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The Little Satellite that Could...
Derek Buzasi
Univ. of Washington
&
Eureka Scientific
Introduction
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“X-ray vision” for stars
A brief history of WIRE
Some Closeups
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Altair
Polaris
Alpha Cir
Procyon
Eclipsing Binaries
Future Prospects
How do we know anything about the internal
structure of the Sun?
Coherent motions on the solar surface
Leighton (1960) uses Doppler effect to observe
oscillatory motion on the Sun
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Amplitudes are ~hundreds of meters/sec
Periods are ~5 min (~3 mHz)
1975: Things start to get interesting…
What are these things?
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They appear to be acoustic waves
traveling in a cavity
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The Sun is ringing like a bell
The study of these waves is called
“helioseismology”
Each unique way in which the Sun can oscillate is called
a “mode”
Modes
OK – so what good is all this?
(Some real physics at last!)
Each mode has a frequency which is characteristic
of some “average” sound speed along its travel
path
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“Low degree” modes penetrate deeply
“High degree” modes sample only the surface
By using information from all of the modes, we can
model the inside of the Sun!
Good news! There are more than a million modes
Bad news! There are more than a million modes
We can even map the internal rotation
…and details of
internal convection
It would be nice to do this for stars, too…
Unfortunately, asteroseismology is much
more difficult
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Less light (and atmospheric interference)
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Unresolved sources
But astronomers are trying!
A little about stars…
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What we’d like to know
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mass, age, composition, rotation rate,
internal structure, activity, etc.
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Ways to find out
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cluster stars (common origin)
 binary stars (interact via gravity)
 field stars (help!)
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Wider applications?
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distance scale, nucleosynthesis, etc.
HertzsprungRussell (HR)
Diagram
Remember this? 
Technical Approaches to Asteroseismology
Two Basic Approaches
1. Look for tiny Doppler shifts (~10 cm/s, or parts per
billion) in spectral lines from the ground, where we
have big telescopes.
2. Look for tiny variations (parts per million) in the
stellar luminosity from space, where the atmosphere
isn’t a problem.
Other Uses for ultra-high-precision photometry
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Rotational Modulation
Granulation (surface signature of
convection)
Eclipses & transits
WIRE = Wide-Field Infrared Explorer
1994: Selected by NASA; IR mission designed to
study extragalactic star formation for 4 mos
1999: Pegasus launch on 4 Mar 1999
1999: Primary mission failure on 8 Mar 1999
1999: Conversion to asteroseismology mission
begins 30 Apr 1999
May 1999 – Sept 2000: Epoch 1
Dec 2003 – 23 Oct 2006: Epoch 2
Launched 4 Mar 1999; failed 8 Mar 1999
The Star Tracker
• Ball Aerospace CT-601
• 52 mm aperture
• 512  512 SITe CCD
•7.8°  7.8° field (1 arcmin/pixel)
•16-bit ADC
•Gain 15e-/ADU
Epoch I
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Original mission was 30
April 1999 - 30 September
2000
28 asteroseismology targets;
10 additional targets
Primary targets only
Mission termination due to
lack of funding
Epoch II
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Mission restart
 New flight software
included field rotation,
making secondary
targets usable
Selected Accomplishments
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Altair
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Polaris
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Interaction of rotation and oscillations
Procyon
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New insights into an old favorite
Alpha Cir
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What don’t we know about the brightest star in the northern
sky?
Granulation in a solar-like star
Eclipsing Binaries
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Old wine in new bottles
Altair
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Brightest star in the northern sky
Part of the “summer triangle”
Sometimes used as a flux standard!
WIRE observed for 22 days...
Each “frame”
represents a
2-day window
Overall envelope
of variability is ~2
ppt
Altair is the brightest δ Scuti star!
Largest peak is at 15.768 d1; amplitude 0.42 mmag
Frequency units are “cycles/day” – 15.76 c/d
corresponds to roughly 1.5 hours.
9 total modes detected; f1 is easily identified as
the fundamental but other IDs are less clear
due to the extremely rapid rotation of the star.
HR diagram showing selected models.
The two evolutionary tracks depicted
correspond to 1.70 solar masses and v =
150 km/s (solid line), and 1.75 solar
masses and v = 200 km/s (dashed line).
Ages in the shaded area range from
500 to 750 Myr
The cross indicates
observations.
Polaris
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“North Star”
Closest and brightest Cepheid
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Amplitude has been dropping for decades
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No longer detectable as variable from the ground
Observed simultaneously using multiple telescopes
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WIRE
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SMEI
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AST
Polaris: Data Quality
Oscillation timing is changing:
stellar evolution in real time!
Alpha Circinis
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Member of class rapidly oscillating peculiar A-type (roAp) stars
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First detected in 1970s with ground-based telescopes
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Typical periods are a few minutes
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Typical amplitudes are a few parts per thousand or less
Alpha Cir has one well-known mode, with a frequency of 2442 mHz
Combination of ground & space-based
observations
Asymmetry in light curve
Amplitude of primary mode is variable:
rotation!
Distance Determination via Asteroseismology
Procyon
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F5 IV (V ~ 0, so one of the brightest stars in the sky)
Historically considered one of the best possible stellar targets for
asteroseismology
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Higher mass, more evolved star is expected to have larger oscillation
amplitudes based on theory
WIRE analysis by Bruntt, Kjeldsen, Buzasi, Bedding
 Two time series totalling ~19 days
The light curves of Procyon as
seen from WIRE in September
1999 (top) and September
2000 (bottom). Data affected
by scattered light have been
removed and the correlation
with FWHM has been
removed. In each panel, only
every fifth data point is plotted.
Comparisons: WIRE x 2, MOST, Solar
The excess power in the MOST data appears to be due to an
as yet not understood noise source.
The plot shows the smoothed
power density spectrum for each
data set. PDSs permit direct
comparison of different time
series, since they take into
account the different lengths and
resolutions of the data sets.
The VIRGO data represent the solar PDS as
viewed from space. Note that hydrodynamic
models predict Procyon to have somewhat
greater granulation “noise” than the Sun does.
The four panels show the power density spectrum of the WIRE 2000 time series along with
different simulations. Each simulation is the mean of five simulations with different seed
numbers. The hatched regions show the 1-σ variation for selected simulations.
Simulations for two different
white noise levels
The timescale of the granulation is 750 s but the
granulation power densities (PDs) are 10, 18, and 64
ppm2/ µHz.
Simulations have timescales
of the granulation of 250,
750, and 1250 s.
The granulation timescale and granulation PDs are
750 s and 18 ppm2/ µHz, while the amplitude of the pmodes are 5, 10, 15 ppm.
WIRE 1999 and 2000 results: Procyon
WIRE spectra are
marked by open
box symbols
Best-fit models
The 1-σ variation of
the simulations is
shown by the
hatched region.
Eclipsing Binaries
A classic astronomer’s tool
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Orbital timing gives stellar masses
Lengths of eclipses give relative stellar radii
Depths of eclipses give relative stellar
temperatures
Shapes of eclipse light curves give
atmospheric structure
Eclipsing Binaries with WIRE
Good (ground-based TT Aur)…
Better (WIRE,  Eri)
Best! (WIRE,  Aur)
But wait, there’s more…
Where do we go from here?
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The future of asteroseismology lies in space!
END
Fourier Transforms Are Your Friends…
(Really!)
Things get worse: here’s a signal-to-noise ratio of 10…
And a signal-to-noise ratio of 0.01…
Real data look like this!
Amazing Appearing Modes!