Simultaneous Observations of H 2 O and SiO Masers toward OH/IR Stars

Abstract

We performed simultaneous observations of SiO v  = 1, 2, 29 SiO v  = 0, J  = 1–0, and H 2 O maser lines toward 252 OH/IR stars using the individual 21 m telescopes of the Korean VLBI Network (KVN). The observations for studying SiO and H 2 O maser properties associated with the different evolutionary stages of OH/IR stars were carried out from 2011 November to 2012 July. Both H 2 O and SiO masers were detected from 50 sources with a detection rate of 20% in one epoch of observation. One-sided SiO maser emissions without H 2 O were detected from 108 sources and H 2 O maser emission was detected from 11 sources, of which the detection rates were 43% and 4%, respectively. The overall detection rate of the SiO maser was 63%, and that of the H 2 O maser was 24%. There were 65 new detections in the SiO maser lines, 22 new detections in the H 2 O maser line, and 4 new detections in the 29 SiO maser line. For the H 2 O and/or SiO maser- detected sources, mutual relations between SiO and H 2 O maser properties (including peak and integrated antenna temperatures, and full widths at zero power, etc.) are investigated based on a statistical analysis. We also investigate these maser properties on an IRAS two-color diagram related to stellar evolutionary sequences. In particular, a large number of SiO v  = 2-only detected sources appear among the SiO-only detected sources compared to those of both H 2 O and SiO maser detected sources and also appear in the later evolutionary stages of asymptotic giant branch AGB stars in the IRAS two-color diagram. These results may be associated with the development of a hot and thick dust envelope at later stages of AGB evolution and the different excitation conditions of SiO v  = 1 and v  = 2 masers. Our observational results will be useful for statistical studies of circumstellar envelopes of OH/IR stars related to their late evolution and future very long baseline interferometry (VLBI) observations.