Mid Infrared Photometry of Mass-Losing AGB Stars

Abstract

We present ground-based mid-infrared 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, ISO, MSX and 2MASS catalogues. Our goal is to establish relations between the IR colors, the effective temperature $T_{eff}$, the luminosity $L$ and the mass loss rate $\dot{M}$, for improving the effectiveness of AGB modelling. 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: i) Values of $L$ and $\dot{M}$ for C stars fit relations previously established by us, with Miras being on average more evolved and mass losing than Semiregulars. ii) 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. iii) A larger reddening characterizes C-rich Miras and post-AGBs. 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. iv) The presence of a long-term variability at mid-IR wavelengths seems to be limited to sources with maximum emission in the 8 -- 20 $\mu$m region, usually Mira variables (1/3 of our sample). Most Semiregular and post-AGB stars studied here remained remarkably constant in mid-IR over the last twenty years.

Accepted for publication in the Astronomical Journal - 35 pages (in preprint), 9 figures, 5 tables

References in corpus (5)

• Near-Infrared Photometry of Carbon Stars
• A Spitzer mid-infrared spectral survey of mass-losing carbon stars in the Large Magellanic Cloud
• Technetium and the third dredge up in AGB stars II. Bulge stars
• Carbon-Rich Mira Variables: Kinematics and Absolute Magnitudes
• Carbon-Rich Mira Variables: Radial Velocities and Distances

Cited by in corpus (3)

• The Mass-Loss Return From Evolved Stars to the LMC: Empirical Relations for Excess Emission at 8 and 24 μm
• The Mass-Loss Return from Evolved Stars to the Large Magellanic Cloud IV: Construction and Validation of a Grid of Models for Oxygen-Rich AGB Stars, Red Supergiants, and Extreme AGB Stars
• The impact of the 18F(a,p)21Ne reaction on asymptotic giant branch nucleosynthesis