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Astron. Astrophys. 360, 196-212 (2000)

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The Henize sample of S stars

III. Uncovering the binary intruders *

S. Van Eck 1,2 and A. Jorissen **  1

1 Institut d'Astronomie et d'Astrophysique (IAA), Université Libre de Bruxelles, C.P.226, Boulevard du Triomphe, 1050 Bruxelles, Belgium
2 European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany

Received 9 December 1999 / Accepted 22 April 2000

Abstract

The properties of S stars are investigated thanks to a large observing program devoted to the well-defined Henize sample (205 S stars south of [FORMULA] and brighter than [FORMULA], covering all galactic latitudes), in order to derive the respective properties of the intrinsic S stars (genuine thermally-pulsing AGB stars) and of the extrinsic S stars (post mass-transfer binaries).

The stellar sample is first cleaned from a few stars misclassified as S thanks to UBV Geneva photometry and low-resolution spectroscopy. These low-resolution spectra also allow to successfully distinguish subclasses within the S star family. Dedicated Geneva photometry and high-resolution spectroscopy have led to the discovery of two symbiotic S stars.

The more stringent difference between extrinsic and intrinsic stars is their technetium content, but several other observational parameters are shown to be efficient to some extent in segregating intrinsic S stars from their extrinsic masqueraders (UBV, JHKL and IRAS photometry, radial-velocity standard deviation, shape of the CORAVEL cross-correlation dip, combination of band strength indices derived from low-resolution spectra). Multivariate classification has been performed on the Henize data sample in order to guarantee a classification as objective as possible and handling at the same time a large number of parameters. The resulting clusters separate efficiently extrinsic and intrinsic S stars, allowing to derive the respective properties of these two distinct stellar classes. The population difference between intrinsic and extrinsic S stars is for the first time clearly demonstrated, since intrinsic S stars are far more concentrated towards the galactic plane than extrinsic S stars ([FORMULA] pc and [FORMULA] pc), and are therefore believed to belong to a younger, more massive population. The frequency of extrinsic and intrinsic S stars in the magnitude-limited Henize sample amounts to 33% and 67%, respectively. In a volume-limited sample, this proportion is subject to large uncertainties mainly because of uncertain luminosities. There are probably as many as 40% extrinsic stars among S stars in a volume-limited sample.

Key words: stars: AGB and post-AGB – stars: binaries: symbiotic – stars: evolution – stars: late-type

* Based on observations carried out at the European Southern Observatory (ESO, La Silla, Chile) and at the Swiss 70 cm telescope (La Silla, Chile)
** Research Associate, National Fund for Scientific Research (FNRS), Belgium

Send offprint requests to: S. Van Eck (svaneck@eso.org)

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© European Southern Observatory (ESO) 2000

Online publication: July 27, 2000
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