4 Observations and data reduction

4.1 Observations

Figure 2: MuSiCoS 98 observations of $\omega$ Ori with breakdown for each site.

$\omega$ Ori was intensively observed in 1998 from November 23 to December 15 with 8 telescopes (see Table 2 and Fig. 2):

• in France, at the Observatoire de Haute Provence (OHP), with the 1.52 m telescope equipped with the spectrograph AURELIE (R=22 000, $\lambda=6520$-6720 Å);
• also at the OHP, with the 1.93 m telescope equipped with the fiber-fed échelle spectrograph ELODIE (R=42 000, $\lambda=3900$-6800 Å);
• in the USA, at Kitt Peak National Observatory, with the 0.9 m coudé feed telescope equipped with the échelle spectrograph (R=65 000, $\lambda= 4070$-5160 or 5300-7000 Å);
• in Chile, at the European Southern Observatory (ESO)/La Silla, with the 0.9 m Dutch telescope equipped with the Landessternwarte Heidelberg échelle spectrograph HEROS (R=20 000, $\lambda=3450$-5600 and 5800-8650 Å simultaneously);
• in China, at the Xinglong Station, with the 2.16 m telescope equipped with the échelle spectrograph (R=43 000 or 35 000, $\lambda=3500$-7000 or 5500-8500 Å respectively);
• in Australia, on Mount Stromlo, with the 1.9 m telescope equipped with the échelle spectrograph (R=35 000, $\lambda=4800$-6800 Å);
• in Brazil, at the Laboratorio Nacional de Astrofisica (MCT/LNA), with the 1.6 m B&C telescope equipped with the coudé spectrograph (R=60 000, $\lambda=6563$ or 6678 Å);
• on the Canary Islands, at La Palma, with the 2.5 m INT telescope equipped with the échelle spectrograph MUSICOS (R=35 000, $\lambda=3900$-6700 Å).

 

 

Table 2: Journal of MuSiCoS 98 observations of $\omega$ Ori.

HJD span(d)	Site	N. of	Exposure	S/N
(2451100+)		spectra	time (min)
39.64-47.72	OHP152	33	15-20	200-500
41.42-47.72	OHP193	29	15-20	300-400
41.66-44.93	Kitt Peak	36	15-25	100-200
43.78-60.88	ESO	47	20	100-300
46.29-55.41	Xinglong	22	20	200-300
47.10-51.08	Stromlo	13	10	150-200
48.74-56.65	LNA	27	18-25	300-500
50.66-58.75	INT	36	7-30	50-150

Bias, flat fields and wavelength calibration exposures produced by Th-Ar lamps were obtained every 2-4 hours. An early-type comparison star with low vsini, $\gamma$ Peg, telluric standard stars such as $\alpha$ Peg and velocity standard stars such as $\beta$ Lep were observed from each site to correct for telluric lines and check for possible problems in the calibration.

4.2 Reduction

The instruments lack homogeneity: gratings, filters, ranges of wavelengths in the orders of échelle spectra and detectors are different, which hampered the data reduction procedure. The OHP152, Kitt Peak, LNA, Mount Stromlo and INT data were reduced using IRAF software. The OHP193, ESO and Xinglong data were reduced with MIDAS software. A correction for the motion of the Earth was applied and the telluric lines were removed when possible. Finally each line in each individual spectrum has been normalized by selecting precise continuum points around the line, and rebinned on a carefully chosen equidistant wavelength grid (0.1 Å) using IRAF. The normalization of the spectra is the most critical part of the reduction procedure of multi-site campaigns to obtain homogeneous data (see Hubert et al. 1997).