Astron. Astrophys. 363, 995-1004 (2000)

2. Observations and data reduction

Our data sample of HD199478 consists of 41 high-resolution H spectra obtained between August 1997 and March 1999 with the coudé spectrograph of the 2 m RCC telescope at the National Astronomical Observatory (Bulgaria). The spectra were collected within an on-going program to study the nature and origin of wind variability in luminous early type stars. The project was started in the spring of 1997 with an ISTA CCD (580x520, 24µ pixel) as a detector. Since October 1998, we have used a PHOTOMETRICS camera equipped with back-illuminated SITe CCD (1024x1024, 24µ pixel). Depending on the configuration used spectra with resolution (R = ) of 15000 and 22000 were obtained. The spectrum coverage ranges from about 60 to about 200Å. Signal to noise ratio, , of 100 to 300 per pixel in the continuum was usually achieved. The distribution of the observations in time is given in Table 2. In this table Column (2) gives the Julian date of observation; Column (3) records the spectral resolution, R=; Column (4) lists the observational window, and Column (5) gives the ratio.

Table 2. Journal of observations

The spectra were uniformly reduced using a series of modules written in IDL by one of us (T.V). The procedure is entirely standard and consists of background subtraction, cosmic ray hits removal, flat-fielding and wavelength calibration. Pixel-to-pixel sensitivity variation is taken into account by dividing the spectra by a mean flat-field, obtained as an average during the relevant night. During the wavelength calibration, the correction for the Earth's motion with respect to the heliocentric rest frame is taken into account. The spectra were normalised by a polynomial fit to the continuum, specified by carefully selected spectral windows free of lines.

The atmospheric water vapour lines were removed by dividing each spectrum of HD199478 with a specially constructed "telluric spectrum". This procedure consists of the following steps. First, we observed a rapidly rotating non-variable B star as a telluric standard in the relevant wavelength domains. Second, using an interactive spline-interpolationg technique we obtained a smooth template of the underlying stellar spectrum and subsequently divided the observed spectrum with this template to derive the "telluric spectrum". This was then scaled in order to obtain the best fit for water vapour line profiles on each of the HD199478 spectra, after which the target spectrum was divided with the derived "telluric spectrum" to remove the water vapour lines. Finally, the spectra were rebinned to a step of 0.2Å per pixel.

© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000
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