Halo White Dwarf Controversy

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Halo White Dwarf Controversy
Ben R. Oppenheimer UC-Berkeley Nigel Hambly,
Andrew Digby University of Edinburgh Simon
Hodgkin Cambridge University Didier
Saumon Vanderbilt University
Painting by Lynette Cook
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WD 0346246
Discovery Hambly et al. (1997)
IR Spectrum Hodgkin et al. (2000)
Oppenheimer et al. (2001)
Parallax Hambly et al. (2001)
L 1.82 x 10-5 L
MR 16.06
?
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LHS 3250
Discovery LHS, Harris et al. (1999)
Oppenheimer et al. (2001)
L 3.26 x 10-5 L
MR 15.8
?
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The New Spectra
38 new, featureless WDs At least 3 are cooler or
as cool as WD 0346.
This is only a sampling of all those spectra,
with WD 0346 thrown in as a point of reference.
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The New Spectra
Bizarre
Na?
Similar to the strange LHS 3250 and SDSS1337, but
with a steeper slope, implying an even lower
temperature.
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Halo Velocity Dispersion
2s
1s
Old Disk Velocity Dispersion
Biases?
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The Density of Halo White Dwarfs
Science paper 38 Halo White Dwarfs
A statistically meaningful sample over a large
fraction of the sky allows one to calculate the
space density
?
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Simulation of Survey
MR 13
Simulation of Survey
12m 14
Simulation of Survey
13m 15
Simulation of Survey
14m 16
Simulation of Survey
15m 17
Simulation of Survey
MR 16m
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Retrograde orbits
Halo Velocity Dispersion
2s
1s
Old Disk Velocity Dispersion
Biases?
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Retrograde orbits
Halo Velocity Dispersion
2s
1s
Old Disk Velocity Dispersion
Thick Disk 3 sig.
Biases?
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Lets accept only the retrogradestars (4 or 8
stars)

• This gives a space density of 2 - 5 x 10-5 pc-3
• Using subdwarf star counts and standard IMF, the
  standard
• stellar halo should have 2 2.5 x 10-5 pc-3
• But, what does this really mean?
• We have found all halo WDs in the solar
  neighborhood.
• The Galaxys halo is less than 10 Gyr old
• The luminosity function has an extremely sharp
  cut off
• Thick disk mass becomes 2-10 times more massive
  with an
• IMF peaked above 2 M?

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3? Cut
7 x 10-5 pc-3, 3 times higher than expected for
the halo Either this represents about .5-.8 of
the dark matter, or they are simply part of the
standard stellar halo, which must (1)
have formed from a strange IMF, (2) is more
massive and (3) probably much younger than
previously thought. The thick disk mass is 2-8
times more massive and must have formed with a
top-heavy IMF.
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Three Choices

• Accept our original analysis. The white dwarfs
  represent 2 of the dark matter. The standard
  picture of the thick disk is unaffected.
  Probably need a peaked IMF for the halo (low Z
  gas).
• Accept the Reid et al. argument that only
  retrograde stars
• are certainly halo members. Halo must be very
  young, the thick disk is far more massive (2-10x)
  than other
• studies show and it must have formed with a
  weird IMF, while the halo and the thin disk did
  not. It also must have had a vastly different
  star formation history than other studies
  indicate.
• 3. Take an intermediate position, i.e. 3?, 0.5
  of the dark matter density is in white dwarfs in
  a halo with a peaked IMF. Or the standard halo
  is only 10 Gyr old and has a weird IMF. The
  thick disk is more massive possibly with a weird
  IMF and SF history.

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Bottom Line(s)

• Our interpretation in Science is the simplest and
  least disruptive of the current, standard
  observations of galactic structure.
• No matter what interpretation you favor, some
  evolution of the IMF is required. (Our
  interpretation makes it a probably a function of
  metallicity. Reid et al.s would make it
  non-standard only for the thick disk.)
• 10-20 new white dwarfs below 4000K

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RPM Diagram
RPM depends only on the absolute mag and the
transverse velocity, not at all on apparent mag
or distance.