valyavin_Magnetic_white_dwarf_stars

Report
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 Ha wings. 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.
WD1953-011
present 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.

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