what we have and what we are missing n.
Skip this Video
Loading SlideShow in 5 Seconds..
What we have, and what we are missing PowerPoint Presentation
Download Presentation
What we have, and what we are missing

What we have, and what we are missing

132 Views Download Presentation
Download Presentation

What we have, and what we are missing

- - - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript

  1. Essential Observations for Stellar Dynamos What we have, and what we are missing Steve Saar (CfA/SAO)

  2. Observations of Stellar Magnetic Variability • Ideally would like high res. vector B! But… • difficult observations, tricky analysis (various ZDI) • results typically low S/N, low spatial res. heavily averaged down B 0. • Still, of use! Only way to see polarity changes… So, typically use proxies for B….

  3. Observations of Magnetic Proxies • Photometry: • observe net differences in light – sum of spots and faculae/plage. (Trick is to disentangle their effects, understand minimum level) • Ca II HK: • totalchromospheric signal (need to calibrate away photospheric background, non-magnetic emission) X-rays: • Not enough data typically… and flares complicate more, but pure B Need long duration (decade+) data with decent coverage

  4. Ca II HK data • see clear cycles, not-so-clear cycles, multiple cycles, chaotic variability, constant emission, trends… • some calibration issues tho, at low S…

  5. Get: • cycle period Pcyc • cycle amplitude Acyc Also: • rotation period Prot(multiple times, most usefully!) • active longitudes • multiple Pcyc(younger stars) • polarity (with ZDI, but few stars, short timeseries) • intermittency (cycle on/off) • pseudo-”butterfly” diagrams (Prot vs FHK over Pcyc ) • background level (turbulent dynamo?)

  6. Ca II HK vs.photometry • AHKvsAphot to see dominance of bright B (plage/faculae) like Sun (positive corr.)dominance of dark B (spots) in more active stars (negative corr.) Lockwood, Radick et al

  7. pseudo-Butterfly diagrams: Prot vs. SHK • See evolution of Prot over the cycle… gets at differential rotation and active latitude migration, which leads to… Donahue 1996 Donahue & Baiunas 1992

  8. Looking under the hood: What makes a dynamo tick? Mean-field αΩ Dynamo number: D ~ αΔΩ R3 /η2 R is easy enough, but the others? Start with differential rotation ΔΩ, can get from: • changes in Prot • Doppler imaging (spots; high vsini) • ZDI (B in plage; high vsini) • line shape (GK, high vsini) Note: this is Surface DR… good enough?

  9. SDR vs. rotation (pre Keper) Key: X=F +=G =K diamond=M boxed=DI large=HK Saar 2009,2011 ∆Ω ~ Ω0.64 =0.25 dex for Ω < 10 d-1 ∆Ω tends to decline for Ω > 10 d-1, , mass dependence (Barnes) ∆Ω does not continue to increase(!) (at least not for all masses)

  10. SDR vs. rotation: Rossby number Key: X=F +=G =K diamond=M boxed=DI * Saar 2009,2011 Fitsare to maximum ∆Ω seen in single dwarfs, F5 and later. For Ro-1 < 90, ∆Ω ~ Ro-1.0 =0.24 dex For Ro-1 > 90, ∆Ω ~ Ro1.3 =0.30 dex

  11. Interestingly, If you aren’t choosy… (Barnes et al 2005; Rheinhold et al 2013) If you don’t screen out binaries, early F stars, evolved stars: Lose most Ω dependence, retain some Teff dependence. Which is right? Know your stars! Many evolved stars & binaries

  12. What makes a dynamo tick? II. Mean-field αΩ Dynamo number again: D ~ αΔΩ R3 /η2 What about α? What is it exactly? α ~ τc/3 < u’ ∨ xu’> Proportional to averaged small-scale kinetic helicity – we can estimate convective velocities, but what about twist? Dimensionally, sometimes estimated from α~ LΩ . Is this good enough??

  13. What makes a dynamo tick? III. Mean-field αΩ Dynamo number again: D ~ αΔΩ R3 /η2 What about η? What is it exactly? η ~ τc/3 < u’ u’> Proportional to averaged small-scale velocity fluctuation – turbulent diffusivity; get from: • Kepler flicker (Bastien et al 2013) ? • Erodes AR – get from AR decay timescales? Dimensionally, sometimes estimated from α~ Lv . Is this good enough??

  14. Lx/Lbol vs. Rotation (Rossby number) Key: diamond=phot box=HK Circle=DI Size~ Lx/Lbol ~Ro-2.3 =0.27 dex for Ro-1 < 80 Lx/Lbol ~ 10-3for Ro-1 > 80, saturation saturated Lx/Lbol begins just where ∆Ω(Ro) peaks!

  15. What makes a dynamo tick? Other items of importance… Stars spin down due to magnetic torque in the stellar wind Spin down in turn effects dynamo B generation, so… Need to know mass loss (or have a good model for it) Data is sparse…. (Wood et al 2005, etc) Helicity losses too (Brandenburg, etc)? maybe from CME rates but almost no data….

  16. What makes a dynamo tick? Other items of importance. II • What drives intermittency (Magnetic grand minima?) - mostly older stars (>1 Gyr), CZ depth dependence? • What are the secondary cycles? • Importance of meridional flows… • How does the spatial distribution of activity evolve? • How does the presence of a binary affect things? … and I’m probably forgetting your favorite!…

  17. Revisit - Data to Use:Be a bit more picky! Any good quality SDR measurement, but only from • Dwarf stars: avoid evolutionary/structural issues • Single stars (or effectively so): avoid tidal effects • Stars ~F5 and cooler: drop stars with thinner CZ which do not follow the “standard” rotation-activity relationships (Walter 87, Bohm-Vitenseetal 05)

  18. New definition for MGM candidates: • Dwarf star, confirmed by high res. spectral fit (Teff , log g) • Low activity: d log R’HK < -5.12 - 0.21 log M/H + dR’HK • Low variability: RMS R’HK variation < 2% (adjust dR’HKto keep optimize separation of potential MGM candidates). Stay flat for > 4 years (> solar minimum) d log R’HK ~ 0.06 gives good results (dashed line, see next slide)… log R’HK box = dwarf; + = evolved log M/H

  19. Are Maunder-like minima rare? III Dwarfs within d log R’HK ≤0.06 (15%) of R’HK(M/H) boundary show low variability (fract. RMSof SHK ≤ 2%). These are our new magnetic grand minimum candidates. • MGM candidates: ~8% of sample dwarfs • Sample: <Teff> = 5610 ± 379 K <[M/H]> = -0.015 ± 0.228 • (but a low activity bias!) MM HK/SHK (%) box = dwarf; + = evolved # years obs.: 4,5,6,7 log R’HK

  20. SDR vs. rotation: Rossby number Key: X=F +=G =K diamond=M boxed=DI * Fits improved if local c is used for ∆Ω(Ω) increasing, global c for∆Ω(Ω) decreasing (from Y-C Kim) (Teff, dCZ dep. into c) For Ro-1 < 90, ∆Ω ~ Ro-0.90 =0.24 dex For Ro-1 > 90, ∆Ω ~ Ro1.31 =0.30 dex (fit to maximum ∆Ω seen)

  21. What about ΔΩ and magnetic flux itself? Not enough B measurements so use X-ray emission as a proxy Should work… (Pevtsov et al 2003; TTauris excepted)

  22. SDR vs. Lx/Lbol (proxy for B, dynamo) Key: white = dMe circle=DI box=HK diamond= phot. Lx/Lbol ~ ∆Ω1.36 =0.48 dex for Lx/Lbol < 6x10-4 (Ω < 10 d-1) Lx/Lbol ~ 10-3 (for Ω > 10 d-1), saturation - for all∆Ω ! Lx/Lbol (and B?) a maximum,independent of ∆Ω !

  23. SDR vs. Lx/Lbol(The Answer is “7”!) Key: white = dMe circle=DI box=HK diamond= phot. Lx/Lbol ~ ∆Ω1.36 =0.48 dex for Lx/Lbol < 6x10-4 (Ω < 10 d-1) Lx/Lbol ~ 10-3 (for Ω > 10 d-1), saturation - for all∆Ω !  Lx/Lbol (and B?) a maximum,independent of ∆Ω !

  24. The Evolution of SDR (combined view) Arrow of time: ∆Ω - Ro Lx/Lbol (B) - ∆Ω Lx/Lbol - Ro ∆Ω increases to a maximum as Ωdeclines, then decreases. Lx/Lbol is steady during the initial ∆Ωincrease, but decays once ∆Ω reaches a maximum and begins to decrease. Initially: ∆Ω~ Ro +1.3 while Lx/Lbol ~10-3(saturated activity)Then ∆Ω ~ Ro -0.9 after Ro-1 ~ 80 or Ω < 10 d-1

  25. SDR vs. age (from gyrochronology) Key: diam.=phot box=HK circle=DI For Ro-1 < 80, ∆Ω ~ t-0.46=0.27 dexstandard Ω spindown For younger stars, ∆Ω increases to this level, F stars by ~30 Myr, G stars by ~60 Myr, early K by ~120 Myr, late M by ~1 Gyr. = the age when thetachocline/shear dynamo “takes over”(?)

  26. Starspot amplitudes/distributions Combine V band spot amplitudes Aspot for >1200 cluster/field single dwarfs Maximum, mean Aspot and distribution all useful. Connect Aspot,max: is there a “wedge” removed (green)?

  27. Starspot amplitudes/distributions. II. Simple models can work: Aspot,max ~ Ro-0.7 < Amax(2 – eβRo ) (no “wedge” missing; dashed) Aspot,max ~ [Ro-0.7 < Amax(2 – eβRo )] - DR(Ro-1) (“wedge”gone; solid) Increased shearing/decay of spots due to DR may explain drop in Aspot,max Data at high Aspot, a bit sparse though…

  28. Starspot amplitudes/distributions. III. 12 bins of 100 stars each; look at moments of distribution: Mean <Aspot> saturates at Ro-1 > ~60 (boxes) RMS σ(Aspot) saturates at Ro-1 > ~60, small drop around Ro-1 ~ 100? Aspot,max binned,shows sharp drop atRo-1 ~ 100, continued rise for larger Ro-1

  29. Starspot amplitudes/distributions. IV. Higher order moments: SkewnessAspot dist. generally rises, sharp break to lower values (more symmetric dist.) at Ro-1 ~100 (boxes) Excess kurtosis Aspot also rises, drops sharply to ~0 (~Gaussian) Ro-1 > 100 (diamonds). Aspot,max , Aspotskewness, and kurtosis all show sharp breaks at Ro-1 ~ 100, at the Aspot “wedge”, where DR slope changes sign and X-rays (and magnetic flux?) saturate. Coincidence?

  30. (so when does he start talking about…) Stellar Activity Cycles TheSDR results help guide how best to explore cycle properties. Previously (Saar & Brandenburg 2001)…. Single dwarfs + binaries, evolved stars

  31. Activity Cycles I. Cycle Period (Work in progress….) Backtrack from Saar & Brandenburg (99,01), use only single dwarfs (vis SDR!) Update data with Frick et al (2004), Messina & Guinan (2001), plus…. Nothing obvious at first…. • cyc ~ 0.0 ? (vis Barnes et al SDR? See also Olah et al 2009: cyc/Ω ~ -1) • But consider where secondaryPcyc(smaller connected symbols) lie

  32. Activity Cycles II. Cycle Period Consider Pcyc(2nd) (connected to main Pcyc by vertical dotted)… • 2 or 3 bands, separated by factors of 4, each with cyc ~ 1.3 • Possible break at  ~ 10 x solar - the same point where  slope changes…. • Multimode dynamo, quantized cyc steps with change in behavior with  at high ? But secondary cycles are key here, bands are fairly wide – Are Pcyc(2nd) true cycles (polarity reversing) or just amplitude modulations? Or just a modulation on the main cycle?

  33. Are secondary Pcyc true cycles? Pcyc(2nd) are often shorter than primary cycle, sometimes just a few (2-6) years. Short, polarity reversing cycles are seen in a few stars: tau Boo (F9V; Donati et al 2008), HD 190771 (G5V; Petit et al 2009) Also: Fractional cycle amplitudes seen in HK of Pcyc(2nd), AHK, have quite different behavior with rotation, suggesting a distinct phenomenon (Moss et al. 2008) = different cycle mode? Main Pcyc: AHK ~ Ro0.3 Pcyc(2nd): AHK ~ Ro-0.4 Transfer of energy to higher order modes as Ro-1 increases?

  34. Magnetic Fields/Geometries How does this all inform recent (ZDI) results on magnetic field strengths/geometries? Ro ~ 0.1 (below) is ~saturation: DR drops off to both sides. Three dynamo modes? Ro<<0.1 poloidal/axisym. Ro ~0.1-2 toroidal/non.-axisym. Ro>2 poloidal/axisymmetric Three regimes? Size ~ B Round/star – axisymmetry Red/blue – poloidal/toroidal Main Pcyc: AHK ~ Ro0.3 Pcyc(2nd): AHK ~ Ro-0.4 Transfer of energy to higher order modes as Ro-1 increases?

  35. Three Regimes(?) Highest Ro-1: DR minimal, convective/turbulent dynamo, poloidal, axisymmetric geometry, low dependence of rotation on activity, uniform generation so Aspot lower. Intermediate Ro-1: DR near maximum, but models (eg, Brown et al.) indicate vmerid tiny, so no flux transport/tachocline dynamo - B production in CZ dynamo with high shear = toroidal. Non-axisymmetric so high Aspot (when DR is low enough). Low Ro-1: DR smaller again, vmerid higher (from models) so here lies solar-like flux-transport/tachocline dynamos. Lower B production and axi-symmetric so Aspot small again. Restores an important role for DR(Ω) in cycles, magnetic field production and geometry

  36. Some side implications • Convective dynamo in rapidly rotating stars could explain (see also Donati et al …): • Low latitude spots (should be high latitude/polar if arising from tachocline dynamo) • Reduced activity changes with Ω on saturation branch • Reduced spindown rate in younger stars • Gradual convective> shear/tachocline dynamo transition could explain lack of activity break in mid M stars

  37. Quick Summary • SDR increases as ~Ro-1 for low , but… • It drops at high ! Stars can have strong B and cycles with little  • Suggestion of dominance change convective dynamos – full CZ dynamos at highest  - tachocline driven at lower  • Cycle period relations more complex/less clear, cycshows evidence for quantized relations with  - some stars show multiple cyc …. Evidence for multimode dynamos? • Amplitudes Acyc increase with increasing CZ depth to mid-K; spot/plage ratio increases with  • Primary/secondary cycles show opposite Acyc trends with ; are secondary cycles different in some way? (not true cycles? Quadrupoles?) • SDR - cyc relations may also show multiple modes… needs more work A loud cry of help!! to theorists out there!

  38. What’s up? Check color - Prot diagram Key: X=F +=G =K =M box=DI bold=FTLP large=HK I branch @ various ages C branch @ various ages Stars with increasing/decreasing shear neatly divide into Barnes’ I branch (Skumanich law Prot ~ age0.5 stars; interface dynamo?) and C branch (Prot ~ eage ; convective dynamo?) stars.

  39. Activity Cycles IIb. Cycle Period Try Rossby number & non-dim. cycle freq. (vis. Brandenburg etal. 1998) • 2 or 3 bands, separated by factors of ~4, but slopes vary a bit cyc/ ~ Ro-A,B,C • Possible break at Ro-1 ~ 60 - the same point where  slope changes…. • Multimode dynamo, quantization(?) of cyc steps less clear here…

  40. Activity Cycles IIc. Cycle Period OR… surrender to a lack of dependence! Fits not as good though… • 2 bands, separated by factor of ~4, cyc/ ~ Ro+1 (ie, no dependence) • Simpler, but many stars are poorly fit. Possible break at Ro-1 ~ 60 - the same point where  slope changes…. • But again, some suggestion of multimode dynamo/quantization(?) of cyc

  41. Magnetic Cycles III. Amplitudes • Ca II HK = plage/network data: Max Acyc increases with B-V, peaksin midK (Saar & Brandenburg 2002) (avg Acyc(spot) increases towards lower masses; Messina et al.) • Acyc decreases with Ro-1; Acyc(2nd) increases with Ro-1 - another sign of multimode dynamo? (Moss ea 2008)

  42. Summary: Two SDR regimes! • ∆Ω increases with Ω at low Ω: standard rotation-activity-age relations, Barnes’ I branch - solar-like tachocline/interface and/or CZ αΩ dynamo (local c best) • ∆Ω decreases with Ω at high Ω: saturated activity, shear dynamo less effective, Barnes’ C branch - so… convective/turbulent dynamo? (global c best) Evolutionary scenario: starting with low ∆Ω and high Ω and a convective dynamo, stars spin down gradually increasing ∆Ω until ∆Ω is large enough to “take over” (at ~60 Myr in G stars, ~120 Myr in early K, ~ 1 Gyr late M). Activity steady. Thereafter, the tachocline/shear/CZ dynamo is more dominant for spindown, and magnetic activity decreases.