Mid-Infrared Photometry of Mass-losing Asymptotic Giant Branch Stars

Abstract

We present ground-based mid-IR imaging for 27 M-, S-, and C-type asymptotic giant branch (AGB) stars. The data are compared with those of the database available thanks to the IRAS, Infrared Space Observatory, Midcourse Space Experiment, and Two Micron All Sky Survey catalogs. Our goal is to establish relations between the IR colors, the effective temperature T eff , the luminosity L, and the mass-loss rateṀ , for improving the effectiveness of AGB modeling. Bolometric (absolute) magnitudes are obtained through distance compilations and by applying previously derived bolometric corrections; the variability is also studied, using data accumulated since the IRAS epoch. The main results are as follows: (1) Values of L andṀ for C stars fit relations previously established by us, with Mira variables being on average more evolved and mass-losing than semiregular variables. (2) Moderate IR excesses (as compared to evolutionary tracks) are found for S and M stars in our sample: they are confirmed to originate from the dusty circumstellar environment. (3) A larger reddening characterizes C-rich Mira variables and post-AGB stars. In this case, part of the excess is due to AGB models overestimating T eff for C stars, as a consequence of the lack of suitable molecular opacities. This has a large effect on the colors of C-rich sources, and sometimes disentangling the photospheric and circumstellar contributions is difficult; better model atmospheres should be used in stellar evolutionary codes for C stars. (4) The presence of a long-term variability at mid-IR wavelengths seems to be limited to sources with maximum emission in the 8Y20 m region, usually Mira variables (one-third of our sample). Most of the semiregular and post-AGB stars studied here have remained remarkably constant in the mid-IR over the last 20 years.

DEEP MIXING IN EVOLVED STARS. I. THE EFFECT OF REACTION RATE REVISIONS FROM C TO Al

“…In their turn, these facts affect the colors, the infrared emission, and the luminosity function of AGB stars. The new models now succeed in reproducing the photometric and spectroscopic properties of their observational counterparts; in particular, the low effective temperatures of C stars at different metallicities (Domínguez et al 2004;Busso et al 2007a) can now be naturally reproduced. The stronger mass-loss rates reduce the number of thermal pulses, but the introduction of an exponentially decaying profile of convective velocities at the base of the envelope increases the TDU efficiency, so that the final enhancement factor of the most significant nuclei produced by AGB stars is roughly the same as before; but now, this is obtained together with a good reproduction of the luminosity functions of AGB stars (S. Cristallo et al 2011, in preparation).…”

The Mid-Infrared Luminosities of Normal Galaxies Over Cosmic Time

“…Using 200 Galactic TP-AGB C stars from Guandalini et al (2006) and Busso et al (2007), and conservatively excluding stars redder than K − [8.8] > 10 mag to reduce any bias towards IR-luminous post-AGB objects, we compute mean colors for the population of dustenshrouded Galactic TP-AGB C stars of K − X C = 5.66 ± 1.16, 6.41 ± 1.15, 6.31 ± 1.06, and 6.87 ± 0.86 mag for the four bandpasses, respectively (Vega magnitudes; zeropoints from Guandalini et al 2006). We do not claim that all stars with sufficient mass have these colors upon joining the TP-AGB, but that they are dusty and IR luminous, such that when averaged over timescales longer than the TP-AGB lifetimes (10 6 yrs; e.g.…”

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

“…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].…”