Annual parallax measurement of extreme OH/IR candidate star OH39.7+1.5

Abstract

OH/IR stars are low- to intermediate-mass stars (about 0.5-8 solar masses) in the AGB phase, and are considered to be in the process of evolving from Mira variables to planetary nebulae. We aim to understand the evolutionary stages of these AGB stars by approaching them from astrometric VLBI observations with VERA. In 2017, we have started VLBI observations of several OH/IR stars including OH39.7+1.5 presented in this poster. We observed the H₂O maser of OH39.7+1.5 and obtained an annual parallax of 0.55 ± 0.03 mas (distance D = 1.81 ± 0.12 kpc). Using this annual parallax, we revealed distribution (about 35 au square), internal motions, and expansion velocities (average about 15.4 ± 5.1 km s^-1) of the maser spots. We compared expansion velocity of H₂O maser with that of OH maser and found to be consistent across the error range. This suggests that the radial velocity of H₂O gas around OH39.7+1.5 has reached a terminal velocity. OH39.7+1.5 has not data on annual parallax or proper motion in Gaia DR3, and this is the first time that annual parallax has been measured.