Emission from the secondary star in the old cataclysmic variable WZ Sagittae

Steeghs, D., Marsh, T.R., Knigge, C., Maxted, P.F.L., Kuulkers, E. and Skidmore, W. (2001) Emission from the secondary star in the old cataclysmic variable WZ Sagittae. Astrophysical Journal, 562 (2), L145-L148. (doi:10.1086/338107).

Abstract

We present the first detection of the mass donor star in the cataclysmic variable WZ Sagittae. Phase-resolved spectroscopy reveals narrow Balmer emission components from the irradiated secondary star during the 2001 outburst. Its radial velocity curve indicates a systemic velocity of -72+/-3 km s-1 and an apparent velocity amplitude of K'2=493+/-10 km s-1. Doppler tomography reveals a highly asymmetric accretion disk including a significant bright spot contribution 20 days into the outburst. We estimate the apparent primary radial velocity using the Halpha tomogram and find K'1=37+/-5 km s-1. Accounting for the likely systematic errors affecting both K1 and K2 measurements, we conservatively derive 493 km s-1<K2<585 km s-1 and K1<37 km s-1. The measured phase offset between bright spot eclipse and inferior conjunction of the secondary star brackets the allowed mass ratio (q=M2/M1) to lie between 0.040 and 0.075. These constraints imply a white dwarf with M1>0.70 Msolar and a low-mass secondary with M2<0.11 Msolar. A nondegenerate mass donor, implying WZ Sge has not yet evolved through its minimum period, is therefore not ruled out by our observations. This would require an improved estimate of K1.

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