Precise mass and radius values for the white dwarf and low mass M dwarf in the pre-cataclysmic binary NN Serpentis

Abstract

Using the high resolution Ultraviolet and Visual Echelle Spectrograph (UVES) mounted on the Very Large Telescope in combination with photometry from the high-speed CCD camera ULTRACAM, we derive precise system parameters for the pre-cataclysmic binary NN Ser. A model fit to the ULTRACAM light curves gives the orbital inclination as and the scaled radii, RWD/a and Rsec/a. Analysis of the He ii 4686 Å absorption line gives a radial velocity amplitude for the white dwarf of KWD= 62.3 ± 1.9 km s-1. We find that the irradiation-induced emission lines from the surface of the secondary star give a range of observed radial velocity amplitudes due to differences in optical depths in the lines. We correct these values to the centre of mass of the secondary star by computing line profiles from the irradiated face of the secondary star. We determine a radial velocity of Ksec= 301 ± 3 km s-1, with an error dominated by the systematic effects of the model. This leads to a binary separation of a= 0.934 ± 0.009 R⊙, radii of RWD= 0.0211 ± 0.0002 R⊙ and Rsec= 0.149 ± 0.002 R⊙ and masses of MWD= 0.535 ± 0.012 M⊙ and Msec= 0.111 ± 0.004 M⊙. The masses and radii of both components of NN Ser were measured independently of any mass-radius relation. For the white dwarf, the measured mass, radius and temperature show excellent agreement with a 'thick' hydrogen layer of fractional mass MH/MWD= 10-4. The measured radius of the secondary star is 10 per cent larger than predicted by models, however, correcting for irradiation accounts for most of this inconsistency, hence the secondary star in NN Ser is one of the first precisely measured very low mass objects(M≲ 0.3 M⊙) to show good agreement with models. ULTRACAMr′, i′ andz′ photometry taken during the primary eclipse determines the colours of the secondary star as(r′-i′)sec= 1.4 ± 0.1 and(i′-z′)sec= 0.8 ± 0.1 which corresponds to a spectral type ofM4 ± 0.5. This is consistent with the derived mass, demonstrating that there is no detectable heating of the unirradiated face, despite intercepting radiative energy from the white dwarf which exceeds its own luminosity by over a factor of 20. © 2010 The Authors. Journal compilation © 2010 RAS.