white dwarfs phys390 astrophysics professor lee carkner lecture 17
Post on 15-Jan-2016
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White Dwarfs
PHYS390 Astrophysics
Professor Lee Carkner
Lecture 17
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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
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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
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White Dwarf Properties
Mass ~ Luminosity ~ 0.03 Lsun
Radius ~ Density ~ 3X109 kg/m3
surface gravity ~ 5X106 m/s2
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Observing White Dwarfs Spectra show strong
pressure broadened H lines
Also produce very low energy X-rays
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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?
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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
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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
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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
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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
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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
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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
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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
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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
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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
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Next Time
Read 16.6-16.7, 17.3 Homework: 16.10, 16.12, 16.14, 17.12