Transcript Document
Asteroseismology with the Kepler Mission Travis Metcalfe (NCAR) 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 STARS Algonquin 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 k1 Ceti • Latitudinal differential rotation pattern has same functional form as Sun Ca HK period Walker et al. (2007) • Kepler will obtain similar rotation measurements for 105 solar-type stars Stellar density and age Elsworth & Thompson (2004) • Large frequency spacing <Dn> scales with average density of the star • Small frequency spacing <dn> sensitive to interior gradients, proxy for age • Probe evolution of activity and rotation as a function of stellar mass and radius Christensen-Dalsgaard (2004) Radial differential rotation Fletcher et al. (2006) • WIRE 50-day time series of a 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) Convection zone depth • Expected seismic signal from a CoRoT 5-month observation of HD 49933 • Second differences (d2n) measure deviations from even frequency spacing • Base of the convection zone and He ionization create oscillatory signals Baglin et al. (2006) Oscillations and magnetic cycles 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 Libbrecht & Woodard (1990) Cycle-induced frequency shifts • 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 Metcalfe et al. (2007) Stellar modeling pipeline • Genetic algorithm probes a broad range of possible model parameters • 0.75 0.002 0.22 1.0 < < < < Mstar Zinit Yinit amlt < < < < 1.75 0.05 0.32 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 mHz 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 Earthlike 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.