2018ApJ...852...27H

The asymptotic giant branch (AGB) star R Sculptoris (R Scl) is one of the most extensively studied stars on the AGB. R Scl is a carbon star with a massive circumstellar shell (M_shell ∼ 7.3 x 10^–3 M_☉) that is thought to have been produced during a thermal pulse event ∼2200 years ago. To study the thermal dust emission associated with its circumstellar material, observations were taken with the Faint Object InfraRed CAMera for the SOFIA Telescope (FORCAST) at 19.7, 25.2, 31.5, 34.8, and 37.1 µm. Maps of the infrared emission at these wavelengths were used to study the morphology and temperature structure of the spatially extended dust emission. Using the radiative-transfer code DUSTY, and fitting the spatial profile of the emission, we find that a geometrically thin dust shell cannot reproduce the observed spatially resolved emission. Instead, a second dust component in addition to the shell is needed to reproduce the observed emission. This component, which lies interior to the dust shell, traces the circumstellar envelope of R Scl. It is best fit by a density profile with n ∝ r^α, where α=0.75_–0.25^+0.45 and a dust mass of M_d=9.0_–4.1^+2.3×10^–6 M_☉. The strong departure from an r^–2 law indicates that the mass-loss rate of R Scl has not been constant. This result is consistent with a slow decline in the post-pulse mass loss that has been inferred from observations of the molecular gas.