Abstract
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 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 ( ) 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 ( , corresponding to 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 ( ) 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 ( ) 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.