Infrared photometry and evolution of mass-losing AGB stars

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.

Mass loss and luminosities of S and C AGB stars with and without Li

“…This could be originated by two concurring phenomena: 1) more luminous Carbon stars are on average more evolved, so had more time to destroy Li; 2) more luminous Carbon stars could be the most massive ones (in a relatively small range of masses), therefore they could destroy Li more efficiently due to higher H-burning temperatures. If we look at the vertical dashed lines, they indicate the range of the luminosity function for Galactic C-rich sources from Guandalini et al (2006). Our stars belong to the typical luminosity range of normal C-stars and cover all the interval, therefore we can conclude that our sample is a good representative for the standard galactic Carbon stars from the disk.…”

“…Guandalini & Busso (2008) [S-type]. The sources richest in Li are all of CJ type: we believe, also thanks to our analysis performed on C-rich galactic sources in Guandalini et al (2006), that they are all source with peculiar evolutionary properties. Their high Li abundance can be produced either by a relatively fast extramixing in low-mass stars or by hot bottom burning in more massive AGB stars.…”

“…We extend it to the Galactic AGB stars of the disk, already examined in their photometrical properties in our previous works (Guandalini et al 2006, Guandalini & Busso 2008, with the aim of creating a homogeneous sub-sample with good estimates for mass and Li-abundances, good measurements of the distance and reliable near-tomid infrared photometry. The resulting list of sources and all the details on the analysis performed are presented in Guandalini et al (2009; Table 1).…”

“…The resulting list of sources and all the details on the analysis performed are presented in Guandalini et al (2009; Table 1). All sources come from large samples of C-rich or S-type AGB stars; the photometric data used and the techniques adopted are from Guandalini & Busso (2008), Busso et al (2007b), and Guandalini et al (2006). Guandalini & Busso (2008) [S-type].…”

“…In order to estimate the rates, we adopted the methods explained in detail in Guandalini et al (2006) and Guandalini (2010). Unfortunately, we didn't find any apparent relation between these two parameters: the data have a too large scatter to draw any conclusion.…”