The detached dust shells around the carbon AGB stars R Sculptoris and V644 Scorpii

Abstract

Context. The morphology of the circumstellar envelopes (CSE) around asymptotic giant branch (AGB) stars gives information on the mass-loss process from the star, its evolution and wind, and on the effect of binary interaction. However, determining the distribution of dust in the circumstellar envelopes is difficult. Observations of polarised, dust-scattered stellar light in the optical have produced images with high-spatial resolution of the envelopes around evolved stars. For sources with detached shells in particular, this method has proven extremely successful. Detached shells are believed to be created during a thermal pulse, and studying them can constrain the time scales and physical properties of one of the main drivers of late stellar evolution. Aims. We aim at determining the morphology of the detached shells around the carbon AGB stars R Scl and V644 Sco. In particular, we attempt to constrain the radii and widths of the detached dust shells around the stars and compare them to observations of the detached gas shells. Methods. We observed the polarised, dust-scattered stellar light around the carbon AGB stars R Scl and V644 Sco using the PolCor instrument mounted on the ESO 3.6 m telescope. Observations were done with a coronographic mask to block out the direct stellar light. The polarised images clearly show the detached shells around R Scl and V644 Sco. Using a dust radiative transfer code to model the dust-scattered polarised light, we constrained the radii and widths of the shells. Results. We determine radii of 19. 5 and 9. 4 for the detached dust shells around R Scl and V644 Sco, respectively. Both shells have an overall spherical symmetry and widths of ≈2 . For R Scl, we can compare the observed dust emission directly with high spatialresolution maps of CO(3−2) emission from the shell observed with ALMA. We find that the dust and gas coincide almost exactly, indicating a common evolution. The data presented here for R Scl are the most detailed observations of the entire dusty detached shell to date. For V644 Sco, these are the first direct measurements of the detached shell. Also here we find that the dust most likely coincides with the gas shell. Conclusions. The observations are consistent with a scenario where the detached shells are created during a thermal pulse. The determined radii and widths will constrain hydrodynamical models describing the pre-pulse mass loss, the thermal pulse, and postpulse evolution of the star.

The wonderful complexity of the Mira AB system

“…In addition, the PACS image only shows the velocity-integrated information, and detailed structures may not be apparent (cf. PolCor and ALMA images of the spiral and shell around R Scl, Maercker et al 2014). The separation between the dust arcs is about 15 -30 , which is significantly larger than in the inner spiral detected with ALMA.…”

Simplified models of stellar wind anatomy for interpreting high-resolution data

“…Several processes may be responsible for the formation of these structures. Shells and arcs may be caused by temporal variations in the mass-loss rate and/or expansion velocity (Maercker et al 2014, and references therein). Aspherical structures may be the result of non-isotropic mass-loss (e.g.…”

Detection of CI line emission from the detached CO shell of the AGB star R Sculptoris

“…Most notable are the geometrically thin, detached CO and dust shells around carbon-rich AGB stars. Presently there are published data on seven CO shells (Schöier et al 2005) and 17 dust shells of this type (Ramstedt et al 2011;Cox et al 2012;Maercker et al 2014;Mečina et al 2014, and references therein) (not all of the dust shells are clearly of the geometrically thin type). We note here that some stars with prominent dust shells, e.g.…”