# SiS in the circumstellar envelope of IRC +10126: maser and quasi-thermal emission

## TL;DR

This study confirms SiS maser action in IRC +10216's circumstellar envelope, revealing detailed spatial and temporal characteristics of both maser and quasi-thermal emissions, and proposes infrared pumping as the excitation mechanism.

## Contribution

First unambiguous confirmation of SiS (1-0) maser in IRC +10216 with high-resolution imaging and analysis of its variability and spatial structure.

## Key findings

- SiS (1-0) maser is unsaturated and variable over time.
- Quasi-thermal SiS emission shows two distinct spatial components.
- Infrared pumping likely excites the SiS maser.

## Abstract

We present new Effelsberg-100 m, ATCA, and VLA observations of rotational SiS transitions in the circumstellar envelope (CSE) of IRC +10216. Thanks to the high angular resolution achieved by the ATCA observations, we unambiguously confirm that the molecule's J=1-0 transition exhibits maser action in this CSE, as first suggested more than thirty years ago. The maser emission's radial velocity peaking at a local standard of rest velocity of -39.862$\pm$0.065 km/s indicates that it arises from an almost fully accelerated shell. Monitoring observations show time variability of the SiS (1-0) maser. The two lowest-$J$ SiS quasi-thermal emission lines trace a much more extended emitting region than previous high-J SiS observations. Their distributions show that the SiS quasi-thermal emission consists of two components: one is very compact (radius<1.5", corresponding to <3$\times 10^{15}$ cm), and the other extends out to a radius >11". An incomplete shell-like structure is found in the north-east, which is indicative of existing SiS shells. Clumpy structures are also revealed in this CSE. The gain of the SiS (1-0) maser (optical depths of about -5 at the blue-shifted side and, assuming inversion throughout the entire line's velocity range, about -2 at the red-shifted side) suggests that it is unsaturated. The SiS (1-0) maser can be explained in terms of ro-vibrational excitation caused by infrared pumping, and we propose that infrared continuum emission is the main pumping source.