Magnetic white dwarf stars a review

Gennady Valyavin

This review discusses the history of isolated magnetic white dwarfs, what is known about them, and problematic questions (mainly those questions, related to origin and evolution of strong magnetic fields in the degenerate stars).

History• 1938: Minkowski, - first spectral observations of the DC white

dwarf GRW 708247 • 1957: Greenstein & Mattews, identifications of spectral features

Polarimetry of GRW 708247

• Kemp, Swedlund, Landstreet, & Angel, 1970: detection of strong circular polarization (magnetic nature of GRW708247).

Quadratic Zeeman effect

• First calculations of the hydrogen transitions in strong magnetic fields: the late of 70th and early 80th (the US and Germany, Tubingen).

• GRW708247 is purely hydrogen WD with field strength of about 300 MG (!)

Atmosphere models of white dwarfs with strongest magnetic fields

Jordan, S. & the Tuebingen group, 1992

90th – massive spectroscopic studies of megagauss white dwarfs. About 100

strong-magnetic white dwarfs have been identified

Spectra of MWDs (the most typical ones)

Properties of MWDs

• 1) Magnetic Field Geometries

• Large scale, roughly dipolar geometries

• A few MWDs have quadrupolar components

Properties of MWDs

• 2) Atmospheric constituents (Putney, 1999 and references therein):

• DA~75%

• DB~15%

• DQ+(DC+DZ(?))~10%

Properties of MWDs• 3) Rotation (Schmidt and

Smith, 1991 and later paper of some other authors)

. The periods are found from minutes to tens of days with one specific fingerprint connected with the group of white dwarfs with strongest magnetic fields: their periods are estimated to be longer than tens of years!

• (Or their periods are shorter than minutes and below the detection limit?)

Properties of MWDs

4) Masses:Basically, all magnetic white dwarfs have masses larger than normal. Besides:

1) According to Valyavin and Fabrika, 1998, 1999, MWDs demonstrate bimodal distribution with two peaks at 0.8 and 1.2 Solar Mass.

2) According to Naleziti and Madej, 2004 the distribution is flat

3) According to Kepler et al., 2012 the distribution is seems nevertheless bimodal.

White dwarfs in the second group definitively have specific origin. And most of them are those from the group of ultra-magnetic white dwarfs, rotation periods of which are estimated to be longer than several tens of years.

Properties of MWDs

• 5) Temperatures (Putney, 1999 and references

therein):

MWDs are found in a wide range of

temperatures.

An Intermediate Conclusion

• Magnetic white dwarf stars comprise about 10% of all known white dwarfs. They are distinguished by the presence of strong, regular, roughly dipolar magnetic fields which cover the range from a few thousand of kilogauss to about one gigagauss.

Origin and evolution of global magnetic fields in isolated white dwarfs

• 1) What is the origin of the white dwarf magnetic fields?

• 2) How do these fields evolve during a WD’s life?

Origin and evolution of white dwarf magnetic ifleds

»Theory• Angel, Borra & Landstreet, 1981: Magnetic fields of

white dwarf stars are fossil remnants of the fields of their progenitor strong-magnetic A/B MS stars.

• Wendell, van Horn, Sargent, 1987: The fields Ohmically decay with the characteristic decay time on the order of about 1010 years and we can not register this process in observations

Evolution of MWD’s magnetic fields with age

• Liebert & Sion, 1978;• Valyavin & Fabrika, 1998;• Liebert, Bergeron & Holberg, 2004

• Kepler et al., 2012 --->

• Relative fraction of cool (old) MWD stars is higher than hot (young) MWDs. This doesn’t contradict to idea of ABL about fossil nature of MWD’s magnetic fields, but definitely, magnetic fields of the strong-magnetic WDs are additionally increasing during evolution.

• To understand this effect it is very important to know does it depend on initial magnetic fields or not (is there a uniform mechanism of the field evolution?).

The incidence of magnetism among weak-field MWDs (detection of MWDs with fields with ~ 1 kG

accuracy)

• Valyavin et al., 2006

• Kawka et al., 2007

• Landstreet et al., 2012

CONCLUSION: The temperature distribution of weak-field MWDs on their surface magnetic fields is flat!

MWD’s field evolution(conclusion)

• There is no any signatures of the field evolution among weak-field MWDs

• The high-field MWDs with fields > 107 G exhibit strong increase in strengths with WD’s age. The nature of this effect is unknown.

Origin of MWDs

• For MWDs of normal masses the fields are likely fossil remnants of the F-B magnetic stars (Angel, Borra, Landstreet, 1980). The ultramassive MWDs can not be originated from isolated A-stars. They could be mergers (Valyavin and Fabrika, 1998,1999; Wickramasinghe and Ferrario, 2010).

Individual MWDs• Some of MWDs exhibit

peculiar atmospheric properties.

• 1. The GD356 star shows all spectral features in emission. Explanation:

• 1. Greenstain and McCarthy, 1985: intracting system or magnetoionic inversion due to interaction between magnetic field and convection

• 2. Wickramasinghe et al., 2010: GD356 has a planetary component

ARE THERE IRREGULAR MAGNETIC STRUCTURES IN WHITE DWARFS?

• A few MWDs demonstrate some evidences for the presence of spots on their surfaces by analogy to sunspots:

• 1. WD1953-011 (Valyavin et al., 2008; 2011)• 2. PG1658+441 (Shtol’ et al., 1997)• 3. Some arguments for the presence of spot

structures in cool white dwarfs are discussed by Vornanen and Berdyugin, 2012

Except PG1658+441, all these stars are comparatively cool and have outer convective layers.

WD1953-011 historical remarks

• Koester, et al., 1998 - detected as an ordinary, weak-field MWD with ~ 100 kG dipole field (from comparison of spectroscopic and polarimetric measuremnets by Schmidt and Smith, 1995)

• Maxted, et al., 2000 - WD1953-011 has add., ~500 kG strong-field component which correspond to a contrast, about 10% projected area with constant magnetic field.

WD1953-011 historical remarks

Wade et al, 2003 – observations with the VLT. They have found complex circular polarization at the Balmer lines: the central classic S-wave of the Stokes-V profile is superposed with additional Stokes-V signature of opposite sign in the Hwings. These signatures well correspond to the weak satellite features observed by Maxted et al., 2000.

They (also Brinkworth et al., 2005) have found photometric variability of the star with a rotational period of about 1.45 days and amplitude of about 0.1 stellar magnitude. And they have made an assumption that the strong-field component could be associated with a dark spot analogous to a Sunspot.

WD1953-011

Valyavin et al., 2008: The strong-field feature has about 400 kG vertical component with the total field strength ~550 kG/ This makes it possible to interpret the strong field feature as a single magnetic flux tube.

WD1953-011present situation

• Modeling the photometric and spectropolarimetric data together we have established, that the brightness and magnetic spots are located at the same latitudes and almost the same longitudes. The difference in temperatures between the dark spot and neighboring areas could be from a few to a few tens of percents!

The presence of the cool, strong-magnetic area suggests as a direct analogy with the sunspots. There are, nevertheless, some problems with such an interpretation.

Unanswered questions are related with:

• 1. Existence of the field-temperature relationship among strong-magnetic white dwarfs and its absence among weak-field degenerates.

• 2. True nature of the group of slowly (fast?) rotating white dwarfs with highest magnetic fields and masses.

• 3. We also need to have more realistic model of MWDs with strongest magnetic fields in order to revise the radius-mass relationship.

• 4. More statistics of magnetic white dwarfs at the subkilogauss level.