White Dwarfs

1
White Dwarfs

• Physical Astronomy
• Professor Lee Carkner
• Lecture 18

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Compact Objects

• Their cores become compact objects
• White dwarf
• Neutron star
• Black hole
• Physically small and thus low luminosity
• Often observable due to mass transfer

3
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

4
White Dwarf Properties

• Mass
• Luminosity
• Radius
• Density 3X109 kg/m3
• surface gravity 5X106 m/s2

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Observing White Dwarfs

• White dwarfs are strong UV emitters

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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

• Occurs at low temperatures or high pressure
• EF (h2/8p2me)(3p2n)2/3
• Where n is the number density of electrons

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Degeneracy

• If the average kinetic energy is less than the
  Fermi energy, the gas is degenerate
• T/r2/3 lt 1261 K m2 kg-2/3

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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

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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

13
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