Download Asteroseismology with the Whole Earth Telescope

Delaware Asteroseismic Research Center
Asteroseismology with
the Whole Earth
Telescope (and More!)
Asteroseismology

Study of interior stellar structure as revealed by global
oscillations.
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Important- - photons we observe come from the surface
Stars pulsate at definite frequencies determined by their
structure
Analogy - bells
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Can be used on any pulsating star.
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What can we learn
?
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Mass, interior structure, composition, interior structure, just like
we learn from earthquakes on earth
For Stars: luminosity, temperature, layering
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Rotation rates – solid body???
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Magnetic fields??
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Heat transport through atmosphere
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How Asteroseismology
Works
On a perfect string, the frequencies are evenly spaced. n
a perfect, uniform string, the frequencies are evenly
spaced, i.e., ! = n¼c/L, n=1,2,3,…
Putting a bead on the string destroys the even spacing
The pattern of the frequency differences is VERY sensitive to
the location of the bead.
Interior Structure  Beads
Signature of Rotation

Doppler Effect
Frequency
What do we need to use
Asteroseismology?
 Monitor
the brightness of a star over time
 Lots of observations using telescopes and
very sensitive cameras.
Telescopes and Cameras
Mt. Cuba 0.6m
SOAR – Chile, 4m
Fancy camera
Aperture Photometry
Monitor variable star
 Monitor “comparison” stars
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Thousands of images each night
Calibrations
Light Curves – Stellar
brightness over time
Time
Fourier Analysis
The process of extracting from a signal the various
frequencies and amplitudes that are present.
Transform a light curve (time domain) into a set of
frequencies in frequency space
Fourier Transforms
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+
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Underlying theorem: Any periodic mathematical function
can expressed as the sum of an infinite number of sine and
cosine functions.
Sinusoids have 3 properties:
 period (frequency)
 Amplitude
 phase
 Fourier
Transform (FT)
 transform from time domain to
frequency domain
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Time
The Whole Earth Telescope
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An international collaboration of astronomers
interested in pulsating stars, particularly pulsating
white dwarf stars
Founded in the 1980’s by R.E. Nather and Don
Winget at the University of Texas
Window Function
Frequency
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Width of peaks – 1/t t=timescale of observations
Separation between peaks – 1/(time between gaps)
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If your light curve
is infinitely long
and has no gaps,
then the FT of a
sine wave
sampled exactly
as your light
curve will be a
delta function (a
single peak.
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Unfortunately,
this rarely
happens. Gaps
introduce
uncertainty,
which appears as
“aliases” in the
FT
Goal of the WET Observations
 Uniform data set – high speed photometry
 Uniform instrumentation – as near as possible
 Interactive headquarters – real time
 Multiple targets
Continuous
coverage –
elimination of aliases
Whole Earth Telescope
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What do we need?
 Good target
 long
lightcurves to accurately identify frequencies
 continuous
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light curves to eliminate aliases
Multi-site observing runs  WET
Spectral
Windows
Delaware Asteroseismic Research Center Mt. Cuba Observatory, DE
Peak Terskol
Peak du Midi France
South Africa
CTIO Chile
McDonald Observatory Texas
McDonald Observatory
Hawaii
WET run:
All telescopes
observing over 1
month period
--must apply for
time individually
--send data to
headquarters each
night
--reduced/analyzed
in real time
Why White Dwarf Stars?
 White Dwarf stars are stellar remnants
 95% of all stars will end up as white dwarfs
 Best way to learn about what is going on in
stars today
Asteroseismology of
White Dwarfs
White dwarfs are faint (mag 12  )
Multiperiodic g-mode pulsators
Periods between 100-1000 s
Amplitudes up to 50 mma
We can uncover information about
• Mass
• Interior chemical composition
• Composition transitions
• Rotation rates
• Magnetic fields
• Pulsating white dwarfs are
“ordinary”
• Mass distribution is ~ 0.6 Mo
• We can apply what we learn to
the population as a whole.
•
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•
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•
8th
WET Workshop Beijing
Sirius A
Sirius B
A Brief Introduction to White
Dwarfs
GW Vir – really hot
DBs – 28,000-22,000 K
DAs – 12,500-11,000 K
GD358
White Dwarfs are Simple
Thin helium layer
Hydrogen – DA
Thin hydrogen layer
Helium – DB
HOT – DO
Size of the Earth
Mass of the Sun
Carbon and
Oxygen core
99.9% Carbon/Oxygen
0.1% Helium
0.0001% Hydrogen
Non-radial Pulsators
l=1, m=0
l=1, m=+1
l=2, m=0
Possible m= -1, 0, +1
Possible m = -2,-1,0,1,2
GD358 – The Prototype Helium
Pulsating White Dwarf
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GD358
GD358 has become the most
studied DB pulsator
 over 1400 hours of
observations
 Periods ~1000-400 s
 Dominant period - ~800 s
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Observed amplitude
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Whole Earth Telescope target
 1990 – 154 hrs
 1991 – 51 hrs
 1994 – 342 hrs
 2000 – 323 hrs
 2006 – 436 hrs
 More data
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Why?
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Model l=1 modes, k=21-7
Current Asteroseismology GD358
• Winget et al. 1990
• Mass=0.61±0.03 Mo
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•
•
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Helium envelope 2.0±1 x 10-6 M*
Luminosity = 0.05±0.12 Mo
Magnetic field = 1300±300 G?????
Differential Rotation – envelop rotating faster
than core?????
• Fontaine & Brassard
• Temperature 22,900 K
• Mass=0.625 Mo (C) 0.660 Mo (C/O))
• Log (Helium envelope mass) = -6.1
• Models depend on input physics – including
convection parameters
• Temperature fits depends on abundances of
hydrogen
There are still mysteries!
GD358 August 1996
What Could Cause This?
 Timescales
were short –
 less
than 2 days
 Amplitude decreased over 3 days
 “Typical” pulsations did not return for ~ 1 month
 Impact?
 Magnetic
Field?
Do We have any Clues?
 Brightness
changes
 High
speed photometry – differential photometry
 Measure the ratio between comparison and GD358
 Indicates 20% increase in brightness
 Spectroscopy
 HST
spectrum taken Aug 16 – after the event
 Indicates temperature was “normal” 4 days after
event
 Light
curves – Light Curve fitting
UV Spectroscopy - Hubble
CII 1335
Ly α
HeII
23900±300 K (He)
Log (H/He)=-5
Log(C/He)=-6
Hybrid spectrum fit
Light Curve Fitting
 Need
High Signal to Noise Light Curve for
actual fitting
 Uses the nonsinusoidal light curve directly
 Need to know frequencies target star is
pulsating at Whole Earth Telescope
 Long term plan –
 map
convection across hydrogen and helium
instability strips
t ~ 320 sec “Typical” GD358
µi ~ 45 degrees
422.561
423.898
463.376
464.209
465.034
571.735
574.162
575.993
699.684
810.291
852.502
962.385
Tau ~ 35 ± 5 sec – “Whoopsie”
µi ~ 52 ±5 degrees
Click
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Period (s)
l
m
422.561
1
1
This implies that GD 358
was ~ 3000 K hotter
during the sforzando
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to edit Master text styles
Second level
Third level
 Fourth level
 Fifth level
Summary and Conclusions??
• Asteroseismology is a powerful tool to study white dwarf stars and other kinds of
pulsations
• There have been 28 WET runs, and numerous smaller “campaigns”
• Delaware Asteroseismic Research Center (DARC)
• This technique has evolved beyond simply generating lists of frequencies and
asteroseismic models
• Explain Mysteries!
• Guess what! We need more observations