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

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 10(3) to 10(4) 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 (corresponding to a linear size of 8 x 10(16) cm) and 7 ''.7 (5 x 10(16) cm), widths of 1 ''.2 (5 x 10(15) cm) and 0 ''.6 (4 x 10(15) cm), and dust masses of 3 x 10(-6) and 3 x 10(-7) M(circle dot), 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 x 10(-9) M(circle dot). Comparisons with CO line interferometer data show that the dust and gas shells coincide spatially, within the errors (<= 1 '' for U Cam and approximate to 2 '' for R Scl). Conclusions. The results are consistent with the interpretation of geometrically thin gas and dust shells formed by amass-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.