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.