Surface magnetic fields across the HR Diagram

Transcription

1 Surface magnetic fields across the HR Diagram John Landstreet University of Western Ontario London, Upper Canada & Armagh Observatory Armagh, Northern Ireland

2 Objective of this talk... This meeting combines specialists in different felds (the Sun, various kinds of stars), with people new to polarimetry Stellar polarimetry reveals scattering material around stars, and their magnetic felds The point of this talk is to give people a kind of simple road map to help to navigate the talks about magnetic felds in stars

3 Why do we need polarimetry to study stellar magnetic fields?? To detect magnetic felds, we use the Zeeman effect. In many hot stars, this is the only detectable symptom of a feld. Zeeman effect splits a single line into multiple components, separated in wavelength and polarised Components are separated by roughly Δλ(A) ~ B(G) 2 λ (A) ~ A/kG

4 Zeeman effect in the intensity spectrum

5 Zeeman splitting in 6kG field of magnetic Ap star HD 94660

6 Weaker fields (HD 96446) show polarisation, but no splitting

7 Recent advances in instruments Recent advances in instrument construction make most of HRD accesible to useful measure Much higher throughput Largely achromatic over wide spectrum Huge spectral range, e.g. all of optical window Several top-end instruments available as facility instruments

8 Improved analysis too! Notice that (circular) V/I polarisation signals are very similar from line to line. Averaging is possible to increase signal-to-noise ratio. Sensitivity to small felds then depends on effcient spectropolarimetry over broad wavelength band high density of fairly deep spectral lines small v sin i (narrow spectral lines) can reach polarimetric sensitivity of

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10 Mapping We fnd that many stars have felds with static or slowly changing structure. Series of polarimetric spectra in all Stokes components, taken as a star rotates, make mapping possible (Piskunov, Kochukhov, Donati, P. Petit). Such maps reveal magnetic feld structure at low to moderate spatial resolution, and often associated temperature or local abundance variations

11 Map of B2V magnetic star HD (Kochukhov et al 11)

12 The overall picture today Improvement in instrumentation has led to many major surveys and at least some feld detections all over HR diagramme!! PMS stars: T Tau and a few Herbig AeBe stars Main sequence (MS): rapidly rotating low mass stars, small fraction of O, B, A (Ap/Bp) stars Giants & AGB stars: a few Ap descendant(?) felds, some weak dynamo felds in both RG, AGB stars Fraction of white dwarfs, many neutron stars

13 Field structures Studying the magnetic felds found, we recognise two main types: dynamo (solar-type) fields, complex topology, changing structure on many short timescales, feld strength larger for shorter rotation periods, in cool stars. Field is currently being generated by a dynamo. fossil fields, roughly dipolar topology, structure virtually constant over tens of years, feld strength independent of rotation rate, in hot stars. Remmnant (fossil) of earlier phase.

14 Fields in pre-main sequence stars Both low and intermediate mass PMS stars pass through "T Tau" (deep convection) phase: rapid rotation, strong dynamo felds, up to ~3 kg (Johns-Krull et al, Donati et al) Intermediate and high mass stars then pass into Herbig AeBe (mostly radiative) phase: a few % show weak fossil felds, 10s or 100s of G at surface (Catala et al, Wade et al, Alecian et al)

15 Classical T Tau star BP Tau: surface & magnetosphere (Donati 2008)

16 Main sequence and evolved stars Low mass main sequence stars have dynamos that depend strongly on rotation rate, <~ 3 kg (Donati et al, P. Petit et al, Morin et al) Small fraction of intermediate and high mass MS stars have fossil felds, Bz ~ kg (Babcock, Preston, Landstreet et al, Mathys, Wade et al (especially MiMeS)) Massive stars can trap stellar wind in closed felds lines - produce emission lines, eclipses...

17 Trapped magnetosphere in σ Ori E

18 Red giants have dynamo felds of a few G or less, depending on rotation, but magnetic Ap star descendants have felds of ~ G even when rotation is very slow (Auriere et al, Konstantinova-Antova et al) Many massive AGB stars have dynamo felds of ~ 1G (Grunhut et al). N.B.: detected felds might be ~1% of actual felds... Fields are detected in most giants that show indirect indicators of magnetism - Ca II H & K line emission, strong X-rays, "rapid" rotation

19 Magnetic fields in red giants

20 Dynamo effect in red giants Normal M giant felds show usual dynamo dependance on rotation velocity (Konstantinova -Antova et al 2013)

21 Collapsed stars White dwarfs reveal felds via usual Zeeman effect and/or continuum polarisation Fields are found in a few % of all white 4 9 dwarfs. Fields range 10 to 10 G. Field structure roughly dipolar, and the felds are fossils Most or all neutron stars have fossil felds for a while (as pulsars), ranging from 109 to 1015 G

22 White dwarf magnetism Intensity and polarisation spectrum of white dwarf GD229 which has a feld of several hundred MG (cf Schmidt et al 1996)

23 Global evolution of fields Now have observational evidence that (some) felds occurs in most major evolution stages In low mass stars, dynamos seem to occur at most stages until fnal collapse to white dwarf In more massive stars, situation is very interesting! T Tau (dynamo) -> Herbig (fossil) -> MS (fossil) -> RG, AGB (dynamo) -> white dwarf or neutron star (fossil). This complex evolution is FAR FROM UNDERSTOOD.

24 Theoretical framework Surface felds are a consequence of internal electric currents and motions In cool stars observed felds may be mainly determined by present convection zone and distribution of angular momentum But we see from strong feld red giants, thought to be descendants of magnetic Ap stars, that earlier feld is also important What happens when giant -> hot white dwarf?

25 Theoretical framework In massive stars, high Teff phases clearly lack contemporary dynamo - insuffcient convection Today's surface feld is the fossil that results from the feld left in star by earlier evolution phases, modifed by Ohmic decay, feld relaxation, instabilities, stellar structure changes and internal shear fows (Braithwaite, Mathis) Fossil felds may be due to evolution in close binary

26 Can we observe field evolution within a single phase? With sample of stars of given mass with welldetermined relative ages (e.g. fraction of main sequence completed, or evolution position on giant branch) we can observe feld evolution statistically This has been carried out for main sequence evolution of stars of 2-5 M0, using a sample of magnetic stars in open clusters of known age

27 Using a cluster Ap star sample, Landstreet et al (2008) showed that RMS magnetic feld declines with stellar age during MS. top: 4-5 Mo; middle 3-4Mo, bottom 2-3Mo.

28 Lots of work left to do. Thanks to my many collaborators, especially Stefano Bagnulo Evelyne Alecian Luca Fossati Oleg Kochukhov Coralie Neiner Gregg Wade Jeffrey Bailey Jessie Silaj Jean-Francois Donati and others! And to you for your attention

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