2007AJ....133.2310B

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 rate M{dot}, 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 and M{dot} 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 8-20 µm 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.