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

1976 ◽  
Vol 209 ◽  
pp. 858 ◽  
Author(s):  
E. F. Borra
Keyword(s):  
Magnetic Fields ◽  
White Dwarfs ◽  
Line Profiles ◽  
Strong Magnetic Fields

Photoionization of the hydrogen atom in strong magnetic fields of White Dwarfs

1999 ◽  
Vol 5 (1) ◽  
pp. 23-31 ◽  
Author(s):  
G. Meinhardt ◽  
W. Schweizer ◽  
H. Herold ◽  
G. Wunner
Keyword(s):  
Hydrogen Atom ◽  
Magnetic Fields ◽  
White Dwarfs ◽  
Strong Magnetic Fields

scholarly journals Long-term evolution of a magnetic massive merger product

2020 ◽  
Vol 495 (3) ◽  
pp. 2796-2812
Author(s):  
F R N Schneider ◽  
S T Ohlmann ◽  
Ph Podsiadlowski ◽  
F K Röpke ◽  
S A Balbus ◽  
...  
Keyword(s):  
Magnetic Fields ◽  
White Dwarfs ◽  
Massive Stars ◽  
Thermal Relaxation ◽  
Long Term Evolution ◽  
Critical Surface ◽  
Single Star ◽  
Strong Magnetic Fields ◽  
Term Evolution

ABSTRACT About 10 per cent of stars more massive than ${\approx}1.5\, {\mathrm{M}}_{\odot }$ have strong, large-scale surface magnetic fields and are being discussed as progenitors of highly magnetic white dwarfs and magnetars. The origin of these fields remains uncertain. Recent three-dimensional (3D) magnetohydrodynamical simulations have shown that strong magnetic fields can be generated in the merger of two massive stars. Here, we follow the long-term evolution of such a 3D merger product in a 1D stellar evolution code. During a thermal relaxation phase after the coalescence, the merger product reaches critical surface rotation, sheds mass and then spins down primarily because of internal mass readjustments. The spin of the merger product after thermal relaxation is mainly set by the co-evolution of the star–torus structure left after coalescence. This evolution is still uncertain, so we also consider magnetic braking and other angular momentum-gain and -loss mechanisms that may influence the final spin of the merged star. Because of core compression and mixing of carbon and nitrogen in the merger, enhanced nuclear burning drives a transient convective core that greatly contributes to the rejuvenation of the star. Once the merger product relaxed back to the main sequence, it continues its evolution similar to that of a genuine single star of comparable mass. It is a slow rotator that matches the magnetic blue straggler τ Sco. Our results show that merging is a promising mechanism to explain some magnetic massive stars and it may also be key to understand the origin of the strong magnetic fields of highly magnetic white dwarfs and magnetars.

scholarly journals Line Band Profiles in the Spectra of Cool Magnetic Helium-Rich White Dwarfs

1985 ◽  
Vol 87 ◽  
pp. 391-395
Author(s):  
I. Bues
Keyword(s):  
Magnetic Fields ◽  
White Dwarfs ◽  
Strong Field ◽  
Abundance Ratio ◽  
Mixed Composition ◽  
Strong Magnetic Fields ◽  
Carbon Abundance ◽  
Helium Stars ◽  
Effective Temperatures

For white dwarfs with effective temperatures smaller than 12000 K, the percentage of objects with a helium-rich atmosphere increases compared to the hydrogen-rich sequence. The carbon abundance, which can be determined from line and band strengths (Bues, 1973; Koester et al., 1982), varies by more than a factor of 1000 within this class. Moreover, for the subclass of white dwarfs with strong magnetic fields, the abundance ratio of H/He differs from that of the DB and DA sequences. The hot star Feige 7, analyzed by Liebert et al. (1977), shows lines of hydrogen and helium at a comparable strength for a moderately strong field of 103 Tesla. If there is any chance of finding white dwarfs which are descendants of hot, non-degenerate helium stars with rotation and magnetic fields, then it should be within these objects of mixed composition.

Polarized Radiation from White Dwarfs and Atoms in Strong Magnetic Fields

1974 ◽  
Vol 53 ◽  
pp. 287-300
Author(s):  
R. F. O'Connell
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Cross Section ◽  
White Dwarfs ◽  
Zeeman Effect ◽  
Initial Response ◽  
Model Atmosphere ◽  
Radiation Absorption ◽  
Strong Magnetic Fields ◽  
Polarized Radiation

We present the recent results of our continuing program of investigation of the behavior of matter in strong to super-strong magnetic fields (B ∼ 106−1012 G). This work was motivated by the discovery of strong magnetic fields (B ∼ 107 G) in some white dwarfs and the likely existence of super-strong fields (B ∼ 1012 G) in pulsars. Magnetic white dwarfs were discovered from observations of the continuous spectrum and one of the most intriguing challenges for the theorist is to provide an explanation for the observed wavelength dependence of the fractional circularly and linearly polarized radiation. Our initial response to this question was the determination of an exact solution of Kemp's harmonic oscillator model. These results are used as input to the ATLAS model atmosphere program and then comparison is made with observations. The disparities still existing between theory and observation convince us of the necessity for developing a new model of the continuum radiation, two likely possibilities being photoionization and free-free absorption. This leads us to present a general formulation of radiation absorption and emission processes in a magnetic field. Next we calculate the cross section for the photoionization, correct to first order in B. For the purpose of obtaining exact results for this cross section, the effect of a magnetic field on the energy spectrum and wave functions of hydrogen, helium, etc. must be obtained. The results for hydrogen are presented here. They will be useful also in determining accurate values for the displacements due to the quadratic Zeeman effect in the line spectra of DA stars, particularly for the higher excited states.

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