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Magnetic activity on rapidly rotating stars I: Surface flux distributions

Magnetic activity on rapidly rotating stars I: Surface flux distributions. Activity proxies Surface coverage of active regions Polar spots Diffusion and advection of surface magnetic fields Filling factors and flux emergence rates. Andrew Collier Cameron, Moira Jardine,

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Magnetic activity on rapidly rotating stars I: Surface flux distributions

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  1. Magnetic activity on rapidly rotating stars I: Surface flux distributions • Activity proxies • Surface coverage of active regions • Polar spots • Diffusion and advection of surface magnetic fields • Filling factors and flux emergence rates. Andrew Collier Cameron, Moira Jardine, John Barnes, Sandra Jeffers, Duncan Mackay, Kenny Wood (University of St Andrews) Jean-François Donati (Obs. Midi-Pyrenees, Toulouse). Meir Semel (Obs. de Paris, Meudon)

  2. Overview • Why are rapidly rotating stars useful? • We can: • map their surfaces! • determine the latitude distribution of active regions • estimate the flux emergence rate and spot lifetime • map the magnetic polarity distribution in the network • What does this all tell us about dynamos? • Spectral-type dependence of surface flux distribution • Spectral-type dependence of differential rotation • Cyclic behaviour: spot coverage, differential rotation • Meridional circulation?

  3. Magnetic activity proxies • Broad-band optical modulation • => dark starspots Collier Cameron et al 1999 Kürster et al 1997

  4. Magnetic activity proxies • Emission cores in strong UV/optical lines • => chromospheres Sun in Ca II 393.3 nm filter Linsky et al 1979

  5. Magnetic activity proxies • Emission cores in strong UV/optical lines •  chromospheres •  rotation periods •  activity cycles Vaughan et al 1981

  6. Magnetic activity proxies • Emission cores in strong UV/optical lines •  chromospheres •  rotation periods •  activity cycles •  differential rotation? • Secular changes in Ω  range of surface rotation rates. • Period-DR relation: • BUT: No reliable latitude information Donahue, Saar & Baliunas 1996

  7. Magnetic activity proxies • Emission cores in strong UV/optical lines •  chromospheres •  rotation periods •  activity cycles •  dynamos? Noyes et al 1984

  8. Magnetic activity proxies • Soft X-ray emission •  magnetically confined coronal plasma XMM spectrum and light-curve of star in IC2391 (Marino et al 2003)

  9. Magnetic activity proxies • Soft X-ray emission •  “Saturation” … Vilhu 1984

  10. Magnetic activity proxies • Soft X-ray emission •  “Saturation” … •  and “super-saturation” Stauffer et al. 1997 Prosser et al. 1996, alpha Persei cluster

  11. Magnetic activity proxies • Decrease in rotation with age • Ultra-fast rotators found in young clusters only • Earlier spectral types spin faster after ~0.3 Gyr • => Hot magnetically channelled winds dΩ/dt ~ -Ω3 Barnes, S. 2001

  12. Barnes, S. 2001 Barnes, S. 2001

  13. Convection and rotation • F, G, K, M spectral types •  outer convective zones • Activity indicators increase with rotation •  Rotation drives activity • Evidence of differential rotation: can we map it? • Spindown rates depend on spectral type •  Convection zone depth is important • Do young stars really have up to 50% starspot occupancy? • For the fastest rotators Lx decreases with Ω !

  14. Evidence for dense spot coverage • TiO bands occur in spots only. O’Neal, Neff & Saar 1996

  15. Evidence for dense spot coverage • TiO bands occur in spots only. • 7055Å/8860Å band ratio gives spot temperature. O’Neal, Neff & Saar 1998

  16. Evidence for dense spot coverage • TiO bands occur in spots only. • 7055Å/8860Å band ratio gives spot temperature. • Band strength gives spot covering fraction. Normalised photospheric spectrum Normalised spot spectrum Composite model spectrum Continuum brightness ratio Spot filling factor O’Neal, Neff & Saar 1998

  17. Evidence for dense spot coverage • TiO bands occur in spots only. • 7055Å/8860Å band ratio gives spot temperature. • Band strength gives spot covering fraction. • Active stars have filling factors fs~20% to 40% O’Neal, Neff & Saar 1998

  18. Measuring spot coverage with HST • Eclipsing binary SV Cam • G0V + K5V • Edge-on orbit • K5V transits primary • Light-curve analysis  radii • Measure missing-flux spectrum at mid eclipse • Use HIPPARCOS parallax to get solid angle  surface brightness Jeffers et al. 2004

  19. Eclipsed-flux deficiency in SV Cam • Eclipsed flux is ~30% less than best-fit Teff indicates. • fS~40% Jeffers et al. 2004

  20. Evolutionary effects of flux blocking • Star expands slightly • Photospheric Teff increases •  significant effects on HR diagrams of young open clusters, e.g. Pleiades Spruit & Weiss 1986 Stauffer et al 2003

  21. Imaging of stellar surfaces • Direct imaging? • Stellar Imager mission concept: • Goal is 50,000 km resolution on a Sunlike star 4 pc away • Requires angular resolution 60-120 µas • 0.5-km space-based UV-optical interferometer array ?

  22. Rotational broadening of photospheric lines Stauffer et al 1997 • Rotational Doppler shift dominates broadening of stellar photospheric lines in rapid rotators. • Rotation profile contains information about surface features (Goncharsky et al 1977, Vogt & Penrod 1983)

  23. A A Intensity Intensity -v sin i v(spot) v sin i -v sin i v(spot) v sin i Velocity Velocity Starspot “bumps” in spectral lines

  24. Imaging of stellar surfaces on a budget • Combine profiles of all recorded photospheric lines to boost S:N. • Compute synthetic line profiles from trial image. • Iterate to target c2 at maximum entropy. • Get simplest image that fits data. • Nearly always get a dark polar cap. -v sin i +v sin i Starspot signatures in photospheric lines

  25. Example: Speedy Mic (K3V) • Spots present at all latitudes including polar regions. Barnes et al 2004

  26. Example: HDE 283572 • Strassmeier et al 1998: WTTS, v sin i = 78 km s–1

  27. Polar fields • Schrijver & Title (2002) modelled flux emergence on stars of different rotation rates. • Rapid rotators develop rings of opposite polarity at poles. • Note reversal of polar fields over cycle. • Also Schüssler (1997) modelled buoyant flux tube emergence. Flux tubes deflected to high latitudes on rapid rotators.

  28. Magnetic activity on rapidly rotating stars II:Temporal evolution • Tracking starspots • Time-varying differential rotation • Differential rotation along the main sequence • Stellar magnetograms • 3D coronal structure Andrew Collier Cameron, Moira Jardine, Duncan Mackay, Kenny Wood, John Barnes, Sandra Jeffers (University of St Andrews) Jean-François Donati (Obs. Midi-Pyrenees, Toulouse). Meir Semel (Obs. de Paris, Meudon)

  29. What else can we learn from stellar surface maps? • Snapshots: • Unpolarized: Latitude distributions of spots • Locations of slingshot prominence complexes • Circularly polarized: Magnetic topology of corona • Days-weeks timescale: • starspots trace surface differential rotation and meridional flows • Weeks-months: • Lifetimes of individual spots and magnetic regions • Years: • Stellar butterfly diagram: Dynamo cycles • Polarity reversals?

  30. Polar spots and convective-zone depth • LQ Lup (G2) • Donati et al (2000)

  31. Polar spots and convective-zone depth • HE 699 (G2-3V; alpha Per G dwarf) • Jeffers et al (2002)

  32. Polar spots and convective-zone depth • HK Aqr (M1) • Barnes et al (2004)

  33. Polar spots and convective-zone depth • RE J1816+541 (M1) • Barnes et al (2001)

  34. Polar spots and convective-zone depth G3V G6V G8V K0V K3V M1V M1V

  35. Surface brightness: 1996 Dec 23 - 29 • Equator rotates faster than pole • solar-like shear • Prot ~ 0.5 d • Equator laps pole by 1 cycleevery ~ 120d

  36. Surface shear: 1996 December 23 - 29 • CCF for surface-brightness images • CCF for magnetic images:

  37. Starspots as flow tracers • Individual spot trails have their own recurrence periods. • Velocity amplitude of sinusoid: Rotation rate at latitude q Axial inclination Stellar radius

  38. Spot velocity amplitude: Matched-filter analysis • Travelling gaussian: Spot rotation rate: Intrinsic line width Foreshortening and limb darkening Spot phase angle relative to observer’s meridian Inclination Latitude

  39. Optimal scaling Scale factor: Badness of fit: xij (phase binned on trial period) c2 gij: equatorial spot at phase 0.5

  40. Optimal scaling Scale factor: Badness of fit: xij (phase binned on trial period) c2 gij: equatorial spot at phase 0.5

  41. Differential rotation: 1988 Dec • Model fit:

  42. Differential rotation: 1992 Jan • Model fit:

  43. Differential rotation: 1993 Nov • Model fit:

  44. Differential rotation: 1995 Dec • Model fit:

  45. Differential rotation: 1996 Dec • Model fit:

  46. Differential rotation: 1998 Dec • Model fit:

  47. Differential rotation: 2000 Dec • Model fit:

  48. Differential rotation: 2001 Dec • Model fit:

  49. Differential rotation 1988-2001 • Differential rotation rate doubled in 3 years from 1988 Dec to 1992 Jan. • As equator speeds up, polar regions slow down. • Rotation rate at q ~ 40o remains ~ constant.

  50. Impact on convective zones • Angular rotation in convective zone • Plot estimates in Ωeq-dΩ plane • Interpret differences as: • distinct anchoring depths • of tracers within CZ • temporal changes in angular • rotation profile within CZ

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