scholarly journals Discovery of kilogauss magnetic fields on the nearby white dwarfs WD 1105–340 and WD 2150+591

2019 ◽  
Vol 623 ◽  
pp. A46 ◽  
Author(s):  
J. D. Landstreet ◽  
S. Bagnulo
Keyword(s):  
Magnetic Fields ◽  
Stellar Evolution ◽  
White Dwarfs ◽  
Weak Field ◽  
Field Structure ◽  
Small Sample ◽  
Dipolar Field ◽  
Surface Field ◽  
Clear Link ◽  
Statistical Studies

Magnetic fields are present in roughly 10% of white dwarfs. These fields affect the structure and evolution of such stars, and may provide clues about their earlier evolution history. Particularly important for statistical studies is the collection of high-precision spectropolarimetric observations of (1) complete magnitude-limited samples and (2) complete volume-limited samples of white dwarfs. In the course of one of our surveys we have discovered previously unknown kG-level magnetic fields on two nearby white dwarfs, WD 1105–340 and WD 2150+591. Both stars are brighter than mV = 15. WD 2150+591 is within the 20 pc volume around the Sun, while WD 1105–340 is just beyond 25 pc in distance. These discoveries increase the small sample of such weak-field white dwarfs from 21 to 23 stars. Our data appear consistent with roughly dipolar field topology, but it also appears that the surface field structure may be more complex on the older star than on the younger one, a result similar to one found earlier in our study of the weak-field stars WD 2034+372 and WD 2359–434. This encourages further efforts to uncover a clear link between magnetic morphology and stellar evolution.

scholarly journals Clues to the origin and properties of magnetic white dwarfs

2019 ◽  
Vol 15 (S357) ◽  
pp. 60-74
Author(s):  
Adela Kawka
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
White Dwarfs ◽  
Field Structure ◽  
Complex Structures ◽  
Surface Mapping ◽  
Time Resolved ◽  
Single Star ◽  
Rotation Rates ◽  
Star Evolution

AbstractA significant fraction of white dwarfs possess a magnetic field with strengths ranging from a few kG up to about 1000 MG. However, the incidence of magnetism varies when the white dwarf population is broken down into different spectral types providing clues on the formation of magnetic fields in white dwarfs. Several scenarios for the origin of magnetic fields have been proposed from a fossil field origin to dynamo generation at various stages of evolution. Offset dipoles are often assumed sufficient to model the field structure, however time-resolved spectropolarimetric observations have revealed more complex structures such as magnetic spots or multipoles. Surface mapping of these field structures combined with measured rotation rates help distinguish scenarios involving single star evolution from other scenarios involving binary interactions. I describe key observational properties of magnetic white dwarfs such as age, mass, and field strength, and confront proposed formation scenarios with these properties.

scholarly journals Magnetic fields in white dwarfs and stellar evolution

2004 ◽  
Vol 355 (3) ◽  
pp. L13-L16 ◽  
Author(s):  
C. A. Tout ◽  
D. T. Wickramasinghe ◽  
L. Ferrario
Keyword(s):  
Magnetic Fields ◽  
Stellar Evolution ◽  
White Dwarfs

scholarly journals Magnetic Fields in AM Her Binaries

1996 ◽  
Vol 158 ◽  
pp. 203-204
Author(s):  
Kinwah Wu ◽  
Paul A. Mason
Keyword(s):  
Magnetic Fields ◽  
White Dwarf ◽  
White Dwarfs ◽  
Field Component ◽  
Quadrupole Field ◽  
Surface Field ◽  
Dipole Component ◽  
Quadrupole Component

There is growing evidence of complex white dwarf magnetic fields in CVs (e.g. Meggitt & Wickramasinghe 1989; Robinson & Cordova 1994). The properties of systems with non-dipolar white dwarf magnetic fields have been investigated, and it was found that a non-negligible quadrupole component is essential for the magnetic locking of AM Her binaries (AM Hers) (Wu & Wickramasinghe 1993). In a recent study the ratio of the quadrupole to dipole component of the surface field of white dwarfs was shown to grow by a factor of 10 during the white dwarf cooling lifetime (Muslimov, van Horn & Wood 1995), thus the quadrupole field component is long lived and significant.

scholarly journals Searching for the weakest detectable magnetic fields in white dwarfs

2018 ◽  
Vol 618 ◽  
pp. A113 ◽  
Author(s):  
S. Bagnulo ◽  
J. D. Landstreet
Keyword(s):  
Magnetic Fields ◽  
White Dwarfs ◽  
Signal To Noise Ratio ◽  
Field Measurements ◽  
Weak Field ◽  
Magnetic Star ◽  
Weak Magnetic Fields ◽  
Medium Resolution ◽  
Magnetic Stars ◽  
Low Sensitivity

Our knowledge of the magnetism in white dwarfs is based on an observational dataset that is biased in favour of stars with very strong magnetic fields. Most of the field measurements available in the literature have a relatively low sensitivity, while current instruments allow us to detect magnetic fields of white dwarfs with sub-kG precision. With the aim of obtaining a more complete view of the incidence of magnetic fields in degenerate stars, we have started a long-term campaign of high-precision spectropolarimetric observations of white dwarfs. Here we report the results obtained so far with the low-resolution FORS2 instrument of the ESO VLT and the medium-resolution ISIS instrument of the WHT. We have considered a sample of 48 stars, of which five are known magnetic or suspected magnetic stars, and obtained new longitudinal magnetic field measurements with a mean uncertainty of about 0.6 kG. Overall, in the course of our survey (the results of which have been partially published in papers devoted to individual stars) we have discovered one new weak-field magnetic white dwarf, confirmed the magnetic nature of another, found that a suspected magnetic star is not magnetic, and suggested two new candidate magnetic white dwarfs. Even combined with data previously obtained in the literature, our sample is not sufficient yet to reach any final conclusions about the actual incidence of very weak magnetic fields in white dwarfs, but we have set the basis to achieve a homogeneous survey of an unbiased sample of white dwarfs. As a by-product, our survey has also enabled us to carry out a detailed characterisation of the ISIS and the FORS2 instruments for the detection of extremely weak magnetic fields in white dwarfs, and in particular to relate the signal-to-noise ratio to measurement uncertainty for white dwarfs of different spectral types. This study will help the optimisation of future observations.

scholarly journals Magnetic Field Evolution in Accreting White Dwarfs

2004 ◽  
Vol 190 ◽  
pp. 58-70
Author(s):  
Andrew Cumming
Keyword(s):  
Magnetic Field ◽  
White Dwarf ◽  
White Dwarfs ◽  
Strong Field ◽  
Weak Field ◽  
Field Structure ◽  
Field Evolution ◽  
Induced Field ◽  
Intermediate Polars ◽  
The Magnetic Field

AbstractI discuss the effect of accretion on the magnetic field of an accreting white dwarf. Whereas the magnetic fields of isolated white dwarfs are not expected to change significantly with time, I show that if an accreting white dwarf increases in mass at a rate > 1–5 × 10−10MΘ yr–1, accretion overcomes ohmic diffusion and has a significant effect on the field structure. I discuss the implications of this result for observed systems. In particular, accretion induced field evolution may provide the missing evolutionary link between the strong field, slowly accreting AM Her systems and the weak field, more rapidly accreting intermediate polars.

scholarly journals Large Scale Variations in Solar Coronal Density and Magnetic Fields

1977 ◽  
Vol 43 ◽  
pp. 17-17
Author(s):  
R. M. McQueen
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Large Scale ◽  
Solar Surface ◽  
Coronal Magnetic Field ◽  
Field Structure ◽  
Surface Field ◽  
Abrupt Changes ◽  
Coronal Density ◽  
Coronal Structures

Observations of the solar equatorial electron scattered coronal radiance made during the Skylab mission show characteristics which dramatically differ with time. The results may be interpreted as indicating a general simplification of the coronal magnetic field, and, in comparison with harmonic analysis of observed solar surface magnetic fields, as indicating a rapid response of outer coronal structures to abrupt changes in the global solar surface field structure.

scholarly journals Confirming known planetary trends using a photometrically selected Kepler sample

2020 ◽  
Author(s):  
Jonah T Hansen ◽  
Luca Casagrande ◽  
Michael J Ireland ◽  
Jane Lin
Keyword(s):  
Sample Size ◽  
Robust Statistics ◽  
Narrow Band ◽  
Small Sample Size ◽  
Small Sample ◽  
Fitting Algorithm ◽  
Statistical Studies ◽  
The Rich ◽  
Host Stars ◽  
High Metallicity

Abstract Statistical studies of exoplanets and the properties of their host stars have been critical to informing models of planet formation. Numerous trends have arisen in particular from the rich Kepler dataset, including that exoplanets are more likely to be found around stars with a high metallicity and the presence of a “gap” in the distribution of planetary radii at 1.9 R⊕. Here we present a new analysis on the Kepler field, using the APOGEE spectroscopic survey to build a metallicity calibration based on Gaia, 2MASS and Strömgren photometry. This calibration, along with masses and radii derived from a Bayesian isochrone fitting algorithm, is used to test a number of these trends with unbiased, photometrically derived parameters, albeit with a smaller sample size in comparison to recent studies. We recover that planets are more frequently found around higher metallicity stars; over the entire sample, planetary frequencies are 0.88 ± 0.12 percent for [Fe/H] < 0 and 1.37 ± 0.16 percent for [Fe/H] ≥ 0 but at two sigma we find that the size of exoplanets influences the strength of this trend. We also recover the planet radius gap, along with a slight positive correlation with stellar mass. We conclude that this method shows promise to derive robust statistics of exoplanets. We also remark that spectrophotometry from Gaia DR3 will have an effective resolution similar to narrow band filters and allow to overcome the small sample size inherent in this study.

scholarly journals Contribution of White Dwarfs to Cluster Masses

1998 ◽  
Vol 11 (1) ◽  
pp. 430-432
Author(s):  
Ted Von Hippel
Keyword(s):  
Stellar Evolution ◽  
Mass Fraction ◽  
Globular Clusters ◽  
White Dwarfs ◽  
Literature Search ◽  
Galactic Disk ◽  
Dynamical Evolution ◽  
Open Clusters ◽  
Solar Neighborhood ◽  
Current Evidence

The study of cluster white dwarfs (WDs) has been invigorated recently bythe Hubble Space Telescope (HST). Recent WD studies have been motivated by the new and independent cluster distance (Renzini et al. 1996), age (von Hippel et al. 1995; Richer et al. 1997), and stellar evolution (Koester & Reimers 1996) information that cluster WDs can provide. An important byproduct of these studies has been an estimate of the WD mass contribution in open and globular clusters. The cluster WD mass fraction is of importance for understanding the dynamical state and history of star clusters. It also bears an important connection to the WD mass fractions of the Galactic disk and halo. Current evidence indicates that the open clusters (e.g. von Hippel et al. 1996; Reid this volume) have essentially the same luminosity function (LF) as the solar neighborhood population. The case for the halo is less clear, despite the number of very good globular cluster LFs down to nearly 0.1 solar masses (e.g. Cool et al. 1996; Piotto, this volume), as the field halo LF is poorly known. For most clusters dynamical evolution should cause evaporation of the lowest mass members, biasing clusters to have flatter present-day mass functions (PDMFs) than the disk and halo field populations. Dynamical evolution should also allow cluster WDs to escape, though not in the same numbers as the much lower mass main sequence stars. The detailed connection between cluster PDMFs and the field IMF awaits elucidation from observations and the new combined N-body and stellar evolution models (Tout, this volume). Nevertheless, the WD mass fraction of clusters already provides an estimate for the WD mass fraction of the disk and halo field populations. A literature search to collect cluster WDs and a simple interpretive model follow. This is a work in progress and the full details of the literature search and the model will be published elsewhere.

Origin of Magnetic Fields in White Dwarfs and Neutron Stars

1971 ◽  
Vol 231 (19) ◽  
pp. 32-33 ◽  
Author(s):  
R. F. O'CONNELL ◽  
K. M. ROUSSEL
Keyword(s):  
Magnetic Fields ◽  
Neutron Stars ◽  
White Dwarfs

scholarly journals Opacities in Strong Magnetic Helds

1995 ◽  
Vol 10 ◽  
pp. 588-590
Author(s):  
Dayal T. Wickramasinghe
Keyword(s):  
Entire Range ◽  
White Dwarfs ◽  
Specific Problem ◽  
Transition Probabilities ◽  
Strong Field ◽  
Energy Levels ◽  
Dipolar Field ◽  
Origin And Evolution ◽  
End Products ◽  
Hydrogen Lines

White dwarfs are one of the most readily studied end products of stellarevolution. Their observed properties have provided and continue to provide important constraints for the theory of stellar evolution. Likewise, a study of magnetism in white dwarfs provides unique insights into the origin and evolution of magnetic fields in stars.Spectacular progress has been made on the specific problem of the structure of the hydrogen atom in strong fields. Energy levels and transition probabilities are now known for all low lying states of hydrogen for the entire range of field strengths appropriate to white dwarfs and neutron stars (104-1013G) (Rosner et al 1984, Forster et al 1984 and Henry and O’Connell 1984). These calculations resulted in the identification of spectral features in the magnetic white dwarf Grw+70°8247 which had remained unidentified for over 50 years (Minkowski 1938), with Zeeman shifted hydrogen lines in a magnetic field of 100 -320 MG ((eg Wickamasinghe and Ferrano 1989). Several other strong field magnetic white dwarfs have since been discovered through hydrogen Zeeman spectroscopy. The data presently at hand show that most hydrogen rich magnetic white dwarfs have complex non-dipolar field structures with strong evidence for higher order multipole components.

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