143 research outputs found
Magnetic white dwarfs in the Early Data Release of the Sloan Digital Sky Survey
We have identified 7 new magnetic DA white dwarfs in the Early Data Release
of the Sloan Digital Sky Survey. Our selection strategy has also recovered all
the previously known magnetic white dwarfs contained in the SDSS EDR,
KUV03292+0035 and HE0330-0002. Analysing the SDSS fibre spectroscopy of the
magnetic DA white dwarfs with our state-of-the-art model spectra, we find
dipole field strengths 1.5<=B_d<=63MG and effective temperatures
8500<=Teff<=39000K. As a conservative estimate, we expect that the complete
SDSS will increase the number of known magnetic white dwarfs by a factor 3.Comment: 7 pages, 4 figures, accepted for publication in Astronomy &
Astrophysic
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
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&
Zeeman tomography of magnetic white dwarfs. IV, The complex field structure of the polars EF Eridani, BL Hydri and CP Tucanae
Context. The magnetic fields of the accreting white dwarfs in magnetic cataclysmic variables (mCVs) determine the accretion geometries, the emission properties, and the secular evolution of these objects.
Aims. We determine the structure of the surface magnetic fields of the white dwarf primaries in magnetic CVs using Zeeman tomography.
Methods. Our study is based on orbital-phase resolved optical flux and circular polarization spectra of the polars EF Eri, BL Hyi, and CP Tuc obtained with FORS1 at the ESO VLT. An evolutionary algorithm is used to synthesize best fits to these spectra from an
extensive database of pre-computed Zeeman spectra. The general approach has been described in previous papers of this series.
Results. The results achieved with simple geometries as centered or offset dipoles are not satisfactory. Significantly improved fits are obtained for multipole expansions that are truncated at degree lmax = 3 or 5 and include all tesseral and sectoral components with
0 ≤ m ≤ l. The most frequent field strengths of 13, 18, and 10MG for EF Eri, BL Hyi, and CP Tuc, and the ranges of field strength covered are similar for the dipole and multipole models, but only the latter provide access to accreting matter at the right locations on the white dwarf. The results suggest that the field geometries of the white dwarfs in short-period mCVs are quite complex, with
strong contributions from multipoles higher than the dipole in spite of a typical age of the white dwarfs in CVs in excess of 1 Gyr.
Conclusions. It is feasible to derive the surface field structure of an accreting white dwarf from phase-resolved low-state circular spectropolarimetry of sufficiently high signal-to-noise ratio. The fact that independent information is available on the strength and
direction of the field in the accretion spot from high-state observations helps in unraveling the global field structure
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
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
Zeeman tomography of magnetic white dwarfs. III, The 70–80 Megagauss magnetic field of PG 1015+014
Aims. We analyse the magnetic field geometry of the magnetic DA white dwarf PG 1015+014 with our Zeeman tomography method.
Methods. This study is based on rotation-phase resolved optical flux and circular polarization spectra of PG 1015+014 obtained with FORS1
at the ESO VLT. Our tomographic code makes use of an extensive database of pre-computed Zeeman spectra. The general approach has been described in Papers I and II of this series.
Results. The surface field strength distributions for all rotational phases of PG 1015+014 are characterised by a strong peak at 70 MG. A separate peak at 80 MG is seen for about one third of the rotation cycle. Significant contributions to the Zeeman features arise from regions
with field strengths between 50 and 90 MG. We obtain equally good simultaneous fits to the observations, collected in five phase bins, for two different field parametrizations: (i) a superposition of individually tilted and off-centred zonal multipole components; and (ii) a truncated multipole expansion up to degree l = 4 including all zonal and tesseral components. The magnetic fields generated by both parametrizations exhibit a similar global structure of the absolute surface field values, but differ considerably in the topology of the field lines. An effective photospheric temperature of T eff = 10 000 ± 1000 K was found.
Conclusions. Remaining discrepancies between the observations and our best-fit models suggest that additional small-scale structure of the magnetic field exists which our field models are unable to cover due to the restricted number of free parameters
Zeeman tomography of magnetic white dwarfs. IV, The complex field structure of the polars EF Eridani, BL Hydri and CP Tucanae
Context. The magnetic fields of the accreting white dwarfs in magnetic cataclysmic variables (mCVs) determine the accretion geometries, the emission properties, and the secular evolution of these objects.
Aims. We determine the structure of the surface magnetic fields of the white dwarf primaries in magnetic CVs using Zeeman tomography.
Methods. Our study is based on orbital-phase resolved optical flux and circular polarization spectra of the polars EF Eri, BL Hyi, and CP Tuc obtained with FORS1 at the ESO VLT. An evolutionary algorithm is used to synthesize best fits to these spectra from an
extensive database of pre-computed Zeeman spectra. The general approach has been described in previous papers of this series.
Results. The results achieved with simple geometries as centered or offset dipoles are not satisfactory. Significantly improved fits are obtained for multipole expansions that are truncated at degree lmax = 3 or 5 and include all tesseral and sectoral components with
0 ≤ m ≤ l. The most frequent field strengths of 13, 18, and 10MG for EF Eri, BL Hyi, and CP Tuc, and the ranges of field strength covered are similar for the dipole and multipole models, but only the latter provide access to accreting matter at the right locations on the white dwarf. The results suggest that the field geometries of the white dwarfs in short-period mCVs are quite complex, with
strong contributions from multipoles higher than the dipole in spite of a typical age of the white dwarfs in CVs in excess of 1 Gyr.
Conclusions. It is feasible to derive the surface field structure of an accreting white dwarf from phase-resolved low-state circular spectropolarimetry of sufficiently high signal-to-noise ratio. The fact that independent information is available on the strength and
direction of the field in the accretion spot from high-state observations helps in unraveling the global field structure
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