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 \to 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 ^ { -1 } indicates that it arises from an almost fully accelerated shell .
Monitoring observations show time variability of the SiS ( 1 \to 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 \hbox to 0.0 pt { . } ^ { \prime \prime } 5 , corresponding to < 3 \times 10 ^ { 15 } cm ) , and the other extends out to a radius > 11 \arcsec .
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 \to 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 \to 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 .