The progenitors of magnetic white dwarfs in open clusters

According to the fossil-field hypothesis magnetic fields are remnants of the previous stages of evolution. However, population synthesis calculations are unable to reproduce the magnetic white dwarf (MWD) sample without binary interaction or inclusion of a population of progenitor with unobservable small-scale fields. One necessary ingredient in population synthesis is the initial-to-final-mass relation (IFMR) which describes the mass-loss processes during the stellar evolution. When white dwarfs are members of open clusters, their evolutionary histories can be assessed through the use of cluster properties. In this work, we assess the cluster membership by correlating the proper-motion of MWDs with the cluster proper-motion and by analyzing the candidates spectroscopically with our magnetic model spectra in order to estimate the effective temperature and radii. We identified SDSS J085523.87+164059.0 to be a proper-motion member of Praesepe. We also included the data of the formerly identified cluster members NGC 6819-8, WD 0836+201 and estimated the mass, cooling age and the progenitor masses of the three probable MWD members of open clusters. According to our analysis, the newly identified cluster member SDSS J085523.87+164059.0 is an ultra-massive MWD of mass 1.12 $\pm$ 0.11 Msolar. We increase the sample of MWDs with known progenitor masses to ten, with the rest of the data coming from the common proper motion binaries. Our investigations show that, when effects of the magnetic fields are included in the diagnostics, the estimated properties of these cluster MWDs do not show evidence for deviations from the IFMR. Furthermore, we estimate the precision of the magnetic diagnostics which would be necessary to determine quantitatively whether magnetism has any effect on the mass-loss.Comment: 8 pages, 4 figures, accepted for publication in A&

Analysis of hydrogen-rich magnetic white dwarfs detected in the Sloan Digital Sky Survey

Context A large number of magnetic white dwarfs discovered in the SDSS have so far only been analyzed by visual comparison of the observations with relatively simple models of the radiation transport in a magnetised stellar atmosphere. Aims We model the structure of the surface magnetic fields of the hydrogen-rich white dwarfs in the SDSS. Methods We calculated a grid of state-of-the-art theoretical optical spectra of hydrogen-rich magnetic white dwarfs (WDs) with magnetic field strengths of between 1 MG and 1200 MG for different angles between the magnetic field vector and the line of sight,and for effective temperatures between 7000 K and 50 000 K. We used a least squares minimization scheme with an evolutionary algorithm to find the best-fit magnetic field geometry of the observed data. We used centered dipoles or dipoles that had been shifted along the dipole axis to model the coadded SDSS fiber spectrum of each object. Result We analyzed the spectra of all known magnetic hydrogen-rich (DA) WDs from the SDSS (97 previously published, plus 44 newly discovered) and also investigated the statistical properties of the magnetic field geometries of this sample. Conclusions The total number of known magnetic white dwarfs has already been more than tripled by the SDSS and more objects are expected after more systematic searches. The magnetic fields have strengths of between ≈1 and 900 MG. Our results further support the claims that Ap/Bp population is insufficient in generating the numbers and field strength distributions of the observed MWDs, and that of either another source of progenitor types or binary evolution is needed. Clear indications of non-centered dipoles exist in about ∼50%, of the objects which is consistent with the magnetic field distribution observed in Ap/Bp stars

Constraints on the origin of the massive, hot, and rapidly rotating magnetic white dwarf RE J 0317-853 from an HST parallax measurement

We use the parallax measurements of RE J 0317-853 to determine its mass, radius, and cooling age and thereby constrain its evolutionary origins. We observed RE J 0317-853 with the the Hubble Space Telescope's Fine Guidance System to measure the parallax of RE J 0317-853 and its binary companion, the non-magnetic white dwarf LB 9802. In addition, we acquired spectra of comparison stars with the Boller & Chivens spectrograph of the SMARTS telescope to correct the parallax zero point. For the corrected parallax, we determine the radius, mass, and the cooling age with the help of evolutionary models from the literature. The properties of RE J 0317-853 are constrained using the parallax information. We discuss the different cases of the core composition and the uncertain effective temperature. We confirm that RE J 0317-853 is close to the Chandrasekhar's mass limit in all cases and almost as old as its companion LB 9802. The precise evolutionary history of RE J 0317-853 depends on our knowledge of its effective temperature. It is possible that it had a single star progenitor possible if we assume that the effective temperature is at the cooler end of the possible range from 30000 to 50000; if T_eff is instead at the hotter end, a binary-merger scenario for RE J 0317-853 becomes more plausible.Comment: 11 pages, 6 figures; language revised edition with added references. Accepted for publication in Astronomy and Astrophysic

Analysis of the Hydrogen-rich Magnetic White Dwarfs in the SDSS

We have calculated optical spectra of hydrogen-rich (DA) white dwarfs with magnetic field strengths between 1 MG and 1000 MG for temperatures between 7000 K and 50000 K. Through a least-squares minimization scheme with an evolutionary algorithm, we have analyzed the spectra of 114 magnetic DAs from the SDSS (95 previously published plus 14 newly discovered within SDSS, and five discovered by SEGUE). Since we were limited to a single spectrum for each object we used only centered magnetic dipoles or dipoles which were shifted along the magnetic dipole axis. We also statistically investigated the distribution of magnetic-field strengths and geometries of our sample.Comment: to appear in the proceedings of the 16th European Workshop on White Dwarfs, Barcelona, 200

Two new young, wide, magnetic + non-magnetic double-degenerate binary systems

We report the discovery of two, new, rare, wide, double-degenerate binaries that each contain a magnetic and a non-magnetic star. The components of SDSS J092646.88+132134.5 + J092647.00+132138.4 and of SDSS J150746.48+521002.1 + J150746.80+520958.0 have angular separations of only 4.6 arcsec (a∼ 650 au) and 5.1 arcsec (a∼ 750 au), respectively. They also appear to share common proper motions. Follow-up optical spectroscopy has revealed each system to consist of a DA and a H-rich high-field magnetic white dwarf (HFMWD). Our measurements of the effective temperatures and the surface gravities of the DA components reveal both to have larger masses than is typical of field white dwarfs. By assuming that these degenerates have evolved essentially as single stars, owing to their wide orbital separations, we can use them to place limits on the total ages of the stellar systems. These suggest that in each case the HFMWD is probably associated with an early-type progenitor (Minit > 2 M⊙). We find that the cooling time of SDSS J150746.80+520958.0 (DAH) is lower than might be expected had it followed the evolutionary path of a typical single star. This mild discord is in the same sense as that observed for two of the small number of other HFMWDs for which progenitor mass estimates have been made, RE J0317-853 and EG 59. The mass of the other DAH, SDSS J092646.88+132134.5, appears to be smaller than expected on the basis of single-star evolution. If this object was/is a member of a hierarchical triple system it may have experienced greater mass loss during an earlier phase of its life as a result of its having a close companion. The large uncertainties on our estimates of the parameters of the HFMWDs suggest that a larger sample of these objects is required to firmly identify any trends in their inferred cooling times and progenitor masses. This should shed further light on their formation and on the impact magnetic fields have on the late stages of stellar evolution. To serve as a starting point, we highlight two further candidate young, wide magnetic + non-magnetic double-degenerate systems within SDSS, CBS 229 and SDSS J074853.07+302543.5 + J074852.95+302543.4, which should be subjected to detailed (resolved) spectroscopic follow-up studie

Analysis of hydrogen-rich magnetic white dwarfs detected in the Sloan Digital Sky Survey

We model the structure of the surface magnetic fields of the hydrogen-rich white dwarfs in the SDSS. We have calculated a grid of state-of-the-art theoretical optical spectra of hydrogen-rich magnetic white dwarfs with magnetic field strengths between 1 MG and 1200 MG for different angles, and for effective temperatures between 7000 K and 50000 K. We used a least-squares minimization scheme with an evolutionary algorithm in order to find the magnetic field geometry best fitting the observed data. We used simple centered dipoles or dipoles which were shifted along the dipole axis to model the coadded SDSS fiber spectrum of each object. We have analysed the spectra of all known magnetic DAs from the SDSS (97 previously published plus 44 newly discovered) and also investigated the statistical properties of magnetic field geometries of this sample. The total number of known magnetic white dwarfs already more than tripled by the SDSS and more objects are expected from a more systematic search. The magnetic fields span a range between ~1 and 900 MG. Our results further support the claim that Ap/Bp population is insufficient in generating the numbers and field strength distributions of the observed MWDs, and either another source of progenitor types or binary evolution is needed. Moreover clear indications for non-centered dipoles exist in about ~50% of the objects which is consistent with the magnetic field distribution observed in Ap/Bp stars.Comment: 15 pages, accepted for publication in A&A. For online version with full appendix figures, see http://www.ari.uni-heidelberg.de/mitarbeiter/bkulebi/papers/12570_online.pd

A magnetic white dwarf in a detached eclipsing binary

SDSS J030308.35+005444.1 is a close, detached, eclipsing white dwarf plus M dwarf binary which shows a large infrared excess which has been interpreted in terms of a circumbinary dust disc. In this paper, we present optical and near-infrared photometric and spectroscopic data for this system. At optical wavelengths, we observe heated pole caps from the white dwarf caused by accretion of wind material from the main-sequence star on to the white dwarf. At near-infrared wavelengths, we see the eclipse of two poles on the surface of the white dwarf by the main-sequence star indicating that the white dwarf is magnetic. Our spectroscopic observations reveal Zeeman-split emission lines in the hydrogen Balmer series, which we use to measure the magnetic field strength as 8 MG. This measurement indicates that the cyclotron lines are located in the infrared, naturally explaining the infrared excess without the need for a circumbinary dust disc. We also detect magnetically confined material located roughly midway between the two stars. Using measurements of the radial velocity amplitude and rotational broadening of the M star, we constrain the physical parameters of the system, a first for a magnetic white dwarf, and the location of the poles on the surface of the white dwarf. SDSS J030308.35+005444.1 is a pre-cataclysmic variable that will likely evolve into an intermediate polar in ∼1 Gyr

The fight for accretion: discovery of intermittent mass transfer in BB Doradus in the low state

Our long-term photometric monitoring of southern nova-like cataclysmic variables with the 1.3-m SMARTS telescope found BB Doradus fading from V ~ 14.3 towards a deep low state at V ~ 19.3 in April 2008. Here we present time-resolved optical spectroscopy of BB Dor in this faint state in 2009. The optical spectrum in quiescence is a composite of a hot white dwarf with Teff = 30000 +- 5000 K and a M3-4 secondary star with narrow emission lines (mainly of the Balmer series and HeI) superposed. We associate these narrow profiles with an origin on the donor star. Analysis of the radial velocity curve of the H-alpha emission from the donor star allowed the measurement of an orbital period of 0.154095 +- 0.000003 d (3.69828 +- 0.00007 h), different from all previous estimates. We detected episodic accretion events which veiled the spectra of both stars and radically changed the line profiles within a timescale of tens of minutes. This shows that accretion is not completely quenched in the low state. During these accretion episodes the line wings are stronger and their radial velocity curve is delayed by ~ 0.2 cycle, similar to that observed in SW Sex and AM Her stars in the high state, with respect to the motion of the white dwarf. Two scenarios are proposed to explain the extra emission: impact of the material on the outer edge of a cold, remnant accretion disc, or the combined action of a moderately magnetic white dwarf (B1 <~ 5 MG) and the magnetic activity of the donor star.Comment: 10 pages, 10 figures, accepted by MNRA

Two new young, wide magnetic + non-magnetic double-degenerate binary systems

Abridged: We report the discovery of two, new, rare, wide, double-degenerate binaries that each contain a magnetic and a non-magnetic star. The components of SDSSJ092646.88+132134.5 + J092647.00+132138.4 and SDSSJ150746.48+521002.1 + J150746.80+520958.0 have angular separations of only 4.6 arcsec (a~650AU) and 5.1 arcsec (a~750AU), respectively. They also appear to share common proper motions. Follow-up optical spectroscopy reveals each system to consist of a DA and a H-rich high-field magnetic white dwarf (HFMWD). Our measurements of the effective temperatures and the surface gravities of the DA components reveal both to have larger masses than are typical of field white dwarfs. By assuming that these degenerates have evolved essentially as single stars, due to their wide orbital separations, we use them to place limits on the total ages of our stellar systems. These argue that in each case the HFMWD is probably associated with an early type progenitor (M_init > 2M_solar). We find that the cooling time of SDSSJ150746.80+520958.0 (DAH) is somewhat lower than might be expected had it followed the evolutionary path of a typical single star. This mild discord is in the same sense as that observed for two of the small number of other HFMWDs for which progenitor mass estimates have been made, REJ0317-853 and EG59. The mass of the other DAH, SDSSJ092646.88+132134.5, appears to be smaller than expected on the basis of single star evolution. If this object was/is a member of a hierarchical triple system it may have experienced greater mass loss during an earlier phase of its life as a result of it having a close companion. The large uncertainties on our estimates of the parameters of the HFMWDs suggest a larger sample of these objects is required to firmly identify any trends in their inferred cooling times and progenitor masses.Comment: Accepted for MNRA