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Astron. Astrophys. 333, 613-618 (1998)

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1. Introduction

The Peculiar red giants with spectral type S were first identified by Merrill (1922). A spectral classification of S stars based on their optical spectra was proposed by Keenan (1954). We know now that S stars are characterized by unusual photospheric abundances due to the enrichment of the stellar surface by nucleosynthesis products. Spectroscopically, S stars are characterized by molecular bands of ZrO and LaO, replacing the TiO bands found in M type stars. The spectra of S stars indicate strong enhancement of s-process elements in the photosphere. Abundance analyses show that in S stars the [FORMULA] ratio is very close to unity (Scalo & Ross 1976; Keenan & Boeshaar 1980), which also reveals the presence of the products of nucleosynthesis at the stellar surface.

Because of the unusual spectral property and element abundance, S stars have been traditionally considered for a long time as intermediate between M type stars and carbon stars as in the evolution sequence M-S-C at the AGB phase. However, in recent years it has become clear that this picture has to be revised, and that there are, actually, two categories of S stars. This dichotomy was first suggested by Iben & Renzini (1983), and has been supported by several observational results (cf. Jorissen & Mayor 1988, 1992; Brown et al. 1990). We now know that one category, the intrinsic S stars that follow the evolution sequence M-S-C are, indeed, lying on the AGB phase. Another category, the extrinsic S stars that have elemental abundances which appear to have been altered by mass transfer from a white dwarf companion, are in binary system. In particular, a defining characteristic which distinguishes these two kinds of stars is that the intrinsic S stars contain the unstable element Technetium (Tc) while the extrinsic S stars do not. Therefore, former ones are also called Tc-rich or with Tc, and later ones Tc-deficient or without Tc as denoted e.g. by Iben & Renzini (1983).

The Tc-deficient S stars can be distinguished from the Tc-rich ones on the following basis (Johnson 1992; Jorissen et al. 1993): (1) they show periodic radial velocity variations as in a binary system; (2) their WD companion can be directly detected in the ultraviolet; (3) they exhibit high excitation line emissions such as He I (10 830 Å); (4) they have different infrared colors. As pointed out by Chen & Kwok (1993), Groenewegen (1993) and Jorissen et al. (1993), S stars with strong infrared excess in the IRAS bands always exhibit lines of the unstable element Tc in their spectrum, a signature of the heavy-element nucleosynthesis associated with the AGB evolution; on the contrary, most of the S stars without infrared excess in the IRAS bands do not exhibit Tc lines. In addition, their [FORMULA], [FORMULA] and [FORMULA] colors are very different (Jorissen et al. 1993; Groenewegen 1993).

In this paper we present the JHK infrared photometry of 24 Tc-rich and 20 Tc-deficient S stars. Several color-color diagrams, including the new data together with the IRAS fluxes from the Point Source Catalog (1988, hereafter PSC) and some magnitudes in V or B from HST Guide Star Catalog (1989, hereafter GSC) are presented to test the best way for distinguishing those two kinds of S stars. Some physical properties are derived from their infrared colors. Their energy distribution and IRAS Low-Resolution Spectra (hereafter LRS) are also discussed.

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© European Southern Observatory (ESO) 1998

Online publication: April 20, 1998