travis metcalfe space science institute + stellar astrophysics centre probing stellar activity with...
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Travis MetcalfeSpace Science Institute +
Stellar Astrophysics Centre
Probing Stellar Activity with Kepler
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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)
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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)
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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)
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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
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• 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)
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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
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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)
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• 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)
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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
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Predictions for Kepler
Metcalfe et al. (2007)
Chaplin et al. (2007)
Karoff et al. (2009)
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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
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Short cycles with Kepler
Mathur et al. (in prep.)
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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.