Astronomers in the Dark

Neill ReidKailash Sahu & Suzanne Hawley

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

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

• 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, days, <V> ~ 200 km/s

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

<M> = 0.5+/- 0.3 solar masses

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

1. MACHOs: <M> ~ 0.5 +/- 0.3 M(sun)

50% 20% of the dark halo

2. HDF proper motion objects – Ibata et al (1999)

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

3. 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

• 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

• 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 / tens of sq. degrees => Luyten’s Palomar surveys (POSSI Luyten E (1963) LHS : 0.5 arcsec/yr, m_r<19.5, > -36 NLTT : 0.18 arcsec/yr, m_rr < 19.5, => 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

• 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

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

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

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

0.33 < 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 (U, V)

Exclude stars within “disk” 2- velocity ellipsoid

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

Finding heavy halo WDs? IV

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

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

where d_max is set by

d_, the distance where 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, lim ~ 3 “/yr V_tan < 1040 km/sec

• What about the disk?…

Galactic Disk kinematics: I

• Velocity dispersions increase as a function of age

^ , = 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

• 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 , 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, > -30 95% complete – probably missing low-velocity stars

Characterising Disk kinematics: I

• Stellar kinematics are usually represented as Schwarzschild velocity ellipsoids:

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

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

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

produce a rank-ordered list, x(i)

determine <x> and std. deviation, plot x(i) vs [ (x(i) - <x>) / ]

A Gaussian distribution produces a straight line, slope combination of 2 Gaussians gives 3 line segments, slope (1), (2)

Disk kinematics: II

Disk kinematics: III

• Results from fitting the M dwarf distribution

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

• 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

• 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 ~ 2 x 10^-4 stars/pc^3

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

Halo white dwarfs? I

• What about the highest velocity white dwarfs?

• In a non-rotating system, N_prograde = N_retrograde

compute 1/V_max for 4 dwarfs with retrograde motion

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

_tot = 2 x _obs ~ 2 x 10^-5 stars / pc^3

=> expected density of halo white dwarfs

• An absence of surprises

Halo white dwarfs? II

• How about the temperature distribution? White dwarfs in a primordial, dark halo should have ~ 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

So why didn’t they….

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

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

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

Summary

• 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

Evidence for heavy halo white dwarfs

1. MACHOs --- but maybe they’re in the LMC/SMC2. HDF proper motions --- but they’re no longer moving

3. High-velocity, cool white dwarfs in the field --- not fast enough or cool enough

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A binary system

White dwarfscan have brown dwarfcompanions

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)

Stellar kinematics are usually modelled as Gaussian distributions (U), (V), (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)

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)

Ultracool M dwarfs have kinematic properties matching M0-M5 dMe dwarfs ~ 2-3 GyrsDoes this make sense?

M7 L0~2600 2100K

Where are the old V LM stars?