2017ApJ...843...54G

We present new Effelsberg 100-m, Australia Telescope Compact Array (ATCA), and Very Large Array 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 30 years ago. The maser emission's radial velocity, peaking at a local standard of rest velocity of -39.862 ± 0.065 km s^–1, 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 3×10¹⁵ cm), and the other extends out to a radius >11''. An incomplete shell-like structure is found in the northeast, 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 blueshifted side and, assuming inversion throughout the entire line's velocity range, about -2 at the redshifted 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.