Detection of a Hot Subdwarf Companion to the Be Star FY Canis Majoris

摘要

The rapid rotation of Be stars may be caused in some cases by past mass andangular momentum accretion in an interacting binary in which the mass donor iscurrently viewed as a small, hot subdwarf stripped of its outer envelope. Herewe report on the spectroscopic detection of such a subdwarf in the Be binarysystem FY Canis Majoris from the analysis of data acquired by the IUEspacecraft and KPNO Coude Feed Telescope over the course of 16 and 21 years,respectively. We present a double-lined spectroscopic orbit for the binarybased upon radial velocities from the IUE spectra and use the orbital solutionswith a Doppler tomography algorithm to reconstruct the components' UV spectra.The subdwarf is hot (T_eff = 45+/-5 kK) and has a mass of about 1.3 M_sun and aradius of about 0.6 R_sun. It contributes about 4% as much flux as the Be stardoes in the FUV. We also present observations of the H-alpha and He I 6678emission features that are formed in the circumstellar disk of the Be star.Orbital flux and velocity variations in the He I 6678 profile indicate thatmuch of the emission forms along the disk rim facing the hot subdwarf where thedisk is probably heated by the incident radiation from the subdwarf. A study ofthe FUV infall shell lines discovered in the 1980s confirms their episodicpresence but reveals that they tend to be found around both quadrature phases,unlike the pattern in Algol binaries. Phase-dependent variations in the UV N Vdoublet suggest the presence of a N-enhanced wind from the subdwarf and apossible shock-interaction region between the stars where the subdwarf's windcollides with the disk of the Be star.