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Astron. Astrophys. 321, L9-L12 (1997)

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Letter to the Editor

The barium stars in the Hertzsprung-Russell diagram * ** ***

J. Bergeat and A. Knapik

Centre de Recherche Astronomique de Lyon (UMR 5574 du CNRS), Observatoire de Lyon, 9 avenue Charles André, F-69561 St-Genis-Laval cedex, France

Received 13 February 1997 / Accepted 27 February 1997

Abstract

We present absolute magnitudes for a sample of 52 barium stars observed by the HIPPARCOS satellite, and their location in the HR diagram. Our plot (Fig. 1) is restricted to stars with parallax accuracies better than 22 %. The luminosity classes range from Ib supergiants down to V dwarfs on the main sequence, as expected from spectral classification. Discrepancies are however noted. No gap is observed in the region extending from the main sequence to the giant branch, exactly as shown by Perryman et al. (1995) for normal stars. This is also true for class II bright giants. A clump is however obvious at G8-K0 IIIb and [FORMULA] which corresponds to the one noted at [FORMULA] and [FORMULA] by Perryman et al. It appears that barium stars on the main sequence are earlier than G4, upward evolution being noticeable for later types. They are also distributed in the subgiant zone following the locus of normal stars, i.e. increasing brightness for later types. A few stars in our sample are also classified as CH stars : four of them are definitely main sequence class V-dwarfs, one is a class IVb faint subgiant while two possible CH-stars are class III-giants. These results are consistent with the currently-admitted model of surface pollution of a normal star through mass transfer in a binary system whose primary has become a white dwarf (WD). HIPPARCOS data show perturbations of the astrometric solution which can be attributed to proved (or possible) binarity for 21 stars out of 121, and 8 of them were already quoted in the CCDM catalogue (not necessarily with a WD component). This low proportion can be explained by the 5-11 magnitudes differences predicted between the two components and/or low angular separations with periods close to one year.

Key words: stars: HR diagram; stars: chemically peculiar; stars: evolution

* This research has made use of the Simbad database operated at CDS, Strasbourg, France.
** Based on data from the ESA HIPPARCOS astrometry satellite
*** Table 2 is only available in electronic form at the CDS via anonymous ftp 130.79.128.5

Send offprint requests to: J. Bergeat

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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