Astronomy and Astrophysics – Astrophysics
Scientific paper
Aug 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000a%26a...360..196v&link_type=abstract
Astronomy and Astrophysics, Vol. 360, p.196-212
Astronomy and Astrophysics
Astrophysics
20
Stars: Agb And Post-Agb, Stars: Binaries: Symbiotic, Stars: Evolution, Stars: Late-Type
Scientific paper
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 δ = -25̂ and brighter than R = 10.5, 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 (zint = 200 ∓ 100 pc and zext = 600 ∓ 100 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. Based on observations carried out at the European Southern Observatory (ESO, La Silla, Chile) and at the Swiss 70 cm telescope (La Silla, Chile)
Jorissen Alain
Van Eck Sophie
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