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Session 12 - Binary Stars.
Display session, Monday, June 08
Atlas Ballroom,
The sdB star KPD 0422+5421 has been found to be a single-lined spectroscopic binary with a period of P=0.0901795\pm (3\times 10^-7) days, or about 2 hours and 10 minutes. The mass function of the unseen companion star is f(M)=0.126\pm 0.028\,M_ødot. The U and B light curves display ellipsoidal modulations with amplitudes \approx 0.02 magnitudes. The lack of a reflection effect in the light curves implies that the radius of the companion star is much less than the radius of the sdB star. Thus the companion star must be a white dwarf. We modelled the light curves using the Wilson-Devinney code and derived the inclination i=78.05\pm 0.50^\circ, and mass ratio q=M_comp/M_sdB=0.87\pm 0.15. With the mass function the derived component masses are then M_sdB=0.72\pm 0.26\,M_ødot and M_comp=0.62\pm 0.18\,M_ødot.
Spectroscopic analysis of the sdB star results in T_eff= 25000\pm 1500 K and \log g = 5.4\pm 0.1, consistent with location on or near the Zero-Age Extended Horizontal Branch. The sdB star's mass thus should be close to 0.50\,M_ødot, with a similar mass for the unseen white dwarf. The sdB atmosphere is depleted in He and C.
Only one detached binary system, WD 0957-666, has a shorter spectroscopically confirmed orbital period than KPD 0422+5421, but the total mass of KPD 0422+5421 is higher. The system is also one of only a few binaries known to contain a subdwarf B star paired with a white dwarf. Within \sim 10^8 yr, the sdB star will complete its core helium burning and either expand to encounter its Roche lobe or shrink, creating a double degenerate system. On the same timescale, gravitational radiation of angular momentum will cause the binary orbit to decay. KPD 0422+5421 represents a rare, short-lived, and poorly observed stage of binary star evolution.
Partially supported by NASA grant NAG 5-3459 and the South African Foundation for Research Development.
