Astronomers in the dark
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Astronomers in the Dark. What you need to know about Galactic Structure before “discovering” Dark Matter. Neill Reid Kailash Sahu & Suzanne Hawley. Outline. Dark matter in the Galaxy – background and definitions Why cool white dwarfs?

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Astronomers in the dark

Astronomers in the Dark

What you need to know about Galactic Structure before “discovering”

Dark Matter

Neill Reid

Kailash Sahu & Suzanne Hawley


Outline

Outline

  • Dark matter in the Galaxy – background and definitions

  • Why cool white dwarfs?

  • Stellar kinematics in the Galactic Disk

  • Heavy halo white dwarfs? Or just boring disk dwarfs?


Galactic dark matter

Galactic dark matter

  • Galaxy rotation curves at large radii are not Keplerian

    - heavy halos (Ostriker, Peebles & Yahil, 1974)

    - Milky Way M ~ 5 x 10^11 solar masses, R < 50 kpc

    visible material (disk + stellar halo) ~ 5 x 10^10 solar masses

    => 90% dark matter – particles? compact objects?

  • Microlensing surveys – MACHO, EROS, DUO,OGLE

    Given timescale, estimated velocity => mass

    MACHO: 13-17 events, t ~ 34-230 days, <V> ~ 200 km/s

    => can account for ~20% of the missing 90%

    <M> = 0.5+/- 0.3 solar masses


Some definitions

Some definitions

  • The Galactic Disk - flattened, rotating population (220 km/sec): Pop I

    - metal-rich, -0.6 < [Fe/H] < 0.15

    - total mass ~ 5 x 10^10 M(sun)

    - complex density structure (old disk, thick disk)

    - local mass density ~ 4.5 x 10^-3 M(sun)/pc^3

    number density ~ 0.1 stars/cubic pc

  • The halo – near-spherical, non-rotating, pressure-supported: Pop II

    - metal-poor, -4 < [Fe/H] < -0.7

    - total mass ~ 3 x 10^9 M(sun)

    - local number density ~ 0.0002 stars/cubic pc (0.2% disk)

  • The dark/heavy halo – near-spherical (?), non-rotating(?): Pop III

    - local mass density ~ 0.01 M(sun)/cubic pc


Why white dwarfs

Why white dwarfs

  • MACHOs: <M> ~ 0.5 +/- 0.3 M(sun)

    50%  20% of the dark halo

  • HDF proper motion objects – Ibata et al (1999)

    2-5 faint, blue sources with apparent motions  100% of dark halo

  • Cool white dwarfs (<3000K) are not black bodies

     molecular hydrogen opacity

    originally highlighted by Mould & Liebert (1978)

    detailed models by Bergeron (1997) and Hansen (1999)

    a few examples have been detected in the field


White dwarf complications

White dwarf complications

  • Cosmic pollution from Population III:

    white dwarfs are remnants – the ejected envelope carries nucleosynthesis products to the field

    How do you preserve a metal-poor Pop II halo?

  • Fiddle the mass function

    - avoid high-mass stars (M > 8 M(sun): no SN

    - avoid low-mass stars (M < 1 M(sun)): no long-lived dwarfs

    - avoid 4-8 M(sun) stars: no carbon stars

  • Require a radically different mode of star formation for Pop III

    - but we have no evidence of significant variations Pop II Pop I

    -3 < [M/H] < 0.2


Finding heavy halo wds i

Finding heavy halo WDs: I

  • We are in the dark halo – local density ~ 10^-2 M_sun/pc^3

    ~4 x 10^-3 MACHOs /pc^3 for 20% in 0.5 M(sun) objects

    if the dark halo is a non-rotating, pressure-supported structure, then we expect high velocities relative to the Sun

    => search for local representatives in proper motion surveys

  • Predicted S ~ 1 / tens of sq. degrees

    => Luyten’s Palomar surveys (POSSI  Luyten E (1963)

    LHS : m > 0.5 arcsec/yr, m_r<19.5, d > -36

    NLTT : m > 0.18 arcsec/yr, m_rr < 19.5, d > -36

    => LHS 3250 (Harris et al, 1999)

    …but dark halo white dwarfs are low luminosity, M( R) > 17

  • Could these dwarfs have been missed in previous surveys?


Finding heavy halo wds ii

Finding heavy halo WDs: II

  • New surveys with deeper plate material

    IIIaJ – B ~ 21.5 – 22 POSS II & UK Schmidt

    IIIaF - R ~ 21 – 21.5 cf Luyten m_r ~ 20

    IVN - I ~ 18 – 19

  • First results: two good halo dwarf candidates

    WD0346+246 (Hodgkin et al (2000))

    T ~ 3500K, velocity ~ 170 km/sec, M_V ~ 17, H/He composition

    F351-50 (Ibata et al, 2000)

    T ~3500K, high velocity, H/He composition

    Oppenheimer et al (2001): IR spectra, comparison with models

  • But the original blue white dwarf isn’t ….

    LHS 3250 – low velocity, over-luminous, binary?


Finding heavy halo wds iii

Finding heavy halo WDs? III

  • Oppenheimer et al. (Science Express, March 23)

    Photographic survey of ~10% of the sky near the SGP

    UK Schmidt plates: Dt ~ 5 to 20 years (IIIaJ, IIIaF, IVN)

    0.33 < m < 10 arcsec/yr; R < 19.8, BRI photometry

  • 105 faint, high motion objects

    Spectroscopic follow-up: 55 confirmed as white dwarfs (DA, DC)

  • Distances from photometric parallaxes

    (B-R)  M_R (+/- 20%)

  • Sample is from South Galactic Cap, so m  (U, V)

    Exclude stars within “disk” 2-s velocity ellipsoid

    [NB <2s includes 86% of a sample for 2 uncorrelated variables]


Finding heavy halo wds iv

Finding heavy halo WDs? IV

  • 38 cool, high-velocity white dwarfs – all DC

    Compute densities using r = S 1/V_max, R < 19.7 mag.

    where d_max is set by

    d_m, the distance where m < 0.33 arcsec/yr, or

    d_m, the distance where R = 19.7

    => local density of 2 x 10^-4 stars/pc^3

    or ~10 times the density of halo white dwarfs

     could account for 2% of dark matter if they’re heavy halo

  • But is the velocity distribution sensible? 34 prograde, 4 retrograde

    Selection effect? <r>=73 pc, m_lim ~ 3 “/yr  V_tan < 1040 km/sec

  • What about the disk?…


Galactic disk kinematics i

Galactic Disk kinematics: I

  • Velocity dispersions increase as a function of age

     s ~ t^b , b = ½  1/3 (orbit diffusion, Wielen )

  • Disk sub-populations – young disk (<10^8 yrs)

    - old disk

    - thick disk

    => discrete kinematic structure


Galactic disk kinematics ii

Galactic Disk kinematics: II

  • Empirical measurements rest on volume-complete samples

     require distances, proper motions, radial velocities, preferably some abundance information

  • M dwarfs are ideal - 80% of disk stars are M dwarfs

    => lots of nearby test particles

    - high m, complex spectra

    => space motions

    - crude abundances from CaH/TiO bands

  • PMSU survey of nearby stars (Reid, Hawley & Gizis, 1995)

    - 2000 M dwarfs potentially within 25 pc

    - volume-limited sample of 514 systems, 8 < M_V < 15, d > -30

    95% complete – probably missing low-velocity stars


Characterising disk kinematics i

Characterising Disk kinematics: I

  • Stellar kinematics are usually represented as Schwarzschild velocity ellipsoids:

    (s(U), s(V), s(W)) centred at (<U>,<V>,<W>)

  • How do we measure s  probability plots (Lutz & Upgren)

    consider a parameter, x, with measurements, x(i)

    produce a rank-ordered list, x(i)

    determine <x> and std. deviation, s

    plot x(i) vs [ (x(i) - <x>) / s ]

    A Gaussian distribution produces a straight line, slope s

  • A combination of 2 Gaussians gives 3 line segments,

  • slope s(1), s(2)


Disk kinematics ii

Disk kinematics: II


Disk kinematics iii

Disk kinematics: III

  • Results from fitting the M dwarf distribution

    <U> <V> <W> s(U) s(V) s(W)

    1 -10 -23 -7 35 21 20

    2 52 36 32

    3? 65

    where ~90% of local stars are in sub-population 1

  • Oppenheimer et al adopt

    0 -35 0 46 50 35

    from Chiba & Beers (2000) analysis of intermediate abundance

    ([Fe/H]~-0.6) dwarfs => overestimate disk kinematics


High velocity disk dwarfs i

High-velocity disk dwarfs I

  • The Galactic disk has a complex kinematic structure

    - poorly represented by single Schwarzschild ellipsoid

  • How many high-velocity disk stars?

     compare the M dwarf velocity distribution against

    Oppenheimer et al.’s halo selection criterion

  • 20 of 514 systems exceed (U+V) velocity limit

    - allowing for incompleteness in PMSU1, ~3.7%

    (note location)


High velocity disk dwarfs ii

High-velocity disk dwarfs: II

  • Disk stars can have high velocities – M dwarfs: 0.2 < M(sun) > 0.6

    3.7% would be classed as dark halo by Oppenheimer et al.

    >1 M(sun) disk stars have experienced the same dynamical evolution

  • High-velocity disk dwarfs are likely to be the oldest disk dwarfs

    => associated with cool white dwarfs

  • Local density: 12 white dwarfs within 8 parsecs, 7 single stars

    + 10 main-sequence dwarfs with M > 1 M(sun)

    => ~ 8 x 10^-3 stars / pc^3

    3.7%  ~3 x 10^-4 white dwarfs / pc^3

  • Oppenheimer et al. calculate r ~ 2 x 10^-4 stars/pc^3

  • High-velocity disk white dwarfs can account for the observed r


Halo white dwarfs i

Halo white dwarfs? I

  • What about the highest velocity white dwarfs?

  • In a non-rotating system, N_prograde = N_retrograde

     compute S 1/V_max for 4 dwarfs with retrograde motion

    F351-50, LHS 147, WD0135-039, WD0300-044

    r_tot = 2 x r_obs ~ 2 x 10^-5 stars / pc^3

    => expected density of halo white dwarfs

  • An absence of surprises


Halo white dwarfs ii

Halo white dwarfs? II

  • How about the temperature distribution?

    White dwarfs in a primordial, dark halo should have t ~ 14 Gyrs

     T < 3000 K

  • Given M_R from (B-R), plot M_R vs (R-I)

     compare with theoretical tracks

    Most have ages < 7 Gyrs

     if they’re dark halo, they have long-lived MS progenitors

    which we don’t observe

  • Most of the Oppenheimer et al. white dwarfs are remnants of the first stars which formed in the thick disk

  • White dwarfs from the stellar halo account for the rest

  • There is no requirement for a dark matter contribution


Questions

Questions

  • So why didn’t they….

  • …calculate how many white dwarfs you get from 5% disk contamination

  • …calculate the (U, V) limits for the appropriate proper motion selection bias

  • …compare the observed temperature distribution with that expected for a 14-Gyr dark halo


Summary

Summary

Evidence for heavy halo white dwarfs

  • MACHOs

  • --- but maybe they’re in the LMC/SMC

  • HDF proper motions

  • --- but they’re no longer moving

  • 3. High-velocity, cool white dwarfs in the field

  • --- not fast enough or cool enough

    • Extraordinary claims require extraordinary evidence

    • Make no unnecessary hypotheses

    • There is no need to invoke dark matter to explain

    • the cool white dwarfs found by Oppenheimer et al


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    A binary system

    A binary system

    White dwarfs

    can have

    brown dwarf

    companions


    A kinematic conundrum 1

    A kinematic conundrum (1)

    Stellar kinematics are correlated with age

     scattering through encounters with molecular clouds

    leads to

    1. Higher velocity dispersions

    2. Lower net rotational velocity, V

    e.g. Velocity distributions of dM (inactive, older)

    and dMe (active, younger)


    A kinematic conundrum 2

    A kinematic conundrum (2)

    Stellar kinematics are usually modelled as Gaussian distributions

     (s(U), s(V), s(W) )

    But disk kinematics are more complex:

     use probability plots

    Composite in V

    2 Gaussian components in (U, W)

    local number ratio high:low ~ 1:10

    thick disk and old disk?


    A kinematic conundrum 3

    A kinematic conundrum (3)

    Kinematics of ultracool dwarfs (M7  L0)

    Hires data for 35 dwarfs

    ~50% trig/50% photo parallaxes

    Proper motions for all

     (U, V, W) velocities

    We expect the sample to be dominated by long-lived

    low-mass stars – although there is at least one BD


    A kinematic conundrum 4

    A kinematic conundrum (4)

    Ultracool M dwarfs have kinematic properties

    matching M0-M5 dMe dwarfs

     t ~ 2-3 Gyrs

    Does this make sense?

    M7  L0

    ~2600  2100K

    Where are the old

    V LM stars?


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