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An Ultraviolet Study of the Short-Period Binary OO Aquilae
OO Aql is a rare W UMa-type eclipsing binary in which the two solar-typestars may have only recently evolved into contact. The binary has anunusually high mass ratio (0.84) and a relatively long orbital period(0.506 days) for its spectral type (mid-G). Twelve ultraviolet spectraof OO Aql were obtained in 1988 with the International UltravioletExplorer satellite, including a series of consecutive observations thatcover nearly a complete orbital cycle. Chromospheric activity is studiedby means of the Mg II h and k emission at 2800 Å. The Mg IIemission is found to vary, even when the emission is normalized to theadjacent continuum flux. This variation may be correlated with orbitalphase in the 1988 observations. It also appears that the normalized MgII emission varies with time, as seen in spectra obtained at twodifferent epochs in 1988 and when compared with two spectra obtainedseveral years earlier. The level of chromospheric activity in OO Aql isless than that of other W UMa-type binaries of similar colors, but thisis attributed to its early stage of contact binary evolution.Ultraviolet light curves were composed from measurements of theultraviolet continuum in the spectra. These were analyzed, along withvisible light curves of OO Aql, to determine the system parameters. Thelarge wavelength range in the light curves enabled a well-constrainedfit to a cool spot in the system.
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