White Dwarfs

PHYS390 Astrophysics

Professor Lee Carkner

Lecture 17

Compact Objects

Their cores become compact objects Neutron star Black hole

Physically small and thus low luminosity Can be responsible for bright outburst

phenomena due to mass transfer

Sirius B In 1844 Bessel determines

Sirius is a 50 year binary via astrometry

In 1862 Alvan G. Clark finds Sirius B in a telescope test

In 1915 Walter Adams uses spectroscopy to get a surface temperature for Sirius B of 27000 K Three times hotter than Sirius A

White Dwarf Properties

Mass ~ Luminosity ~ 0.03 Lsun

Radius ~ Density ~ 3X109 kg/m3

surface gravity ~ 5X106 m/s2

Observing White Dwarfs Spectra show strong

pressure broadened H lines

Also produce very low energy X-rays

Classification

About 2/3 of white dwarfs are in the DA class

Strong gravity creates a density gradient

Only thin surface layer of H can exist

Other white dwarfs show no H lines or no lines at all Stripped of H in giant phase?

Fermi Energy

A gas where all of the low energy states are filled is called degenerate

The maximum energy of a degenerate electron is called the Fermi energy (EF)

EF = (h2/82me)(32n)2/3

Where n is the number density of electrons

Degeneracy

The degree of degeneracy depends on temperature and density

T/2/3 < 1261 K m2 kg-2/3

The smaller the value of T/2/3 the more degenerate the gas

Pressure

P = ((32)2/3/5)(h2/42me)[(Z/A)(/mH)]5/3

where Z is the number of protons and A is the number of nucleons (~0.5 for white dwarfs)

For relativistic electrons:

P = ((32)1/3/4)(hc/2)[(Z/A)(/mH)]4/3

Mass-Volume Relation

We find that Mass X Volume = constant

In order the support a greater mass, we need more electron degeneracy pressure which requires a greater density

Chandrasekhar Limit

As the radius goes to zero the mass goes to a maximum

Mass greater than Chandrasekhar limit cannot be supported by electron degeneracy

White Dwarf Binaries

If the second star is not a compact object

and is close enough, it will transfer mass onto the white dwarf

Can produce an accretion disk and variability

Generally referred to as novae or cataclysmic variables

Dwarf Novae

Quiescent for months then get brighter for a week or two

Caused by an increase in mass flow through the accretion disk

Friction in the disk causes the disk to heat up

Dwarf novae are periodic, reoccurring every few months

Classical Novae

Very large brightness increase over a few days

Caused by build up of hydrogen on the surface of the white dwarf

The luminosity quickly exceeds the Eddington limit

Takes thousands of years to build

back up

Type Ia Supernova

If enough mass falls onto a white dwarf that it exceeds the Chandrasekhar limit, it will collapse violently

Very bright (M=-19.3) with brightness well correlated to light curve

Next Time

Read 16.6-16.7, 17.3 Homework: 16.10, 16.12, 16.14, 17.12