Download powerpoint file

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
Discovery: Hambly et al. (1997)
IR Spectrum: Hodgkin et al. (2000)
Oppenheimer et al. (2001)
WD 0346+246
Parallax: Hambly et al. (2001)
L = 1.82 x 10-5 L
MR = 16.06
Discovery: LHS, Harris et al. (1999)
LHS 3250
L = 3.26 x 10-5 L
MR = 15.8
Oppenheimer et al. (2001)
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.
The New Spectra
Bizarre:
Na?
Similar to the strange
LHS 3250 and SDSS1337,
but with a steeper slope,
implying an even lower
temperature.
Halo Velocity Dispersion
2s
1s
Biases?
Old Disk
Velocity
Dispersion
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:
  1.3 10 M pc
4

-3
MR < 12m
Simulation of Survey
12m < MR < 13m
Simulation of Survey
13m < MR < 14m
Simulation of Survey
14m < MR < 15m
Simulation of Survey
15m < MR < 16m
Simulation of Survey
MR > 16m
Simulation of Survey
Retrograde orbits
Halo Velocity Dispersion
2s
1s
Old Disk
Velocity
Dispersion
Biases?
Retrograde orbits
Halo Velocity Dispersion
2s
1s
Old Disk
Velocity
Dispersion
Thick
Disk
3 sig.
Biases?
Let’s accept only the retrograde
stars (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 Galaxy’s 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
3s 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.
Three Choices:
1. 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).
2. 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. 3s, >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.
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 nonstandard only for the thick disk.)
• ~10-20 new white dwarfs below 4000K
RPM Diagram
H R  R  5 log   5
vtan

4.74d
H R  M R  5 log vtan  3.38
RPM depends only on the
absolute mag and the transverse
velocity, not at all on apparent
mag or distance.