Session 12 - Binary Stars.
Display session, Monday, June 08
Atlas Ballroom,

## [12.11] Attempted Resolution of the HD 173764 Binary System

S. B. Parsons (CSC/STScI), O. G. Franz, L. H. Wasserman (Lowell Obs.)

TRANS and POS mode FGS observations of the G4 II star \beta Scuti (HD 173764) are being made with HST to resolve the secondary stellar component and infer the orbital inclination, distance, and masses. Since G-type bright giants and supergiants are relatively rare, the primary's actual mass and luminosity are especially important for comparison with theory for rapid post main sequence evolution. Of several such spectroscopic binaries not yet resolved by speckle or other techniques, \beta Scuti with a spectroscopic period of 832.5 days and eccentricity of 0.37 offered the best hope of resolution on a time scale comparable to HST observing cycles. The HD 173764 components are sufficiently separated physically to avoid the complications of mass exchange. From analysis of IUE observations and the total UV-optical energy distribution, the secondary has a spectral class of B9 and a difference in visual luminosity of about 3 mag. Isochrone fitting using this delta V and temperatures from the spectral types yields 23 \pm 3 mas for the anticipated projected major axis of the system. The TRANS mode data from our GO proposals 6495 and 7486 demonstrate duplicity in the signal but fail to provide actual separations and position angles even near apastron. Analysis of the scans is compromised by irregular temporal changes in FGS3 behavior. The anticipated separation of 32 mas near apastron should be recoverable even at the 3 mag visual contrast; instead this separation can be no more than 22 mas. This is consistent however with the Hipparcos parallax of 4.7 mas relative to the expected 7 mas from isochrone fitting. Thus \beta Scuti is more luminous than expected, by an amount which exceeds the preliminary error analysis in using isochrones from 1993 Geneva models. Analysis of POS mode data is in progress and should at least reveal the orientation of the orbit on the sky. It is hoped that one remaining observation can be taken with the superior FGS1R instrument near the next apastron and provide one good separation measurement.