Transcript Reid_may

Extrasolar planets: a
Galactic perspective
I. Neill Reid
STScI
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?
STScI 2005 May Symposium
Outline
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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)
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
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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
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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
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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
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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
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:
1.
2.
3.
Disk
Thick disk
Halo
But not the Bulge ….
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The inner & outer halos
Inner halo forms
through rapid ELS-style
collapse of the protoGalactic 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
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
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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
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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)
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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)
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Host star properties
1.
2.
3.
Clear correlation between
[m/H] and planetary
frequency
No obvious correlation
between M* and MP ,
(although Gl 436 & 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?
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Stellar kinematics
1. Generally characterised as Schwarzschild velocity ellipsoid,
with Gaussian dispersions in cardinal directions: U, V, W
2. Dispersions (sU,, sV,, sW,, stot), are expected to increase with
age:
stot a t1/3 (diffusion theory)
3. A composite population – use probability plots
Cumulative velocity
distribution as a function
of inverse probability:
Gaussian  straight line
2 Gaussians  3 line
segments
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Host star kinematics
U
V
W
sU
sV
sW
sTot
field
-9.6
-20.2
-7.6
38.8
31.0
17.7
52.7
hosts
-4.0
-25.5
-20.4
37.7
22.9
20.4
48.6
Planetary hosts are
representative of the
local field stars
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Kinematics & sampling
Even though the hosts reside in the
Solar Neighbourhood now, they are
likely drawn from R0±1.5 kpc
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And the thick disk
All three thick-disk
candidates have
significant motions
w.r.t. the Sun –
notably HD 37124 &
HD 114762
Kinematics and planetary
properties
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No obvious correlations
between space motions
and planetary
characteristics
(cf. Santos et al, 2003).
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Corotation and planets
Is the Sun’s near-LSR
velocity important? (the
issue of long-term
habitability)
 apparently not, at
least for planet
formation
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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:
1. Thin disk density distribution:
r(R) = r0 e-(R-R0)/h e-z/z0 where h ~ 2500 pc, z0 ~ 300 pc
2. The abundance distribution as f(R) :
both <[m/H]> and N ([m/H])
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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?
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Abundance distributions
 SN –
Haywood
(2002)
Bulge 
Ferreras,
Wyse & Silk
(2003)
How metal-rich is the underlying
older population in the inner disk?
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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
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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
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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
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Summary
1. Planetary host stars are remarkably unremarkable
(once one allows for the preference for high [m/H])
2. Several of the known systems are probably
members of the thick disk
3. 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 >3.5 x 107 “RV-detectable” systems in the
6 to 10 kpc Solar Ring.
Never mind the planets,
where’s the food?
STScI 2005 May Symposium