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

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

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

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

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

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 A K3 star: HD S

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

Convection zones (where the fields come from) Schwartzschild crit. violated for r < R – d ce Heat x-port by turbulent convection Velocity & size of convection eddies (v ml & l ml ) from mixing length theory d ce

Convection zones R>R> 2 R > F0 F5G0G5K0K5 R*R* d ce Fully radiative

Explaining Activity Levels Individual Variation Variance within class

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

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

Rotation determines activity Periods P obs measured from HK index S(t) (from Noyes et al. 1984) Less variance than vs. B-V

Why Rotation Matters t c = l ml /v ml a ml = l ml /H p (from Gilman via Noyes et al. 1984) l ml = 0 for B-V < 0.4 (i.e. earlier than ~F3) Ro = P obs / t c Significance of rotation on convection: Mixing Length Theory Rossby Number

Defining the Rossby Number (from Noyes et al. 1984) a ml =1 (great variance) a ml =2 (small variance) Ro = P obs / t c

The Dynamo Number Parker’s Dynamo # Dynamo is linear inst- ability for N D > N crit Dynamo a -effect:

The Dynamo Number Parker’s Dynamo # Dynamo is linear inst- ability for N D > N crit Dynamo a -effect:

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

(from Patten and Simon 1996) Stars in open cluster 2391 (30My old) R X from ROSAT observations Rotation periods P obs from optical photometry N R = Ro = P obs / t c Activity vs. Rossby Number

Evolution of Activity (from Skumanich 1972)

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)

Evolution of Activity (from MWO HK Project)

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

from MWO HK Project Cyclic variation in activity

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

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

Cycle periods (from Baliunas & Vaughan 1985)

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

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?