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Astron. Astrophys. 361, 629-640 (2000)

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2. Observations

HS 0209+0832 was observed on January 10, 1999 with the Space Telescope Imaging Spectrograph (STIS) onboard HST. Four exposures in three different wavelengths regions have been carried out during two orbits. The spectra cover the entire range from 1150 Å to 5700 Å with a resolution of [FORMULA] 0.03 Å in the far ultraviolet and [FORMULA] 5 Å in the optical. Table 1 contains an overview of the observations. The data were reduced with the standard procedures of the STSDAS package in IRAF using the latest available reference files (February 2000).


Table 1. HST/STIS observations from January 10, 1999

The observed spectral range contains the hydrogen Balmer lines (H[FORMULA] and higher) which can be used to determine effective temperature and gravity. The continuum slope from the optical to the ultraviolet and L[FORMULA] can serve as additional temperature indicators. The helium abundance can be determined from the He I line at 4471 Å and the He II line at 1640 Å.

At the time of the planning of the HST observation it was not known that HS 0209+0832 may have variable helium features. Therefore, the exposures were only taken on a single date and were not prepared to search for variability. The E140M (FUV) observation is, however, splitted into two exposures but the He II line at 1640 Å does not vary in strengths between these exposures. The gap between the exposures was 47 minutes which is much shorter than the time scale of eight months by which the line strength varied according to Heber et al. (1997).

We have also compared the new optical observations of the He I line at 4471 Å with the observations from 1990 but did not find significant variations. Note, however, that both observations have only medium resolution (7 Å and 5 Å) and medium signal-to-noise ([FORMULA]). For the HST observations, the main focus of the optical spectrum aimed at a precise determination of the overall energy distribution.

The FUV spectra contain besides L[FORMULA] and He II 1640 Å about 250 interstellar and photospheric lines. In particular, interstellar features from C II, N I, O I, Si II, and S I as well as stellar lines from C III, C IV, Al III, Si III, Si IV, Ca III, Ti III, Ti IV, Ni III, Ni IV, Zn III, and Zn IV can be identified.

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© European Southern Observatory (ESO) 2000

Online publication: October 2, 2000