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The Kinematics of High Proper Motion Halo White Dwarfs

Koopmans, L. V. E. and Blandford, R. D. (2001) The Kinematics of High Proper Motion Halo White Dwarfs. . (Unpublished)

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We analyse the kinematics of the entire spectroscopic sample of 99 recently discovered high proper-motion white dwarfs by Oppenheimer et al. using a maximum-likelihood analysis, and discuss the claim that the high-velocity white dwarfs are members of a halo population with a local density at least ten times greater than traditionally assumed. We argue that the observations, as reported, are consistent with the presence of an almost undetected thin disc plus a thick disc, with densities as conventionally assumed. In addition, there is a kinematically distinct, flattened, halo population at the more than 99% confidence level. Surprisingly, the thick disc and halo populations are indistinguishable in terms of luminosity, color and apparent age (1–10 Gyr). Adopting a bimodal, Schwarzschild model for the local velocity ellipsoid, with the ratios σ_U: σ_V: σ_W=1:2/3:1/2, we infer radial velocity dispersions of σ_U=62^(+8)_(−10) kms^(−1) and 150^(+80)_(−40) kms^(−1) (90% C.L.) for the local thick disc and halo populations, respectively. The thick disc result agrees with the empirical relation between asymmetric drift and radial velocity dispersion, inferred from local stellar populations. The local thick-disc plus halo density of white dwarfs is n^(td+h)_(0,WD)=(1.9 ± 0.5)×10^(−3) pc^(−3) (90% C.L.), of which n^h_(0,WD)=1.1^(+2.1)_(−0.7)×10^(−4) pc^(−3) (90% C.L.) belongs to the halo, a density about five times higher than previously thought. Adopting a mean white-dwarf mass of 0.6 M_⊙, the latter amounts to 0.8^(+1.6) _(−0.5)×10^(−2) (90% C.L.) of the nominal local halo density. Assuming a simple spherical logarithmic potential for the Galaxy, we infer from our most-likely model an oblate halo white-dwarf density profile with n(r) ∝ r^(−α) and α ≈ 3.0. The halo white dwarfs contributes ~2.6 × 10^9 M_⊙, i.e. a mass fraction of ~0.004, to the total mass inside 50 kpc (Ω_(WD) ~10^(−4)). The halo white dwarf population has a microlensing optical depth towards the LMC of τ^h_(WD) ≈1.3 × 10^(−9). The thick-disc white dwarf population gives _τ^(td)_(WD) ≈ 4 × 10^(−9). The integrated Galactic optical depth from both populations is 1–2 orders of magnitude below the inferred microlensing optical depth toward the LMC. If a similar white-dwarf population is present around the LMC, then self-lensing can not be excluced as explanation of the MACHO observations. We propose a mechanism that could preferentially eject disc white dwarfs into the halo with the required speeds of ~200 kms^(−1), through the orbital instability of evolving triple star systems. Prospects for measuring the density and velocity distribution of the halo population more accurately using the Hubble Space Telescope Advanced Camera for Surveys (ACS) appear to be good.

Item Type:Report or Paper (Discussion Paper)
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URLURL TypeDescription Paper
Koopmans, L. V. E.0000-0003-1840-0312
Blandford, R. D.0000-0002-1854-5506
Additional Information:The authors thank Ben Oppenheimer for many valuable discussions and providing tables with their results prior to publication. LVEK thanks Dave Chernoff for several discussions. The authors are indebted to David Graff and Andy Gould for bringing our attention to a mistake in the normalisation of the likelihood function. This research has been supported by NSF AST-9900866 and STScI GO-06543.03-95A.
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Subject Keywords:stars: white dwarfs – galaxy: halo – stellar content – structure – gravitational lensing – dark matter
Record Number:CaltechAUTHORS:20190605-082146604
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:96129
Deposited By: Tony Diaz
Deposited On:06 Jun 2019 04:47
Last Modified:03 Oct 2019 21:19

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