White Dwarf Stars as Probes of Physical and Astrophysical Processes

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1 White Dwarf Stars as Probes of Physical and Astrophysical Processes M I K E M O N T G O M E R Y D E P A R T M E N T O F A S T R O N O M Y, M C D O N A L D O B S E R V A T O R Y A N D T H E T E X A S C O S M O L O G Y C E N T E R, U N I V E R S I T Y O F T E X A S D I R E C T O R O F S C I E N C E O P E R A T I O N S, D E L A W A R E A S T E R O S E I S M I C R E S E A R C H C E N T E R M A R C H 7,

2 White Dwarfs are very faint Sirius A Sirius B

3 What are White Dwarf Stars? Macroscopic demonstration of QM Endpoint of evolution for most stars, 98% of all stars, including our sun Homogeneous in mass and surface composition: essentially monoelemental photospheres Simple internal structure and composition; evolution is just cooling

4 The Physics of White Dwarfs The Physics of White Dwarfs White dwarfs are supported by electron degeneracy pressure (the Pauli Exclusion Principle) Cooling is controlled by the heat capacity of the ions, and the surface temperature When hot ( > 25,000 K) they emit more energy in neutrinos than in photons As they get very cool (about 7000 K), the ions in the core settle into a crystalline lattice, i.e., they freeze or crystallize Gravity is high (g» 10 8 cm/s 2 ), so heavy elements sink, producing nearly pure H and/or He layers Normal mass (» 0.6 M ) white dwarfs have C/O cores

5 Mono-elemental Surface Layers H He C carbon surface DQ Three White Dwarf Flavors

6 Typical White Dwarf Structure Thin helium layer Thinner hydrogen layer Carbon and Oxygen core 99% Carbon/Oxygen DA= hydrogen atmosphere DB= helium atmosphere DQ= carbon atmosphere 1% Helium 0.01% Hydrogen

7 Science with White Dwarfs Pulsating white dwarfs allow us to: Constrain their core chemical profiles Constrain the physics of crystallization Probe the physics of convection Test the properties of exotic particles such as plasmon neutrinos and axions Look for extra-solar planets White dwarf evolution allows us to measure the ages of Thin disk Globular clusters Open clusters Thick disk Halo

8 White Dwarf Evolution Fowler and Chandrasekhar provided the first mechanical description of white dwarf structure, with support due to degenerate electrons: Mestel (1952) provided first the thermal description of white dwarf evolution:

9 The White Dwarf Luminosity Function (WDLF) Winget et al. (1987) The downturn is due to the finite Galactic age: 9 2 Gyr

10 A Modern Update of the WDLF Data of Harris et al. (2006) Low-mass C/O models of Salaris et al. (2010) High-mass O/Ne models of Althaus et al. (2007) Main sequence lifetimes from Dartmouth database Similar WDLFs can now be made for Globular Clusters

11 The white dwarfs in globular clusters can be used to test crystallization physics Globular Cluster NGC 6397

12 The Physics of Crystallization A one-component plasma (OCP) -- all the particles are identical ) regular lattice structure in 3D, Crys = 178

13 Number of stars Number of stars excess of stars at this luminosity brighter/hotter dimmer/cooler

14 Number of stars Number of stars fall-off is due to Debye cooling bump is due to crystallization in the models (Winget et al., 2009, ApJ Letters, 693, L6) brighter/hotter dimmer/cooler

15 summary: either these WDs have no significant Oxygen or Crys of a C/O mixture» 220 Schneider et al. (2012) have recently used direct Molecular Dynamics Simulations to calculate the C/O phase diagram. They indeed find higher values of Crys for mixtures, explaining our findings

16 Shortest Period WD Binary Discovered The White Dwarfs are orbiting around each other at a very rapid speed: 600 km/s The orbit should be shrinking rapidly so the WDs should come in contact due to loss of energy through gravitational wave radiation. The change in orbital period, coupled with direct gravity wave measurements will provide a fundamental test of Einstein s General Relativity. This object should have a high signal to noise and be easily detected with LISA or ELISA.

17 This system exhibits: Large radial velocities ( 600 km/s) eclipses of each star by the other ellipsoidal variations Doppler boosting Orbital Phase

18 Rate of Orbital Decay Measured: dp/dt = ( 9.8 ± 2.8) s/s

19

20 Two new classes of pulsating white dwarf recently found: DQV- atmospheres dominated by C and O (Montgomery et al. 2008) ELM V Extremely Low Mass (ELM) White dwarf, M < 0.2 M, log g» 6 (Hermes et al 2012) DQV ELM V

21 What we (think) we know about Extremely Low-Mass (ELM) WDs 0.25 M M < 0.25 M He-core Stripped of material before much He fusion to C/O can occur Identified spectroscopically (hydrogen-atmosphere WDs with narrower lines) Must form in binaries Single-star evolution would take too long to form a 0.2 M WD So far, 18 of 18 WDs with masses < 0.25 M have detected RV companions (Kilic et al. 2011, ApJ 727 3) 0.82 M

22 Extremely Low Mass White Dwarfs Thin hydrogen layer Helium core For M < 0.2 Msun, residual nuclear burning still occurring at base of H envelope Mostly Helium 0.1-1% Hydrogen (Panei et al. 2007, Steinfadt et al. 2010)

23 THE FIRST PULSATING EXTREMELY LOW MASS WHITE DWARF : SDSS J J1840 comparison star Hermes et al. (2012)

24 Pulsating white dwarfs allow us to: Constrain their core chemical profiles Constrain the physics of crystallization Probe the physics of convection Look for extra-solar planets Test the properties of exotic particles such as plasmon neutrinos and axions Constrain accretion in CV systems Constraining their structure makes WDs more reliable as age indicators for the Galaxy and star clusters

25 The Main Obstacle of Time-Series Measurements: The Window Function 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. Width of peaks 1/t t=timescale of observations Separation between peaks 1/(time between gaps) Unfortunately, this rarely happens. Gaps introduce uncertainty, which appears as aliases in the FT

26 The ideal tool to study pulsating WDs is The Whole Earth Telescope (WET) Xinglong Station (NAOC)

27 Goal of the WET Observations Uniform data set high speed photometry Uniform instrumentation as near as possible Uniform reduction procedures Interactive headquarters data reduced in real time Multiple targets Continuous coverage elimination of aliases

28 What do we need? Good target Whole Earth Telescope long lightcurves to accurately identify frequencies continuous light curves to eliminate aliases Multi-site observing runs WET Spectral Windows

29 Single Site

30 Full WET Run

31 Fractional Amplitude Xinglong Station (NAOC) There will be a WET run on An ELM WD in May 2013!

32 What We Hope to Learn about the ELMS Where they come from Single star evolution would take > 100 Gyr ) must be product of binary star evolution Are there residual nuclear reactions? How much residual hydrogen is there? What were their progenitors? How much mass did the system lose? How does convection operate in these stars? We can study this by modelling their light curves

33 Example: Convection in GD358 Convection is a fundamental problem in astrophysics. Light Curve Fitting Montgomery, 2005 Idea: Underlying pulsations are sinusoidal Convection zone changes as surface temperature changes Delays and attenuates the pulsations Result: Nonlinear pulse shapes in light curve Use pulse shapes to determine convection zone parameters thermal response time (depth)

34 Light curve fit of the multi-periodic DBV GD358 0 ~ sec µ i ~ degrees Period (s) ell m Montgomery et al. (2010)

35 GD358 during the May 2006 WET Run

36 Simultaneously fit 29 high S/N runs: nonlinear fit (only 3 additional parameters)

37 Asteroseismology of GD358 Mass = 0.630±0.015 M o Log (M He )= -2.79±0.06 L= 0.05±0.012 L o Convective adjustment timescale ~ 600 seconds Rotation rate: Period ~ 1 2 days Magnetic Field > 1200 G

38 Summary and Conclusions White dwarfs can be used to answer many fundamental questions. For example: Ages of clusters The physics of crystallization Gravitational radiation Pulsations allow us to: Determine structural parameters of stars, e.g., the ELM WDs Constrain how convection operates in normal mass and ELM WDs Thanks!

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