High-resolution HST/ACS images of detached shells around carbon stars

Abstract

Context. Overall spherically symmetric, geometrically thin gas and dust shells have been found around a handful of asymptotic giant branch (AGB) carbon stars. Their dynamical ages lie in the range of 103 to 104 years. A tentative explanation for their existence is that they have formed as a consequence of mass-loss-rate modulations during a He-shell flash. Aims: The detached shells carry information on their formation process, as well as on the small-scale structure of the circumstellar medium around AGB stars due to the absence of significant line-of-sight confusion. Methods: The youngest detached shells, those around the carbon stars R Scl and U Cam, are studied here in great detail in scattered stellar light with the Advanced Survey Camera on the Hubble Space Telescope. Quantitative results are derived assuming optically thin dust scattering. Results: The detached dust shells around R Scl and U Cam are found to be consistent with an overall spherical symmetry. They have radii of 19.2 arcsec (corresponding to a linear size of 8 × 1016 cm) and 7.7 arcsec (5 × 1016 cm), widths of 1.2 arcsec (5 × 1015 cm) and 0.6 arcsec (4 × 1015 cm), and dust masses of 3 × 10-6 and 3 × 10-7 M_⊙, respectively. The dynamical ages of the R Scl and U Cam shells are estimated to be 1700 and 700 yr, respectively, and the shell widths correspond to time scales of 100 and 50 yr, respectively. Small-scale structure in the form of less than arcsec-sized clumps is clearly seen in the images of the R Scl shell. Average clump dust masses are estimated to be about 2 × 10-9 M_⊙. Comparisons with CO line interferometer data show that the dust and gas shells coincide spatially, within the errors (≤1´´ for U Cam and ≈2´´ for R Scl). Conclusions: The results are consistent with the interpretation of geometrically thin gas and dust shells formed by a mass-loss eruption during a He-shell flash, and where interaction with a previous wind plays a role as well. The mass loss responsible for the shells must have been remarkably isotropic, and, if wind interaction plays a role, this also applies to the mass loss prior to the eruption. Clumpy structure is present in the R Scl shell, possibly as a consequence of the mass loss itself, but more likely as a consequence of instabilities in the expanding shell. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Insitute, which is operated by the AURA, Inc., under NASA contract NAS5-26555.