Astronomy and Astrophysics – Astronomy
Scientific paper
May 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998aas...192.1216o&link_type=abstract
American Astronomical Society, 192nd AAS Meeting, #12.16; Bulletin of the American Astronomical Society, Vol. 30, p.836
Astronomy and Astrophysics
Astronomy
Scientific paper
The sdB star KPD 0422+5421 has been found to be a single-lined spectroscopic binary with a period of P=0.0901795+/- (3x 10(-7) ) days, or about 2 hours and 10 minutes. The mass function of the unseen companion star is f(M)=0.126+/- 0.028 Msun. The U and B light curves display ellipsoidal modulations with amplitudes ~ 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+/- 0.50(deg) , and mass ratio q=M_comp/M_sdB=0.87+/- 0.15. With the mass function the derived component masses are then M_sdB=0.72+/- 0.26 Msun and M_comp=0.62+/- 0.18 Msun. Spectroscopic analysis of the sdB star results in Teff= 25000+/- 1500 K and log g = 5.4+/- 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 Msun, 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 ~ 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.
Koen Chris
Orosz Jerome A.
Wade Richard A.
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