Open AccessKPD 0422 + 5421: a new short‐period subdwarf B/white dwarf binary
Author(s) -
Koen C.,
Orosz Jerome A.,
Wade Richard A.
Publication year - 1998
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1046/j.1365-8711.1998.01961.x
Subject(s) - physics , subdwarf , astrophysics , white dwarf , effective temperature , orbital period , astronomy , stars
The subdwarf B (sdB) star KPD 0422 + 5421 was discovered to be a single‐lined spectroscopic binary with a period of P = 0.090 1795 ± (3 × 10 −7 ) d (2 h 10 min). The U B light curves display an ellipsoidal modulation with amplitudes of ≈ 0.02 mag. The sdB star contributes nearly all of the observed flux. This and the absence of any reflection effect suggest that the unseen companion star is small (i.e. R comp ≈ 0.01 R ⊙ ) and therefore degenerate. We modelled the U B light curves and derived i = 78.05° ± 0.50° and a mass ratio of q = M comp / M sdB = 0.87 ± 0.15. The sdB star fills 69 per cent of its Roche lobe. These quantities may be combined with the mass function of the companion [ f ( M ) = 0.126 ± 0.028 M ⊙ ] to derive M sdB = 0.72 ± 0.26 M ⊙ and M comp = 0.62 ± 0.18 M ⊙ . We used model spectra to derive the effective temperature, surface gravity and helium abundance of the sdB star. We found T eff = 25 000 ± 1500 K, log g = 5.4 ± 0.1 and [He/H] = −1.0. With a period of 2 h 10 min, KPD 0422 + 5421 has one of the shortest known orbital periods of a detached binary. This system is also one of only a few known binaries that contain a subdwarf B star and a white dwarf. Thus KPD 0422 + 5421 represents a relatively unobserved, and short‐lived, stage of binary star evolution.