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Extrasolar planets: a Galactic perspective

Extrasolar planets: a Galactic perspective. I. Neill Reid STScI. The questions. Over 150 extrasolar planets have been discovered since 1995 -this includes several multiplanet systems

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Extrasolar planets: a Galactic perspective

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  1. Extrasolar planets: a Galactic perspective I. Neill Reid STScI

  2. STScI 2005 May Symposium The questions • Over 150 extrasolar planets have been discovered since 1995 • -this includes several multiplanet systems • 1. Are there any properties (besides [m/H]) that set the parent stars apart from the average disk star? • 2. Given the statistical properties of the parent stars, coupled with our current knowledge of Galactic structure, • How common are planetary systems in the Milky Way?

  3. STScI 2005 May Symposium Outline • The Extrasolar Planetary Systems • Resolved systems? • The host stars • Local stellar populations • Kinematics and planets • Filling the Galaxy • Summary and conclusions See also papers by the Geneva group (Udry et al; Santos et al; Halbwachs, Mayor & Udry; Bodaghee et al)

  4. STScI 2005 May Symposium The planets • ~143 planets in conventional systems: • ~17MJ > M > ~0.067MJ • at least 17 multi-planet systems • 1.2 days < P < 8 years • 0.015 AU < a < 4.2 AU • Most systems have high eccentricity orbits • How do we know that these are really planets? • Brown dwarf/M-dwarf desert Both low-mass stars and brown dwarfs are extremely rare as close (a < 10 AU) companions of solar-type stars Solar-type stars at d < 25 pc

  5. STScI 2005 May Symposium 2M1207 & GQ Lupi TW Hydrae member –t ~ 10 Myrs MP ~ 35 MJ , MS ~ 2-5 MJ D ~ 60 AU Lupus I member –t ~ 1 Myrs MP ~ 0.45 M⊙, MS ~ 3-40 MJ D ~ 100 AU

  6. STScI 2005 May Symposium Brown dwarfs or exoplanets? • Both companions lie in the outer regions of the disk – even for 1.7 M⊙b Pic • 2M 1207A/B has high mass ratio, q ~ 0.2 • Both are more likely to be brown dwarf companions than exoplanets – can provide crucial insight on BD binary formation

  7. STScI 2005 May Symposium The planetary hosts Most hosts are late-F, G or early-K main-sequence stars – exceptions: 2 M dwarfs ~10 giants ~8 subgiants 128 from RV surveys 1 microlensing 6 transit surveys

  8. STScI 2005 May Symposium Completeness • Valenti/Fisher (Keck) sample matched against Hipparcos dataset • ~45% complete to 25pc for solar-type stars (4 < MV < 6) Most of the `missing’ stars are included in the Geneva sample

  9. STScI 2005 May Symposium The host stars: distances Overwhelming majority lie within 50 pc  bright stars from radial velocity surveys. Stars are drawn from local representatives of the Galactic stellar populations: • Disk • Thick disk • Halo • But not the Bulge ….

  10. STScI 2005 May Symposium The inner & outer halos • Inner halo forms through rapid ELS-style collapse of the proto-Galactic cloud at t ~11-13 Gyrs Outer halo forms  through subsequent (and continuing) accretion of satellite galaxies Old, non-rotating, metal-poor ( [m/H] < -1) population Local density of halo stars ~ 2.6 x 10-4 stars pc-3 , or 1:400 relative to the disk ~60% of local subdwarfs contributed by inner halo No known planetary systems

  11. STScI 2005 May Symposium The thick disk: densities Flattened, rotating, mildly metal-poor population, identified from analysis of starcounts perpendicular to the Plane. Some ambiguity in deriving r0 & z0, but current analyses favour z0~900 pc & r0 ~ 1.0 x 10-2 stars pc-3 , or 1:10 relative to the disk

  12. STScI 2005 May Symposium Thick disk: kinematics & [m/H] • Joint kinematic/abundance analyses by Fuhrmann, Prochaska, Bernsby & others indicate that thick disk stars have enhanced [a/Fe] • Limited enrichment from Type I SN • Origin in short-lived (1-2 Gyr) star-forming episode Current concensus favours formation through disruption and inflation of the initial Galactic disk by a major merger • Hiatus in star formation • Addition of low [m/H[ gas • Thin disk reforms

  13. STScI 2005 May Symposium Planets from the thick disk? [Ti], [Fe] from Valenti & Fisher (2005) and Bodaghee et al (2003) 3 TD candidates: HD 6434 (0.48 MJ), HD 37124 (0.75 MJ), HD 114762 (11 MJ) 3 intermediate: HD 114729 (0.82 MJ), r CrB (1.04 MJ), HD 168746 (0.23 MJ)

  14. STScI 2005 May Symposium The disk Haywood (2002) Flattened, rotating population with r0 ~ 9.0 x 10-2 stars pc-3 (90% of SN) Double exponential density law: z0~300 pc, h0~2,500 pc Substantial dispersion in [m/H] at any given age – indication of broad age-metallicity relation Valenti & Fisher (2005)

  15. STScI 2005 May Symposium Host star properties • Clear correlation between [m/H] and planetary frequency • No obvious correlation between M*and MP , (although Gl 836 & 876 have low-mass planets). • No obvious correlation between [M/H] and MP or orbital properties (a, e) • How about the kinematics of the (sub-)sample?

  16. STScI 2005 May Symposium Stellar kinematics • Generally characterised as Schwarzschild velocity ellipsoid, with Gaussian dispersions in cardinal directions: U, V, W • Dispersions (sU,, sV,, sW,, stot), are expected to increase with age: stot a t1/3 (diffusion theory) • A composite population – use probability plots Cumulative velocity distribution as a function of inverse probability: Gaussian  straight line 2 Gaussians  3 line segments

  17. STScI 2005 May Symposium Host star kinematics field hosts U -9.6 -4.0 V -20.2 -25.5 W -7.6 -20.4 sU 38.8 37.7 sV 31.0 22.9 sW 17.7 20.4 sTot 52.7 48.6 Planetary hosts are representative of the local field stars

  18. STScI 2005 May Symposium Kinematics & sampling Even though the hosts reside in the Solar Neighbourhood now, they are likely drawn from R0±1.5 kpc

  19. STScI 2005 May Symposium And the thick disk All three thick-disk candidates have significant motions w.r.t. the Sun – notably HD 37124 & HD 114762

  20. STScI 2005 May Symposium Kinematics and planetary properties No obvious correlations between space motions and planetary characteristics (cf. Santos et al, 2003).

  21. STScI 2005 May Symposium Corotation and planets Is the Sun’s near-LSR velocity important? (the issue of long-term habitability)  apparently not, at least for planet formation

  22. STScI 2005 May Symposium Covering the Galaxy Planetary formation apparently depends only on stellar metallicity. Most planets are likely to be associated with thin disk stars  Predicting the overall frequency throughout the Galaxy requires: • Thin disk density distribution: • r(R) =r0 e-(R-R0)/h e-z/z0 where h ~ 2500 pc, z0 ~ 300 pc • The abundance distribution as f(R) : • both <[m/H]> and N ([m/H])

  23. STScI 2005 May Symposium Radial abundance gradients Data for HII regions (Shaver et al, 1983) and Cepheids (Andrievsky et al, 2002) suggest a broad plateau in <[m/H]> from 6-10 kpc; mild decline at >10 kpc; steep rise at R< 6 kpc. These are all relatively young tracers – what about the older stars?

  24. STScI 2005 May Symposium Abundance distributions • SN – Haywood (2002) Bulge  Ferreras, Wyse & Silk (2003) How metal-rich is the underlying older population in the inner disk?

  25. STScI 2005 May Symposium Counting planets • As a partial estimate: • limit analysis to 6 < R < 10 kpc • assume the Solar Neighbourhood metallicity distribution • adopt r(R) =r0 e-(R-R0)/h e-z/z0, where h ~ 2500 pc, z0 ~ 300 pc • use the nearby-star luminosity function to set the density • normalisation of solar-type stars • 4 < MV < 6  4.4 x 10-3 stars pc-3  2.74 stars pc-2 • assign 90% to disk; 10% to thick disk

  26. STScI 2005 May Symposium Planets in the Solar Ring • Density wins over area  NP decreases with R • Numbers compensate for frequency • solar-metallicity systems are almost as common as metal-rich systems In total, expect NP ~ 3.5 x 107 (6%) for 6 < R < 10 kpc a < 4 AU, M < ~1MJ

  27. STScI 2005 May Symposium Are there carbon planets in the inner Galaxy? • Hypothesis: if C/O > 1, CO binds [O], preventing silicate formation • Carbides dominate to give C-rich planet (Kuchner & Seagar) C originates in intermediate-mass stars (AGB) and high mass WC stars (~equal proportions)  C/O increases with [Fe/H] (time) • [C] = 8.39, [O] = 8.66 •  Require [C/O] ~ +0.3, suggesting [Fe/H] > 0.4 Gustafsson et al, 1999 – fine analyses of nearby FG stars

  28. STScI 2005 May Symposium Summary • Planetary host stars are remarkably unremarkable (once one allows for the preference for high [m/H]) • Several of the known systems are probably members of the thick disk • Integrating planetary frequency across the Galaxy is currently limited by our knowledge of the abundance distribution in the inner and outer Galaxy – but there are likely >n x 107 “RV-detectable” systems in the 6 to 10 kpc Solar Ring.

  29. STScI 2005 May Symposium Never mind the planets, where’s the food?

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