Astrometry with the Hubble Space Telescope: Improved Trigonometric Parallaxes of Selected Hyads

We present absolute parallaxes and proper motions for seven members of the Hyades open cluster, pre-selected to lie in the core of the cluster. We obtain Hyad parallaxes with archival astrometric data from Fine Guidance Sensor (FGS) 3, combined with newer data for 3 Hyads from FGS 1R, both white-light interferometers on the Hubble Space Telescope. Proper motions, spectral classifications and VJHK photometry of the stars comprising the astrometric reference frames provide spectrophotometric estimates of reference star absolute parallaxes. Introducing these into our model as observations with error, we determine absolute parallaxes for each Hyad. Compared to our original (van Altena et al. 1997) determinations, our combination of new data and improved analysis lowered the individual parallax errors by an average factor of 2.9. Comparing with the four stars contained in the Hipparcos catalog, we obtain an average factor of seven improvement. Our parallax values now more closely agree with Hipparcos for the four stars in common. Additionally, we measure a perturbation orbit for vA 627, previously identified as a spectroscopic binary. Incorporating a perturbation in the modeling improves the parallax accuracy for this Hyad.

We gratefully acknowledge support from NASA, through the Space Telescope Science Institute, grants AR-11746 and GO-11942.