On the reflection effect in three sdOB binary stars

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Abstract
Echelle spectroscopy is presented of three binary systems containing hot subdwarf primary stars and low-mass, low-luminosity secondary stars — HW Vir, AA Dor and KV Vel. The semi-amplitudes of the radial-velocity curves of the primary components are determined to an accuracy of 1 – 2 per cent. The secondary component of KV Vel displays a rich set of weak emission lines due to Cii, C iii, C iv, N ii, N iii and O iv, arising from the hemisphere that is irradiated by the hot primary star. For HW Vir and AA Dor, an extensive search, via proven cross-correlation procedures, fails to reveal any spectral signatures attributable to the secondary components. The published multi-colour light curves of these three systems are analysed to confirm some previous results, and to demonstrate that the amplitude and detailed shape of the reflection effect can be matched provided that the irradiated hemispheres of the (otherwise) cool secondary stars have bolometric albedos of unity, and have limb-darkening coefficients much reduced over normal values, or even exhibit limb-brightening. Astrophysical parameters are determined, based upon these analyses and assuming, in the cases of HW Vir and AA Dor, a mass of 0.5 M⊙ for the subdwarf stars. The three hot subdwarfs lie in the region of the HR diagram appropriate for evolution models for binary systems undergoing late case B mass transfer followed by a common-envelope phase. For HW Vir and AA Dor, the cool secondary stars are entirely consistent with ZAMS models at 0.09–0.14 M⊙, whilst the secondary in KV Vel has log R/R⊙ ∼ 0.2 dex larger than a ZAMS model for its mass of 0.23 M⊙.

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