the surface magnetic fields of white dwarf...
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14 June 2017 Physics of White Dwarf Stars
The surface magnetic fields ofwhite dwarf stars
John Landstreet
University of Western OntarioLondon, Upper Canada
&Armagh Observatory
Armagh, Province of Ulster
14 June 2017 Physics of White Dwarf Stars
Why study magnetic fields instars ?
● Magnetic fields alter spectral lines, greatly changepulsation modes, and produce « activity ». Fieldsstrongly affect interpretation of observations
● A magnetic field can stabilise a stellar atmosphereand substantially alter its physical structure (e.g. bysupressing convection, or by Lorentz forces)
● Fields greatly affect transport of angular momentumand mixing – during accretion or mass loss phases,and inside the star at any time
14 June 2017 Physics of White Dwarf Stars
What to expect in WDs ?
● In 1960s Lo Woltjer pointed out that WDswith same magnetic flux as MS magnetic Apstars would have B ~ 107 G (103 T)
● Discovery of pulsars in 1967, with similarmagnetic flux and B ~ 1012 G, gave this ideaplausibility (and excitement)
● Could we find fields in WDs ?
14 June 2017 Physics of White Dwarf Stars
How are magnetic fields detectedand measured ?
● To detect most stellar magnetic fields, we usethe Zeeman effect. In many hot stars, this isthe only detectable symptom of a field.
● Zeeman effect splits a single spectral line intomultiple components, separated in wavelengthand polarised
● Components are separated by roughly
Δλ(A) ~ 5 10-13 B(G) λ2(A) ~ 13 A/MG
14 June 2017 Physics of White Dwarf Stars
Search for WD fields● B >~ 5 MG fields should produce easily seen
Zeeman splitting of >~50 A● Among ~100 WDs known in 1970, no Zeeman
splitting seen => fields are rare, or small, or«unrecognisable», or buried inside WDs
● First really sensitive search using polarimetrygave NO detections (Angel & Landstreet 1970)
● Jim Kemp searched for continuum circularpolarisation. I proposed Grw+70 8247….
14 June 2017 Physics of White Dwarf Stars
Fields ARE present, but not easilyrecognised
● Grw+70 8247shows stronglycircularlypolarised flux
● No obviousexplanation otherthan REALLYSTRONGmagnetic field(Kemp et al 1970)
14 June 2017 Physics of White Dwarf Stars
Megagauss magnetic fields !● Quickly interpreted as resulting from field of
B~107 G (now known to be 3 108 G)● Further searches during next 20 yr, mostly
for broad-band circular polarisation,discovered ~1 or 2 new MWD/yr, usuallyexotic, puzzling objects
● Eventually a few MWDs found that showexpected simple Zeeman effect
14 June 2017 Physics of White Dwarf Stars
Strange spectra of polarised MWDs● We only recognise exotic
stars like this as WDs byastrometry !
● Spectrum of GD229typical of some earlyMWDs : completelyunfamiliar lines andfeatures in all Stokescomponents I, Q, U, V
14 June 2017 Physics of White Dwarf Stars
Survey field detections ● Most of ~250 MWDs
known are from singlelow-resolution I spectra(e.g. Gianninas et al 11,SDSS)
● R~2000 and S/N~10 =>lower limit to fielddetection is ~2 MG.Upper limit ?
14 June 2017 Physics of White Dwarf Stars
How can we measure these fields ?
● For fields below ~20 MG, Zeeman splittinggives direct measure of <|B|> from separationof pi and sigma components
● For such fields polarimetry can measure <Bz>
from separation between mean spectral lineposition in left and right circularly polarisedlight
14 June 2017 Physics of White Dwarf Stars
Zeeman splitting and polarisation
● With high spectralresolution, detectsuper-weak fields(WD2047+372,<|B|> = 60 kG,Landstreet + inprep)
● CFHT ESPaDOnSpolarised spectra
14 June 2017 Physics of White Dwarf Stars
Fields above ~10 MG
● In MG fields, perturbation of energy levels byfield more complex than simple Zeeman effect
● To measure larger fields, new atomic physicscalculations were required, to determinespectral line wavelengths as function of B
● Major physics contributions, e.g. from Wunnerand collaborators from 1980s on –> thespaghetti diagrams
14 June 2017 Physics of White Dwarf Stars
B measures for strong fields● For strong fields, above
Zeeman range, with tablesof lambda vs B for Balmerlines, we can identifyabsorption features in Ispectrum, estimate <|B|>(e.g. HE0241-0155, Reimerset al 04)
● V spectrum is usuallyproblematic, so no <B
z>
14 June 2017 Physics of White Dwarf Stars
MWDs brighter than V = 14 NB : 5/8 have fields below 1 MG !
WD2153-512 Teff = 6100 K, C + H <Bz> ~ 1.3 MG <|B|> ~ 5 MG ??
WD2047+372 14700 K, H -12 to +15 kG 57 kG
WD2359-434 8650 K, H ~4 to 12 kG ~100 kG
WD1900+705 11850 K, H 320 MG
WD0446-789 24450 K, H 2.5 to 6.2 kG 20 kG ?
WD2105-820 10800 K, H 4 to 11 kG 43 kG
WD1953-011 7900 K, H ~40 kG 100 to 500 kG
WD0912+536 7250 K, He ? ~100 MG
14 June 2017 Physics of White Dwarf Stars
Models of surface field geometry● Models give us better measurement of
surface fields, possibly info about interior B
● May simply fit <|B|> and <Bz> variations, or be
based on comparisons of computed I, (Q, U), Vspectra with observed ones.
● These computed spectra must include a lot ofphysics – radiative transfer (of four coupledStokes parameters), continuous & lineopacities, magnetic effects in lines….
14 June 2017 Physics of White Dwarf Stars
Map of 70 MG field of PG1015+014(Euchner, Jordan et al 06)
14 June 2017 Physics of White Dwarf Stars
Prominent unresolved issues
● Balmer line broadening in a magnetic field● Polarised radiative transfer in non-LTE line
core of Halpha for model line computations● Polarised continuous H opacity : absorption
coefficients, dichroism and how these expressthemselves in producing continuum polarisation
● All of the above for He, for magnetic DB stars
14 June 2017 Physics of White Dwarf Stars
Balmer line broadening in B field
● In non-magnetic atmosphere, levels of H aredegenerate, so collisions with ions and freeelectrons lead to first-order Stark shifts andhence to very broad line wings.
● In field of many kG or more, degeneracy isbroken by B. No complete theory or accuratecomputations available of line wing shape inpresence of large B level splitting.
14 June 2017 Physics of White Dwarf Stars
Non-LTE in Balmer Halpha core
● Halpha has deep non-LTE core. Currentmagnetic line synthesisprogrammes are in LTEand compute coreincorrectly.
● How to incorporate therequired physics intosuch a program ?
WD 2047+372
14 June 2017 Physics of White Dwarf Stars
Bound-free absorption coefficientof H in large magnetic field
● Observed continuumpolarisation of Grw+708247 compared tocomputed polarisation(Jordan 92)
● Major discrepancy is asymptom of inaccuratebound-free opacity(Zhao & Stancil 07)
14 June 2017 Physics of White Dwarf Stars
All of the above for He
● All the problems withspectrum synthesis ofDA stars are moresevere for DB stars(He-rich atmospheres),as shown byWickramasinghe + 02
14 June 2017 Physics of White Dwarf Stars
Some conclusions
● We now know how to identify WDs withsurface fields between 10 kG and 1000 MG
● We can estimate field strengths for most ofthis range, but such estimates are ratherinhomogeneous
● Modelling is difficult, especially for high fields,due to limitations of tools and physics, and tothe complex structure of the fields
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