Astron. Astrophys. 362, 1020-1040 (2000)

3. Observational data and reduction

The optical spectroscopic multi-site campaign was carried out at six observatories: Haute Provence (OHP), France, Kitt Peak (KPNO), USA, Dominion (DAO), Canada, Okayama (OKAO), Japan, Crimean (CRAO), Crimea, and Ondejov (Ond), Czech Republic, from 1993 August 30 to September 8.

The He I 6678 line was monitored at six sites and the H line at two sites. Technical observational details are given in Table 2 and the log of observations in Table 3. Bias, flat fields and wavelength calibration exposures produced by Th-Ar lamps were obtained regularly. Reference stars were observed nightly on each site for accurate radial velocity (RV) linking and a better determination of stellar continuum. Luckily, due to good weather conditions at most sites, we obtained a very good temporal coverage in the campaign. The He I line was continuously monitored over two 2.77 and 2.72 day time spans, only interrupted for 7 hours during the 8.6 day campaign.

Table 2. Technical observational conditions in optical spectroscopy

Table 3. Log of spectroscopic observations

For all sites except Crimea and Ondejov, observations were preliminarily reduced with IRAF ¹ by each group of observers. For Crimea and Ondejov data, reduction was done with local facilities. Kitt Peak spectra were also corrected in the vicinity of H for atmospheric contamination. The final reduction steps were done with IRAF by the Meudon group using the one-dimensional spectra provided by observers. Reference regions were carefully selected for satisfactory determination of a pseudo-continuum over about 100 Å around He I 6678. A one piece cubic spline function fitting was applied to these selected regions to determine the continuum level. 778 and 507 spectra were available for the study of He I 6678 and H respectively, the S/N ratio varying between 150 and 900. Kitt Peak spectra have the shortest time exposures, especially the first night, due to the combination of gain and simultaneous observation of H and He I 6678. We chose not to average the spectra because we preferred to be able to search for high frequencies if they were present. All spectra were corrected for heliocentric velocity; for the He I 6678 study, they were rebinned in the 6665-6690 Å wavelength range with the step  Å.

Wavelength stability is generally good except for the Crimean spectra which each night show global shifts with respect to other sites. However, these shifts were easily corrected because of overlapping observations on sites. It has to be stressed that good agreement was generally found between different quantities measured on data sets provided by each site when they overlap. However equivalent widths of Ondejov are smaller by 10% for He I absorption and H emission lines obtained at other sites. The small number of spectra obtained at Ondejov and the lack of overlap in time coverage (only 3 Ondejov profiles could be compared with others) did not allow us to include them in the search for periodicities in equivalent width and peak intensity; nevertheless they could be used for the study of V/R ratio and RV centroids, these quantities being only weakly affected by discrepancies in equivalent width.

© European Southern Observatory (ESO) 2000

Online publication: October 30, 2000
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