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Aims: The detached shells carry information on their formation process, aswell as on the small-scale structure of the circumstellar medium around AGBstars due to the absence of significant line-of-sight confusion. Methods: Theyoungest detached shells, those around the carbon stars R Scl and U Cam, arestudied here in great detail in scattered stellar light with the AdvancedSurvey Camera on the Hubble Space Telescope. Quantitative results are derivedassuming optically thin dust scattering. Results: The detached dust shellsaround R Scl and U Cam are found to be consistent with an overall sphericalsymmetry. They have radii of 19.2" (corresponding to a linear size of 8x10^16cm) and 7.7" (5x10^16 cm), widths of 1.2" (5x10^15 cm) and 0.6" (4x10^15 cm),and dust masses of 3x10^-6 and 3x10^-7 M(Sun), respectively. The dynamical agesof 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-sizedclumps is clearly seen in the images of the R Scl shell. Average clump dustmasses are estimated to be about 2x10^-9 M(Sun). Comparisons with CO lineinterferometer data show that the dust and gas shells coincide spatially,within the errors (<=1" for U Cam and approx. 2" for R Scl). Conclusions: Theresults are consistent with the interpretation of geometrically thin gas anddust shells formed by a mass-loss eruption during a He-shell flash, and whereinteraction with a previous wind plays a role as well. Clumpy structure ispresent 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.

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