Stellar Activity in the Kepler Era
Tom Ayres (CASA)
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!
Activity
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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 multi-kilogauss 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 195 (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(and rotation)
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. 2008)
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; Lockwood et al. 2007)
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.
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.