The Fine Guidance Sensor Orbit of the G4 Bright Giant HD 173764

Abstract

TRANS and POS mode observations of the G4 IIa star β Scuti (HD 173764) have been made with a Hubble Space Telescope (HST) Fine Guidance Sensor (FGS3). This spectroscopic binary with a period of 833 days and an eccentricity of 0.36 offered hope of resolving the secondary stellar component and inferring the orbital inclination, distance, and masses, thus contributing to comparisons with theory for rapid post–main-sequence evolution. 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 ~3 mag. Isochrone fitting using this ΔV and temperatures from the spectral types yields ~26 mas for the anticipated projected separation at apastron. The TRANS mode data demonstrate duplicity in the signal but fail to provide reliable separations and position angles even near apastron. This separation is instead no more than 22 mas. However, the parallax is probably smaller, by a similar factor, than that expected from initial isochrone fitting, and β Sct is more luminous than originally estimated. Analysis of POS mode data reveals the orbital inclination and the orientation on the sky for a semimajor axis of only 5.7 ± 0.4 mas for the primary's orbit around the center of mass. The parallax from POS mode observations is quite small, only consistent with isochrones at the 2 σ level. From all of the evidence, the system lies at 210 ± 23 pc and the evolved G-type primary has Mv -2.9.