Detection of a Hot Subdwarf Companion
to the Be Star FY Canis Majoris
Geraldine J. Peters1, Douglas R. Gies2,
Erika D. Grundstrom3, and M. Virginia McSwain4
1
Space Sciences Center and Department
of Physics & Astronomy, University of Southern California,
Los Angeles, CA 90089-1341; gjpeters@mucen.usc.edu
2
Department of Physics and Astronomy, Georgia State University,
P.O. Box 4106, Atlanta, GA 30302-4106; gies@chara.gsu.edu
3
Department of Physics and Astronomy, Vanderbilt University,
Nashville, TN 37206; erika.grundstrom@vanderbilt.edu
4
Department of Physics, Lehigh University,
Bethlehem, PA 18105; mcswain@lehigh.edu
The rapid rotation of Be stars may be caused in some cases by past
mass and angular momentum accretion in an interacting binary in which
the mass donor is currently viewed as a small, hot subdwarf stripped
of its outer envelope. Here we report on the spectroscopic detection
of such a subdwarf in the Be binary system FY Canis Majoris from the
analysis of data acquired by the IUE spacecraft and KPNO
Coudé Feed Telescope over the course of 16 and 21 years,
respectively. We present a double-lined spectroscopic orbit for the
binary based upon radial velocities from the IUE spectra and use
the orbital solutions with a Doppler tomography algorithm to
reconstruct the components' UV spectra. The subdwarf is hot
(Teff = 45±5 kK)
and has a mass of about 1.3 M and a radius
of about 0.6 R. It contributes about 4% as
much flux as the Be star does in the FUV. We also present observations of the
Hα and He I λ 6678 emission
features that are formed in the circumstellar disk of the Be star. Orbital flux and
velocity variations in the He I λ 6678 profile indicate
that much of the emission forms along the disk rim facing the hot
subdwarf where the disk is probably heated by the incident radiation
from the subdwarf. A study of the FUV infall shell lines discovered in
the 1980s confirms their episodic presence 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 V doublet
suggest the presence of a N-enhanced wind from the subdwarf and a
possible shock-interaction region between the stars where the
subdwarf's wind collides with the disk of the Be star.
Accepted by ApJ
Preprints from
gjpeters@mucen.usc.edu
or on the web at
http://arxiv.org/abs/0806.3004
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