white dwarf stars as probes of physical and astrophysical...

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!