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