Astron. Astrophys. 352, 287-296 (1999)

2. Observations and data reduction

BD +39 3226 is a sdO star displaying a pure helium line spectrum in the optical (Giddings 1980) in a distance of about 270 pc ¹ at the galactic coordinates , . Its high radial velocity of 279 km s^-1 makes this star suitable for studies of the local ISM, since stellar and interstellar absorption lines are well separated. We analyse echelle spectra obtained during the ORFEUS II mission in Nov./Dec. 1996 and NEWSIPS reduced long- and short-wavelength high dispersion spectra taken from the IUE Final Archive (IUEFA ) ². The total wavelength range thus covered is about 900 to 3200 Å.

The ORFEUS spectrum was obtained in 4 pointings with a total of 5000 s integration time. The basic reduction of the data was performed by the ORFEUS team in Tübingen. Details about the ORFEUS instrumentation and the basic data handling and calibration are given in Barnstedt et al. (1999). We repeat the main features here.

The wavelength calibration of the ORFEUS spectra is based on the interstellar spectrum in the ORFEUS target HD 93521. The accuracy of this calibration is estimated as better than 0.05 Å, but small systematic effects within the spectrum cannot be excluded. The zero point of the wavelength calibration is based on the geocoronal Ly emission. Since the ORFEUS aperture measures 20" in diameter, errors in the pointing may shift the wavelength zero point. Optical spectra of BD +39 3226 give the stellar heliocentric radial velocity as  km s^-1, based on values by Dworetsky et al. (1982) and Giddings (1980) and one derived from observations with FOCES at the Calar Alto 2.2 m telescope. In the ORFEUS spectrum the mean radial velocity of 22 sharp stellar metal lines is  km s^-1. We corrected the ORFEUS spectrum for this shift. The new velocity zero point should be accurate within  km s^-1.

The spectral resolution is intrinsically better than 10⁴, but pointing jitter () and the addition of spectra from several pointings may cause some deterioration. We have filtered the spectra as provided by Tübingen with a wavelet algorithm (Fligge & Solanki 1997) effectively leading to a 3 pixel boxcar filter. We derive from our spectrum a resolution of .

ORFEUS spectra are influenced by scattered light, which is implicitly corrected for by subtracting the intensities in the interorder space from the echelle order intensities. Near strong features residual effects may still be present (Barnstedt et al. 1999). However, since the ORFEUS spectra are slightly tilted with respect to the detector grid, affected areas lie normally at some distance from such features.

The resolution of the IUE spectra is also about , which is equivalent to km s^-1. While only one long wavelength spectrum (LWR 11789, exp. time 5040 s) can be found in the IUEFA , several short wavelength spectra are available (SWP 15275, 48312, 48313, 48314, each with 3600 s exp. time). To improve the S/N ratio we added the four SWP spectra. The IUE spectra were smoothed with a 3 pixel boxcar filter.

© European Southern Observatory (ESO) 1999

Online publication: November 23, 1999
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