Context. The asymptotic giant branch (AGB) phase marks the end of the evolution for low- and intermediate-mass stars, which are fundamental contributors to the mass return to the interstellar medium and to the chemical evolution of galaxies. The deta
iled understanding of mass loss processes is hampered by the poor knowledge of the luminosities and distances of AGB stars. Aims. In a series of papers we are trying to establish criteria permitting a more quantitative determination of luminosities for the various types of AGB stars, using the infrared (IR) fluxes as a basis. An updated compilation of the mass loss rates is also required, as it is crucial in our studies of the evolutionary properties of these stars. In this paper we concentrate our analysis on the study of the mass loss rates for a sample of galactic S stars. Methods. We reanalyze the properties of the stellar winds for a sample of galactic MS, S, SC stars with reliable estimates of the distance on the basis of criteria previously determined. We then compare the resulting mass loss rates with those previously obtained for a sample of C-rich AGB stars. Results. Stellar winds in S stars are on average less efficient than those of C-rich AGB stars of the same luminosity. Near-to-mid infrared colors appear to be crucial in our analysis. They show a good correlation with mass loss rates in particular for the Mira stars. We suggest that the relations between the rates of the stellar winds and both the near-to-mid infrared colors and the periods of variability improve the understanding of the late evolutionary stages of low mass stars and could be the origin of the relation between the rates of the stellar winds and the bolometric magnitudes.
The study of the evolutionary properties of Asymptotic Giant Branch stars still presents unresolved topics. Progress in the theoretical understanding of their evolution is hampered by the difficulty to empirically explain key physical parameters like
their luminosity, mass loss rate and chemical abundances. We are performing an analysis of Galactic AGB stars trying to find constraints for these parameters. Our aim is of extending this analysis to the AGB stars of the Magellanic Clouds and of the Dwarf Spheroidal Galaxies using also mid-infrared observations from the Antarctic telescope IRAIT. AGB sources from the Magellanic Clouds will be fundamental in our understanding of the AGB evolution because they are all at a well defined distance (differently from the Galactic AGBs). Moreover, these sources present different values of metallicity: this fact should permit us of examining in a better way their evolutionary properties comparing their behaviour with the one from Galactic sources.
We present an analysis of Li abundances in low mass stars (LMS) during the Red Giant Branch (RGB) and Asymptotic Giant Branch (AGB) stages, based on a new determination of their luminosities and evolutionary status. By applying recently suggested mod
els for extra-mixing, induced by magnetic buoyancy, we show that both Li-rich and Li-poor stars can be accounted for. The simplest scenario implies the development of fast instabilities on the RGB, where Li is produced. When the fields increase in strength, buoyancy slows down and Li is destroyed. 3He is consumed, at variable rates. The process continues on the AGB, where however moderate mass circulation rates have little effect on Li due to the short time available. O-rich and C-rich stars show different histories of Li production/destruction, possibly indicative of different masses. More complex transport schemes are allowed by magnetic buoyancy, with larger effects on Li, but most normal LMS seem to show only the range of Li variation discussed here.
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 chemical evolution of Galaxies are hampered by po
or knowledge of their Luminosities and mass loss rates,both for C-rich and for O-rich sources.We plan to establish criteria permitting a more quantitative determination of luminosities for the various types of AGB stars on the basis of IR fluxes.In this paper we concentrate on O-rich and s-element-rich MS, S stars and include a small sample of SC stars.We reanalyze the absolute bolometric magnitudes and colors of MS, S, SC stars on the basis of a sample of intrinsic and extrinsic long period variables.We derive bolometric corrections as a function of near- and mid-IR colors,adopting as references a group of stars for which the SED 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 of C-rich AGB stars. Luminosity estimates are also verified on the basis of existing Period-Luminosity relations valid for O-rich Miras.S star bolometric luminosities are almost indistinguishable from those of C-rich AGB stars.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 bands.Based on Period-Luminosity relations for O-rich Miras and on Magnitude-color relations for the same variables we show how approximate distances for sources of so far unknown parallax can be inferred. We argue that most of the sources have a rather small mass(<2Msun);dredge-up might then be not effective enough to let the C/O ratio exceed unity.
We present here a study of the Infrared properties of Asymptotic Giant Branch stars (hereafter AGB) based on existing databases, mainly from space-borne experiments. Preliminary results about C and S stars are discussed, focusing on the topics for wh
ich future Infrared surveys from Antarctica will be crucial. This kind of surveys will help in making more quantitative our knowledge of the last evolutionary stages of low mass stars, especially for what concerns luminosities and mass loss.
As part of a reanalysis of Galactic Asymptotic Giant Branch stars (hereafter AGB stars) at infrared wavelengths, we discuss here two samples (the first of carbon-rich stars, the second of S stars) for which photometry in the near- and mid-IR and dist
ance estimates are available. Whenever possible we searched also for mass-loss rates. The observed spectral energy distributions extended in all cases up to 20 $mu$m and for the best-observed sources up to 45 $mu$m. The wide wavelength coverage allows us to obtain reliable bolometric corrections, and hence bolometric magnitudes. We show that mid-IR fluxes are crucial for estimating bolometric magnitudes for stars with dusty envelopes and that the so-called luminosity problem of C stars (i.e. the suggestion that they are less luminous than predicted by models) does not appear to exist.
In this note I present an outline of infrared (IR) photometric AGB properties, based on two samples of Galactic Long Period Variables (C- and S-type respectively). I show the various selection criteria used during the choice of the sources and descri
be the motivations of observing them at near- and mid-IR wavelengths. I discuss the problems encountered in estimating their luminosity and distance and motivate the methods I choose for this purpose. Properties of the luminosity functions and of the Hertzsprung-Russell (HR) diagrams obtained from the analysis are discussed. Finally, the choices made for estimating of the mass loss rates are described and preliminary results concerning them are shown.
