An Ultraviolet Study of the Short-Period Binary OO Aquilae
February 2001, AJ, 121, 1084
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B.J. Hrivnak, E.F. Guinan, L.E. DeWarf & I. Ribas
Abstract:
OO Aql is a rare W UMa-type eclipsing binary in which the two
solar-type stars may have only recently evolved into contact.
The binary has an unusually high mass ratio (0.84) and a
relatively long orbital period (0.506 days) for its spectral
type (mid-G). Twelve ultraviolet spectra of OO Aql
were obtained in 1988 with the International Ultraviolet
Explorer satellite, including a series of consecutive
observations that cover nearly a complete orbital cycle.
Chromospheric activity is studied by means of the Mg II
h and k emission at 2800 Å. the Mg II
emission is found to vary, even where the emission is
normalized to the adjacent continuum flux. This variation
may be correlated with orbital phase in the 1988 observations.
It also appears that the normalized Mg II emission varies with
time, as seen in spectra obtained at two different epochs in
1988 and when compared with two spectra obtained several years
earlier. The level of chromospheric activity in OO Aql is
less than that of other W UMa-type binaries of similar colors,
but this is attributed to its early stage of contact binary
evolution. Ultraviolet light curves were composed from
measurements of its ultraviolet continuum in the spectra.
These were analyzed, along with visible light curves of OO
Aql, to determine the system parameters. the large wavelength
range in the light curves enabled a well-constrained fit to a
cool spot in the system.