Travis Metcalfe (NCAR)
Asteroseismology with theKepler Mission
We are the stars which sing,
We sing with our light;
We are the birds of fire,
We fly over the sky.
SONG OF THE STARSAlgonquin Mythology
• Why is asteroseismology important to the primary science goal of Kepler?
• Transit only gives radius of planet relative to the unknown stellar radius
• Asteroseismology will measure the stellar radius with a precision of 2-3%
• Why is asteroseismology important to the primary science goal of Kepler?
• Transit only gives radius of planet relative to the unknown stellar radius
• Asteroseismology will measure the stellar radius with a precision of 2-3%
Kepler mission overview
• NASA mission currently scheduled for launch in mid-February 2009
• 105 square degrees just above galactic plane in the constellation Cygnus
• Single field for 4-6 years, 100,000 stars 30 minute sampling, 512 at 1 minute
Surface differential rotation
• Three seasons of precise MOST photometry for the solar-type star 1 Ceti
• Latitudinal differential rotation pattern has same functional form as Sun
• Kepler will obtain similar rotation measurements for 105 solar-type stars
Walker et al. (2007)
Ca HK period
Stellar density and age
• Large frequency spacing <> scales with average density of the star
• Small frequency spacing <> sensitive to interior gradients, proxy for age
• Probe evolution of activity and rotation as a function of stellar mass and radius
Christensen-Dalsgaard (2004)
Elsworth & Thompson (2004)
• WIRE 50-day time series of Cen A has resolved the rotational splitting
• Splitting as a function of radial order can indirectly probe differential rotation
• Even low-degree modes allow rough inversions of the inner 30% of radius
Gough & Kosovichev (1993)
Radial differential rotation
Fletcher et al. (2006)
Convection zone depth
• Expected seismic signal from a CoRoT 5-month observation of HD 49933
• Second differences (2) measure deviations from even frequency spacing
• Base of the convection zone and He ionization create oscillatory signals
Baglin et al. (2006)
• 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
Oscillations and magnetic cycles
Libbrecht & Woodard (1990)
Salabert et al. (2004)
• Solar p-mode shifts show spread with degree and frequency dependence
• Normalizing shifts by our parametrization removes most of the dependencies
• Kepler will document similar shifts in hundreds of solar-type stars
Cycle-induced frequency shifts
Metcalfe et al. (2007)
Stellar modeling pipeline
• Genetic algorithm probes a broad range of possible model parameters
• 0.75 < Mstar < 1.75
0.002 < Zinit < 0.05
0.22 < Yinit < 0.32
1.0 < mlt < 3.0
• Finds optimal balance between asteroseismic and other constraints
Application to BiSON data
• Fit to 36 frequencies with l = 0-2 and constraints on temperature, luminosity
• Matches frequencies with scaled surface correction better than 0.6 Hz r.m.s.
• Temperature and age within +0.1%, luminosity and radius within +0.4%
TeraGrid portal
• Web interface to specify observations with errors, or upload as a text file
• Specify parameter values to run one instance of the model, results archived
• Source code available for those with access to large cluster or supercomputer
Summary
• Kepler needs asteroseismology to determine the absolute sizes of any potentially habitable Earth-like planets that may be discovered.
• The mission will yield a variety of data to calibrate dynamo models, sampling many different sets of physical conditions and evolutionary phases.
• A uniform analysis of the asteroseismic data will help minimize the systematic errors, facilitated by a TeraGrid-based community modeling tool.
