M. Zorotovic

Post-common-envelope binaries from SDSS - IX: Constraining the common-envelope efficiency

Context. Reconstructing the evolution of post-common-envelope binaries (PCEBs) consisting of a white dwarf and a main-sequence star can constrain current prescriptions of common-envelope (CE) evolution. This potential could so far not be fully exploited due to the small number of known systems and the inhomogeneity of the sample. Recent extensive follow-up observations of white dwarf/main-sequence binaries identified by the Sloan Digital Sky Survey (SDSS) paved the way for a better understanding of CE evolution. n nAims. Analyzing the new sample of PCEBs we derive constraints on one of the most important parameters in the field of close compact binary formation, i.e. the CE efficiency α. n nMethods. After reconstructing the post-CE evolution and based on fits to stellar evolution calculations as well as a parametrized energy equation for CE evolution, we determine the possible evolutionary histories of the observed PCEBs. In contrast to most previous attempts we incorporate realistic approximations of the binding energy parameter λ. Each reconstructed CE history corresponds to a certain value of the mass of the white dwarf progenitor and – more importantly – the CE efficiency α. We also reconstruct CE evolution replacing the classical energy equation with a scaled angular momentum equation and compare the results obtained with both algorithms. n nResults. We find that all PCEBs in our sample can be reconstructed with the energy equation if the internal energy of the envelope is included. Although most individual systems have solutions for a broad range of values for α, only for α = 0.2–0.3 do we find simultaneous solutions for all PCEBs in our sample. If we adjust α to this range of values, the values of the angular momentum parameter γ cluster in a small range of values. In contrast if we fix γ to a small range of values that allows us to reconstruct all our systems, the possible ranges of values for α remains broad for individual systems. n nConclusions. The classical parametrized energy equation seems to be an appropriate prescription of CE evolution and turns out to constrain the outcome of the CE evolution much more than the alternative angular momentum equation. If there is a universal value of the CE efficiency, it should be in the range of α = 0.2–0.3. We do not find any indications for a dependence of α on the mass of the secondary star or the final orbital period.

Post common envelope binaries from SDSS - VIII. Evidence for disrupted magnetic braking

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. nAims. 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. nMethods. 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. nResults. Among the 670 WDMS binaries we find 205 strong PCEB candidates. The fraction of PCEBs among WDMS binaries peaks around Msec ∼ 0.25 M and steeply drops towards higher mass secondary stars in the range of Msec = 0.25−0.4 M. nConclusions. The decrease of the number of PCEBs at the fully convective boundary strongly suggests that the evolutionary timescales 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 ndisrupted magnetic braking scenario.

Post Common Envelope Binaries from SDSS. XV: Accurate stellar parameters for a cool 0.4-solar mass white dwarf and a 0.16-solar mass M-dwarf in a 3 hour eclipsing binary

We identify SDSSJ121010.1+334722.9 as an eclipsing post-common-envelope binary, with an orbital period of P ~ 3 hrs, containing a very cool, low-mass, DAZ white dwarf and a low-mass main-sequence star of spectral type M5. A model atmosphere analysis of the metal absorption lines detected in the blue part of the optical spectrum, along with the GALEX near-ultraviolet flux, yields a white dwarf temperature of 6000 +/- 200 K and a metallicity value of log(Z/H)= -2.0 +/- 0.3. The sodium absorption doublet is used to measure the radial velocity of the secondary star, K2 ~ 252 km/s and iron absorption lines in the blue part of the spectrum provide the radial velocity of the white dwarf, K1 ~ 95 km/s, yielding a mass ratio of q ~ 0.38. Light curve model fitting, using the Markov Chain Monte Carlo (MCMC) method, gives the inclination angle as i = (79.05 - 79.36) +/- 0.15 degrees, and the stellar masses as M1 = 0.415 +/- 0.010 solar-masses and M2 = 0.158 +/- 0.006 solar-masses. Systematic uncertainties in the absolute calibration of the photometric data influence the determination of the stellar radii. The radius of the white dwarf is found to be R1 = (0.0157 - 0.0161) +/- 0.0003 solar-radii and the volume-averaged radius of the tidally distorted secondary is R2 = (0.197 - 0.203) +/- 0.003 solar-radii. The white dwarf in J1210+3347 is a very strong He-core candidate.

Post-common envelope binaries from SDSS - XVI. Long orbital period systems and the energy budget of CE evolution

Virtually all close compact binary stars are formed through common-envelope (CE) evolution. It is generally accepted that during this crucial evolutionary phase a fraction of the orbital energy is used to expel the envelope. However, it is unclear whether additional sources of energy, such as the recombination energy of the envelope, play an important role. Here we report the discovery of the second and third longest orbital period post-common envelope binaries (PCEBs) containing white dwarf (WD) primaries, i.e. SDSSJ121130.94-024954.4 (Porb = 7.818 +- 0.002 days) and SDSSJ222108.45+002927.7 (Porb = 9.588 +- 0.002 days), reconstruct their evolutionary history, and discuss the implications for the energy budget of CE evolution. We find that, despite their long orbital periods, the evolution of both systems can still be understood without incorporating recombination energy, although at least small contributions of this additional energy seem to be likely. If recombination energy significantly contributes to the ejection of the envelope, more PCEBs with relatively long orbital periods (Porb >~ 1-3 day) harboring massive WDs (Mwd >~ 0.8 Msun) should exist.

Observations of three pre-cataclysmic variables from the Edinburgh-Cape blue object survey

Aims: This study aims at determining the parameters of the three candidate pre-cataclysmic binaries ECu200912477–1738, ECu200913349–3237, and ECu200914329–1625, most importantly their orbital period. nMethods: Time-series photometry reveals orbital modulation in the form of sinusoidal variation due to the reflection effect. Photometric observations are complemented by time-resolved spectroscopy that yields radial velocities of the H

White dwarf post common envelope binaries from the SDSS

alpha

A large HST program: effective temperatures of cataclysmic variable white dwarfs

emission line. The combination of both methods allows us to unambiguously determine the orbital periods. The average spectra are used to estimate physical parameters of the primary and secondary stellar components. nResults: We determine the orbital period for ECu200912477–1738 as 0.362 d, thus confirming the value previously reported. A similar period, P=0.350u2009d, is found for ECu200914329–1625. Both systems incorporate a medium-hot white dwarf (

Post-common envelope binaries from SDSS - XV. Accurate stellar parameters for a cool 0.4 M⊙ white dwarf and a 0.16 M⊙ M dwarf in a 3 h eclipsing binary: SDSS 1210+3347

T = 15,000{-}20,000

Monte Carlo Simulations of the Post‐Common‐Envelope White‐Dwarf Main‐Sequence Binary Population

K) and an M3V secondary star. The third pre-CV, ECu200913349–3237, is the youngest of the three, with a hot WD (

Binary population models of CVs

T sim 35,000