H alpha polarization and line profiles in white dwarfs with strong magnetic fields.

Isolated cool white dwarf stars more often have strong magnetic fields than young, hotter white dwarfs, which has been a puzzle because magnetic fields are expected to decay with time but a cool surface suggests that the star is old. In addition, some white dwarfs with strong fields vary in brightness as they rotate, which has been variously attributed to surface brightness inhomogeneities similar to sunspots, chemical inhomogeneities and other magneto-optical effects. Here we describe optical observations of the brightness and magnetic field of the cool white dwarf WD1953-011 taken over about eight years, and the results of an analysis of its surface temperature and magnetic field distribution. We find that the magnetic field suppresses atmospheric convection, leading to dark spots in the most magnetized areas. We also find that strong fields are sufficient to suppress convection over the entire surface in cool magnetic white dwarfs, which inhibits their cooling evolution relative to weakly magnetic and non-magnetic white dwarfs, making them appear younger than they truly are. This explains the long-standing mystery of why magnetic fields are more common amongst cool white dwarfs, and implies that the currently accepted ages of strongly magnetic white dwarfs are systematically too young.

In only three of the 61 known magnetic white dwarfs helium has been identified unambiguously while about 20% of all non-magnetic stars of this class are known to contain He<ns0:sc>I</ns0:sc> or He<ns0:sc>II</ns0:sc>. One reason for this discrepancy is that the identification of peculiar objects as magnetic white dwarfs is based either on the presence of hydrogen line components in strong magnetic fields -for which atomic data exist since 1984 -or the polarization of the corresponding radiation which has not been measured for many objects. Until recently, data for He<ns0:sc>I</ns0:sc> data were available only for magnetic fields below 20 MG. This changed with the publication of extensive data by the group in Heidelberg. The corresponding calculations have now been completed for the energetically lowest five states of singlet and triplet symmetry for the subspaces with $|m| \le 3$; selected calculations have been performed for even higher excitations. In strongly magnetized white dwarfs only line components are visible whose wavelengths vary slowly with respect to the magnetic field, particularly stationary components which have a wavelength minimum or maximum in the range of the magnetic fields strengths on the stellar surface. In view of the many ongoing surveys finding white dwarfs we want to provide the astronomical community with a tool to identify helium in white dwarfs for fields up to 5.3 GG. To this end we present all calculated helium line components whose wavelengths in the UV, optical, and near IR vary slowly enough with respect to the field strength to produce visible absorption features. We also list all stationary line components in this spectral range. Finally, we find series of minima and maxima which occur as a result of series of extremal transitions to increasingly higher excitations. We estimated the limits for 8 series which can possibly give rise to additional absorption in white dwarf spectra; one strong absorption feature in GD229 which is yet unexplained by stationary components is very close to two estimated series limits.

Many magnetic white dwarfs exhibit a polarised spectrum that periodically varies as the star rotates because the magnetic field is not symmetric about the rotation axis. In this work, we report the discovery that while weakly magnetic white dwarfs of all ages with M ≤ 1 M⊙ show polarimetric variability with a period between hours and several days, the large majority of magnetic white dwarfs in the same mass range with cooling ages older than 2 Gyr and field strengths ≥ 10 MG show little or no polarimetric variability. This could be interpreted as extremely slow rotation, but a lack of known white dwarfs with measured periods longer than two weeks means that we do not see white dwarfs slowing their rotation. We therefore suggest a different interpretation: old strongly magnetic white dwarfs do not vary because their fields are roughly symmetric about the rotation axes. Symmetry may either be a consequence of field evolution or a physical characteristic intrinsic to the way strong fields are generated in older stars. Specifically, a strong magnetic field could distort the shape of a star, forcing the principal axis of maximum inertia away from the spin axis. Eventually, as a result of energy dissipation, the magnetic axis will align with the angular momentum axis. We also find that the higher-mass strongly magnetised white dwarfs, which are likely the products of the merging of two white dwarfs, may appear as either polarimetrically variable or constant. This may be the symptom of two different formation channels or the consequence of the fact that a dynamo operating during a merger may produce diverse magnetic configurations. Alternatively, the massive white dwarfs with constant polarisation may be rotating with periods much shorter than the typical exposure times of the observations.

White Dwarfs for Physicists D. Koester. Magnetic White Dwarfs Observations in Cosmic Laboratories S. Jordan. Hydrogen in Strong Electric and Magnetic Fields and Its Application to Magnetic White Dwarfs S. Friedrich, et al. Helium Data for Strong Magnetic Fields Obtained by Finite Element Calculations M. Braum, et al.. The Spectrum of Atomic Hydrogen in Magnetic and Electric Fields of White Dwarf Stars P. Fassbinder, W. Schweizer. Neutron Star Atmospheres G. Pavlov. Hydrogen Atoms in Neutron Star Atmospheres: Analytical Approximations for Binding Energies A.Y. Potekhin. Absorption of Normal Modes in a Strongly Magnetized Hydrogen Gas T. Bulik, G. Pavlov. Electronic Structure of Light Elements in Strong Magnetic Fields P. Pouree, et al. From Field-Free Atoms to Finite Molecular Chains in Very Strong Magnetic Fields M.R. Godefroid. The National High Magnetic Field Laboratory - a Precis J.E. Crow. Self-Adaptive Finite Element Techniques for Stable Bound Matter-Antimatter Systems in Crossed Electric and Magnetic Fields J. Ackermann. A Computational Method for Quantum Dynamics of a Three-Dimensional Atom in Strong Fields V.S. Melezhik. 25 Additional Articles. Index.

Magnetic fields in isolated and interacting white dwarfs

The formation of spectral features of hydrogen and helium atoms in white dwarfs having magnetic fields on the order of gauss is investigated using a Milne-Eddington type schematic model atmosphere. Several examples are presented to illustrate the effects of varying the magnitude and orientation of the stellar dipole magnetic field. Results indicate that dipofe fields on the order of 108 gauss are possible mechanisms for the production of DC-type white-dwarf spectra. While the spectrum of the magnetic white dwarf Grw + cannot be identified as that of either hydrogen or helium in a field of less than 108 gauss, preliminary results indicate that a somewhat larger field may be responsible. Subject headings: line profiles - magnetic stars - white-dwarf stars - Zeeman effect

Despite thousands of spectroscopic detections, only four isolated white dwarfs exhibit Balmer emission lines. The temperature inversion mechanism is a puzzle over 30 years old that has defied conventional explanations. One hypothesis is a unipolar inductor that achieves surface heating via ohmic dissipation of a current loop between a conducting planet and a magnetic white dwarf. To investigate this model, new time-resolved spectroscopy, spectropolarimetry, and photometry of the prototype GD 356 are studied. The emission features vary in strength on the rotational period, but in antiphase with the light curve, consistent with a cool surface spot beneath an optically thin chromosphere. Possible changes in the line profiles are observed at the same photometric phase, potentially suggesting modest evolution of the emission region, while the magnetic field varies by 10 per cent over a full rotation. These comprehensive data reveal neither changes to the photometric period, nor additional signals such as might be expected from an orbiting body. A closer examination of the unipolar inductor model finds points of potential failure: the observed rapid stellar rotation will inhibit current carriers due to the centrifugal force, there may be no supply of magnetospheric ions, and no antiphase flux changes are expected from ohmic surface heating. Together with the highly similar properties of the four cool, emission-line white dwarfs, these facts indicate that the chromospheric emission is intrinsic. A tantalizing possibility is that intrinsic chromospheres may manifest in (magnetic) white dwarfs, and in distinct parts of the Hertzsprung–Russell diagram based on structure and composition.

Three aspects of magnetic white dwarf stars are studied to aid in the understanding of stellar evolution. A survey of ∼ 50 DC white dwarf stars was conducted in circular spectropolarimetry to search for magnetic fields >~ 30 kG. Four DC stars were discovered with magnetic fields above 30 kG: G 111-49 with Be ~ -220 MG, G 183-35 with Be= +6.8 ± 0.5 MG, G 256-7 with Be= +4.9 ± 0.5 MG, and G 234-4 with Be=+39.6 ± 11.6 kG. A new magnetic DB white dwarf was also discovered, LB 8827 with Be= 1.0 ± 0.5 MG. A total of 15% of the white dwarfs in the survey have a magnetic field > 30 kG. This value is far larger than the 2% of DA stars, but more than half of the DC stars were originally misclassified. Only 5% of the re-classified DC stars have magnetic fields above 30 kG. Three magnetic stars from the DC white dwarf survey were re-observed to investigate the possibility of rotation. Two are definitely rotating: LHS 1734 with 16 min <~ P < 1 yr and G 158-45 (=LHS 1044) with a probable period P ~ 4.44hr but a definite period P The isolated magnetic white dwarfs G 99-47, KUV 813-14 (KUV 23162-1220), and G 227-35 were observed in linear and circular spectropolarimetry and then compared to calculated theoretical spectra to find a model for the magnetic field strength and structure. The comparisons were to Stokes' V/I (circular polarization) spectra in addition to total flux Flambda, and these add many constraints to the possible solutions. An off-centered dipole or a dipole+quadrupole configuration best fits the observations. The results of the survey and the modeling are consistent with the theory that the magnetic Ap stars are the predecessors of magnetic white dwarfs.

When a star exhausts its nuclear fuel, it either explodes as a supernova or more quiescently becomes a white dwarf, an object about half the mass of our Sun with a radius of about that of the Earth. About one-fifth of white dwarfs exhibit the presence of magnetic fields, whose origin has long been debated as either the product of previous stages of evolution or of binary interactions. We here report the discovery of two massive and magnetic white-dwarf members of young star clusters in the Gaia second data release (DR2) database, while a third massive and magnetic cluster white dwarf was already reported in a previous paper. These stars are most likely the product of single-star evolution and therefore challenge the merger scenario as the only way to produce magnetic white dwarfs. The progenitor masses of these stars are all above 5 solar masses, and there are only two other cluster white dwarfs whose distances have been unambiguously measured with Gaia and whose progenitors’ masses fall in this range. This high incidence of magnetic white dwarfs indicates that intermediate-mass progenitors are more likely to produce magnetic remnants and that a fraction of magnetic white dwarfs forms from intermediate-mass stars.

The sample of white dwarfs included in the local 20 pc volume documents, fairly accurately, the total production of white dwarfs over roughly 10 Gyr of stellar evolution in this part of the Milky Way Galaxy. In this sample, we have been systematically searching for magnetic white dwarfs. Here we report the discovery of six new magnetic white dwarfs, with a field strength from a few MG to about 200 MG. Two of these stars show H lines that are split and polarised by the magnetic field. One star shows extremely weak spectral lines in intensity, to which highly polarised narrow features correspond. The three other stars have featureless flux spectra, but show continuum polarisation. These new discoveries support the view that at least 20% of all white dwarfs in the local 20 pc volume have magnetic fields, and they fully confirm the suspicion that magnetism is a common rather than a rare characteristic of white dwarfs. We discuss the level and the handedness of the continuum polarisation in the presence of a magnetic field in cool white dwarfs. We suggest that a magnetic field with a 15 MG longitudinal component produces 1% of continuum circular polarisation. We have also shown that the problem of cross-talk from linear to circular polarisation of the FORS2 instrument, used in our survey, represents an obstacle to accurate measurements of the circular polarisation of faint white dwarfs when the background is illuminated, and polarised, by the moon.

White dwarfs (WDs) are the most abundant compact objects, and recent surveys have suggested that over a third of WDs in accreting binaries host a strong (B ≳ 1 MG) magnetic field. However, the origin and evolution of WD magnetism remain under debate. Two WD pulsars, AR Sco and J191213.72–441045.1 (J1912), have been found, which are non-accreting binaries hosting rapidly spinning (1.97 minutes and 5.30 minutes, respectively) magnetic WDs. The WD in AR Sco is slowing down on a P / P ̇ ≈ 5.6 × 1 0 6 yr timescale. It is believed they will eventually become polars, accreting systems in which a magnetic WD (B ≈ 10−240 MG) accretes from a Roche lobe-filling donor spinning in sync with the orbit (≳78 minutes). Here, we present multiwavelength data and analysis of Gaia22ayj, which outbursted in 2022 March. We find that Gaia22ayj is a magnetic accreting WD that is rapidly spinning down ( P / P ̇ = 6 . 1 − 0.2 + 0.3 × 1 0 6 yr) like WD pulsars, but shows clear evidence of accretion, like polars. Strong linear polarization (40%) is detected in Gaia22ayj; such high levels have only been seen in the WD pulsar AR Sco and demonstrate the WD is magnetic. High speed photometry reveals a 9.36 minutes period accompanying a high amplitude (∼2 mag) modulation. We associate this with a WD spin or spin–orbit beat period, not an orbital period as was previously suggested. Fast (60 s) optical spectroscopy reveals a broad “hump,” reminiscent of cyclotron emission in polars, between 4000 and 8000 Å. We find an X-ray luminosity of L X = 2 . 7 − 0.8 + 6.2 × 1 0 32 ergs − 1 in the 0.3–8 keV energy range, while two very large array radio campaigns resulted in a non-detection with a F r &lt; 15.8 μJy 3σ upper limit. The shared properties of both WD pulsars and polars suggest that Gaia22ayj is a missing link between the two classes of magnetic WD binaries.

view Abstract Citations (62) References (29) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the Evolution of Magnetic White Dwarfs Fontaine, Gilles ; Thomas, John H. ; van Horn, H. M. Abstract We show that the magnetic DC white dwarfs cannot have evolved from stars of spectral type DA. A separate origin is necessary, and a direct evolutionary connection between the magnetic DC stars and the nuclei of planetary nebulae seems plausible. Such a connection implies photospheric field strengths 10 -10 gauss in planetary nuclei, which may be large enough to be detectable. In addition, we find that magnetic white dwarfs must be hotter than some (weakly massdependent) limiting effective temperature. This prediction is potentially susceptible to observational verification. Subject headings: magnetic stars - stellar evolution - white dwarf stars Publication: The Astrophysical Journal Pub Date: September 1973 DOI: 10.1086/152381 Bibcode: 1973ApJ...184..911F full text sources ADS |

Super-massive white dwarf (WD) stars in the mass range 2.4 -2.8 solar masses are believed to be the progenitors of "super-luminous" Type Ia supernovae according to a hypothesis proposed by some researchers. They theorize such a higher mass of the WD due to the presence of a very strong magnetic field inside it. We revisit their first work on magnetic WDs (MWDs) and present our theoretical results that are very different from theirs. The main reason for this difference is in the use of the equation of state (EoS) to make stellar models of MWDs. An electron gas in a magnetic field is Landau quantized and hence, the resulting EoS becomes non-polytropic. By constructing models of MWDs using such an EoS, we highlight that a strong magnetic field inside a WD would make the star super-massive. We have found that our stellar models do indeed fall in the mass range given above. Moreover, we are also able to address an observational finding that the mean mass of MWDs are almost double that of non-magnetic WDs. Magnetic field changes the momentumspace of the electrons which in turn changes their density of states (DOS), and that in turn changes the EoS of matter inside the star. By correlating the magnetic DOS with the non-polytropic EoS, we were also able to find a physical reason behind our theoretical result of super-massive WDs with strong magnetic fields. In order to construct these models, we have considered different equations of state with at most three Landau levels occupied and have plotted our results as mass-radius relations for a particular chosen value of maximum Fermi energy. Our results also show that a multiple Landau-level system of electrons leads to such an EoS that gives multiple branches in the mass-radius relations, and that the super-massive MWDs are obtained when the Landau-level occupancy is limited to just one level. Finally, our theoretical results can be explained solely on the basis of quantum and statistical mechanics that warrant no assumptions regarding stars.

The origin of strong ($\stackrel{\gt }{\scriptstyle \sim }1\,\mathrm{ MG}$) magnetic fields in white dwarfs has been a puzzle for decades. Recently, a dynamo mechanism operating in rapidly rotating and crystallizing white dwarfs has been suggested to explain the occurrence rates of strong magnetic fields in white dwarfs with close low-mass main-sequence star companions. Here, we investigate whether the same mechanism may produce strong magnetic fields in close double white dwarfs. The only known strongly magnetic white dwarf that is part of a close double white dwarf system, the magnetic component of NLTT 12758, is rapidly rotating and likely crystallizing and therefore the proposed dynamo mechanism represents an excellent scenario for the origin of its magnetic field. Presenting a revised formation scenario for NLTT 12758, we find a natural explanation for the rapid rotation of the magnetic component. We furthermore show that it is not surprising that strong magnetic fields have not been detected in all other known double white dwarfs. We therefore conclude that the incidence of magnetic fields in close double white dwarfs supports the idea that a rotation- and crystallization-driven dynamo plays a major role in the generation of strong magnetic fields in white dwarfs.

view Abstract Citations (7) References (9) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Quadratic Zeeman effect in the hydrogen Balmer lines from magnetic white dwarfs. Surmelian, G. L. ; Oconnell, R. F. Abstract Motivated by Preston's suggestion of using the quadratic Zeeman effect to detect large magnetic fields in DA white dwarfs, we use a variational method for an accurate calculation of the total Zeeman wavelength shifts of the hydrogen Balmer absorption lines (Ha-H10) for magnetic fields from 106 to 5 x 106 gauss. Interference effects between upper values of the principal quantum number n become significant in that range. The quadratic shifts are smaller than those given by perturbation theory, leading to an increase in the estimated magnetic field. Subject headings: magnetic stars - white dwarf stars - Zeeman effect Publication: The Astrophysical Journal Pub Date: November 1974 DOI: 10.1086/153209 Bibcode: 1974ApJ...193..705S Keywords: Balmer Series; H Lines; Magnetic Stars; White Dwarf Stars; Zeeman Effect; Absorption Spectra; Frequency Shift; H Alpha Line; Hydrogen Atoms; Magnetic Flux; Stellar Spectra; Astrophysics full text sources ADS |