the surface magnetic fields of white dwarf stars · 14 june 2017 physics of white dwarf stars...
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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
Zeeman effect in the intensityspectrum
14 June 2017 Physics of White Dwarf Stars
Zeeman effect also leads to linepolarisation
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
Spectrum of H in a large field(Schimiczek & Wunner 2014)
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
14 June 2017 Physics of White Dwarf Stars
Thank you for your interest
And thanks to my collaboratorsStefano Bagnulo, Armagh Obs.Gena Valyavin, SAO of the RAS