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Session 44 - Double Stars.
Display session, Thursday, January 08
Exhibit Hall,

[44.07] Hubble FGS Astrometric Measures of the Halo Population spectroscopic binary HD 20039

W. Osborn (Central Michigan U.), J. Hershey (CSC/STScI)

Until recently no halo population binaries were known. Consequently, no direct mass determinations have been made for this important class of object. High precision radial velocities obtained in the course of recent kinematical studies of halo stars have now identified about one hundred spectroscopic binaries. One of these is the rather bright double-lined system HD 20039. The spectroscopic orbital elements together with its photometric distance indicated that the HST Fine Guidance Sensors might be able to resolve this system. If so, the relative astrometric measures of the components combined with the accurate spectroscopic data would allow the orbit to be completely determined and the individual masses obtained.

We have obtained high signal-to-noise FGS TRANSfer-mode observations of 20039. Pairs of observations were made at six epochs around the spectroscopically predicted times of maximum separation. Three epochs cover about 0.2 in orbital phase around periastron and the other three about 0.2 in phase near apastron. Analysis of the data indicates that the system has been resolved, but just at the FGS resolution threshold. This poor resolution has led to significant uncertainties in the derived separations, position angles, magnitude difference, and astrometric orbit solution. There are also inconsistencies between the astrometric results and the spectroscopic orbit. We suggest that observations with the new, higher resolution FGS1R would provide a more reliable astrometric orbit.

Program listing for Thursday