A Review of Magnetic Activity in Sun-like stars
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Transcript of 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
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?