Asteroseismology with the Whole Earth Telescope (and More!)
Delaware Asteroseismic Research Center
Asteroseismology Study of interior stellar structure as revealed by global
oscillations. Important- - photons we observe come from the surface Stars pulsate at definite frequencies determined by their
structure Analogy - bells
Can be used on any pulsating star.
What can we learn? Mass, interior structure, composition, interior structure, just like
we learn from earthquakes on earth For Stars: luminosity, temperature, layering Rotation rates – solid body???
Magnetic fields??
Heat transport through atmosphere
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
FrequencyDoppler Effect
What do we need to use Asteroseismology?
Monitor the brightness of a star over timeLots of observations using telescopes and
very sensitive cameras.
Telescopes and Cameras
SOAR – Chile, 4mMt. Cuba 0.6m
Fancy camera
Aperture Photometry
Thousands of images each night
Monitor variable star Monitor “comparison” stars
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
Sinusoids have 3 properties: period (frequency) Amplitude phase
Underlying theorem: Any periodic mathematical function can expressed as the sum of an infinite number of sine and cosine functions.
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Fourier Transform (FT) transform from time domain to
frequency domain
Time
The Whole Earth Telescope 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
Width of peaks – 1/t t=timescale of observations Separation between peaks – 1/(time between gaps)
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.
Unfortunately, this rarely happens. Gaps introduce uncertainty, which appears as “aliases” in the FT
Frequency
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 What do we need?
Good target
long lightcurves to accurately identify frequencies continuous light curves to eliminate aliases
Multi-site observing runs WET
Delaware Asteroseismic Research Center Mt. Cuba Observatory, DE
Spectral Windows
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 remnants95% of all stars will end up as white dwarfsBest way to learn about what is going on in
stars today
Asteroseismology of White Dwarfs
Sirius B
• 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.
Sirius A
8th WET Workshop Beijing
A Brief Introduction to White Dwarfs
GW Vir – really hot
DBs – 28,000-22,000 KDAs – 12,500-11,000 K
GD358
Carbon and Oxygen core
Thin helium layer
Thin hydrogen layer
White Dwarfs are Simple
99.9% Carbon/Oxygen0.1% Helium0.0001% Hydrogen
Hydrogen – DA
Helium – DB
HOT – DO
Size of the Earth
Mass of the Sun
Non-radial Pulsators
l=1, m=0l=1, m=+1
Possible m= -1, 0, +1
l=2, m=0
Possible m = -2,-1,0,1,2
GD358 – The Prototype Helium Pulsating White Dwarf
GD358 GD358 has become the most
studied DB pulsator over 1400 hours of
observations Periods ~1000-400 s Dominant period - ~800 s Observed amplitude
Whole Earth Telescope target 1990 – 154 hrs 1991 – 51 hrs 1994 – 342 hrs 2000 – 323 hrs 2006 – 436 hrs More data
Why?
Model l=1 modes, k=21-7
Current Asteroseismology GD358
8th WET Workshop Beijing
• Winget et al. 1990• Mass=0.61±0.03 Mo• 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
23900±300 K (He) Log (H/He)=-5Log(C/He)=-6
Hybrid spectrum fit
Ly α
CII 1335
HeII
Light Curve Fitting
Need High Signal to Noise Light Curve for actual fitting
Uses the nonsinusoidal light curve directlyNeed to know frequencies target star is
pulsating at Whole Earth TelescopeLong term plan –
map convection across hydrogen and helium instability strips
t~ 320 sec “Typical” GD358µi ~ 45 degrees
422.561423.898463.376464.209465.034571.735574.162575.993699.684810.291852.502962.385
Period (s) l m422.561 1 1
Tau ~ 35 ± 5 sec – “Whoopsie”µi ~ 52 ±5 degrees
This implies that GD 358 was ~ 3000 K hotter during the sforzando
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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