ASTROMETRICALLY REGISTERED SIMULTANEOUS OBSERVATIONS OF THE 22 GHz H2O AND 43 GHz SiO MASERS TOWARD R LEONIS MINORIS USING KVN AND SOURCE/FREQUENCY PHASE REFERENCING

摘要

Oxygen-rich asymptotic giant branch (AGB) stars can be intense emitters of SiO (v =1 and 2, J=1→0) and H_2O maser lines at 43 and 22 GHz, respectively. Very long baseline interferometry (VLBI) observations of the maser emission provide a unique tool to probe the innermost layers of the circumstellar envelopes in AGB stars. Nevertheless, the difficulties in achieving astrometrically aligned H_2O and v =1 and v =2 SiO maser maps have traditionally limited the physical constraints that can be placed on the SiO maser pumping mechanism. We present phase-referenced simultaneous spectral-line VLBI images for the SiO v =1 and v =2, J=1→0, and H_2O maser emission around the AGB star R LMi, obtained from the Korean VLBI Network (KVN). The simultaneous multichannel receivers of the KVN offer great possibilities for astrometry in the frequency domain.With this facility, we have produced images with bona fide absolute astrometric registration between high-frequency maser transitions of different species to provide the positions of the H_2O maser emission and the center of the SiO maser emission, hence reducing the uncertainty in the proper motions for R LMi by an order of magnitude over that from Hipparcos. This is the first successful demonstration of source frequency phase referencing for millimeter VLBI spectral-line observations and also where the ratio between the frequencies is not an integer.