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Stellar Activity in
the Kepler Era
Tom Ayres (CASA)
Activity
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Talk deals with lower
rungs of Drake’s
Ladder, where,
sadly, sexiness is
low, but on positive
side, knowledge
content is high
Note that main focus
of Kepler mission is
much higher on ladder!
Outline
• What is “Activity?” (Sun-like stars)
• Stellar Cycles & Irradiances
• Spots & Rotation (A.Brown: Fri;
Meibom: Tues; Garcia: Fri;
Soderblom: Wed [flares])
• Seismology (T.Brown: Thur [LCOGT])
Guiding question:
Is Sun ‘normal’ in cosmic scheme of
things?
Solar Activity
Solar minimum at left; maximum
at right; dramatic changes in
sunspot counts over 11 yr cycle
Sunspots are regions of multikilogauss magnetic fields
Sunspot numbers vary (erratically) over
11-yr period, with polarity reversals over
22-yr Hale cycle, and migrate from high
latitudes to low (“butterfly diagram”)
Surface magnetic activity strongly
influences solar outer atmosphere,
including 1 MK corona
Fe XII l195 (1 MK) coronal emission persists
at spot minimum (left ; max at right). ‘Fuzzy
ball’ devolves from magnetic carpet: small
clumps of flux built by local dynamo, independent
of deep seated el jefe dynamo responsible for
sunspots and their decadal cycling: lack of spots
doesn’t mean lack of activity
Activity H-R Diagram
Activity appears
confined to ‘cool
stars’, in
convective half of
H-R diagram
Not coincidence!
Originally thought
to signal lack of
acoustic energy,
but dynamo needs
convection too
Stellar Activity Cycles
Long term Ca II emissions of nearby field star closely
mimic Sun’s cycle. Visible brightness changes of Sun
only few milli-mags, in positive correlation with sunspots
(Radick, Lockwood, Skiff, & Baliunas 1998)
More examples (from SSS: Hall et al.
Most late-type stars of near-solar
color show long term variations in Ca II
emission, many cyclic. Others,
typically low RHK and often subgiants,
are ‘flat activity’ (Radick et al. 1998)
Solar variations on
long (and short)
timescales fall close
to stars of similar
activity
(Radick et al. ’98;
Case Study: Alpha Cen AB
Alpha Centauri triple
system. Two solar-like
stars about 20 au apart
(Sun-Uranus); dim red
dwarf 10,000 au away
Slightly metal rich
compared with Sun,
slightly older by ~1 Gyr.
G2V primary (“A”) is
near twin of our own star
In 2005, XMM reported complete disappearance
of Alpha Cen A corona. Since ‘00 Alpha Cen
orbital separation closing rapidly: no longer easily
resolvable by XMM, still trivial for Chandra. HRC
campaign (since Oct ‘05) captures both stars.
Coronal histories:
B=red Sun=black
(yellow: Cyc 21-23 shifted in time); bottom panel:
Chandra (dots) & XMM (shaded, scaled). B has
~8 yr cycle, rising to new max. No clear period for
A, modest cycle depth (‘flat activity’ star?)
Rotation-Age-Activity Connection
’Skumanich laws’ confirm importance of
dynamo, creating high levels of activity in
fast rotating stars, but also root of
magnetic braking, which ultimately quenches
activity.
Week in life of Sun
as seen by
SDO/HMI.
Sunspot passages
create dimmings in
visible light curves,
but surrounding
plage regions cause
overall increase in
irradiance.
Kepler can
measure rotation
periods, diff
rotation, and
photometric
changes
associated with
starspot
evolution…trick is
to disentangle
latter two
effects.
Evolution of sunspot fields often accompanied by
significant super-rotational motions
Starspot modeling for two Kepler objects
Key issue in contemporary solar physics:
“flux emergence”…will be tricky for Kepler
to make any progress in this area.
What about
seismology?
Here are 1-min
measurements
from Virgo
instrument on
SOHO (vs. ~50
mmag changes
seen in spotted
stars).
Asteroseismology Impact
• Mode frequencies sensitive to magnetic
structure: cycle induced frequency shifts?
(Garcia et al. 2010, Science, CoRot)
• Rotation/differential rotation from mode
splittings (Karnoff et al. 2009, MNRAS)
• Convection zone depth (ibid); age (Mathur)
• Highly demanding on precision of acoustic
frequencies; requires bright stars; Sun in
the “sour spot”
GONG Solar
torsional
oscillations;
equatorial
branch
originates
several yrs
before
appearance
of first
sunspots of
new cycle;
Cycle 25??
Measurements
of large scale
coronal topology
using green line
(forbidden
iron); “rush to
the poles” seen
in previous
cycles is late
or absent.
Left: Mean field strength of
sunspots, and temperature contrast,
declining: all this together suggests
approaching Maunder minimum?
Right: solar oxygen in trouble too
Conclusions
Kepler
will make fundamental
contributions to the understanding of
stellar activity by exploiting two
complementary aspects of the precision
photometry: spot modulations (surface
activity, rotation & diff rotation, flares)
and asteroseismology (rotation, diff
rotation [?], internal structure, age,
cycles). A third possibility – correlations
of luminosity with spottedness – will be
possible if absolute accuracy is achieved.