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A Review of Magnetic Activity in Sun-like stars

A Review of Magnetic Activity in Sun-like stars. Magnetic Stars Seminar Oct. 30, 2002. Who’s active, Who’s not. Evidence of magnetic activity. Activity on main sequence: types F  M B-V > 0.4. (From Linsky 1985). Inferring activity. (CaII H line).

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A Review of Magnetic Activity in Sun-like stars

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  1. A Review of Magnetic Activity in Sun-like stars Magnetic Stars Seminar Oct. 30, 2002

  2. Who’s active, Who’s not Evidence of magnetic activity Activity on main sequence: types F  M B-V > 0.4 (From Linsky 1985)

  3. Inferring activity (CaII H line) Variation in CaII K along solar slit (from MWO HK Project) S ~ integral under (CaII K line)

  4. Measuring activity Luminosity: [ ergs sec-1 ] Avg. surface flux: [ ergs sec-1 cm-3 ] Fundamental Flux ratio: dimensionless

  5. Measuring activity Luminosity: [ ergs sec-1 ] Avg. surface flux: [ ergs sec-1 cm-3 ] Fundamental Flux ratio: dimensionless CaII lines HK-index: continuum Observed color-dependant conversion factor

  6. Inferring activity • MWO HK project: • 111 stars F2-M2 • Observed monthly • in CaII H & K • Continuing since • 1966 (O. Wilson) • Presently led by • S. Baliunas S http://www.mtwilson.edu/Science/HK_Project/ A K3 star: HD160346

  7. MWO survey of nearby* stars Who’s active, Who’s not log(FHK) Cutoff at B-V ~ 0.4 F3 Vaughan Preston Gap * d < 25 pc (from Vaughan & Preston 1980)

  8. Convection zones (where the fields come from) • Schwartzschild crit. • violated for • r < R – dce • Heat x-port by • turbulent convection • Velocity & size • of convection • eddies (vml & lml ) • from mixing length • theory dce

  9. Convection zones 2 R> R* R> Fully radiative dce 8000 7000 6000 5000 4000 F0 F5 G0 G5 K0 K5

  10. Explaining Activity Levels Individual Variation Variance within class

  11. Measuring Rotation Periods Pobs=0.527 d Photometric measurements over time Lomb periodogram … identify peak frequency (from Patten and Simon 1996)

  12. Measuring Rotation Periods Pobs=0.527 d Photometric measurements over time Re-sampled at putative period Pobs (from Patten and Simon 1996)

  13. Rotation determines activity <R’HK> <F’HK> Less variance than vs. B-V Periods Pobs measured from HK index S(t) (from Noyes et al. 1984)

  14. Why Rotation Matters Significance of rotation on convection: Rossby Number Mixing Length Theory tc = lml/vml aml = lml /Hp Ro = Pobs/tc lml= 0 for B-V < 0.4 (i.e. earlier than ~F3) (from Gilman via Noyes et al. 1984)

  15. Defining the Rossby Number Ro = Pobs/tc aml=1 (great variance) aml=2 (small variance) (from Noyes et al. 1984)

  16. The Dynamo Number Parker’s Dynamo # Dynamo is linear inst-ability for ND > Ncrit Dynamo a-effect:

  17. The Dynamo Number Parker’s Dynamo # Dynamo is linear inst-ability for ND > Ncrit Dynamo a-effect:

  18. Activity vs. Rossby Number 41 Local stars Pobs from S(t) young stars old stars (from Noyes et al. 1984)

  19. Activity vs. Rossby Number (from Patten and Simon 1996) • Stars in open cluster 2391 (30My old) • RX from ROSAT observations • Rotation periods Pobs from optical photometry • NR = Ro = Pobs/tc

  20. Evolution of Activity (from Skumanich 1972)

  21. Evolution of Rotation Rate • Spin-up during • collapse to ZAMS • Spin down by mag- • netic braking: wind • carries away angular • momentum • Slow rotation  • less field  • less wind (from Hartmann & Noyes 1987)

  22. Evolution of Activity (from MWO HK Project)

  23. Evolution of Activity IC2391 IC2602 Skumanich-law Pleiades Hyades Sun (from Patten & Simon 1996)

  24. Cyclic variation in activity from MWO HK Project

  25. Cycle Types L V V-P gap C F (from Baliunas et al. 1995)

  26. Cycle Types V-P gap: High-S Low-S (from Baliunas et al. 1995)

  27. Cycle periods (from Baliunas & Vaughan 1985)

  28. Differential Rotation(?) Cases where Pobs varies w/ cycle… Interpretation: activity belt is migrating (cf butterfly) sampling rot. rates at diff. latitudes from MWO HK Project

  29. Summary • Stars in classes F3 – K are magnetic • Magnetic activity depends on Ro • Some (~50%) have activity cycles* • Cycle periods range from 2 yrs to > 20 yrs • Younger stars tend to be more variable or have shorter periods *Q: what fraction of time is Sun cyclic?

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