Astrometric-Spectroscopic Determination of the Absolute Masses of the HgMn Binary Star φ Herculis

Abstract

The mercury-manganese star φ Her is a well-known spectroscopic binary that has been the subject of a recent study by Zavala and coworkers in which they resolved the companion using long-baseline interferometry. The total mass of the binary is now fairly well established, but the combination of spectroscopy with astrometry has not resulted in individual masses consistent with the spectral types of the components. The motion of the center of light of φ Her was clearly detected by the Hipparcos satellite. Here we make use of the Hipparcos intermediate data (``abscissa residuals'') and show that by combining them in an optimal fashion with the interferometry the individual masses can be obtained reliably using only astrometry. We reexamine and then incorporate existing radial velocity measurements into the orbital solution, obtaining improved masses of 3.05+/-0.24 and 1.614+/-0.066 M_solar that are consistent with the theoretical mass-luminosity relation from recent stellar evolution models. These mass determinations provide important information for the understanding of the nature of this peculiar class of stars.