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Astron. Astrophys. 333, 613-618 (1998) 1. IntroductionThe 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
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 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. ![]() ![]() ![]() ![]() © European Southern Observatory (ESO) 1998 Online publication: April 20, 1998 ![]() |