UBC Faculty Research and Publications

Lives of White Dwarf Stars Richer, Harvey Mar 17, 2008

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Lives of White Dwarf StarsH. Richer (UBC)Collaborators  z J. Brewer, S. Davis, H. Richer, A. Ruberg - UBCz J. Anderson - Space Telescope Science Institutez B. Hansen, D. Reitzel, M. Rich - UCLAz A. Dotter - Dartmouthz G. Fahlman, P. Stetson - HIA/NRCz J. Kalirai - UCSCz J. Hurley - Monashz I. King - UWz M. Shara - AMNHSELECTED MOMENTS IN LIVES OF WDsz Demonstrate very exciting physics can be explored using WDs - from stellar dynamics, to neutrino physics, to condensed matter physics, through to cosmology.INTRODUCTION TO WDs AND HST DATAz Short history of white dwarfs - what are they?z Some physics of white dwarfs - basic ideas.zWhy did we need the Hubble Space Telescope to do this work?z The data and the reductions.Today’s TalkShort History of WDsSirius1844: ‘Sirius Wobbles’ as it moves through space -invisible companion1862: Sirius B discovered & photographed - blue not red1930’s: Structure explained with quantum mechanicsSirius A and Bz Faint star  is blue!!New type of starz A & B similar colour∴ same temperature (~ 104 K)z Luminosities differ by 104   ∴ radii differ by 102z A & B about same mass    ∴ densities differby 106z Thus for B, ρ ≈ 106gm/cm3z For Sun ρ ≈ 1 gm/cm3Sirius ASirius BHR Diagram and Stellar EvolutionHR Diagram and Stellar Evolution(Unfortunately NOT Harvey Richer Diagram)l    i  iHR Diagram and Stellar Evolution Stars – Basic IdeasFor normal stars (Sun)Therefore, normal stars are very close to hydrostatic equilibrium  Normal starsWhite dwarfs   Degeneracy Pressurez When matter is compressed, the Uncertainty Principle  comes into playz There is zero point energyz With we derivez This leads toz More massive stars are smaller, for 1 solar mass Cooling of white dwarfszHeat content of corezLuminosity of the corezThis leads toor  zWhite dwarfs cool with time - a clock!     White Dwarf Cooling ModelsGlobular Star Clustersz ~160 in our Galaxyz >100,000 stars (many WDs)zAll stars at same distancez Low in heavy elements - oldz Images contaminated by:Ê(a) stars in the disk Ê(b) stars in the bulge and halo and Ê(c) background galaxiesSunWDs faint and in crowded field of globular clusterz Need superb imaging telescopez Need to distinguish stars from faint galaxiesz Need to distinguish cluster stars from fieldz Best is Hubble Space TelescopeObservational ConsiderationsHubble Space Telescopez 2.4m Reflectorz Launched 1990z Orbits at 575 kmz Period 97 minz UV to IRSpace - High Strehl PSFE. Martin (2005)Find faint objectsMeasure  positions wellDistinguish stars from almost unresolved galaxiesSpace - Stable PSFNICMOS PSF Over 9 Month Period - (STScI 2004)HST Observations 2005NGC 6397 (8400 Lyr2nd closest)126 Orbits (4.7 days)4% all time in 20053σ detection limit 4 x 10-23 Sun (candleon Moon)Archival data back 10yearsProcessed Colour ImageF814W Peak Image (left) - Stacked Image (right)Single Image (left) Stacked Image (right)   - arrow indicates very faint cluster WDFinding Faint Stars in CCD ImagesStar generated peak in 165/252 images - faint cluster WD z Images undersampled -large fraction of light in central pixelzDefine local maximum - any pixel higher than 8 neighboursz Examine each pixel in 252 images - make peak map - 1 or 0 at position of every pixel on every frame - add - PEAK MAPzRandom peaks add background level (252/9 = 28)zDemocratic finding techniquePeak Map      Stacked ImageMeasuring PhotometryExtract 7x7 box around each star on each frame. Outer pixels set background. Inner 9 pixels used to find flux from star. Model PSF tells fraction of star’s light at each pixel at offset (Δx,Δy). Flux each pixel Pij,n = f*ψij,n + sn (f* star’s flux, ψ fraction light in that pixel and s the sky. Equation is a straight line with slope f* and intercept s.HR Diagram Globular ClusterRicher et al. 2006, 200848,785 objects 8,537 starsExaggerated Motion of a Globular Clusteri    l lProper Motion Selectionz Field observed had archival data back 10 yearsz PMs scaled to 10 yearsz All stars left of red line (2σ cut) are cluster membersz Clean separation from field stars at Δr ~ 3 pixelsz Archival data not deep, only 60% overlapProper Motion Cleaned HR Diagram2317 starsSelected Moments in Lives of WDsÊWD get their kicks.zWhat happens at birth?Ê Testing electroweak theory.Ê Making diamonds in the sky.z Neutrino cooling and WDs.z Crystals and WDs. ÊWDs sing the blues.ÊWhen the stars get turned on.zWDs and the CIA.z Old WDs and cosmology.Pulsar B1508+551100 km/secEscaping Milky Way l   /i il   Neutron stars kicked at birthOff-centre SN explosionNeutron Star Birth Not QuiescentHobbs et al. 2005NRAO/AUI/NSFBirth is not exactly quiescentMass loss not always symmetric -can provide a “kick”.QuiescentWhite Dwarf Birth - Not Always WDs at Birth Examine radial distribution in clusterSuggests “kick” at birth 3 - 5 km/secDavis et al. 2008May explain why low mass star clusters have too few white dwarfs. Stars and Neutrinos z Only from Sun and SN 1987A have direct detections of stellar neutrinos been made.z Conditions favourable to produce neutrinos in interiors hot WDsz Mechanism is   γ --> ν + ν- (plasmon ν process)(process forbidden for free γ but γ can couple to plasma in WD interior)z Important test of universality of weak interactionz Can measure indirectly in WD stars.WD Birthrate and Neutrinosν fraction <10% after 3x107 yrs (25,000K)  1 such star in our field∴ Current birthrate of WDs in our field ~3x10-8/yr - agrees with # stars leaving main sequenceVariable WDs and Neutrinos More interesting is to measure rate of cooling as a direct test of the ν processes (eg at 30,000K drops 10-3 K/yr due to ν cooling)Winget et al. 2003But period change in variable WD is measurable (10-13 s/s)Variable WDs and Neutrinos P ~100 - 2000 secCrystallization in WD Sequence  Hansen et al 2007Onset crystallization near ~5000KCauses a jump in WD number counts as ratecooling slows down dueto release latent heat.Stellar seismology on aWD yielded ~90% of starcrystallized  (lattice C-O)Crystallization in WD Sequence  Hansen et al 2007Onset crystallization near ~5000KCauses a jump in WD number counts as ratecooling slows down dueto release latent heat.Stellar seismology on aWD yielded ~90% of starcrystallized  (lattice C-O)Named: Lucy in the Sky with DiamondsWDs and CIA Change in WD  colours due to Collision Induced AbsorptionDotted - Planck curve for4000K, solid pure H,dashed 1% H 99% He.Cooling of Old WDsTheory ObservationsTruncation in WD Cooling Sequencez Limit to which BULKof WDs cooled to givenage of clusterz Important age diagnostic not due to incompletenessModeling the WD Cooling AgeModeling includes:z# stars on main sequencezMain Sequence ModelszMi – MfzWD Cooling ModelszDistancezExtinctionHansen et al 2007Model Fit to Dataz Age = 11.51 +/-0.47 Gyrz χ2 = 1.27zMain difference: data too blue at F814W ~ 27.25z Systematic errors due to different models not includedLα red wing important - age 12.21 +/-0.35 with this opacity includedCosmology and WD AgesWMAP Model z vs age z and Cosmic Star FormationareeoneHR Diagram and Stellar EvolutionSpectrum Cool DA WD  At high T, WD spectra  almost black bodyAt low T, H2 & CIA Spectrum CIA extends below 1μSeen weakly in spectra some cool field WDsNever seen in cluster cooling sequenceHansen 2000


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