Claus Tappert

Post common envelope binaries from SDSS - XII. The orbital period distribution

Context. The complexity of the common-envelope phase and of magnetic stellar wind braking currently limits our understanding of close binary evolution. Because of their intrinsically simple structure, observational population studies of white dwarf plus main sequence (WDMS) binaries can potentially test theoretical models and constrain their parameters. Aims. The Sloan Digital Sky Survey (SDSS) has provided a large and homogeneously selected sample of WDMS binaries, which we characterise in terms of orbital and stellar parameters. Methods. We have obtained radial velocity information for 385 WDMS binaries from follow-up spectroscopy and for an additional 861 systems from the SDSS subspectra. Radial velocity variations identify 191 of these WDMS binaries as post common-envelope binaries (PCEBs). Orbital periods of 58 PCEBs were subsequently measured, predominantly from time-resolved spectroscopy, bringing the total number of SDSS PCEBs with orbital parameters to 79. Observational biases inherent to this PCEB sample were evaluated through extensive Monte Carlo simulations. Results. We find that 21-24% of all SDSS WDMS binaries have undergone common-envelope evolution, which is in good agreement with published binary population models and high-resolution HST imaging of WDMS binaries unresolved from the ground. The bias-corrected orbital period distribution of PCEBs ranges from 1.9 h to 4.3 d and approximately follows a normal distribution in log (Porb), peaking at ∼10.3 h. There is no observational evidence for a significant population of PCEBs with periods in the range of days to weeks. Conclusions. The large and homogeneous sample of SDSS WDMS binaries provides the means to test fundamental predictions of binary population models, hence to observationally constrain the evolution of all close compact binaries.

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. 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 Msec ∼ 0.25 M and steeply drops towards higher mass secondary stars in the range of Msec = 0.25−0.4 M. Conclusions. 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 disrupted magnetic braking scenario.

Post common envelope binaries from SDSS - XIII. Mass dependencies of the orbital period distribution

Context. Post-common-envelope binaries (PCEBs) consisting of a white dwarf (WD) and a main-sequence secondary star are ideal systems to constrain models of common-envelope (CE) evolution. Until very recently, observed samples of PCEBs have been too small to fully explore this potential, however the recently identified large and relatively homogenous sample of PCEBs from the Sloan Digital Sky Survey (SDSS) has significantly changed this situation. Aims. We here analyze the orbital period distributions of PCEBs containing He- and C/O-core WDs separately and investigate whether the orbital period of PCEBs is related to the masses of their stellar components. Methods. We performed standard statistical tests to compare the orbital period distributions and to determine the confidence levels of possible relations. Results. The orbital periods of PCEBs containing He-core WDs are significantly shorter than those of PCEBs containing C/O-core WDs. While the He-core PCEB orbital period distribution has a median value of Porb ~ 0.28 d, the median orbital period for PCEBs containing C/O-core WDs is Porb ~ 0.57 d. We also find that systems containing more massive secondaries have longer post-CE orbital periods, in contradiction to recent predictions. Conclusions. Our observational results provide new constraints on theories of CE evolution. However we suggest future binary population models to take selection effects into account that still affect the current observed PCEB sample.

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.

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

A stellar prominence in the white dwarf/red dwarf binary QS Vir: evidence for a detached system

Using high-resolution Ultraviolet and Visual Echelle Spectrograph (UVES) spectra of the eclipsing post-common envelope binary QS Vir, we detect material along the line of sight to the white dwarf at orbital phase ϕ= 0.16. We ascribe this to a stellar prominence originating from the M dwarf secondary star which passes in front of the white dwarf at this phase. This creates sharp absorption features in the hydrogen Balmer series and Ca ii H&K lines. The small size of the white dwarf allows us to place tight constraints on the column density of hydrogen in the n= 2 level of log 10 N2= 14.10 ± 0.03 cm−2 and, assuming local thermodynamical equilibrium, the temperature of the prominence material of ∼9000 K. The prominence material is at least 1.5 stellar radii from the surface of the M dwarf. The location of the prominence is consistent with emission features previously interpreted as evidence for Roche lobe overflow in the system. We also detect Mg ii 4481 A absorption from the white dwarf. The width of the Mg ii line indicates that the white dwarf is not rapidly rotating, in contrast to previous work, hence our data indicate that QS Vir is a pre-cataclysmic binary, yet to initiate mass transfer, rather than a hibernating cataclysmic variable as has been suggested.

The pre-cataclysmic variable, LTT 560

Aims. System parameters of the object LTT560 are determined in order to clarify its nature and evolutionary status. Methods. We apply time-series photometry to reveal orbital modulations of the light curve, time-series spectroscopy to measure radial velocities of features from both the primary and the secondary star, and flux-calibrated spectroscopy to derive temperatures of both components. Results. We find that LTT 560 is composed of a low temperature (T ∼ 7500 K) DA white dwarf as the primary and an M5.5±1 mainsequence star as the secondary component. The current orbital period is Porb = 3.54(07) h.We derive a mass ratio Msec/Mwd = 0.36(03) and estimate the distance to d = 25–40 pc. Long-term variation of the orbital light curve and an additional Hα emission component on the white dwarf indicate activity in the system, probably in the form of flaring and/or accretion events.

Accretion in the detached post-common-envelope binary LTT 560

In a previous study, we found that the detached post-common-envelope binary LTT 560 displays an Hα emission line consisting of two anti-phased components. While one of them was clearly caused by stellar activity from the secondary late-type main-sequence star, our analysis indicated that the white dwarf primary star is potentially the origin of the second component. However, the low resolution of the data means that our interpretation remains ambiguous. We here use time-series UVES data to compare the radial velocities of the Hα emission components to those of metal absorption lines from the primary and secondary stars. We find that the weaker component most certainly originates in the white dwarf and is probably caused by accretion. An abundance analysis of the white dwarf spectrum yields accretion rates that are consistent with mass loss from the secondary due to a stellar wind. The second and stronger Hα component is attributed to stellar activity on the secondary star. An active secondary is likely to be present because of the occurrence of a flare in our time-resolved spectroscopy. Furthermore, Roche tomography indicates that a significant area of the secondary star on its leading side and close to the first Lagrange point is covered by star spots. Finally, we derive the parameters for the system and place it in an evolutionary context. We find that the white dwarf is a very slow rotator, suggesting that it has had an angular-momentum evolution similar to that of field white dwarfs. We predict that LTT 560 will begin mass transfer via Rochelobe overflow in ∼3.5 Gyr, and conclude that the system is representative of the progenitors of the current population of cataclysmic variables. It will most likely evolve to become an SU UMa type dwarf nova.

Spectroscopic analysis of tremendous-outburst-nova candidates

In the course of a long-term project investigating classical novae with large outburst amplitudes, we have performed optical spectroscopy of several old-nova candidates. We here present the spectra of the candidates V630 Sgr, XX Tau, CQ Vel, V842 Cen, and V529 Ori, that hitherto lacked such classification. While the first four show spectra typical of cataclysmic variables and can thus be identified as such, V529 Ori is probably misclassified. Of special interest are the two systems XX Tau and V842 Cen, which show signs of being low mass transfer systems. As such they can be used to judge the evolution scenarios for novae. In particular, given the rather young age of their outbursts, it appears more likely that these systems are not on their way into hibernation (i.e., cutting off mass transfer for a longer period of time), but are simply settling down towards their original configuration of comparatively low, but steady, mass transfer, such as for dwarf novae.

K-band spectroscopy of pre-cataclysmic variables ⋆

Aims. There exists now substantial evidence for abundance anomalies in a number of cataclysmic variables (CVs), indicating that the photosphere of the secondary star incorporates thermonuclear processed material. However, the spectral energy distribution in CVs is usually dominated by the radiation produced by the accretion process, severely hindering an investigation of the stellar components. On the other hand, depending on how the secondary star has acquired such material, the above mentioned abundance anomalies could also be present in pre-CVs, i.e. detached white/red dwarf binaries that will eventually evolve into CVs, but have not yet started mass transfer, and therefore allow for an unobstructed view on the secondary star at infrared wavelengths. Methods. We have taken K-band spectroscopy of a sample of 13 pre-CVs in order to examine them for anomalous chemical abundances. In particular, we study the strength of the 12CO and 13CO absorption bands that have been found diminished and enhanced, respectively, in similar studies of CVs. Results. All our systems show CO abundances that are within the range observed for single stars. The weakest 12CO bands with respect to the spectral type are found in the pre-CV BPM 71214, although on a much smaller scale than observed in CVs. Furthermore there is no evidence for enhanced 13CO. Taking into account that our sample is subject to the present observational bias that favours the discovery of young pre-CVs with secondary stars of late spectral types, we can conclude the following: 1) our study provides observational proof that the CO anomalies discovered in certain CVs are not due to any material acquired during the common envelope phase, and 2) if the CO anomalies in certain CVs are not due to accretion of processed material during nova outburst, then the progenitors of these CVs are of a significantly different type than the currently known sample of pre-CVs.