The detached dust shells around the carbon AGB stars R Scl and V644 Sco

TL;DR

This study uses polarised light observations to analyze the morphology of detached dust shells around two carbon AGB stars, R Scl and V644 Sco, revealing their structure, size, and relation to gas shells, thus informing stellar evolution models.

Contribution

It provides the first direct measurements of the detached dust shell around V644 Sco and detailed observations of R Scl's shell, enhancing understanding of thermal pulse effects.

Findings

Detached shells are spherical with radii of 19.5 and 9.4 arcsec.

Dust and gas shells coincide almost exactly around R Scl.

First direct measurements of V644 Sco's detached shell.

Abstract

Detached shells are believed to be created during a thermal pulse, and constrain the time scales and physical properties of one of the main drivers of late stellar evolution. We aim at determining the morphology of the detached dust shells around the carbon AGB stars R Scl and V644 Sco, and compare this to observations of the detached gas shells. We observe the polarised, dust-scattered stellar light around these stars using the PolCor instrument mounted on the ESO 3.6m telescope. Observations were done with a coronographic mask to block out the direct stellar light. The polarised images clearly show the detached shells. Using a dust radiative transfer code to model the dust-scattered polarised light, we constrain the radii and widths of the shells to 19.5 arcsec and 9.4 arcsec for the detached dust shells around R Scl and V644 Sco, respectively. Both shells have an overall spherical symmetry and widths of approx. 2 arcsec. For R Scl we can compare the observed dust emission directly with high spatial-resolution 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. 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 post-pulse evolution of the star.