph is thermally controlled, so wavelength drifts due to temperature variations are very small.

The main influence on the wavelength stability of the spectrograph is a change in atmospheric pressure, which changes the air refractive index and shifts the zero point of the calibration (100 m/s per mm/Hg). All of the observations were performed in stable conditions, so the wavelength shift during the night was typically around 0.1 pixel, i.e. 50 m/s. There is also a small mechanical flexure of the dewar which moves the echelle orders.

Our main source of error was due to the fact that we determined the radial velocity with only one absorption line (Fe II 4923.921 Å), which was observed with a relatively small signal-to-noise ratio in order to have good temporal resolution. Depending on weather conditions, this ratio is between 40 and 60. A good idea the accuracy is given by the dispersion of the radial velocity between phases 0.2 and 0.5. During this interval the infalling motion of the atmosphere occurs. Consequently, we expect an almost linear variation of the radial velocity and the dispersion of velocities around this straight line gives a good estimate of the accuracy. Depending on the night, it was between 117 and 299 m/s (standard deviation), so the true error is somewhere between these two numbers.

In this paper we compare radial velocity curves over an interval of three years. Observations of standard stars since ELODIE was put into operation at the end of 1993, show that, in normal observational conditions, the fluctuation of the spectrograph "zero point" is between 5 and 10 m/s (Naef 1999 private communication).

© European Southern Observatory (ESO) 2000

Online publication: July 13, 2000
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