Locating dust and molecules in the inner circumstellar environment of R Sculptoris with MATISSE

Abstract

Context. Asymptotic giant branch (AGB) stars are one of the main sources of dust production in the Galaxy. However, it is not yet clear what this process looks like and where the dust happens to be condensing in the circumstellar environment. Aims. By characterizing the location of the dust and the molecules in the close environment of an AGB star, we aim to achieve a better understanding the history of the dust formation process. Methods. We observed the carbon star R Scl with the thermal-infrared VLTI/MATISSE instrument in L-and N-bands. The high angular resolution of the VLTI observations (as small as 4.4 mas in the L-band and 15 mas in the N-band with ATs), combined with a large uv-plane coverage allowed us to use image reconstruction methods. To constrain the dust and molecules' location, we used two different methods: one using MIRA image reconstruction algorithm and the second using the 1D code RHAPSODY. Results. We found evidence of C 2 H 2 and HCN molecules between 1 and 3.4 R ⋆ which is much closer to the star than the location of the dust (between 3.8 and 17.0 R ⋆ ). We also estimated a mass-loss rate of 1.2 ± 0.4 × 10 −6 M ⊙ yr −1 . In the meantime, we confirmed the previously published characteristics of a thin dust shell, composed of amorphous carbon (amC) and silicon carbide (SiC). However, no clear SiC feature has been detected in the MATISSE visibilities. This might be caused by molecular absorption that can affect the shape of the SiC band at 11.3 µm. Conclusions. The appearance of the molecular shells is in good agreement with predictions from dynamical atmosphere models. For the first time, we co-located dust and molecules in the environment of an AGB star. We confirm that the molecules are located closer to the star than the dust. The MIRA images unveil the presence of a clumpy environment in the fuzzy emission region beyond 4.0 R ⋆ . Furthermore, with the available dynamic range and angular resolution, we did not detect the presence of a binary companion. To solve this problem, additional observations combining MATISSE and SAM-VISIR instrument should enable this detection in future studies.

RHAPSODY: Reconstructing Hankel rAdial Profiles in centro-Symmetric Objects with Discrete rings for astrophYsics

“…5, by varying the parameters a k using a gradient descent or any other optimization algorithm (in our case: the Cobyla minimization routine from the Lmfit package † library that can be found in Python). This description of the algorithm corresponds exactly to the version of RHAPSODY the has been been used in 9…”

“…In this paragraph we test the robustness of the RHAPSODY against a realistic model used in our paper on R Scl. 9 We built a toy model mimicking the R Scl environment using DUSTY and a Gaussian layer at 15 mas with an intensity ratio with respect to the central source starting from 1/1000 and reaching up to 1/10 (Fig 8 in appendix). Using ASPRO2, we generate VLTI/MATISSE visibilities, add error noise to the data, and use the same (u,v)plane coverage for each of our different models and for both LM -and N -bands.…”

“…We used RHAPSODY to analyze this dataset. 9 Thanks to this tool, we have discovered interesting structures in the vicinity of the star. As we can see in Fig 6, RHAPSODY has spotted two interesting structures in L-band: one between 5 and 7 mas, above the Rosseland radius (dashed blue line) and one between the two green lines between 12 and 18 mas.…”