Delaware Asteroseismic Research Center Asteroseismology with the Whole Earth Telescope (and More!)
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 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 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 + + 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 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 Frequency 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
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 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. 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
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 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 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 Period (s) l m 422.561 1 1 This implies that GD 358 was ~ 3000 K hotter during the sforzando 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