Infrared photometry and evolution of mass-losing AGB stars - II. Luminosity and colors of MS and S stars

Context. Asymptotic giant branch (AGB) phases mark the end of the evolution for low- and intermediate- mass stars. Our understanding of the mechanisms through which they eject the envelope and our assessment of their contribution to the mass return to the interstellar medium and to the chemical evolution of Galaxies are hampered by poor knowledge of their luminosities and mass loss rates, both for C-rich and for O-rich sources. Aims. We plan to establish criteria permitting a more quantitative determination of luminosities (and subsequently of mass loss rates) for the various types of AGB stars on the basis of infrared fluxes. In this paper, in particular, we concentrate on O-rich and s-elementrich MS, S stars and include a small sample of SC stars. Methods. We reanalyze the absolute bolometric magnitudes and colors of MS, S, SC stars on the basis of a sample of intrinsic (single) and extrinsic (binary) long period variables. We derive bolometric corrections as a function of near- and mid-infrared colors, adopting as references a group of stars for which the spectral energy distribution could be reconstructed in detail over a large wavelength range. We determine the absolute HR diagrams, and compare luminosities and colors of S-type giants with those, previously derived, of C-rich AGB stars. Luminosity estimates are also verified on the basis of existing period- luminosity relations valid for O- rich Miras. Results. S star bolometric luminosities are almost indistinguishable from those of C-rich AGB stars. On the contrary, their circumstellar envelopes are thinner and less opaque. Despite this last property the IR wavelengths remain dominant, with the bluest stars having their maximum emission in the H or K(short) bands. Near- to- mid infrared color di. erences are in any case smaller than for C stars. Based on period- luminosity relations for O-rich Miras and on Magnitude- color relations for the same variables we show how approximate distances (hence intrinsic parameters) for sources of so far unknown parallax can be inferred. We argue that most of the sources have a rather small mass (< 2 M-circle dot); dredge- up might then be not e. ective enough to let the C/O ratio exceed unity.