Stellar Surface Structures
• Surface features associated with activity
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Chromospheres
Spots
Plage
Coronal holes
Flares
• Types of stellar activity
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– Solar-type stars
– M dwarfs
– Active stars
Star spots
Doppler imaging
Spots on A stars
Stellar oscillations
Star Spot Properties
• Size – large, medium, small
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Inactive, weak-dynamo stars (small spots only)
Active, strong-dynamo stars (big spots)
Light curve amplitudes may be 0.5 mag
Nature of spots indeterminate
• Temperatures
– Do all of a star’s spots have the same temperature?
– Do spots have umbra/penumbra structure?
– How does a spot temperature evolve as it forms and
vanishes?
• Magnetic fields
– How do fields correlate with other spot properties?
• Spot locations – polar, equatorial, both?
– Most have dark polar spots with strong B
• Spot lifetimes
Plage
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What is plage? - Regions of strong (~340 G), vertical magnetic field
Seen in white light as bright regions around sunspots
Much higher contrast in Ca II K
Feature of upper photosphere and lower chromosphere
Sun varies by ~15% in Ca II K line emission over a solar cycle
Fields replenished from sunspot fields, drift poleward to merge with
intranetwork fields
Ca II K emission is rotationally modulated
What are Coronal Holes?
• Regions of the solar corona where the magnetic
field lines diverge outward from the Sun
• Develop in regions adjacent to areas of similar
polarity
• Low density, material flows outward – source of
much of the solar wind
Solar Flares
• Magnetic flux tubes “reconnect” in the corona (coronal
loops)
• Electrons accelerate down the magnetic field lines toward
the lower atmosphere, producing microwave emission
• Electrons collide with ions, producing hard x-rays, white
light emission from chromosphere
• Chromospheric plasma heated to coronal temperatures, hot
plasma flows up into the corona
• Shock front moves downward to heat the photospheric base
• As the density of the corona increases, it is further heated
by the energetic electrons. Soft x-rays from the corona
then heat the chromosphere
M Dwarf Flares
• The mechanism for M dwarf flares is different than in the
Sun
• Blue and UV continuum increase by several magnitudes in
seconds (unlike the Sun, where the contrast to the
photospheric background is less)
• Typically a black body of 8,000 –10,000K
• The source of the white light is still unknown
• Strong,broad emission lines in the UV and optical – Balmer,
Ca II K + He I, He II, Ca II IR triplet, numerous singly and
doubly ionized metals
• UV emission lines also stronger
• Broadening mechanism unknown
• Soft x-ray emission rises more slowly
• M dwarf flux dims just before the outburst
• Coronal mass ejections? Mass loss rates from flares
estimated at 10-13 MSun per year
Activity Cycles
Young Stars
Old Stars
The Sun
Main Sequence
Age
1 Gyr
Few Gyr
4.6 Gyr
Mean
chromospheric
flux ratio
0.31
0.17
0.17
Mean rotation
period
9.1d
27 d
25 d
Cycle behavior
Periodic or
erratic; none
are flat
Periodic, ¼ are
flat
Periodic, 1/3
are flat
Correlation of
magnetic
activity and
flux?
inverse
correlated
correlated
Activity
Cycles
• Long term
chromospheric
activity indices
for several stars
showing different
patterns of
activity cycles
Age-Activity Relation
• In solar-type stars, age-activity relation is
well defined
• Young stars have stronger Ca II K line
emission (flux proportional to t-1/2)
• M dwarfs don’t fit the solar-type relation
• Activity is more prolonged;
• Activity is a function of both age AND mass
• dMe stars are kinematically younger than
dM stars
• In older clusters, activity “turns on” at later
spectral type
Activity Distributions
Rotation
• In survey for rotation, 25% of stars have rotation
rates above 2 km/sec
• Later type dwarfs MORE LIKELY to have
measurable rotation
• Earlier type M dwarfs that rotate are usually
young
• Rotation takes longer to decay in the later M
dwarfs
• No strong correlation between activity level and
the rate of rotation
• A low threshold for rotation to maintain activity in
M dwarfs
• Among very late M dwarfs, some rapid rotators DO
NOT show activity
Effects of Activity on M Dwarfs
• Activity affects color, luminosity, TiO
band strength
• Active stars are redder/brighter
• TiO stronger or weaker depending on
the particular band
Mapping Starspots
• Direct imaging – limited application
• Photometric light curves
– Intensity vs. time
– Equatorial spots
• Eclipsing binaries
 Doppler imaging
– Intensity vs. radial velocity vs. time
– See Vogt and Penrod 1983 (PASP, 95, 565)
Doppler Imaging
from Vogt &
Penrod 1983
As a spot moves across
the star the line profile
changes. From an
observed line profile, one
can construct an image of
the surface of the star.
This technique has been
applied to many different
types of stars.
Surfaces of T Tauri Stars
• Cool circumstellar disk around a late-type,
magnetically active star
• Light variations at all wavelengths, timescales
– Mass accretion
– Magnetic fields
• Periodic light variations – rotational modulation
– Amplitudes 0.05 – 0.5 mag
– Periods stable over several years
• Cool spots cover a large fraction of the surface,
typically polar, similar to RS CVn’s
• Some evidence for warm spots as well
• Magnetic fields ~1kG
Spots on the
ZAMS
• Two Pleiades dwarfs
– K5V, M0V
• Vsini=60-70 km/sec
• Periods ~10 hours
• Inclinations ~ 50-60
degrees
• Again, dark polar
spots
Starspots on Active Stars?
• A few dozen stars with Doppler images
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RS CVn
T Tauri
FK Comae
W Uma
Young single dwarfs
BY Dra
Rotation periods from 0.31 to 19 days (165 to 25 km/sec)
Radii from 0.77 to 16 RSun
Temperatures from 4000 K to 6000 K
PMS to class III giants
Generally show dark polar spots – unlike the Sun. Why do
they differ from the solar paradigm?
– Faster rotation
– Mostly deeper convective zones
• Presence of polar spots remains controversial
Strassmeier’s Spot
• Doppler images of HR 1099
(RS CVn star) from 1981-1989
• Star dominated by a large
polar spot
• Smaller spots form in
equatorial regions and migrate
toward pole
• Spots merge together and may
merge into polar spot
• Polar rotation fixed with
orbital period
• Equatorial rotation slightly
faster
• Some spots persist over years
• Spot patterns reminiscent of
solar coronal holes
Spots in HR 1099
(Vogt & Hatzes)
Activity on Active Stars – NOT “Solar-like”
• Starspot latitude
– Solar-type – mostly equatorial
– Active stars – mostly polar
• Chromospheres
– Solar type - ~ 0.01 solar radii
– Active stars - ~ one stellar radius
• Rotation activity relations
– Solar type – strong correlation
– Active stars – activity saturates at 15 km/sec
• Activity cycles
– Solar type – long term periodicities in Ca II K
– Active stars – most show no evidence for cycles
Types of Stellar Activity
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Solar type stars
Young stars/Active stars
M dwarfs
T Tauri stars
The Sun is not representative
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Different spot locations and sizes
Different migration patterns
Filling factors
Cycles or no cycles
Different dependences on rotation rates
Spots on Ap Stars
• 10-15% of late B-early F stars have magnetic fields (Sr-CrEu stars, Si stars)
• Oblique rotator model – dipole field inclined to rotation axis
(and also decentered)
• High Teff and stable atmospheres
• Radiative and gravitational forces push atoms up or down
– Length scales ~104 km
– Time scales 102-104 years
• Magnetic fields suppress motion of ions across field lines
• Element may accumulate where field horizontal, deplete
where field is vertical (or vice versa)
• Expect polar spots, equatorial rings
• Si depleted in polar spots, enhanced in rings
• Cr enhanced in polar spots, depleted in rings
• Element diffusion along horizontal field lines may cause
surface abundance distributions to evolve with time (time
scale ~108 years)
Si II Spots on Cu Vir (Si Ap Star)
Stellar Oscillations
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Solar acoustic (p-mode)
oscillations ~5 min, 107 modes
Stellar obs. limited to lowest
order modes in integrated light
About 15 min, amplitudes a few
parts per million
Radial velocity vs. photometric
techniques
p-modes vs. g-modes
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p-modes: Pressure is restoring
force
g-modes: Buoyancy is restoring
force
White dwarfs, delta Scuti’s,
roAp stars, etc.