matthias schreiber - Profile on Academia.edu (original) (raw)

Papers by matthias schreiber

Research paper thumbnail of The White Dwarf Binary Pathways Survey −III. Contamination from hierarchical triples containing a white dwarf

Monthly Notices of the Royal Astronomical Society, 2020

The White Dwarf Binary Pathways Survey aims at increasing the number of known detached A, F, G, a... more The White Dwarf Binary Pathways Survey aims at increasing the number of known detached A, F, G, and K main-sequence stars in close orbits with white dwarf companions (WD+AFGK binaries) to refine our understanding about compact binary evolution and the nature of Supernova Ia progenitors. These close WD+AFGK binary stars are expected to form through common envelope evolution, in which tidal forces tend to circularize the orbit. However, some of the identified WD+AFGK binary candidates show eccentric orbits, indicating that these systems are either formed through a different mechanism or perhaps they are not close WD+AFGK binaries. We observed one of these eccentric WD+AFGK binaries with SPHERE and find that the system TYC 7218-934-1 is in fact a triple system where the WD is a distant companion. The inner binary likely consists of the G-type star plus an unseen low-mass companion in an eccentric orbit. Based on this finding, we estimate the fraction of triple systems that could contam...

Research paper thumbnail of Understanding white dwarf binary evolution with white dwarf/main sequence binaries: first results from SEGUE

arXiv (Cornell University), Sep 1, 2007

Close white dwarf binaries make up a wide variety of objects such as double white dwarf binaries,... more Close white dwarf binaries make up a wide variety of objects such as double white dwarf binaries, which are possible SN Ia progenitors, cataclysmic variables, super soft sources, or AM CVn stars. The evolution and formation of close white dwarf binaries crucially depends on the rate at which angular momentum is extracted from the binary orbit. The two most important sources of angular momentum loss are the common envelope phase and magnetic braking. Both processes are so far poorly understood. Observational population studies of white dwarf/main sequence binaries provide the potential to significantly progress with this situation and to clearly constrain magnetic braking and the CE-phase. However, the current population of white dwarf/main sequence binaries is highly incomplete and heavily biased towards young systems containing hot white dwarfs. The SDSSII/SEGUE collaboration awarded us with 5 fibers per plate pair in order to fill this gap and to identify the required unbiased sample of old white dwarf/main sequence binaries. The success rate of our selection criteria exceeds 65% and during the first 10 months we have identified 41 new systems, most of them belonging to the missed old population.

Research paper thumbnail of Disc accretion onto white dwarfs

Disc accretion onto white dwarfs In non-magnetic cataclysmic variables (CVs) a white dwarf accret... more Disc accretion onto white dwarfs In non-magnetic cataclysmic variables (CVs) a white dwarf accretes matter from a mainsequence secondary star via an accretion disc. The dynamical behaviour of the accretion disc determines the accretion rate onto the white dwarf. Thermal instabilities in the accretion disc associated with the ionisation of hydrogen can lead to a limit-cycle behaviour in which the disc switches quasi-periodically between high and low accretion states. This thermal limit-cycle model is the generally accepted explanation for dwarf nova outbursts observed in many CVs. The process of disc accretion in non-magnetic CVs is subject to a number of external conditions, namely mass transfer variations of the secondary star, stream overflow and irradiation by the white dwarf. In this thesis I develop a model for time-dependent disc accretion onto white dwarfs and analyse the influence of these external conditions on the accretion process. I examine the effects of mass transfer variations by deriving real mass transfer variations from light curve monitoring of the disc-less CV AM Her. These mass transfer variations I include in simulations of disc accretion onto white dwarfs in non-magnetic systems and find that the mass accretion rate of the disc relaxes to an equilibrium with the prevailing mass transfer rate on a rather short timescale. I conclude that the observed changes in outburst duration and outburst magnitude are caused by nearly simultaneous variations of the mass loss rate from the secondary. I also present a new model for the stripping of the stream by the accretion disc, and find that stream overflow can have subtle effects on the evolution of the accretion disc only if the amount of overflowing stream material exceeds 25% of the mass transfer rate. I conclude that solely very large stream overflow fractions can change the outbursts of dwarf novae. For realistic amounts of stream overflow the overall outburst behaviour is marginally changed. The accretion disc is mostly influenced by the white dwarf irradiation. I present a selfconsistent model for irradiated accretion discs and find that efficient irradiation in dwarf nova systems causes small "echo" outbursts following the larger ones immediately. This result contrasts with the observations of dwarf nova outbursts. As an explanation for this discrepancy I suggest that the reprocessing efficiency of disc irradiation is rather small. This is in agreement with results I obtain from detailed simulations of irradiated discs in post novae. These systems are excellent laboratories for studying the effects of disc irradiation because the white dwarf heated during the nova eruption provides a much stronger irradiation of the disc than in normal dwarf novae. I derive time-limits for the occurrence of dwarf nova outbursts in post novae and present detailed simulations of the evolution of irradiated discs in post novae. In addition to the developed theory of irradiated discs around white dwarfs, I show preliminary results of an intensive observing campaign on the post nova system V446 Her. Finally, discussing the influence of disc irradiation on the post nova evolution in the light of the current working hypothesis leads me to put forward a new scenario.

Research paper thumbnail of Understanding the evolution of close white dwarf binaries

arXiv: Solar and Stellar Astrophysics, 2019

Interacting binaries containing white dwarfs can lead to a variety of outcomes that range from po... more Interacting binaries containing white dwarfs can lead to a variety of outcomes that range from powerful thermonuclear explosions, which are important in the chemical evolution of galaxies and as cosmological distance estimators, to strong sources of low frequency gravitational wave radiation, which makes them ideal calibrators for the gravitational low-frequency wave detector LISA mission. However, current theoretical evolution models still fail to explain the observed properties of the known populations of white dwarfs in both interacting and detached binaries. Major limitations are that the existing population models have generally been developed to explain the properties of sub-samples of these systems, occupying small volumes of the vast parameter space, and that the observed samples are severely biased. The overarching goal for the next decade is to assemble a large and homogeneous sample of white dwarf binaries that spans the entire range of evolutionary states, to obtain prec...

Research paper thumbnail of Cold Giant Planets Evaporated by Hot White Dwarfs

The Astrophysical Journal, 2019

Atmospheric escape from close-in Neptunes and hot Jupiters around Sun-like stars driven by extrem... more Atmospheric escape from close-in Neptunes and hot Jupiters around Sun-like stars driven by extreme ultraviolet (EUV) irradiation plays an important role in the evolution of exoplanets and in shaping their ensemble properties. Intermediate and low mass stars are brightest at EUV wavelengths at the very end of their lives, after they have expelled their envelopes and evolved into hot white dwarfs. Yet the effect of the intense EUV irradiation of giant planets orbiting young white dwarfs has not been assessed. We show that the giant planets in the solar system will experience significant hydrodynamic escape caused by the EUV irradiation from the white dwarf left behind by the Sun. A fraction of the evaporated volatiles will be accreted by the solar white dwarf, resulting in detectable photospheric absorption lines. As a large number of the currently known extrasolar giant planets will survive the metamorphosis of their host stars into white dwarfs, observational signatures of accretion from evaporating planetary atmospheres are expected to be common. In fact, one-third of the known hot single white dwarfs show photospheric absorption lines of volatile elements, which we argue are indicative of ongoing accretion from evaporating planets. The fraction of volatile contaminated hot white dwarfs strongly decreases as they cool. We show that accretion from evaporating planetary atmospheres naturally explains this temperature dependence if more than 50% of hot white dwarfs still host giant planets.

Research paper thumbnail of No cataclysmic variables missing: higher merger rate brings into agreement observed and predicted space densities

Monthly Notices of the Royal Astronomical Society, 2018

The predicted and observed space density of cataclysmic variables (CVs) has been for a long time ... more The predicted and observed space density of cataclysmic variables (CVs) has been for a long time discrepant by at least an order of magnitude. The standard model of CV evolution predicts that the vast majority of CVs should be period bouncers, whose space density has been recently measured to be ρ 2 × 10 −5 pc −3. We performed population synthesis of CVs using an updated version of the Binary Stellar Evolution (BSE) code for single and binary star evolution. We find that the recently suggested empirical prescription of consequential angular momentum loss (CAML) brings into agreement predicted and observed space densities of CVs and period bouncers. To progress with our understanding of CV evolution it is crucial to understand the physical mechanism behind empirical CAML. Our changes to the BSE code are also provided in details, which will allow the community to accurately model mass transfer in interacting binaries in which degenerate objects accrete from low-mass main-sequence donor stars.

Research paper thumbnail of Three in one go: consequential angular momentum loss can solve major problems of CV evolution

Monthly Notices of the Royal Astronomical Society: Letters, 2015

The average white dwarf (WD) masses in cataclysmic variables (CVs) have been measured to signific... more The average white dwarf (WD) masses in cataclysmic variables (CVs) have been measured to significantly exceed those of single WDs, which is the opposite of what is theoretically expected. We present the results of binary population synthesis models taking into account consequential angular momentum loss (CAML) that is assumed to increase with decreasing WD mass. This approach cannot only solve the WD mass problem but also brings in agreement theoretical predictions and observations of the orbital period distribution and the space density of CVs. We speculate that frictional angular momentum loss following nova eruptions might cause such CAML and could be the missing ingredient of CV evolution.

Research paper thumbnail of On the occurrence of dwarf nova outbursts in post novae

We show that irradiation of the accretion disc by the white dwarf limits the occurrence of dwarf ... more We show that irradiation of the accretion disc by the white dwarf limits the occurrence of dwarf nova outbursts in post nova accretion discs. After the nova explosion, the white dwarf has to cool for up to ∼ 100 yr-depending on the orbital period (i.e., disc size) and the temperature of the white dwarf after the nova-eruption-before the disc can begin producing dwarf nova outbursts. During this time the inner disc is maintained in a hot, ionised state. Using these calculations, we interpret the longterm photometric variability of the post nova V446 Her (Nova Her 1960) which shows today regular dwarf nova outbursts. As the white dwarf in V446 Her continues to cool over the next ∼10-20 yr, we predict an increase in the amplitude of outbursts and a decrease of the outburst frequency, because the decreasing irradiation of the accretion disc should allow an increasing annular extent of the accretion disc to participate in limit cycle oscillations.

Research paper thumbnail of The response of a dwarf nova disc to real mass transfer variations

The response of a dwarf nova disc to real mass transfer variations

Astronomy & Astrophysics - ASTRON ASTROPHYS, 2000

We present simulations of dwarf nova outbursts taking into account realistic variations of the ma... more We present simulations of dwarf nova outbursts taking into account realistic variations of the mass loss rate from the secondary. The mass transfer variation has been derived from 20 years of visual monitoring and from X-ray observations covering various accretion states of the discless cataclysmic variable AM Herculis. We find that the outburst behaviour of a fictitious dwarf nova with the same system parameters as AM Her is strongly influenced by these variations of the mass loss rate. Depending on the mass loss rate, the disc produces either long outbursts, a cycle of one long outburst followed by two short outbursts, or only short outbursts. The course of the transfer rate dominates the shape of the outbursts because the mass accreted during an outburst cycle roughly equals the mass transferred from the secondary over the outburst interval. Only for less than 10% of the simulated time, when the mass transfer rate is nearly constant, the disc is in a quasi-stationary state during...

Research paper thumbnail of MULTI-SITE OBSERVATIONS OF PULSATION IN THE ACCRETING WHITE DWARF SDSS J161033.64–010223.3 (V386 Ser)

MULTI-SITE OBSERVATIONS OF PULSATION IN THE ACCRETING WHITE DWARF SDSS J161033.64–010223.3 (V386 Ser)

The Astrophysical Journal, 2010

Research paper thumbnail of Cataclysmic Variables from Sdss. VII. The Seventh Year (2006)

Cataclysmic Variables from Sdss. VII. The Seventh Year (2006)

The Astronomical Journal, 2009

Research paper thumbnail of Post-common-envelope binaries from SDSS - I. 101 white dwarf main-sequence binaries with multiple Sloan Digital Sky Survey spectroscopy

Monthly Notices of the Royal Astronomical Society, 2007

We present a detailed analysis of 101 white dwarf main-sequence binaries (WDMS) from the Sloan Di... more We present a detailed analysis of 101 white dwarf main-sequence binaries (WDMS) from the Sloan Digital Sky Survey (SDSS) for which multiple SDSS spectra are available. We detect significant radial velocity variations in 18 WDMS, identifying them as post-commonenvelope binaries (PCEBs) or strong PCEB candidates. Strict upper limits to the orbital periods are calculated, ranging from 0.43 to 7880 d. Given the sparse temporal sampling and relatively low spectral resolution of the SDSS spectra, our results imply a PCEB fraction of 15 per cent among the WDMS in the SDSS data base. Using a spectral decomposition/fitting technique we determined the white dwarf effective temperatures and surface gravities, masses and secondary star spectral types for all WDMS in our sample. Two independent distance estimates are obtained from the flux-scaling factors between the WDMS spectra, and the white dwarf models and main-sequence star templates, respectively. Approximately one-third of the systems in our sample show a significant discrepancy between the two distance estimates. In the majority of discrepant cases, the distance estimate based on the secondary star is too large. A possible explanation for this behaviour is that the secondary star spectral types that we determined from the SDSS spectra are systematically too early by one to two spectral classes. This behaviour could be explained by stellar activity, if covering a significant fraction of the star by cool dark spots will raise the temperature of the interspot regions. Finally, we discuss the selection effects of the WDMS sample provided by the SDSS project.

Research paper thumbnail of Post-common envelope binaries from SDSS - VII. A catalogue of white dwarf-main sequence binaries

Monthly Notices of the Royal Astronomical Society, 2009

We present a catalogue of 1602 white-dwarf-main-sequence (WDMS) binaries from the spectroscopic S... more We present a catalogue of 1602 white-dwarf-main-sequence (WDMS) binaries from the spectroscopic Sloan Digital Sky Survey Data Release 6 (SDSS DR6). Among these, we identify 440 as new WDMS binaries. We select WDMS binary candidates by template fitting all 1.27 million DR6 spectra, using combined constraints in both χ 2 and signal-to-noise ratio. In addition, we use Galaxy Evolution Explorer (GALEX) and UKIRT Infrared Sky Survey (UKIDSS) magnitudes to search for objects in which one of the two components dominates the SDSS spectrum. We use a decomposition/fitting technique to measure the effective temperatures, surface gravities, masses and distances to the white dwarfs, as well as the spectral types and distances to the companions in our catalogue. Distributions and density maps obtained from these stellar parameters are then used to study both the general properties and the selection effects of WDMS binaries in the SDSS. A comparison between the distances measured to the white dwarfs and the main-sequence companions shows d sec > d wd for approximately one-fifth of the systems, a tendency already found in our previous work. The hypothesis that magnetic activity raises the temperature of the inter-spot regions in active stars that are heavily covered by cool spots, leading to a bluer optical colour compared to inactive stars, remains the best explanation for this behaviour. We also make use of SDSS-GALEX-UKIDSS magnitudes to investigate the distribution of WDMS binaries, as well as their white-dwarf effective temperatures and companion star spectral types, in ultraviolet to infrared colour space. We show that WDMS binaries can be very efficiently separated from single main-sequence stars and white dwarfs when using a combined ultraviolet, optical and infrared colour selection. Finally, we also provide radial velocities for 1068 systems measured from the Na I λλ8183.27, 8194.81 absorption doublet and/or the Hα emission line. Among the systems with multiple SDSS spectroscopy, we find five new systems exhibiting significant radial velocity variations, identifying them as post-common-envelope binary candidates.

Research paper thumbnail of 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

Monthly Notices of the Royal Astronomical Society, 2011

We identify SDSS J121010.1+334722.9 as an eclipsing post-common-envelope binary, with an orbital ... more We identify SDSS J121010.1+334722.9 as an eclipsing post-common-envelope binary, with an orbital period of P orb = 2.988 h, 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 Galaxy Evolution Explorer near-ultraviolet flux, yields a white dwarf temperature of T eff,WD = 6000 ± 200 K and a metallicity value of log [Z/H] = −2.0 ± 0.3. The Na I λλ8183.27, 8194.81 absorption doublet is used to measure the radial velocity of the secondary star, K sec = 251.7 ± 2.0 km s −1 , and Fe I absorption lines in the blue part of the spectrum provide the radial velocity of the white dwarf, K WD = 95.3 ± 2.1 km s −1 , yielding a mass ratio of q = 0.379 ± 0.009. Light-curve model fitting, using the Markov chain Monte Carlo method, gives the inclination angle as i = (79. • 05-79. • 36) ± 0. • 15, and the stellar masses as M WD = 0.415 ± 0.010 M and M sec = 0.158 ± 0.006 M. 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 R WD = (0.0157-0.0161) ± 0.0003 R and the volume-averaged radius of the tidally distorted secondary is R sec,vol.aver. = (0.197-0.203) ± 0.003 R. The white dwarf in SDSS J121010.1+334722.9 is a very strong He-core candidate.

Research paper thumbnail of The shortest period detached white dwarf + main-sequence binary

Monthly Notices of the Royal Astronomical Society, 2011

We present high-speed ULTRACAM and SOFI photometry and X-shooter spectroscopy of the recently dis... more We present high-speed ULTRACAM and SOFI photometry and X-shooter spectroscopy of the recently discovered 94-min orbital period eclipsing white dwarf/main-sequence binary SDSS J085746.18+034255.3 (CSS 03170) and use these observations to measure the system parameters. We detect a shallow secondary eclipse and hence are able to determine an orbital inclination of i = 85. • 5 ± 0. • 2. The white dwarf has a mass of 0.51 ± 0.05 M and a radius of 0.0247 ± 0.0008 R. With a temperature of 35 300 ± 400 K the white dwarf is highly overinflated if it has a carbon-oxygen core; however, if it has a helium core then its mass and radius are consistent with evolutionary models. Therefore, the white dwarf in SDSS J085746.18+034255.3 is most likely a helium core white dwarf with a mass close to the upper limit expected from evolution. The main-sequence star is an M8 dwarf with a mass of 0.09 ± 0.01 M and a radius of 0.110 ± 0.004 R placing it close to the hydrogen burning limit. The system emerged from a common envelope ∼20 million years ago and will reach a semidetached configuration in ∼400 million years, becoming a cataclysmic variable with a period of 66 min, below the period minimum.

Research paper thumbnail of Post-common envelope binaries from SDSS - III. Seven new orbital periods

Monthly Notices of the Royal Astronomical Society, 2008

We present follow-up spectroscopy and photometry of 11 post common envelope binary (PCEB) candida... more We present follow-up spectroscopy and photometry of 11 post common envelope binary (PCEB) candidates identified from multiple Sloan Digital Sky Survey (SDSS) spectroscopy in an earlier paper. Radial velocity measurements using the Na I λλ 8183.27,8194.81 absorption doublet were performed for nine of these systems and provided measurements of six orbital periods in the range P orb = 2.7 − 17.4 h. Three PCEB candidates did not show significant radial velocity variations in the follow-up data, and we discuss the implications for the use of SDSS spectroscopy alone to identify PCEBs. Differential photometry confirmed one of our spectroscopic orbital periods and provided one additional P orb measurement. Binary parameters are estimated for the seven objects for which we have measured the orbital period and the radial velocity amplitude of the low-mass companion star, K sec. So far, we have published nine SDSS PCEBs orbital periods, all of them P orb < 1 d. We perform Monte-Carlo simulations and show that 3σ SDSS radial velocity variations should still be detectable for systems in the orbital period range of P orb ∼ 1 − 10 days. Consequently, our results suggest that the number of PCEBs decreases considerably for P orb > 1 day, and that during the common envelope phase the orbital energy of the binary star is maybe less efficiently used to expell the envelope than frequently assumed.

Research paper thumbnail of A ZZ Ceti white dwarf in SDSS J133941.11+484727.5

Monthly Notices of the Royal Astronomical Society, 2006

We present time-resolved spectroscopy and photometry of the cataclysmic variable (CV) SDSS J13394... more We present time-resolved spectroscopy and photometry of the cataclysmic variable (CV) SDSS J133941.11+484727.5 (SDSS 1339) which has been discovered in the Sloan Digital Sky Survey (SDSS) Data Release 4. The orbital period determined from radial velocity studies is 82.524(24) min, close to the observed period minimum. The optical spectrum of SDSS 1339 is dominated to 90 per cent by emission from the white dwarf (WD). The spectrum can be successfully reproduced by a three-component model (white dwarf, disc, secondary) with T WD =12 500 K for a fixed log g = 8.0, d = 170 pc, and a spectral type of the secondary later than M8. The mass-transfer rate corresponding to the optical luminosity of the accretion disc is very low, 1.7 × 10 −13 M yr −1. Optical photometry reveals a coherent variability at 641 s with an amplitude of 0.025 mag, which we interpret as non-radial pulsations of the white dwarf. In addition, a long-period photometric variation with a period of either 320 or 344 min and an amplitude of 0.025 mag is detected, which bears no apparent relation with the orbital period of the system. Similar long-period photometric signals have been found in the CVs SDSS J123813.73−033933.0, SDSS J204817.85−061044.8, GW Lib and FS Aur, but so far no working model for this behaviour is available.

Research paper thumbnail of Post-common-envelope binaries from SDSS - V. Four eclipsing white dwarf main-sequence binaries

Monthly Notices of the Royal Astronomical Society, 2009

We identify SDSS 011009.09+132616.1, SDSS 030308.35+005444.1, SDSS 143547.87+ 373338.5 and SDSS 1... more We identify SDSS 011009.09+132616.1, SDSS 030308.35+005444.1, SDSS 143547.87+ 373338.5 and SDSS 154846.00+405728.8 as four eclipsing white dwarf plus main-sequence (WDMS) binaries from the Sloan Digital Sky Survey (SDSS), and report on follow-up observations of these systems. SDSS 0110+1326, SDSS 1435+3733 and SDSS 1548+4057 contain DA white dwarfs, while SDSS 0303+0054 contains a cool DC white dwarf. Orbital periods and ephemerides have been established from multiseason photometry. SDSS 1435+3733, with P orb = 3 h has the shortest orbital period of all known eclipsing WDMS binaries. As for the other systems, SDSS 0110+1326 has P orb = 8 h, SDSS 0303+0054 has P orb = 3.2 h and SDSS 1548+4057 has P orb = 4.4 h. Time-resolved spectroscopic observations have been obtained and the Hα and Ca II λλ8498.02, 8542.09, 8662.14 triplet emission lines, as well as the Na I λλ8183.27, 8194.81 absorption doublet were used to measure the radial velocities of the secondary stars in all four systems. A spectral decomposition/fitting technique was then employed to isolate the contribution of each of the components to the total spectrum, and to determine the white dwarf effective temperatures and surface gravities, as well as the spectral types of the companion stars. We used a light-curve modelling code for close binary systems to fit the eclipse profiles and the ellipsoidal modulation/reflection effect in the light curves, to further constrain the masses and radii of the components in all systems. All three DA white dwarfs have masses of M WD ∼ 0.4-0.6 M , in line with the expectations from close binary evolution. The DC white dwarf in SDSS 0303+0054 has a mass of M WD 0.85 M , making it unusually massive for a post-common-envelope system. The companion stars in all four systems are M dwarfs of spectral type M4 and later. Our new additions raise the number of known eclipsing WDMS binaries to 14, and we find that the average white dwarf mass in this sample is M WD =0.57 ± 0.16 M , only slightly lower than the average mass of single white dwarfs. The majority of all eclipsing WDMS binaries contain low-mass (<0.6 M) secondary stars, and will eventually provide valuable observational input for the calibration of the mass-radius relations of low-mass main-sequence stars and of white dwarfs.

Research paper thumbnail of Post-common envelope binaries from SDSS-X: the origin of low-mass white dwarfs

Monthly Notices of the Royal Astronomical Society, 2011

We present the first white dwarf mass distributions of a large and homogeneous sample of post-com... more We present the first white dwarf mass distributions of a large and homogeneous sample of post-common envelope binaries (PCEBs) and wide white dwarf main-sequence (WDMS) binaries directly obtained from observations. Both distributions are statistically independent, with PCEBs showing a clear concentration of systems towards the low-mass end of the distribution and the white dwarf mass distribution of wide WDMS binaries being similar to that of single white dwarfs. Our results provide evidence that the majority of low-mass (M wd 0.5 M) white dwarfs are formed in close binaries.

Research paper thumbnail of Stream overflow and dwarf nova eruptions

Monthly Notices of the Royal Astronomical Society, 1998

We consider the effects of accretion stream overflow on the viscous dynamics of accretion discs i... more We consider the effects of accretion stream overflow on the viscous dynamics of accretion discs in dwarf novae. If the stream from the secondary star is geometrically thick enough, some fraction of its material can flow over and under the disc. The mass and specific angular momentum of the stream are then deposited not only at the point of collision with the outer disc, but also at those radii in the inner disc with geometric heights that are large enough to intercept the residual stream, or near the radius where the disc has the same specific angular momentum as the stream. The overflowing stream can alter the behaviour of heating fronts and cooling fronts in the disc. If the mass fraction of the overflowing stream is of order tens of per cent, the deposition of mass in the inner parts of the disc is sufficient to change the character of the eruption light curves significantly.

Research paper thumbnail of The White Dwarf Binary Pathways Survey −III. Contamination from hierarchical triples containing a white dwarf

Monthly Notices of the Royal Astronomical Society, 2020

The White Dwarf Binary Pathways Survey aims at increasing the number of known detached A, F, G, a... more The White Dwarf Binary Pathways Survey aims at increasing the number of known detached A, F, G, and K main-sequence stars in close orbits with white dwarf companions (WD+AFGK binaries) to refine our understanding about compact binary evolution and the nature of Supernova Ia progenitors. These close WD+AFGK binary stars are expected to form through common envelope evolution, in which tidal forces tend to circularize the orbit. However, some of the identified WD+AFGK binary candidates show eccentric orbits, indicating that these systems are either formed through a different mechanism or perhaps they are not close WD+AFGK binaries. We observed one of these eccentric WD+AFGK binaries with SPHERE and find that the system TYC 7218-934-1 is in fact a triple system where the WD is a distant companion. The inner binary likely consists of the G-type star plus an unseen low-mass companion in an eccentric orbit. Based on this finding, we estimate the fraction of triple systems that could contam...

Research paper thumbnail of Understanding white dwarf binary evolution with white dwarf/main sequence binaries: first results from SEGUE

arXiv (Cornell University), Sep 1, 2007

Close white dwarf binaries make up a wide variety of objects such as double white dwarf binaries,... more Close white dwarf binaries make up a wide variety of objects such as double white dwarf binaries, which are possible SN Ia progenitors, cataclysmic variables, super soft sources, or AM CVn stars. The evolution and formation of close white dwarf binaries crucially depends on the rate at which angular momentum is extracted from the binary orbit. The two most important sources of angular momentum loss are the common envelope phase and magnetic braking. Both processes are so far poorly understood. Observational population studies of white dwarf/main sequence binaries provide the potential to significantly progress with this situation and to clearly constrain magnetic braking and the CE-phase. However, the current population of white dwarf/main sequence binaries is highly incomplete and heavily biased towards young systems containing hot white dwarfs. The SDSSII/SEGUE collaboration awarded us with 5 fibers per plate pair in order to fill this gap and to identify the required unbiased sample of old white dwarf/main sequence binaries. The success rate of our selection criteria exceeds 65% and during the first 10 months we have identified 41 new systems, most of them belonging to the missed old population.

Research paper thumbnail of Disc accretion onto white dwarfs

Disc accretion onto white dwarfs In non-magnetic cataclysmic variables (CVs) a white dwarf accret... more Disc accretion onto white dwarfs In non-magnetic cataclysmic variables (CVs) a white dwarf accretes matter from a mainsequence secondary star via an accretion disc. The dynamical behaviour of the accretion disc determines the accretion rate onto the white dwarf. Thermal instabilities in the accretion disc associated with the ionisation of hydrogen can lead to a limit-cycle behaviour in which the disc switches quasi-periodically between high and low accretion states. This thermal limit-cycle model is the generally accepted explanation for dwarf nova outbursts observed in many CVs. The process of disc accretion in non-magnetic CVs is subject to a number of external conditions, namely mass transfer variations of the secondary star, stream overflow and irradiation by the white dwarf. In this thesis I develop a model for time-dependent disc accretion onto white dwarfs and analyse the influence of these external conditions on the accretion process. I examine the effects of mass transfer variations by deriving real mass transfer variations from light curve monitoring of the disc-less CV AM Her. These mass transfer variations I include in simulations of disc accretion onto white dwarfs in non-magnetic systems and find that the mass accretion rate of the disc relaxes to an equilibrium with the prevailing mass transfer rate on a rather short timescale. I conclude that the observed changes in outburst duration and outburst magnitude are caused by nearly simultaneous variations of the mass loss rate from the secondary. I also present a new model for the stripping of the stream by the accretion disc, and find that stream overflow can have subtle effects on the evolution of the accretion disc only if the amount of overflowing stream material exceeds 25% of the mass transfer rate. I conclude that solely very large stream overflow fractions can change the outbursts of dwarf novae. For realistic amounts of stream overflow the overall outburst behaviour is marginally changed. The accretion disc is mostly influenced by the white dwarf irradiation. I present a selfconsistent model for irradiated accretion discs and find that efficient irradiation in dwarf nova systems causes small "echo" outbursts following the larger ones immediately. This result contrasts with the observations of dwarf nova outbursts. As an explanation for this discrepancy I suggest that the reprocessing efficiency of disc irradiation is rather small. This is in agreement with results I obtain from detailed simulations of irradiated discs in post novae. These systems are excellent laboratories for studying the effects of disc irradiation because the white dwarf heated during the nova eruption provides a much stronger irradiation of the disc than in normal dwarf novae. I derive time-limits for the occurrence of dwarf nova outbursts in post novae and present detailed simulations of the evolution of irradiated discs in post novae. In addition to the developed theory of irradiated discs around white dwarfs, I show preliminary results of an intensive observing campaign on the post nova system V446 Her. Finally, discussing the influence of disc irradiation on the post nova evolution in the light of the current working hypothesis leads me to put forward a new scenario.

Research paper thumbnail of Understanding the evolution of close white dwarf binaries

arXiv: Solar and Stellar Astrophysics, 2019

Interacting binaries containing white dwarfs can lead to a variety of outcomes that range from po... more Interacting binaries containing white dwarfs can lead to a variety of outcomes that range from powerful thermonuclear explosions, which are important in the chemical evolution of galaxies and as cosmological distance estimators, to strong sources of low frequency gravitational wave radiation, which makes them ideal calibrators for the gravitational low-frequency wave detector LISA mission. However, current theoretical evolution models still fail to explain the observed properties of the known populations of white dwarfs in both interacting and detached binaries. Major limitations are that the existing population models have generally been developed to explain the properties of sub-samples of these systems, occupying small volumes of the vast parameter space, and that the observed samples are severely biased. The overarching goal for the next decade is to assemble a large and homogeneous sample of white dwarf binaries that spans the entire range of evolutionary states, to obtain prec...

Research paper thumbnail of Cold Giant Planets Evaporated by Hot White Dwarfs

The Astrophysical Journal, 2019

Atmospheric escape from close-in Neptunes and hot Jupiters around Sun-like stars driven by extrem... more Atmospheric escape from close-in Neptunes and hot Jupiters around Sun-like stars driven by extreme ultraviolet (EUV) irradiation plays an important role in the evolution of exoplanets and in shaping their ensemble properties. Intermediate and low mass stars are brightest at EUV wavelengths at the very end of their lives, after they have expelled their envelopes and evolved into hot white dwarfs. Yet the effect of the intense EUV irradiation of giant planets orbiting young white dwarfs has not been assessed. We show that the giant planets in the solar system will experience significant hydrodynamic escape caused by the EUV irradiation from the white dwarf left behind by the Sun. A fraction of the evaporated volatiles will be accreted by the solar white dwarf, resulting in detectable photospheric absorption lines. As a large number of the currently known extrasolar giant planets will survive the metamorphosis of their host stars into white dwarfs, observational signatures of accretion from evaporating planetary atmospheres are expected to be common. In fact, one-third of the known hot single white dwarfs show photospheric absorption lines of volatile elements, which we argue are indicative of ongoing accretion from evaporating planets. The fraction of volatile contaminated hot white dwarfs strongly decreases as they cool. We show that accretion from evaporating planetary atmospheres naturally explains this temperature dependence if more than 50% of hot white dwarfs still host giant planets.

Research paper thumbnail of No cataclysmic variables missing: higher merger rate brings into agreement observed and predicted space densities

Monthly Notices of the Royal Astronomical Society, 2018

The predicted and observed space density of cataclysmic variables (CVs) has been for a long time ... more The predicted and observed space density of cataclysmic variables (CVs) has been for a long time discrepant by at least an order of magnitude. The standard model of CV evolution predicts that the vast majority of CVs should be period bouncers, whose space density has been recently measured to be ρ 2 × 10 −5 pc −3. We performed population synthesis of CVs using an updated version of the Binary Stellar Evolution (BSE) code for single and binary star evolution. We find that the recently suggested empirical prescription of consequential angular momentum loss (CAML) brings into agreement predicted and observed space densities of CVs and period bouncers. To progress with our understanding of CV evolution it is crucial to understand the physical mechanism behind empirical CAML. Our changes to the BSE code are also provided in details, which will allow the community to accurately model mass transfer in interacting binaries in which degenerate objects accrete from low-mass main-sequence donor stars.

Research paper thumbnail of Three in one go: consequential angular momentum loss can solve major problems of CV evolution

Monthly Notices of the Royal Astronomical Society: Letters, 2015

The average white dwarf (WD) masses in cataclysmic variables (CVs) have been measured to signific... more The average white dwarf (WD) masses in cataclysmic variables (CVs) have been measured to significantly exceed those of single WDs, which is the opposite of what is theoretically expected. We present the results of binary population synthesis models taking into account consequential angular momentum loss (CAML) that is assumed to increase with decreasing WD mass. This approach cannot only solve the WD mass problem but also brings in agreement theoretical predictions and observations of the orbital period distribution and the space density of CVs. We speculate that frictional angular momentum loss following nova eruptions might cause such CAML and could be the missing ingredient of CV evolution.

Research paper thumbnail of On the occurrence of dwarf nova outbursts in post novae

We show that irradiation of the accretion disc by the white dwarf limits the occurrence of dwarf ... more We show that irradiation of the accretion disc by the white dwarf limits the occurrence of dwarf nova outbursts in post nova accretion discs. After the nova explosion, the white dwarf has to cool for up to ∼ 100 yr-depending on the orbital period (i.e., disc size) and the temperature of the white dwarf after the nova-eruption-before the disc can begin producing dwarf nova outbursts. During this time the inner disc is maintained in a hot, ionised state. Using these calculations, we interpret the longterm photometric variability of the post nova V446 Her (Nova Her 1960) which shows today regular dwarf nova outbursts. As the white dwarf in V446 Her continues to cool over the next ∼10-20 yr, we predict an increase in the amplitude of outbursts and a decrease of the outburst frequency, because the decreasing irradiation of the accretion disc should allow an increasing annular extent of the accretion disc to participate in limit cycle oscillations.

Research paper thumbnail of The response of a dwarf nova disc to real mass transfer variations

The response of a dwarf nova disc to real mass transfer variations

Astronomy & Astrophysics - ASTRON ASTROPHYS, 2000

We present simulations of dwarf nova outbursts taking into account realistic variations of the ma... more We present simulations of dwarf nova outbursts taking into account realistic variations of the mass loss rate from the secondary. The mass transfer variation has been derived from 20 years of visual monitoring and from X-ray observations covering various accretion states of the discless cataclysmic variable AM Herculis. We find that the outburst behaviour of a fictitious dwarf nova with the same system parameters as AM Her is strongly influenced by these variations of the mass loss rate. Depending on the mass loss rate, the disc produces either long outbursts, a cycle of one long outburst followed by two short outbursts, or only short outbursts. The course of the transfer rate dominates the shape of the outbursts because the mass accreted during an outburst cycle roughly equals the mass transferred from the secondary over the outburst interval. Only for less than 10% of the simulated time, when the mass transfer rate is nearly constant, the disc is in a quasi-stationary state during...

Research paper thumbnail of MULTI-SITE OBSERVATIONS OF PULSATION IN THE ACCRETING WHITE DWARF SDSS J161033.64–010223.3 (V386 Ser)

MULTI-SITE OBSERVATIONS OF PULSATION IN THE ACCRETING WHITE DWARF SDSS J161033.64–010223.3 (V386 Ser)

The Astrophysical Journal, 2010

Research paper thumbnail of Cataclysmic Variables from Sdss. VII. The Seventh Year (2006)

Cataclysmic Variables from Sdss. VII. The Seventh Year (2006)

The Astronomical Journal, 2009

Research paper thumbnail of Post-common-envelope binaries from SDSS - I. 101 white dwarf main-sequence binaries with multiple Sloan Digital Sky Survey spectroscopy

Monthly Notices of the Royal Astronomical Society, 2007

We present a detailed analysis of 101 white dwarf main-sequence binaries (WDMS) from the Sloan Di... more We present a detailed analysis of 101 white dwarf main-sequence binaries (WDMS) from the Sloan Digital Sky Survey (SDSS) for which multiple SDSS spectra are available. We detect significant radial velocity variations in 18 WDMS, identifying them as post-commonenvelope binaries (PCEBs) or strong PCEB candidates. Strict upper limits to the orbital periods are calculated, ranging from 0.43 to 7880 d. Given the sparse temporal sampling and relatively low spectral resolution of the SDSS spectra, our results imply a PCEB fraction of 15 per cent among the WDMS in the SDSS data base. Using a spectral decomposition/fitting technique we determined the white dwarf effective temperatures and surface gravities, masses and secondary star spectral types for all WDMS in our sample. Two independent distance estimates are obtained from the flux-scaling factors between the WDMS spectra, and the white dwarf models and main-sequence star templates, respectively. Approximately one-third of the systems in our sample show a significant discrepancy between the two distance estimates. In the majority of discrepant cases, the distance estimate based on the secondary star is too large. A possible explanation for this behaviour is that the secondary star spectral types that we determined from the SDSS spectra are systematically too early by one to two spectral classes. This behaviour could be explained by stellar activity, if covering a significant fraction of the star by cool dark spots will raise the temperature of the interspot regions. Finally, we discuss the selection effects of the WDMS sample provided by the SDSS project.

Research paper thumbnail of Post-common envelope binaries from SDSS - VII. A catalogue of white dwarf-main sequence binaries

Monthly Notices of the Royal Astronomical Society, 2009

We present a catalogue of 1602 white-dwarf-main-sequence (WDMS) binaries from the spectroscopic S... more We present a catalogue of 1602 white-dwarf-main-sequence (WDMS) binaries from the spectroscopic Sloan Digital Sky Survey Data Release 6 (SDSS DR6). Among these, we identify 440 as new WDMS binaries. We select WDMS binary candidates by template fitting all 1.27 million DR6 spectra, using combined constraints in both χ 2 and signal-to-noise ratio. In addition, we use Galaxy Evolution Explorer (GALEX) and UKIRT Infrared Sky Survey (UKIDSS) magnitudes to search for objects in which one of the two components dominates the SDSS spectrum. We use a decomposition/fitting technique to measure the effective temperatures, surface gravities, masses and distances to the white dwarfs, as well as the spectral types and distances to the companions in our catalogue. Distributions and density maps obtained from these stellar parameters are then used to study both the general properties and the selection effects of WDMS binaries in the SDSS. A comparison between the distances measured to the white dwarfs and the main-sequence companions shows d sec > d wd for approximately one-fifth of the systems, a tendency already found in our previous work. The hypothesis that magnetic activity raises the temperature of the inter-spot regions in active stars that are heavily covered by cool spots, leading to a bluer optical colour compared to inactive stars, remains the best explanation for this behaviour. We also make use of SDSS-GALEX-UKIDSS magnitudes to investigate the distribution of WDMS binaries, as well as their white-dwarf effective temperatures and companion star spectral types, in ultraviolet to infrared colour space. We show that WDMS binaries can be very efficiently separated from single main-sequence stars and white dwarfs when using a combined ultraviolet, optical and infrared colour selection. Finally, we also provide radial velocities for 1068 systems measured from the Na I λλ8183.27, 8194.81 absorption doublet and/or the Hα emission line. Among the systems with multiple SDSS spectroscopy, we find five new systems exhibiting significant radial velocity variations, identifying them as post-common-envelope binary candidates.

Research paper thumbnail of 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

Monthly Notices of the Royal Astronomical Society, 2011

We identify SDSS J121010.1+334722.9 as an eclipsing post-common-envelope binary, with an orbital ... more We identify SDSS J121010.1+334722.9 as an eclipsing post-common-envelope binary, with an orbital period of P orb = 2.988 h, 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 Galaxy Evolution Explorer near-ultraviolet flux, yields a white dwarf temperature of T eff,WD = 6000 ± 200 K and a metallicity value of log [Z/H] = −2.0 ± 0.3. The Na I λλ8183.27, 8194.81 absorption doublet is used to measure the radial velocity of the secondary star, K sec = 251.7 ± 2.0 km s −1 , and Fe I absorption lines in the blue part of the spectrum provide the radial velocity of the white dwarf, K WD = 95.3 ± 2.1 km s −1 , yielding a mass ratio of q = 0.379 ± 0.009. Light-curve model fitting, using the Markov chain Monte Carlo method, gives the inclination angle as i = (79. • 05-79. • 36) ± 0. • 15, and the stellar masses as M WD = 0.415 ± 0.010 M and M sec = 0.158 ± 0.006 M. 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 R WD = (0.0157-0.0161) ± 0.0003 R and the volume-averaged radius of the tidally distorted secondary is R sec,vol.aver. = (0.197-0.203) ± 0.003 R. The white dwarf in SDSS J121010.1+334722.9 is a very strong He-core candidate.

Research paper thumbnail of The shortest period detached white dwarf + main-sequence binary

Monthly Notices of the Royal Astronomical Society, 2011

We present high-speed ULTRACAM and SOFI photometry and X-shooter spectroscopy of the recently dis... more We present high-speed ULTRACAM and SOFI photometry and X-shooter spectroscopy of the recently discovered 94-min orbital period eclipsing white dwarf/main-sequence binary SDSS J085746.18+034255.3 (CSS 03170) and use these observations to measure the system parameters. We detect a shallow secondary eclipse and hence are able to determine an orbital inclination of i = 85. • 5 ± 0. • 2. The white dwarf has a mass of 0.51 ± 0.05 M and a radius of 0.0247 ± 0.0008 R. With a temperature of 35 300 ± 400 K the white dwarf is highly overinflated if it has a carbon-oxygen core; however, if it has a helium core then its mass and radius are consistent with evolutionary models. Therefore, the white dwarf in SDSS J085746.18+034255.3 is most likely a helium core white dwarf with a mass close to the upper limit expected from evolution. The main-sequence star is an M8 dwarf with a mass of 0.09 ± 0.01 M and a radius of 0.110 ± 0.004 R placing it close to the hydrogen burning limit. The system emerged from a common envelope ∼20 million years ago and will reach a semidetached configuration in ∼400 million years, becoming a cataclysmic variable with a period of 66 min, below the period minimum.

Research paper thumbnail of Post-common envelope binaries from SDSS - III. Seven new orbital periods

Monthly Notices of the Royal Astronomical Society, 2008

We present follow-up spectroscopy and photometry of 11 post common envelope binary (PCEB) candida... more We present follow-up spectroscopy and photometry of 11 post common envelope binary (PCEB) candidates identified from multiple Sloan Digital Sky Survey (SDSS) spectroscopy in an earlier paper. Radial velocity measurements using the Na I λλ 8183.27,8194.81 absorption doublet were performed for nine of these systems and provided measurements of six orbital periods in the range P orb = 2.7 − 17.4 h. Three PCEB candidates did not show significant radial velocity variations in the follow-up data, and we discuss the implications for the use of SDSS spectroscopy alone to identify PCEBs. Differential photometry confirmed one of our spectroscopic orbital periods and provided one additional P orb measurement. Binary parameters are estimated for the seven objects for which we have measured the orbital period and the radial velocity amplitude of the low-mass companion star, K sec. So far, we have published nine SDSS PCEBs orbital periods, all of them P orb < 1 d. We perform Monte-Carlo simulations and show that 3σ SDSS radial velocity variations should still be detectable for systems in the orbital period range of P orb ∼ 1 − 10 days. Consequently, our results suggest that the number of PCEBs decreases considerably for P orb > 1 day, and that during the common envelope phase the orbital energy of the binary star is maybe less efficiently used to expell the envelope than frequently assumed.

Research paper thumbnail of A ZZ Ceti white dwarf in SDSS J133941.11+484727.5

Monthly Notices of the Royal Astronomical Society, 2006

We present time-resolved spectroscopy and photometry of the cataclysmic variable (CV) SDSS J13394... more We present time-resolved spectroscopy and photometry of the cataclysmic variable (CV) SDSS J133941.11+484727.5 (SDSS 1339) which has been discovered in the Sloan Digital Sky Survey (SDSS) Data Release 4. The orbital period determined from radial velocity studies is 82.524(24) min, close to the observed period minimum. The optical spectrum of SDSS 1339 is dominated to 90 per cent by emission from the white dwarf (WD). The spectrum can be successfully reproduced by a three-component model (white dwarf, disc, secondary) with T WD =12 500 K for a fixed log g = 8.0, d = 170 pc, and a spectral type of the secondary later than M8. The mass-transfer rate corresponding to the optical luminosity of the accretion disc is very low, 1.7 × 10 −13 M yr −1. Optical photometry reveals a coherent variability at 641 s with an amplitude of 0.025 mag, which we interpret as non-radial pulsations of the white dwarf. In addition, a long-period photometric variation with a period of either 320 or 344 min and an amplitude of 0.025 mag is detected, which bears no apparent relation with the orbital period of the system. Similar long-period photometric signals have been found in the CVs SDSS J123813.73−033933.0, SDSS J204817.85−061044.8, GW Lib and FS Aur, but so far no working model for this behaviour is available.

Research paper thumbnail of Post-common-envelope binaries from SDSS - V. Four eclipsing white dwarf main-sequence binaries

Monthly Notices of the Royal Astronomical Society, 2009

We identify SDSS 011009.09+132616.1, SDSS 030308.35+005444.1, SDSS 143547.87+ 373338.5 and SDSS 1... more We identify SDSS 011009.09+132616.1, SDSS 030308.35+005444.1, SDSS 143547.87+ 373338.5 and SDSS 154846.00+405728.8 as four eclipsing white dwarf plus main-sequence (WDMS) binaries from the Sloan Digital Sky Survey (SDSS), and report on follow-up observations of these systems. SDSS 0110+1326, SDSS 1435+3733 and SDSS 1548+4057 contain DA white dwarfs, while SDSS 0303+0054 contains a cool DC white dwarf. Orbital periods and ephemerides have been established from multiseason photometry. SDSS 1435+3733, with P orb = 3 h has the shortest orbital period of all known eclipsing WDMS binaries. As for the other systems, SDSS 0110+1326 has P orb = 8 h, SDSS 0303+0054 has P orb = 3.2 h and SDSS 1548+4057 has P orb = 4.4 h. Time-resolved spectroscopic observations have been obtained and the Hα and Ca II λλ8498.02, 8542.09, 8662.14 triplet emission lines, as well as the Na I λλ8183.27, 8194.81 absorption doublet were used to measure the radial velocities of the secondary stars in all four systems. A spectral decomposition/fitting technique was then employed to isolate the contribution of each of the components to the total spectrum, and to determine the white dwarf effective temperatures and surface gravities, as well as the spectral types of the companion stars. We used a light-curve modelling code for close binary systems to fit the eclipse profiles and the ellipsoidal modulation/reflection effect in the light curves, to further constrain the masses and radii of the components in all systems. All three DA white dwarfs have masses of M WD ∼ 0.4-0.6 M , in line with the expectations from close binary evolution. The DC white dwarf in SDSS 0303+0054 has a mass of M WD 0.85 M , making it unusually massive for a post-common-envelope system. The companion stars in all four systems are M dwarfs of spectral type M4 and later. Our new additions raise the number of known eclipsing WDMS binaries to 14, and we find that the average white dwarf mass in this sample is M WD =0.57 ± 0.16 M , only slightly lower than the average mass of single white dwarfs. The majority of all eclipsing WDMS binaries contain low-mass (<0.6 M) secondary stars, and will eventually provide valuable observational input for the calibration of the mass-radius relations of low-mass main-sequence stars and of white dwarfs.

Research paper thumbnail of Post-common envelope binaries from SDSS-X: the origin of low-mass white dwarfs

Monthly Notices of the Royal Astronomical Society, 2011

We present the first white dwarf mass distributions of a large and homogeneous sample of post-com... more We present the first white dwarf mass distributions of a large and homogeneous sample of post-common envelope binaries (PCEBs) and wide white dwarf main-sequence (WDMS) binaries directly obtained from observations. Both distributions are statistically independent, with PCEBs showing a clear concentration of systems towards the low-mass end of the distribution and the white dwarf mass distribution of wide WDMS binaries being similar to that of single white dwarfs. Our results provide evidence that the majority of low-mass (M wd 0.5 M) white dwarfs are formed in close binaries.

Research paper thumbnail of Stream overflow and dwarf nova eruptions

Monthly Notices of the Royal Astronomical Society, 1998

We consider the effects of accretion stream overflow on the viscous dynamics of accretion discs i... more We consider the effects of accretion stream overflow on the viscous dynamics of accretion discs in dwarf novae. If the stream from the secondary star is geometrically thick enough, some fraction of its material can flow over and under the disc. The mass and specific angular momentum of the stream are then deposited not only at the point of collision with the outer disc, but also at those radii in the inner disc with geometric heights that are large enough to intercept the residual stream, or near the radius where the disc has the same specific angular momentum as the stream. The overflowing stream can alter the behaviour of heating fronts and cooling fronts in the disc. If the mass fraction of the overflowing stream is of order tens of per cent, the deposition of mass in the inner parts of the disc is sufficient to change the character of the eruption light curves significantly.