Abstract
Massive stars profoundly influence the evolution
of the Universe, from galactic dynamics and structure to
star formation. They are often found with bound companions.
However, our knowledge of O-type multiple systems with
periods in the range from years to thousands of years is
incomplete due their great distances. We present results
from a high angular resolution survey to find such angularly
resolved companions using the Fine Guidance Sensor on the
Hubble Space Telescope and using ground-based adaptive
optics at Gemini North. We observed 75 O- and early B-type
stars in Cyg OB2 and determined that 42% of the sample have
at least one companion that meets a statistical criterion
for gravitationally bound status. As a case study, we
present an examination of high resolution, ultraviolet
spectroscopy from Hubble Space Telescope of the photospheric
spectrum of the O-supergiant in the massive X-ray binary HD
226868 = Cyg X-1. We analyzed this and ground-based optical
spectra to determine the effective temperature and gravity
of the O9.7 Iab supergiant. Using non-LTE, line blanketed,
plane parallel models from the TLUSTY grid, we obtain Teff =
28.0 ±2.5 kK and log g 3.00 ± 0.25, both lower than in
previous studies. The optical spectrum is best fit with
models that have enriched He and N abundances. We fit the
model spectral energy distribution for this temperature and
gravity to the UV, optical, and IR fluxes to determine the
angular size of and extinction towards the binary. The
angular size then yields relations for the stellar radius
and luminosity as a function of distance. B assuming that
the supergiant rotates synchronously with the orbit, we can
use the radius -- distance relation to find mass estimates
for both the supergiant and black hole as a function of the
distance and the ratio of stellar to Roche radius. Fits of
the orbital light curve yield an additional constraint that
limits the solutions in the mass plane. Our results indicate
masses of 23(+8/-6)
solar masses for the supergiant and 11(+5/-3) solar masses
for the black hole.The results of this survey provide
fundamental information on the impact of environment on
massive binaries and also the role multiplicity has on
massive star formation and evolution.