Astrometry of SiO Masers: First detections of circumstellar SiO maser emission at λ = 7 mm using three European VLBI stations

Abstract

The positions of circumstellar molecular masers relative to each other, to the central star, and to the compact extragalactic sources are of astrophysical and astrometrical interest. The SiO masers at λ = 7 mm are especially interesting as the maser spots are located very close to the photosphere of the star. For astrometry we would like to see a proper distribution of the maser spots and several epochs of observations in order to estimate the position of the central star and eventual motions of the spots.The optical position of this star can be determined with ground-based astrometric facilities, and many of the stars showing SiO emission are in theHipparcoscatalogue. A link between the Optical and Radio Reference Frames is achieved in a second step relating the maser spot positions to the Extragalactic Reference Frame (EGRF) established with VLBI (Baudryet al., 1984).