Observing and modeling the dynamic atmosphere of the low mass-loss C-star R Sculptoris at high angular resolution

TL;DR

This study combines high-resolution infrared observations with dynamic modeling to explore the close circumstellar environment of the carbon-rich star R Sculptoris, revealing insights into the processes initiating mass loss in AGB stars.

Contribution

It provides the first detailed comparison of high-resolution interferometric data with a self-consistent dynamic model for R Scl, highlighting the role of dust formation and molecular structures.

Findings

Model agrees well with spectrophotometric data from 0.4 to 25 μm.

Interferometric visibilities match the model in the K-band.

Discrepancies suggest the importance of non-equilibrium dust formation processes.

Abstract

We study the circumstellar environment of the carbon-rich star R Scl using the near- and mid-infrared high spatial resolution observations from the ESO-VLTI instruments VINCI and MIDI. These observations aim at increasing our knowledge of the dynamic processes in play within the very close circumstellar environment where the mass loss of AGB stars is initiated. Data are interpreted using a self-consistent dynamic model. Interferometric observations do not show any significant variability effect at the 16 m baseline between phases 0.17 and 0.23 in the K band, and for both the 15 m baseline between phases 0.66 and 0.97 and the 31 m baseline between phases 0.90 and 0.97 in the N band. We find fairly good agreement between the dynamic model and the spectrophotometric data from 0.4 to 25 $\mu$m. The model agrees well with the time-dependent flux data at 8.5 $\mu$m, whereas it is too faint at 11.3 and 12.5 $\mu$m. The VINCI visibilities are reproduced well, meaning that the extension of the model is suitable in the K-band. In the mid-infrared, the model has the proper extension to reveal molecular structures of C2H2 and HCN located above the stellar photosphere. However, the windless model used is not able to reproduce the more extended and dense dusty environment. Among the different explanations for the discrepancy between the model and the measurements, the strong nonequilibrium process of dust formation is one of the most probable. The complete dynamic coupling of gas and dust and the approximation of grain opacities with the small-particle limit in the dynamic calculation could also contribute to the difference between the model and the data.