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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
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0.22
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Zinit
Yinit
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1.75
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0.32
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• 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.