the surface magnetic fields of white dwarf stars · 14 june 2017 physics of white dwarf stars...

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


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


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 ?


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


Zeeman effect in the intensityspectrum


Zeeman effect also leads to linepolarisation


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….


Fields ARE present, but not easilyrecognised

● Grw+70 8247shows stronglycircularlypolarised flux

● No obviousexplanation otherthan REALLYSTRONGmagnetic field(Kemp et al 1970)


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


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


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 ?


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


Zeeman splitting and polarisation

● With high spectralresolution, detectsuper-weak fields(WD2047+372,<|B|> = 60 kG,Landstreet + inprep)

● CFHT ESPaDOnSpolarised spectra


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


Spectrum of H in a large field(Schimiczek & Wunner 2014)


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>


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


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….


Map of 70 MG field of PG1015+014(Euchner, Jordan et al 06)


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


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.


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


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)


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


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


Thank you for your interest

And thanks to my collaboratorsStefano Bagnulo, Armagh Obs.Gena Valyavin, SAO of the RAS

the surface magnetic fields of white dwarf stars · 14 june 2017 physics of white dwarf stars...

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