Infrared photometry and evolution of mass-losing AGB stars

I. Carbon stars revisited

¹ Dipartimento di Fisica, Università di Perugia, via A. Pascoli, 06123 Perugia, Italy e-mail: roald.guandalini@fisica.unipg.it
² Tuorla Astronomical Observatory, University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland e-mail: stefano.ciprini@utu.fi
³ Dipartimento di Fisica Generale, Università di Torino, via P. Giuria 1, 10125 Torino, Italy e-mail: silvestro@ph.unito.it
⁴ Istituto di Astrofisica Spaziale e Fisica Cosmica, 00100 Roma, Italy e-mail: persi@rm.iasf.cnr.it

Received: 7 April 2005
Accepted: 12 September 2005

Abstract

As part of a reanalysis of galactic Asymptotic Giant Branch (AGB) stars at infrared (IR) wavelengths, we discuss a sample (357) of carbon stars for which mass loss rates, near-IR photometry and distance estimates exist. For 252 sources we collected mid-IR fluxes from the MSX (6C) and the ISO-SWS catalogues. Most stars have spectral energy distributions up to 21 μm, and some (1/3) up to 45 μm. This wide wavelength coverage allows us to obtain reliable bolometric magnitudes. The properties of our sample are discussed with emphasis on ~70 stars with astrometric distances. We show that mid-IR fluxes are crucial to estimate the magnitude of stars with dusty envelopes. We construct HR diagrams and show that the luminosities agree fairly well with model predictions based on the Schwarzschild's criterion, contrary to what is widely argued in the literature. A problem with the brightness of C stars does not appear to exist. From the relative number of Mira and Semiregular C-variables, we argue that the switch between these classes is unlikely to be connected to thermal pulses. The relevance of the two populations varies with the evolution, with Miras dominating the final stages. We also analyze mass loss rates, which increase for increasing luminosity, but with a spread that probably results from a dependence on a number of parameters (like e.g. different stellar masses and different mechanisms powering stellar winds). Instead, mass loss rates are well monitored by IR colours, especially if extended to 20 μm and beyond, where AGB envelopes behave like black bodies. From these colours the evolutionary status of various classes of C stars is discussed.