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Lives of White Dwarf Stars 2008

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Lives of White Dwarf Stars H. Richer (UBC) Collaborators z J. Brewer, S. Davis, H. Richer, A. Ruberg - UBC z J. Anderson - Space Telescope Science Institute z B. Hansen, D. Reitzel, M. Rich - UCLA z A. Dotter - Dartmouth z G. Fahlman, P. Stetson - HIA/NRC z J. Kalirai - UCSC z J. Hurley - Monash z I. King - UW z M. Shara - AMNH SELECTED MOMENTS IN LIVES OF WDs z 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 DATA z 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 Talk Short History of WDs Sirius 1844: ‘Sirius Wobbles’ as it moves through space - invisible companion 1862: Sirius B discovered & photographed - blue not red 1930’s: Structure explained with quantum mechanics Sirius A and B z Faint star  is blue!! New type of star z A & B similar colour∴ same temperature (~ 104 K) z Luminosities differ by 104   ∴ radii differ by 102 z A & B about same mass    ∴ densities differ by 106 z Thus for B, ρ ≈ 106 gm/cm3 z For Sun ρ ≈ 1 gm/cm3 Sirius A Sirius B HR Diagram and Stellar Evolution HR Diagram and Stellar Evolution (Unfortunately NOT Harvey Richer Diagram)l    i  i HR Diagram and Stellar Evolution Stars – Basic Ideas For normal stars (Sun) Therefore, normal stars are very close to hydrostatic equilibrium Normal stars White dwarfs Degeneracy Pressure z When matter is compressed, the Uncertainty Principle  comes into play z There is zero point energy z With we derive z This leads to z More massive stars are smaller, for 1 solar mass Cooling of white dwarfs zHeat content of core zLuminosity of the core zThis leads to or zWhite dwarfs cool with time - a clock! White Dwarf Cooling Models Globular Star Clusters z ~160 in our Galaxy z >100,000 stars (many WDs) zAll stars at same distance z Low in heavy elements - old z Images contaminated by: Ê(a) stars in the disk Ê(b) stars in the bulge and halo and Ê(c) background galaxies Sun WDs faint and in crowded field of globular cluster z Need superb imaging telescope z Need to distinguish stars from faint galaxies z Need to distinguish cluster stars from field z Best is Hubble Space Telescope Observational Considerations Hubble Space Telescope z 2.4m Reflector z Launched 1990 z Orbits at 575 km z Period 97 min z UV to IR Space - High Strehl PSF E. Martin (2005) Find faint objects Measure  positions well Distinguish stars from almost unresolved galaxies Space - Stable PSF NICMOS PSF Over 9 Month Period - (STScI 2004) HST Observations 2005 NGC 6397 (8400 Lyr 2nd closest) 126 Orbits (4.7 days) 4% all time in 2005 3σ detection limit 4 x 10-23 Sun (candle on Moon) Archival data back 10 years Processed Colour Image F814W Peak Image (left) - Stacked Image (right) Single Image (left) Stacked Image (right) - arrow indicates very faint cluster WD Finding Faint Stars in CCD Images Star generated peak in 165/252 images - faint cluster WD z Images undersampled - large fraction of light in central pixel zDefine local maximum - any pixel higher than 8 neighbours z Examine each pixel in 252 images - make peak map - 1 or 0 at position of every pixel on every frame - add - PEAK MAP zRandom peaks add background level (252/9 = 28) zDemocratic finding technique Peak Map      Stacked Image Measuring Photometry Extract 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 Cluster Richer et al. 2006, 2008 48,785 objects 8,537 stars Exaggerated Motion of a Globular Clusteri    l l Proper Motion Selection z Field observed had archival data back 10 years z PMs scaled to 10 years z All stars left of red line (2σ cut) are cluster members z Clean separation from field stars at Δr ~ 3 pixels z Archival data not deep, only 60% overlap Proper Motion Cleaned HR Diagram 2317 stars Selected Moments in Lives of WDs ÊWD 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+55 1100 km/sec Escaping Milky Way l  / i il Neutron stars kicked at birth Off-centre SN explosion Neutron Star Birth Not Quiescent Hobbs et al. 2005 NRAO/AUI/NSF Birth is not exactly quiescent Mass loss not always symmetric - can provide a “kick”. Quiescent White Dwarf Birth - Not Always WDs at Birth Examine radial distribution in cluster Suggests “kick” at birth 3 - 5 km/sec Davis et al. 2008 May 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 WDs z 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 interaction z Can measure indirectly in WD stars. WD Birthrate and Neutrinos ν 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 sequence Variable 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. 2003 But period change in variable WD is measurable (10-13 s/s) Variable WDs and Neutrinos P ~100 - 2000 sec Crystallization in WD Sequence Hansen et al 2007 Onset crystallization near ~5000K Causes a jump in WD number counts as rate cooling slows down due to release latent heat. Stellar seismology on a WD yielded ~90% of star crystallized  (lattice C-O) Crystallization in WD Sequence Hansen et al 2007 Onset crystallization near ~5000K Causes a jump in WD number counts as rate cooling slows down due to release latent heat. Stellar seismology on a WD yielded ~90% of star crystallized  (lattice C-O) Named: Lucy in the Sky with Diamonds WDs and CIA Change in WD  colours due to Collision Induced Absorption Dotted - Planck curve for 4000K, solid pure H, dashed 1% H 99% He. Cooling of Old WDs Theory Observations Truncation in WD Cooling Sequence z Limit to which BULK of WDs cooled to given age of cluster z Important age diagnostic not due to incompleteness Modeling the WD Cooling Age Modeling includes: z# stars on main sequence zMain Sequence Models zMi – Mf zWD Cooling Models zDistance zExtinctionHansen et al 2007 Model Fit to Data z Age = 11.51 +/-0.47 Gyr z χ2 = 1.27 zMain difference: data too blue at F814W ~ 27.25 z Systematic errors due to different models not included Lα red wing important - age 12.21 +/-0.35 with this opacity included Cosmology and WD Ages WMAP Model z vs age z and Cosmic Star Formation are e one HR Diagram and Stellar Evolution Spectrum Cool DA WD At high T, WD spectra almost black body At low T, H2 & CIA Spectrum CIA extends below 1μ Seen weakly in spectra some cool field WDs Never seen in cluster cooling sequence Hansen 2000


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