Letter to the Editor

Post common envelope binaries from SDSS^*

VIII. Evidence for disrupted magnetic braking

¹ Departamento de Física y Astronomía, Facultad de Ciencias, Universidad de Valparaíso, Valparaíso, Chile e-mail: matthias@dfa.uv.cl
² Department of Physics, University of Warwick, Coventry, CV4 7AL, UK
³ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
⁴ Department of Physics and Chemistry, Keele University, Staffordshire, ST5 5BG, UK
⁵ Institute of Astronomy & Astrophysics, National Observatory of Athens, 15236 Athens, Greece
⁶ Isaac Newton Group of Telescopes, Apartado de correos 321, S/C de la Palma, 38700, Canary Islands, Spain
⁷ Instituto de Astrofísica de Canarias, Vía Láctea, s/n, 38205 La Laguna, Spain
⁸ Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
⁹ European Southern Observatory, Alonso de Cordova 3107, Santiago, Chile
¹⁰ Departamento de Física y Astronomía, Facultad de Ciencias, Pont. Universidad Catolica, Santiago, Chile

Received: 4 January 2010
Accepted: 15 March 2010

Abstract

Context. The standard prescription of angular momentum loss in compact binaries assumes magnetic braking to be very efficient as long as the secondary star has a radiative core, but to be negligible if the secondary star is fully convective. This prescription has been developed to explain the orbital period gap observed in the orbital period distribution of cataclysmic variables but has so far not been independently tested. Because the evolutionary time-scale of post common envelope binaries (PCEBs) crucially depends on the rate of angular momentum loss, a fundamental prediction of the disrupted magnetic braking theory is that the relative number of PCEBs should dramatically decrease for companion-star masses exceeding the mass that corresponds to the fully-convective boundary.

Aims. We present the results of a large survey of PCEBs among white dwarf/main sequence (WDMS) binaries that allows us to determine the fraction of PCEBs as a function of secondary star mass and therewith to ultimately test the disrupted magnetic braking hypothesis.

Methods. We obtained multiple spectroscopic observations spread over at least two nights for 670 WDMS binaries. Systems showing at least 3σ radial velocity variations are considered to be strong PCEB candidates. Taking into account observational selection effects we compare our results with the predictions of binary population simulations.

Results. Among the 670 WDMS binaries we find 205 strong PCEB candidates. The fraction of PCEBs among WDMS binaries peaks around M_sec ~ 0.25  and steeply drops towards higher mass secondary stars in the range of M_sec = 0.25–0.4 .

Conclusions. The decrease of the number of PCEBs at the fully convective boundary strongly suggests that the evolutionary time scales of PCEBs containing fully convective secondaries are significantly longer than those of PCEBs with secondaries containing a radiative core. This is consistent with significantly reduced magnetic wind braking of fully convective stars as predicted by the disrupted magnetic braking scenario.