A Review of Magnetic Activity in Sunlike stars

1
A Review of Magnetic Activity in Sun-like stars

• Magnetic Stars Seminar
• Oct. 30, 2002

2
Whos active, Whos not
Evidence of magnetic activity
Activity on main sequence types F ? M B-V
gt 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
Whos active, Whos not
MWO survey of nearby stars
log(FHK)
Cutoff at B-V 0.4 F3
Vaughan Preston Gap
d lt 25 pc
(from Vaughan Preston 1980)
8
Convection zones
(where the fields come from)

• Schwartzschild crit.
• violated for
• r lt R dce
• Heat x-port by
• turbulent convection
• Velocity size
• of convection
• eddies (vml lml )
• from mixing length
• theory

dce
9
Convection zones
2 Rgt
R
Rgt
Fully radiative
dce
7000
6000
5000
4000
8000
F0
F5
G0
G5
K0
K5
10
Explaining Activity Levels
Individual Variation
Variance within class
11
Measuring Rotation Periods
Pobs0.527 d
Photometric measurements over time
?Lomb periodogram identify peak frequency
(from Patten and Simon 1996)
12
Measuring Rotation Periods
Pobs0.527 d
Photometric measurements over time
Re-sampled at putative period Pobs
(from Patten and Simon 1996)
13
Rotation determines activity
ltRHKgt
ltFHKgt
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 lt 0.4 (i.e. earlier than F3)
(from Gilman via Noyes et al. 1984)
15
Defining the Rossby Number
Ro Pobs/tc
aml1 (great variance)
aml2 (small variance)
(from Noyes et al. 1984)
16
The Dynamo Number
Parkers Dynamo
Dynamo is linear inst-ability for ND gt Ncrit
Dynamo a-effect
17
The Dynamo Number
Parkers Dynamo
Dynamo is linear inst-ability for ND gt 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 Interpretati
on 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 gt 20 yrs
• Younger stars tend to be more variable
• or have shorter periods

Q what fraction of time is Sun cyclic?