IRC+10216 mass loss properties through the study of $\lambda$3mm emission: Large spatial scale distribution of SiO, SiS, and CS

TL;DR

This study uses high-resolution ALMA and IRAM-30m data to analyze the spatial distribution and abundances of SiO, SiS, and CS in IRC+10216's circumstellar envelope, revealing complex, clumpy structures and episodic mass loss.

Contribution

It provides detailed spatial mapping and chemical modeling of molecular emissions in IRC+10216, highlighting the non-homogeneous, episodic nature of its mass loss process.

Findings

Extended emission shells for SiO, SiS, and CS

Fractional abundances estimated as ~10^{-7} for SiO and ~10^{-6} for SiS and CS

Evidence of clumpy, non-spherical wind structures and variable mass loss

Abstract

The study of the gas in the envelopes surrounding asymptotic giant branch (AGB) stars through observations in the millimetre wavelength range provides information about the history and nature of these molecular factories. Here we present ALMA observations at subarsecond resolution, complemented with IRAM-30m data, of several lines of SiO, SiS, and CS towards the best-studied AGB circumstellar envelope, IRC+10216. We aim to characterise their spatial distribution and determine their fractional abundances mainly through radiative transfer and chemical modelling. The three species display extended emission with several enhanced emission shells. CS displays the most extended distribution reaching distances up to approximately 20''. SiS and SiO emission have similar sizes of approximately 11'', but SiS emission is slightly more compact. We have estimated fractional abundances relative to H$_2$, which on average are equal to f(SiO)$\sim$10$^{-7}$, f(SiS)$\sim$10$^{-6}$, and f(CS)$\sim$10$^{-6}$ up to the photo-dissociation region. The observations and analysis presented here show evidence that the circumstellar material displays clear deviations from an homogeneous spherical wind, with clumps and low density shells that may allow UV photons from the interstellar medium (ISM) to penetrate deep into the envelope, shifting the photo-dissociation radius inwards. Our chemical model predicts photo-dissociation radii compatible with those derived from the observations, although it is unable to predict abundance variations from the starting radius of the calculations ($\sim$10$R_{*}$), which may reflect the simplicity of the model. We conclude that the spatial distribution of the gas proves the episodic and variable nature of the mass loss mechanism of IRC+10216, on timescales of hundreds of years.