The Henize sample of S stars --- III. Uncovering the binary intruders

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 = -25^̂ 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 (z_int = 200 ∓ 100 pc and z_ext = 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)