Compact vs. scattered: Suitability of southern 6.7 GHz methanol masers for VLBI astrometry

Abstract The 6.7 GHz methanol maser transition is exclusively associated with young, high-mass stars and represents a potential target for astrometric studies, including accurate determination of their distance through trigonometric parallax measurements. There are more than 1 000 known 6.7 GHz methanol maser sources in the Milky Way; however, not all are suitable targets for astrometric measurements. We have used the Long Baseline Array to observe 187 southern 6.7 GHz methanol masers and identify 69 sources with one or more maser spots that are sufficiently compact and intense to be suitable targets for very long baseline interferometry astrometry with current instruments. Maser compactness appears to be a strong function of Galactic position, with masers that are likely in nearby spiral arms being more compact, while those associated with distant arms or the central Galactic region being less compact – a relationship we associate with scatter broadening. This has implications for astrophysical masers, especially distant ones employed for Galactic astrometry.