Travis MetcalfeSpace 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 (Prot / 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 ~ A0 (R / M) Qj(Dc) and fit MDI p-mode data

• A0 ~ activity level, while the depth of the source Dc ~ Hp ~ L1/4 R3/2 / 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 49933

• 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.