SiO AND H{sub 2}O MASER SURVEY TOWARD POST-ASYMPTOTIC GIANT BRANCH AND ASYMPTOTIC GIANT BRANCH STARS
We performed simultaneous observations of SiO v = 1, 2, {sup 29}SiO v = 0, J = 1-0 and H{sub 2}O 6{sub 16}-5{sub 23} maser lines toward 143 AGB and 164 post-asymptotic giant branch (AGB) stars in order to investigate how evolutionary characteristics from AGB to post-AGB stars appear in both SiO and H{sub 2}O maser emissions. The observations were carried out from 2011 February to 2012 March using the Korean VLBI Network single-dish telescopes. We have detected SiO and/or H{sub 2}O maser emission from 21 sources out of 164 post-AGB stars including 12 new detections. Of 143 AGB stars, we detected SiO and/or H{sub 2}O maser emission from 44 stars including 24 new detections. SiO v = 2, J = 1-0 maser emission without a SiO v = 1 maser was detected from 7 sources among 14 SiO-detected post-AGB stars, and the intensity of the SiO v = 2, J = 1-0 maser tends to be much stronger than that of SiO v = 1, which is different from those of AGB stars. This may be related to the development of hot dust shells according to the evolutionary processes of post-AGB stars. We also found that both SiO and H{sub 2}O masers were detected in the blue group (LI, or Left of IRAS), while only the H{sub 2}O maser was detected in the red group (RI, or Right of IRAS) for post-AGB stars. These different detection rates between SiO and H{sub 2}O masers may originate from the different abundances of masing molecules in the circumstellar envelope according to the different mass and expansion velocity between LI and RI regions together with their evolutionary stages.