Dust temperature and density profiles in the envelopes of AGB and post-AGB carbon stars from mid-infrared observations^*

¹ Dpt Cassiopée, CNRS-UMR 6202, Observatoire de la Côte d'Azur, BP 4229, 06304 Nice Cedex 4, France e-mail: lagadec@obs-nice.fr
² Dpt Artémis, CNRS-UMR 6162, Observatoire de la Côte d'Azur, BP 4229, 06304 Nice Cedex 4, France
³ Laboratoire d'Astrophysique, Observatoire de Grenoble, 414 rue de la Piscine, 38400 Saint-Martin d'Hyères, France

Received: 29 June 2004
Accepted: 18 November 2004

Abstract

First mid-infrared images of a sample of AGB and post-AGB carbon stars (V Hya, IRC +10216, CIT 6 and Roberts 22) obtained at the ESO-La Silla Observatory (Chile) are reported. CIT 6 presents a cometary-like feature clearly seen in the 9.7 μm image, Roberts 22 shows an envelope slightly elongated in the north-east direction while images of V Hya and IRC +10216 are roughly spherically symmetric. Using an inversion technique, the dust emissivity was derived from the observed intensity profiles, allowing a determination of the grain temperature and density distributions inside the envelope for these stars. Dust masses and mass-loss rates were estimated for V Hya and IRC +10216. Our results are comparable to those obtained in previous studies if dust grains have dimensions in the range ~0.01–0.2 μm. Color maps suggest the presence of temperature inhomogeneities in the central regions of the dust envelopes. In the case of V Hya, an eccentric hot point, whose direction coincides with the jet previously seen in [S II] emission, suggests that we are observing material ejected in a previous mass-loss event. Bipolar lobes are clearly seen in the color maps of Roberts 22 and IRC +10216.