We have successfully determined positions, relative to Sgr A*, for 2 SiO masers that are associated with AGB stars. The actual measurement is difficult because of the very rapid phase fluctuations at 43 GHz, and to a lesser extent to the variability and low quality a-priori positions of the masers. However, we have demonstrated that it is possible to obtain the positions accurately with VLBI observations, enabling future proper motion measurements. The positional errors remain within the intrinsic positional uncertainty of the location of the SiO maser in the circumstellar shell: 1 mas.
For future measurements, the Fast-Switching VLA mode will improve the sensitivity as well as reducing the phase errors compared to our experiments. However, observations need to be done under very good conditions for observing at these high frequencies and the phased VLA is essential to obtain sufficient sensitivity and short interferometer spacings. It would be advantageous to use higher bandwidth recording in the future.
© European Southern Observatory (ESO) 1998
Online publication: October 22, 1998