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A. Wootten (NRAO), M. Claussen (NRAO), K.B. Marvel (AAS), B. Wilking (U. of Missouri-St. Louis)
Water masers trace gas motions in astrophysical flows; their tremendous brightness temperatures make them perfect targets for very long baseline radio interferometry.
Long term monitoring ( Claussen, et al. 1996 ApJS, 106, 111) suggests they have lifetimes sufficiently long for proper motion measurements if the observations are spaced on one month scales.
Space velocities (> 50 km s-1) in and along outflows are sufficient to produce measurable motion (~ 1 AU) over a month or so, easily resolved by the VLBA's 0.1 AU beam in the closest regions (d~200pc). These expectations are borne out by our observations of IRAS05413-0104, where masers show expansion at 64 ± 27 km s-1 (Claussen et al. 1998 ApJ 507, L79). We present multi-epoch (monthly) VLBA images of the water masers in the nearest known rich maser source, IRAS16293-2422 (IR16293 hereafter).
The masers, in addition to being powerful enough to map with the VLBA, reveal the kinematics of the postshock region on sub-AU scales. The maser which is strongest at VLSR ~4 km s-1 (the velocity of ambient material in the core) lies coincident with the strongest centimeter continuum component of the flow. The maser structure of the 4 km s-1 IR16293 masers, defines an ellipse with a diameter of roughly one astronomical unit, with about the same geometry as is seen on scales up to one thousand times larger. This structure may define the locus of a shock within a disk around the young star. The maser radial velocities are an order of magnitude below the expected orbital speed for material at 1 AU from a 1 M\odot \ star (see Mundy et al. 1992 ApJ 352, 159), such as IR16293 is currently believed to be. Hence if they represent the velocity of ambient material, that material will accrete onto the central star. The velocities might also reflect a gentle expansion of the flow into the inner region of the disk, or beaming effects that mask the true kinematics in the region.
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