Tomographic Separation of Composite Spectra. III. Ultraviolet Detection of the Hot Companion of phi Persei

Abstract

We have used archival lUE high-dispersion UV spectra of the Be binary φ Per and published spectroscopic radial velocity curves to reconstruct the individual primary and secondary spectra using Doppler tomography. The primary's spectrum has rotationally broadened photospheric lines (consistent with a spectral type B0.5 III-Ve) and narrow "shell" lines formed in its circumstellar disk. The recovered secondary spectrum (which contributes only ≈12% of the UV flux) has a very different appearance, with strong emission in C IV λ1550 and many narrow, weak absorption lines (mainly Fe v) similar to those found in the spectra of hot 0-type subdwarfs (in particular the sdO6 star HD 49798). These results strongly support Poeckert's (1981) model in which the secondary is the stripped-down core of a once massive star. Such objects could be the progenitors of unusual supernovae.