Halo White Dwarf Controversy

Painting by Lynette Cook

Ben R. OppenheimerUC-Berkeley

Nigel Hambly, Andrew DigbyUniversity of Edinburgh

Simon HodgkinCambridge University

Didier SaumonVanderbilt University

WD 0346+246Discovery: 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

LHS 3250Discovery: LHS, Harris et al. (1999)

Oppenheimer et al. (2001)

L = 3.26 x 10-5 L

MR = 15.8

The New Spectra38 new, featureless WDsAt least 3 are cooler or ascool as WD 0346.

This is only a samplingof all those spectra, withWD 0346 thrown in as apoint of reference.

The New Spectra

Bizarre:

Similar to the strangeLHS 3250 and SDSS1337,

but with a steeper slope,implying an even lower

temperature.

Na?

Halo Velocity Dispersion

Old DiskVelocityDispersion

Biases?

The Density of Halo White DwarfsScience paper: Halo White Dwarfs

A statistically meaningful sample over a large fraction of the sky allows one to calculate the space density:

34 pc M 103.1 -

Simulation of SurveyMR < 12m

Simulation of Survey12m < MR < 13m

Simulation of Survey13m < MR < 14m

Simulation of Survey14m < MR < 15m

Simulation of Survey15m < MR < 16m

Simulation of SurveyMR > 16m

Halo Velocity Dispersion

Old DiskVelocityDispersion

Biases?

Retrograde orbits

Halo Velocity Dispersion

Old DiskVelocityDispersion

Biases?

ThickDisk3 sig.

Retrograde orbits

Let’s 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 standardstellar 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

3 Cut

7 x 10-5 pc-3, ~ 3 times higher than expected for the haloEither this represents about .5-.8% of the dark matter, or they are simply part of the “standard” stellar halo, which must (1) haveformed 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 starsare certainly halo members. Halo must be very young, the thick disk is far more massive (2-10x) than otherstudies 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.

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

RPM Diagram5log5 RH R

d.

v

744tan

38.3log5 tan vMH RR

RPM depends only on theabsolute mag and the transversevelocity, not at all on apparent

mag or distance.