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

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.