Transcript Stellar Activity with SONG

Stars and magnetic activity
Heidi Korhonen
Astrophysikalisches Institut Potsdam
NORDFORSK Summer school 2006
Outline
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Stars
 General properties
 Rotation
Stellar activity
 Solar
 Stellar
Light curve inversions
Doppler imaging
 Principle
 Requirements
 Results
Spectral classification 1
Morgan-Keenan spectral classification
Class
T
Colour
M
R
L
Spectral lines
O
30 000-60 000 K
Blue
60
15
1 400 000
Ionized atoms, especially helium
B
10 000-30 000 K
Bluewhite
18
7
20 000
Neutral helium, some hydrogren
A
7500-10 000 K
White
3.2
2.3
80
Strong hydrogen, some ionized
metals
F
6000-7500 K
Yellowwhite
1.7
1.3
6
Hydrogen and ionized metals,
such as calcium and iron
G
5000-6000 K
Yellow
1.0
1.0
1.0
Ionized calcium and both neutral
and ionized metals
K
3500-5000 K
Orange
0.9
0.9
0.4
Neutral metals
M
2500-3500 K
Red
0.4
0.4
0.04
Strong molecules, e.g., titanium
oxide and some neutral calcium
E
A
R
L
Y
--L
A
T
E
Spectral classification 2
Luminosity class 1
A number of different luminosity classes are distinguished:
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Ia most luminous supergiants
Ib less luminous supergiants;
II bright giants
III normal giants
IV subgiants
V main sequence stars (dwarfs)
Luminosity class 2
Internal structure of the Sun
Other stars have different
configurations in their
interiors
Stellar rotation
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Main sequence:
 Early-type stars (O, B and A) have large rotational
velocities, typically between 100 and 200 km/sec
 At spectral type F, there is a rapid decline from about
100 km/sec at F0 to 10 km/sec at G0.
 The Sun (G2) rotates at about 2 km/sec
 Reder stars have usually rorational velocity of 1 km/sec
and slower
At ZAMS all stars rotate rapidly, but the late type stars
brake fast during the first ~ tens of millions of years
At main sequence cool stars enter a weak braking face that
lasts billions of years
-dynamo
The -effect
SOHO
Solar magnetic field is generated by
a magnetic dynamo within the Sun
•In the -effect the fields are
stretched out and wound around the
Sun by differential rotation
•Twisting of the magnetic filed lines
is caused by the effects of the Sun‘s
rotation, so called -effect
Magnetic activity
SOHO
Solar photosphere
SST
Solar cycle
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Solar activity has on average 11
year cycle, with polarity change
22 years
First spots of the new cycle at
latitude 30-35, last spots of
the cycle within 10 of the
equator  butterfly diagram
Solar chromosphere
NOAA/SEL/USAF
Flares
SOHO
Solar corona
SOHO
Stellar photosphere
Long-term V band photometry of FK
Com (Korhonen et al. 2001)
Phased light-curves for 1990-1993 and results
from light-curve inversions
Korhonen et al. 2002
Stellar chromosphere
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Observed for example with:
 H
 Ca II H&K
 Mg II h & k
 certain ion lines
Most famous study of stellar chromospheres is the Mt. Wilson HK
survey
 Started by Olin Wilson in mid 1960‘s
 Has monitored hundred stars continuosly since then
 In total more than 400 stars monitored, both dwarfs and giants
HK Survey 1
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If we place the slit of a
spectrograph across the surface
of the Sun, we can trace the
change in the emission of the
calcium K line.
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The size and extent of
chromospheric active regions
on the Sun varies dramatically
over the course of the activity
cycle.
HK Survey 2
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HD 16160
 Spectral type K3V
 Cycle length 13.2 yrs
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HD 101501
 Spectral type G8V
 No cycles, but variability
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HD 9562
 Spectral type G2V
 Flat
Stellar corona
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Surface fields from the Zeeman-Doppler imaging maps
Coronal fields extrapolated from the surface fields
Here example for AB Dor:
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ZDI Donati et al
 Coronal extrapolation Jardine et al
Light-curve inversions
Doppler imaging
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In Doppler imaging the
distortions appearing in the
observed line profile due to the
presence of spots and moving
due to the stellar rotation
Ill-posed inversion problem
Many methods for solving:
aximum Entropy Method (e.g.,
Vogt et al 1987 ), Tikhonov
Regularization (e.g., Piskunov
et al 1990), Occamian
Approach (Berdyugina 1998),
Principal Components Analysis
(Savanov & Strassmeier 2005)
From Svetlana Berdyugina
Requirements
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Instrumentation
 High spectral resolution
 High signal-to-noise ratio
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Object
 Good phase coverage (convenient rotation period)
 Rapid rotation
 Not too long exposure time (bright)
 Something to map!
Simulations by Silva Järvinen
Vsini = 45 km/s
Vsini = 30 km/s
Vsini = 17 km/s
vsini
Resolution: 250 000
S/N: infinite
Phases: 20
Phase smearing
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During the observations the star rotates and the bump
moves in the lineprofile
The bump signal will be smeared
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Integration time should be as short as possible:
t0.01Prot
Examples:
Prot = 20 days
Prot = 5 days
Prot = 2 days
Prot = 0.5 days
t > 4.5 hours
t  70 minutes
t  30 minutes
t  7 minutes
Results
FK Com: Korhonen et al 2004
² CrB: Strassmeier & Rice 2003
II Peg: Berdyugina et al 1998