标题： 高分辨率HST/ACS拍摄的碳星周围的孤立壳层 显示英文标题

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

摘要： 目标：分离的壳层携带了其形成过程的信息，以及由于视线方向上无显著混淆，使其能够提供AGB恒星周围小尺度结构的信息。 方法：在这里，利用哈勃空间望远镜上的高级巡天相机，对最年轻的分离壳层，即围绕碳星R Scl和U Cam的壳层进行了详细研究。 定量结果是假设尘埃散射为光学薄的情况下得出的。 结果：发现R Scl和U Cam周围的分离尘埃壳层与整体球面对称性一致。 它们的半径分别为19.2"（对应线性尺寸为8x10^16 cm）和7.7"（5x10^16 cm），宽度分别为1.2"（5x10^15 cm）和0.6"（4x10^15 cm），尘埃质量分别为3x10^-6和3x10^-7 M(Sun)。 R Scl和U Cam壳层的动力学年龄分别估计为1700年和700年，壳层宽度分别对应100年和50年的时标。 在R Scl壳层的图像中，明显可见小于角秒级的结块状小尺度结构。 平均结块尘埃质量估计约为2x10^-9 M(Sun)。 与CO线干涉仪数据的比较显示，在误差范围内（U Cam的误差为<=1"，R Scl的误差约为2"），尘埃和气体壳层在空间上是一致的。 结论：这些结果与几何上薄的气体和尘埃壳层的解释一致，这些壳层是由氦壳闪光期间的质量损失爆发形成的，并且与先前风的相互作用也起了一定作用。 R Scl壳层中存在结块结构，可能是质量损失本身的结果，但更可能是扩展壳层中不稳定性导致的结果。 显示英文摘要

摘要： 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 8x10^16 cm) 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 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 2x10^-9 M(Sun). Comparisons with CO line interferometer data show that the dust and gas shells coincide spatially, within the errors (<=1" for U Cam and approx. 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. 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.