Spectroscopy and eclipse-mapping of the mass-exchanging binary star V361 Lyr

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Abstract
We present the first spectroscopic investigation of the short-period (P = 0.309614 d) mass-exchanging binary system V361 Lyr. From observations over two complete orbits, we show that the system is double-lined, with the stars having minimum masses of m1 sin3i = 1.25 ± 0.03 M⊙ and m2 sin3i = 0.87 ± 0.03 M⊙. The mass ratio m2/m1 = 0.69 ± 0.02 and the projected semimajor axis of the relative orbit is a sin i = 2.47 ± 0.05 R⊙. The spectral type of the primary component is estimated to be F8-G0. The H± line profile clearly shows two absorption components when the system is at its brightest and bluest stage, around orbital phases 0.35–0.45. The published V and I light curves are analysed with light2 to show that the primary component (T = 6200 K) fills its Roche lobe, whilst the secondary (T = 4500 K) is detached and fills ≈ 57 per cent of the available Roche volume. Eclipse maps of both stellar surfaces are derived via dots from light curves obtained in 1988, 1989 and 1992. These show that the secondary has a persistent elongated hot structure on its equator with an estimated temperature of 10000 K that is consistent with the expectations of a theoretical mass transfer stream leaving the primary at its inner Lagrangian point and falling on to the secondary. The accretion luminosity of ≈ 0.5 L⊙ from that impacting stream, as estimated from the observed decrease in orbital period and the derived stellar masses, is about twice the luminosity of the structure inferred from the light curves and the surface-image reconstructions. The eclipse maps for the primary illustrate small changes in the cool spot structures that are typical of magnetic activity observed in other rapidly-rotating solar-type stars that are single or are members of binary systems (e.g. AB Dor, XY UMa). This model for V361 Lyr predicts successfully the phase-dependent variations observed in the Hα line. The properties of this mass-exchanging semidetached binary are compared with the near-contact and the contact systems of the W UMa type for the purpose of establishing whether or not it represents the short-lived phase of evolution into contact for the first time, or the broken-contact stage of a thermal-relaxation oscillation. The derived locations of both components on the zero-age main sequence would favour the former interpretation. V361 Lyr is reddened by E(B − V) = 0.43 and lies at a distance of 520 α 40 pc.

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