Ultraviolet Eclipse Observations and Fundamental Parameters of the Binary HR 2554 (G6 II+A1 V)

Abstract

HR 2554 is an eclipsing binary system that contains a G6 II primary star and an A dwarf secondary. We have obtained ultraviolet spectra during two eclipses of HR 2554 in 1994 March–April and 1995 May using the HST Goddard High Resolution Spectrograph (GHRS) to study the atmospheric structure of the primary and to better establish the properties of the binary system. We have determined accurate orbital elements for both components, using new and existing optical spectra for the primary and the GHRS spectra for the secondary, in conjunction with eclipse photometry presented in 1992 by Schroder & Hunsch. The derived orbital period is 195.258 ± 0.003 days, the derived radii are 31.3 ± 0.9 R⊙ and 1.9 ± 0.1 R⊙, and the derived masses are 3.14 ± 0.17 M⊙ and 1.98 ± 0.11 M⊙, respectively. The resulting positions of HR 2554 in the Hertzsprung-Russell (H-R) diagram are somewhat inconsistent with the independent evolution of the component stars; in particular, the G star primary is both overluminous and too blue for its mass. The difficulties might be overcome if the metallicity of the system were unusually low ([Fe/H] ~ -1), but this possibility appears to be ruled out by the [Fe/H] = -0.04 determination of Eggen. We suspect HR 2554 has undergone a more complicated evolutionary history involving mass transfer than has been considered previously.