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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 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? • 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, 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 • 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 • 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 • 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 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 • 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: 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 • 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 • 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 • 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 • 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: 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 • 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 r ~ 2 x 10^-4 stars/pc^3 • High-velocity disk white dwarfs can account for the observed r
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 • 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 • 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 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 White dwarfs can have brown dwarf companions
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 (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) 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 t ~ 2-3 Gyrs Does this make sense? M7 L0 ~2600 2100K Where are the old V LM stars?