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.