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 Discovery of weak magnetic fields in four DZ white dwarfs in the local 20 pc volume

2019 ◽  
Vol 630 ◽  
pp. A65 ◽  
Author(s):  
S. Bagnulo ◽  
J. D. Landstreet
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
White Dwarfs ◽  
Field Measurements ◽  
Weak Magnetic Fields ◽  
Polluted Atmosphere ◽  
Highly Sensitive ◽  
Cool White Dwarfs ◽  
The Magnetic Field ◽  
Made In

We report the discovery of weak magnetic fields in three white dwarfs within the local 20 pc volume (WD 0816−310, WD 1009−184, and WD 1532+129), and we confirm the magnetic nature of a fourth star (WD 2138−332) in which we had previously detected a field at a 3σ level. The spectra of all these white dwarfs are characterised by the presence of metal lines and lack of H and He lines, that is, they belong to the spectral class DZ. The polarisation signal of the Ca II H+K lines of WD 1009−184 is particularly spectacular, with an amplitude of 20% that is due to the presence of a magnetic field with an average line-of-sight component of 40 kG. We have thus established that at least 40% of the known DZ white dwarfs with an He-rich atmosphere contained in the 20 pc volume have a magnetic field, while further observations are needed to establish whether the remaining DZ white dwarfs in the same volume are magnetic or not. Metal lines in the spectra of DZ white dwarfs are thought to have originated by accretion from rocky debris, and it might be argued that a link exists between metal accretion and higher occurrence of magnetism. However, we are not able to distinguish whether the magnetic field and the presence of a polluted atmosphere have a common origin, or if it is the presence of metal lines that allows us to detect a higher frequency of magnetic fields in cool white dwarfs, which would otherwise have featureless spectra. We argue that the new highly sensitive longitudinal field measurements that we have made in recent years are consistent with the idea that the magnetic field appears more frequently in older than in younger white dwarfs.

scholarly journals A novel and sensitive method for measuring very weak magnetic fields of DA white dwarfs

2015 ◽  
Vol 580 ◽  
pp. A120 ◽  
Author(s):  
J. D. Landstreet ◽  
S. Bagnulo ◽  
G. G. Valyavin ◽  
D. Gadelshin ◽  
A. J. Martin ◽  
...  
Keyword(s):  
Magnetic Fields ◽  
White Dwarfs ◽  
Weak Magnetic Fields ◽  
Sensitive Method

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 Dissipation of Magnetic Fields in Very Dense Interstellar Clouds and the Magnetic Flux of a Newborn Star

1987 ◽  
Vol 115 ◽  
pp. 454-455
Author(s):  
Toyoharu Umebayashi ◽  
Takenori Nakano
Keyword(s):  
Magnetic Fields ◽  
Magnetic Flux ◽  
Magnetic Star ◽  
Number Density ◽  
Mass Ratio ◽  
Interstellar Clouds ◽  
Neutral Particles ◽  
Surface Field ◽  
Magnetic Stars ◽  
Dominant Process

The magnetic flux to mass ratio of an interstellar cloud is 104 to 105 times the ratio in a typical magnetic star with a surface field of 1kG, and is at least several hundred times the ratio in most strongly magnetic stars. This excess magnetic flux must be lost in some stage of star formation. The dominant process of magnetic flux loss in ordinary clouds is the drift of charged particles and magnetic fields in the sea of neutral particles (plasma drift, also called ambipolar diffusion). However, even this process is inefficient in a cloud of hydrogen number density nH ≲ 1010 cm−3.

scholarly journals Magnetic white dwarfs: Observations, theory and future prospects

2016 ◽  
Vol 25 (01) ◽  
pp. 1630005 ◽  
Author(s):  
Enrique García–Berro ◽  
Mukremin Kilic ◽  
Souza Oliveira Kepler
Keyword(s):  
Magnetic Fields ◽  
White Dwarfs ◽  
Current Status ◽  
Future Prospects ◽  
Physical Mechanisms ◽  
Weak Magnetic Fields ◽  
Interesting Class ◽  
The Subject ◽  
Points Of View ◽  
The Magnetic Field

Isolated magnetic white dwarfs have field strengths ranging from [Formula: see text][Formula: see text]G to [Formula: see text][Formula: see text]G, and constitute an interesting class of objects. The origin of the magnetic field is still the subject of a hot debate. Whether these fields are fossil, hence the remnants of original weak magnetic fields amplified during the course of the evolution of the progenitor of white dwarfs, or on the contrary, are the result of binary interactions or, finally, other physical mechanisms that could produce such large magnetic fields during the evolution of the white dwarf itself, remains to be elucidated. In this work, we review the current status and paradigms of magnetic fields in white dwarfs, from both the theoretical and observational points of view.

scholarly journals On the incidence of weak magnetic fields in DA white dwarfs

2012 ◽  
Vol 545 ◽  
pp. A30 ◽  
Author(s):  
J. D. Landstreet ◽  
S. Bagnulo ◽  
G. G. Valyavin ◽  
L. Fossati ◽  
S. Jordan ◽  
...  
Keyword(s):  
Magnetic Fields ◽  
White Dwarfs ◽  
Weak Magnetic Fields

On the possibility of detecting weak magnetic fields in variable white dwarfs

1989 ◽  
Vol 336 ◽  
pp. 403 ◽  
Author(s):  
Philip W. Jones ◽  
Carl J. Hansen ◽  
W. Dean Pesnell ◽  
Steven D. Kawaler
Keyword(s):  
Magnetic Fields ◽  
White Dwarfs ◽  
Weak Magnetic Fields

scholarly journals Pulsation of Rotating and Magnetic Stars

1993 ◽  
Vol 137 ◽  
pp. 497-511 ◽  
Author(s):  
Sergei V. Vorontsov
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Spherical Symmetry ◽  
Slow Rotation ◽  
Theoretical Studies ◽  
Pulsating Stars ◽  
Magnetic Effects ◽  
Weak Magnetic Fields ◽  
Magnetic Stars ◽  
Oscillation Frequencies

AbstractRotation and a magnetic field break the spherical symmetry of a star viewed as a pulsating system, lifting the degeneracy of oscillation frequencies, and leading to (sometimes prominent) observational consequences. Theoretical studies of rotational and magnetic effects in pulsating stars are reviewed, starting with simple configurations with slow rotation and weak magnetic fields.

scholarly journals Strong Gradients in Weak Magnetic Fields Induce DOLLOP Formation in Tap Water

Water ◽  
2016 ◽  
Vol 8 (3) ◽  
pp. 79 ◽  
Author(s):  
Martina Sammer ◽  
Cees Kamp ◽  
Astrid Paulitsch-Fuchs ◽  
Adam Wexler ◽  
Cees Buisman ◽  
...  
Keyword(s):  
Magnetic Fields ◽  
Tap Water ◽  
Weak Field ◽  
Electric Impedance ◽  
Laser Scattering ◽  
Magnetic Inhomogeneities ◽  
Weak Magnetic Fields ◽  
Absolute Strength ◽  
Electric Impedance Spectroscopy ◽  
The Magnetic Field

In 2012 Coey proposed a theory on the mechanism of magnetic water treatment based on the gradient of the applied field rather than its absolute strength. We tested this theory by measuring the effect of very weak field magnets (≤ 10 G) containing strong magnetic inhomogeneities (ΔB = 770 G·m−1 (WCM 62081992) and 740 G·m−1 (WCM 62083545)) on tap water samples by the use of electric impedance spectroscopy (EIS) and laser scattering. Our results show an increased formation of nm-sized prenucleation clusters (dynamically ordered liquid like oxyanion polymers or “DOLLOPs”) due to the exposure to the magnetic field and thus are consistent with Coey’s theory which is therefore also applicable to very weak magnetic fields as long as they contain strong gradients.

scholarly journals Magnetic observations of pulsating B and Be stars with ESPaDOnS and Narval

2008 ◽  
Vol 4 (S259) ◽  
pp. 391-392
Author(s):  
James Silvester ◽  
C. Neiner ◽  
H. F. Henrichs ◽  
G. A. Wade ◽  
E. Alecian ◽  
...  
Keyword(s):  
Magnetic Field ◽  
High Resolution ◽  
Magnetic Fields ◽  
Critical Analysis ◽  
Be Stars ◽  
Early Type ◽  
Weak Magnetic Fields ◽  
Magnetic Stars ◽  
Early Type Stars ◽  
New Generation

AbstractDiscoveries of magnetic fields in pulsating B and Be stars have been claimed from low-resolution spectropolarimetric observations with FORS1 at VLT. We used the new generation of high-resolution spectropolarimeters, ESPaDOnS at CFHT and NARVAL at TBL, to check for the existence of these fields. We find that most of the claimed magnetic stars do not host a magnetic field. This work shows the importance of a critical analysis of FORS1 data when searching for weak magnetic fields in early-type stars and the advantage of using ESPaDOnS and NARVAL to study such type of stars.

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