Snapshot VLBI observations of SiO v = 1 and v = 2 (J = 1-0) maser sources

Abstract

We have made snapshot VLBI observations of SiO masers of the J = 1-0 transition at two vibrational levels v = 1 and v = 2 using the Kashima-Nobeyama Interferometer (KNIFE). Out of 37 sources observed, we detected both of the lines from 16 sources with a typical detection limit of 10 Jy. Most of the SiO masers were significantly spatially resolved with a KNIFE baseline (200 km), indicating extended/complicated structures of the SiO masers with a typical angular size of a few milliarcseconds. Similar fringe-phase profiles along a radial velocity spanning larger than 1.0 km s-1 between the two lines indicate a similar spatial structure along the velocity spacings. This tendency, shown from 13 SiO maser sources except Orion KL, supports spatial coincidence of the two lines found previously for M-type stars VY CMa and W Hya. The spatial coincidence should enable us to measure group-delays of the SiO masers for stellar astrometry.