Download Dwarf novae

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 = (h2/8p2me)(3p2n)2/3
 Where n is the number
density of electrons
Degeneracy
 The degree of
degeneracy depends on
temperature and density

T/r2/3 < 1261 K m2 kg-2/3
 The smaller the value of
T/r2/3 the more degenerate
the gas
Pressure

P = ((3p2)2/3/5)(h2/4p2me)[(Z/A)(r/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 = ((3p2)1/3/4)(hc/2p)[(Z/A)(r/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