Presentation on theme: "Halo White Dwarf Controversy Painting by Lynette Cook Ben R. Oppenheimer UC-Berkeley Nigel Hambly, Andrew Digby University of Edinburgh Simon Hodgkin Cambridge."— Presentation transcript:
Halo White Dwarf Controversy Painting by Lynette Cook Ben R. Oppenheimer UC-Berkeley Nigel Hambly, Andrew Digby University of Edinburgh Simon Hodgkin Cambridge University Didier Saumon Vanderbilt University
WD 0346+246 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 M R = 16.06
LHS 3250 Discovery: LHS, Harris et al. (1999) Oppenheimer et al. (2001) L = 3.26 x 10 -5 L M R = 15.8
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
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.
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. 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 Diagram RPM depends only on the absolute mag and the transverse velocity, not at all on apparent mag or distance.