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 0 1 3
White Dwarf Stars as Probes of Physical and Astrophysical Processes
White Dwarfs are very faint
Sirius A
Sirius B
•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
What are White Dwarf Stars?
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 » 108 cm/s2), so heavy elements sink,
producing nearly pure H and/or He layers
“Normal” mass (» 0.6 M )̄ white dwarfs have C/O cores
The Physics of White Dwarfs
Mono-elemental Surface Layers
DQ
H He C
Three White Dwarf Flavors
carbon surface
Carbon and Oxygen core
Thin helium layer
Thinner hydrogen layer
―Typical‖ White Dwarf Structure
99% Carbon/Oxygen
1% Helium
0.01% Hydrogen
DA= hydrogen atmosphere DB= helium atmosphere DQ= carbon atmosphere
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
Science with White Dwarfs
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:
The White Dwarf Luminosity Function (WDLF)
Winget et al. (1987) The downturn is due to the finite Galactic age: 9 § 2 Gyr
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
Globular Cluster
NGC 6397
The white dwarfs in globular clusters can
be used to test crystallization physics
The Physics of Crystallization
A one-component plasma (OCP) -- all the particles are identical
) regular lattice structure
in 3D, ¡Crys = 178
excess of stars at
this luminosity
Nu
mb
er
of
star
s
brighter/hotter dimmer/cooler
Nu
mb
er
of
star
s
bump is due to crystallization
in the models (Winget et al.,
2009, ApJ Letters, 693, L6)
Nu
mb
er
of
star
s
fall-off is due to
Debye cooling
dimmer/cooler brighter/hotter
Nu
mb
er
of
star
s
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
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.
This system exhibits:
Large radial velocities (§ 600 km/s)
eclipses of each star by the other
ellipsoidal variations
Doppler boosting
Orbital Phase
Rate of Orbital Decay Measured:
dP/dt = (−9.8 ± 2.8) × 10−12 s/s
Two “new” classes of pulsating white dwarf recently found:
DQV
ELM V
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)
What we (think) we know about Extremely Low-Mass (ELM) WDs…
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)
0.25 M
0.82 M
Helium core
Thin hydrogen layer
Extremely Low Mass White Dwarfs
―Mostly‖ Helium
0.1-1% Hydrogen
For M < 0.2 Msun, residual nuclear burning still occurring at base of H envelope
(Panei et al. 2007 , Steinfadt et al. 2010)
THE FIRST PULSATING EXTREMELY LOW MASS WHITE DWARF : SDSS J184037.78+642312.3
Hermes et al. (2012)
comparison star
J1840
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
The Main Obstacle of Time-Series Measurements: The 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
The ideal tool to study pulsating WDs is…
The Whole Earth Telescope (WET)
Xinglong Station (NAOC)
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
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
Single Site
Full WET Run
Xinglong Station (NAOC)
Fra
ctio
na
l A
mp
litu
de
There will be a WET run on An ELM WD in May 2013!
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
What We Hope to Learn about the ELMS
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)
Period (s) ell m
422.561 1 1
423.898 1 -1
463.376 1 1
464.209 1 0
465.034 1 -1
571.735 1 1
574.162 1 0
575.933 1 -1
699.684 1 0
810.291 1 0
852.502 1 0
962.385 1 0
Montgomery et al. (2010)
¿0 ~ 586§ 12 sec µi ~ 47.5 § 2.2 degrees
Light curve fit of the multi-periodic DBV GD358
GD358 during the May 2006 WET Run
Simultaneously fit 29 high S/N runs:
nonlinear fit (only 3 additional parameters)
Asteroseismology of GD358
Mass = 0.630±0.015 Mo
Log (MHe)= -2.79±0.06
L= 0.05±0.012 Lo
Convective adjustment timescale ~ 600 seconds
Rotation rate: Period ~ 1—2 days
Magnetic Field > 1200 G
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!