Hipparcosand MAP Studies of the Triple Star π Cephei

Abstract

We reexamine the visual and radial velocity data for the π Cephei triple star system and derive the masses of the three components. Our study is based upon the analysis of Hipparcos Intermediate Astrometric Data (IAD) measurements obtained with the red light Thaw refractor's Multichannel Astrometric Photometer (Thaw/MAP) and positions obtained from photographic plates acquired with the Thaw refractor's original photographic objective. The weighted mean parallax of the star system is now 13.8 ± 0.41 mas, corresponding to a distance modulus of 4.30 ± 0.065, yielding absolute visual magnitudes of 0.24 ± 0.065 and 2.50 ± 0.070 for the A and B components, respectively, and a total system mass of 8.81 ± 0.87 M☉. The 58 yr span of the plate collection makes possible the first detection of the photocentric motion caused by the 160 yr orbit of π Cep A/B yielding masses of 6.88 ± 0.69 M☉ and 1.93 ± 0.23 M☉, respectively, for the central spectroscopic giant binary star and the late A secondary component. Although of shorter time span, the higher precision of the IAD and Thaw/MAP data allow the first detection of the astrometric motion caused by the spectroscopic companion of the A component. A comparison of the astrometric, spectroscopic, and photometric studies of the π Cep suggests that the spectroscopic binary is composed of a pair of red giants with similar masses, Aa = 3.63 ± 0.53 M☉, Ab = 3.27 ± 0.48 M☉, and a red magnitude difference of approximately 1.7 mag. The importance of long-term astrometric coverage is pointed out by the fact that the motion of both the A/B system and Aa/Ab system were missed during the compilation of the Hipparcos catalog and are only revealed in those data after their detection in the MAP/Thaw measurements.