Travis Metcalfe Space Science Institute + Stellar Astrophysics Centre Probing Stellar Activity with Kepler

Detecting stellar activity Sun-as-a-star data show rotational modulation and long-term magnetic cycle Spectroscopic data track emission from faculae relative to the continuum Photometric data show integrated effect of dark spots and bright faculae Lockwood et al. (2007)

Activity index (spectroscopic) Magnetic network on the Sun (faculae/plage) bright in the Ca II H & K lines Measure ratio of total emission in the line cores to flux in the continuum Use disk-integrated time series measurements to track magnetic cycles Schröder et al. (2009)

Mount Wilson survey Mount Wilson survey found magnetic activity cycles in many stars Mean activity level and cycle period scale with Rossby number (P rot /  c ) Survey ended in 2000’s after more than 30 years of Ca HK observations Baliunas et al. (1995)

Activity and rotation Böhm-Vitense (2007) Saar & Brandenburg (99) sample of stars with clear rotation and cycle periods Two distinct relationships between cycle period and rotation (100 & 300/cycle) Some stars on the Active branch show secondary cycles on Inactive branch

Intensity variations in the Sun over cycle 23 using Virgo data from SOHO Brightening from faculae outweighs the dimming from additional spots Range of variation and measurement dispersion both grow near maximum Activity index (photometric) Basri et al. (2013)

Influence on amplitudes Chaplin et al. (2011) Survey for solar-like oscillations during first 10 months of Kepler mission Lower detection rate for stars with higher levels of magnetic activity Magnetism inhibits convection, reducing the oscillation amplitudes

Salabert et al. (2004) Solar p-mode shifts first detected in 1990, depend on frequency and degree Even the lowest degree solar p-modes are shifted by the magnetic cycle Unique constraints on the mechanism could come from asteroseismology Influence on frequencies Libbrecht & Woodard (1990)

Magnetic perturbations modify the near-surface propagation speed Also leads to decreased convective velocity and change in temperature Distinct behavior for solar f-modes and p-modes confirms these sources Theoretical interpretation Goldreich et al. (1991) Dziembowski & Goode (2005)

Scaling for other stars Metcalfe et al. (2007) Parameterize shifts with  ~ A 0 (R / M) Q j (D c ) and fit MDI p-mode data A 0 ~ activity level, while the depth of the source D c ~ H p ~ L 1/4 R 3/2 / M Normalizing shifts by this parametrization removes most of the dependencies

Predictions for Kepler Metcalfe et al. (2007) Chaplin et al. (2007) Karoff et al. (2009)

Garcia et al. (2010) Salabert et al. (2011) Solar pattern of frequency and amplitude changes observed in HD Frequency dependence of shifts also similar to (but larger than) solar Larger shifts for an F star supports Dziembowski scaling for other stars CoRoT: HD 49933

Short cycles with Kepler Mathur et al. (in prep.)

Overview Precise time-series photometry from Kepler (+ground-based spectroscopy) can track stellar activity in a large sample for up to four years. Magnetic activity reduces oscillation amplitudes (by inhibiting convection) and induces shifts in the oscillation frequencies over the stellar cycle. The shortest cycles and the largest frequency shifts are expected for F stars, making them ideal targets for probing stellar activity with Kepler.