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White Dwarfs

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.

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White Dwarfs

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  1. White Dwarfs PHYS390 Astrophysics Professor Lee Carkner Lecture 17

  2. 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

  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 • Three times hotter than Sirius A

  4. White Dwarf Properties • Mass ~ • Luminosity ~ 0.03 Lsun • Radius ~ • Density ~ 3X109 kg/m3 • surface gravity ~ 5X106 m/s2

  5. Observing White Dwarfs • Spectra show strong pressure broadened H lines • Also produce very low energy X-rays

  6. 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?

  7. 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

  8. 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

  9. 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

  10. 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

  11. 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

  12. 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

  14. 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

  15. 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

  16. Next Time • Read 16.6-16.7, 17.3 • Homework: 16.10, 16.12, 16.14, 17.12

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