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 ofmagnetic 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

obs24 IdLi

2*4 R

LF ii

bol4

eff L

L

T

FR iii

Luminosity:

Avg. surface flux:

Flux ratio:

[ ergs sec-1 cm-3 ]

[ ergs sec-1 ]

dimensionless

Fun

dam

enta

l

Measuring activity

obs24 IdLi

2*4 R

LF ii

bol4

eff L

L

T

FR iii

Luminosity:

Avg. surface flux:

Flux ratio:

[ ergs sec-1 cm-3 ]

[ ergs sec-1 ]

dimensionless

HK-index:RV

KHS

CaII lines

continuum

SVBCRHK )(cf

Fun

dam

enta

lO

bser

ved

color-dependant conversion factorphot' RRR HKHK

Inferring activityMWO HK project:• 111 stars F2-M2• Observed monthly in CaII H & K• Continuing since 1966 (O. Wilson)• Presently led by S. Baliunas

http://www.mtwilson.edu/Science/HK_Project/

A K3 star: HD160346

S

Who’s active, Who’s not

MWO survey ofnearby* stars

(from Vaughan & Preston 1980)

log(

FH

K)

Cutoff atB-V ~ 0.4 F3

Vaughan Preston Gap

* d < 25 pc

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

Convection zones

7000 6000 5000 40008000

R>

2 R>

F0 F5 G0 G5 K0 K5

R*

dce

Fullyradiative

Explaining Activity LevelsIndividualVariation

Variancewithinclass

Measuring Rotation Periods

(from Patten and Simon 1996)

Pobs=0.527 d

Photometric measurements over time

Lomb periodogram … identify peak frequency

Measuring Rotation Periods

(from Patten and Simon 1996)

Pobs=0.527 d

Photometric measurements over time

Re-sampled at putative period Pobs

Rotation determines activity

Periods Pobs measured from HK index S(t)

<R’HK> <F’HK>

(from Noyes et al. 1984)

Less variance than vs. B-V

Why Rotation Matters

c = lml/vml

ml = lml /Hp

(from Gilman via Noyes et al. 1984)

lml = 0 forB-V < 0.4(i.e. earlierthan ~F3)

Ro = Pobs/c

Significance of rotation on convection:

Mixing Length TheoryRossby Number

Defining the Rossby Number

(from Noyes et al. 1984)

ml=1 (great variance) ml=2 (small variance)

Ro = Pobs/c

The Dynamo Number

2

4dyn '

d

ND

d ~3

1dyn vv

c

d

2

turb ~

Parker’sDynamo #

Dynamo is linear inst-ability for ND > Ncrit

Dynamo -effect:

ddr

d ~'

The Dynamo Number

2

4dyn '

d

ND

d ~dyn vv

c

d

2

turb ~

Parker’sDynamo #

Dynamo is linear inst-ability for ND > Ncrit

Dynamo -effect:

ddr

d ~'

22obs

2 )/(~)(~ RoPN ccD

Activity vs. Rossby Number

(from Noyes et al. 1984)

41 Local stars Pobs from S(t)

young stars

old stars

(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/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-SLow-S

(from Baliunas et al. 1995)

Cycle periods

(from Baliunas & Vaughan 1985)

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

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?