Transcript On the Frequency of Giant Planets in the Metal

The Planet-Host Star Connection:
Probing Structural and Evolutionary
Properties of Exoplanets
Alessandro Sozzetti
(INAF-OATo, CfA)
SAIT 2008 – Teramo, 8/5/2008
The Cabal
Cabal: a secret political clique or faction
• M.G. Lattanzi, A. Spagna, R. Pannunzio, R. Morbidelli
(INAF-OATo)
• S. Casertano (STScI)
• D. Pourbaix, S. Jancart (ULB)
• D. Queloz (Geneva)
• D.W. Latham, G. Torres, D. Charbonneau, M.J. Holman,
S. Korzennik (CfA)
• J.N. Winn (MIT)
• F.T. O’Donovan (Caltech)
• At least a dozen others…
SAIT 2008 – Teramo, 8/5/2008
The Age of
Comparative Exoplanetology
Statistics of planetary systems reveal many surprises, such as:
 Orbital elements distributions
 Correlations among planet properties and stellar host characteristics
In order to discriminate between:
•
•
•
•
Competing planet formation theories
Orbital migration models
Dynamical interactions mechanisms
Planet interiors and atmospheres models
One should:
Carry out experiments specifically designed to test theoretical models!
The comparison is hampered by biased samples, small number
statistics & uncertainties in stellar/planetary parameters.
SAIT 2008 – Teramo, 8/5/2008
The Planet-Host Star Connection:
Testing Planet Formation Models
SAIT 2008 – Teramo, 8/5/2008
Core Accretion & Disk Instability
* Core Accretion: Bottom-Up!
Accumulate a 10 M core (dust to
planetesimals to runaway accretion),
which accretes a massive gaseous
envelope from the disk.
* Disk Instability: Top-Down!
Local gravitational collapse of a
gaseous portion of the disk leads to a
Jupiter-mass (or larger) protoplanet.
The rocky core is formed almost
simultaneously by sedimentation of
dust grains to the center.
Boss (SSRv, 2005)
SAIT 2008 – Teramo, 8/5/2008
The Fp – [Fe/H] Relation
Do giant planets form by Core Accretion, Disk Instability, or both?
Ida & Lin (ApJ, 2004),
Kornet et al. (A&A, 2006):
“The probability of forming
gas giant planets by core
accretion is roughly a
linear function of Z”
Boss (ApJL, 2002):
“The probability of forming gas
giant planets by disk instability
is remarkably insensitive to Z”
N/A
SAIT 2008 – Teramo, 8/5/2008
The Fp-M* Relation
Ida & Lin 2005:
“Giant Planets are more likely
To form around massive stars”
Kennedy & Kenyon 2007:
“We fully agree with them”
Boss 2006: “I can form them anyways, but have no plots to show”
SAIT 2008 – Teramo, 8/5/2008
The Mp-M* Relation
Laughlin et al. 2004:
“Only small planets can
form around M dwarfs”
Boss 2006:
“I can form them anyways,
but have no plots to show”
Ida & Lin 2005:
“We fully agree with them”
SAIT 2008 – Teramo, 8/5/2008
Fp vs [Fe/H] revisited
Fischer & Valenti 2005
Fp  102.0[ Fe/ H ]
?
Sozzetti et al. (ApJ, 2008):
K > 100 m/s, P < 3 yr, -1.0<[Fe/H]<0.5:
SAIT 2008 – Teramo, 8/5/2008
Fp  102.0[ Fe/ H ]  C ?
Doppler Surveys:
Massive/Evolved/Young Hosts
Johnson et al. 2007
Lovis & Mayor 2007
Combined database: a few hundred stars
SAIT 2008 – Teramo, 8/5/2008
Setiawan et al. 2008
The Gaia Legacy
How do Planet Properties and Frequencies Depend
Upon the Characteristics of the Parent Stars?
Johnson 2007
Gaia:
an unbiased, magnitude-limited
astrometric giant planet survey
of 3x105 stars within < 200 pc
?
Casertano et al. (A&A 2008)
Gaia will test the fine structure of giant planet
parameters distributions and frequencies, and
investigate their possible changes as a function
of stellar mass, metallicity, and age with
unprecedented resolution
104 stars per 0.1 MSun bin!
SAIT 2008 – Teramo, 8/5/2008
The Planet-Host Star Connection:
Testing Planet Structural Models
SAIT 2008 – Teramo, 8/5/2008
Irradiated Giant Planets
THEORETICAL INPUT
- internal properties
- structure and heat content
- atmospheric properties
OBSERVABLES
Evolutionary properties as a function
of irradiation conditions and orbital
distance:
- mass,
- radius,
- temperature,
- age,
- emergent spectrum
SAIT 2008 – Teramo, 8/5/2008
The Mp-Rp Relation
Coreless??
Transiting planets
come in many flavors
What are their
actual interiors?
How did they form?
Very large core?
Roughly OK
SAIT 2008 – Teramo, 8/5/2008
Default models
have trouble!
The Mc – [Fe/H] Connection
Do inferred exoplanets core masses depend on metallicity?
Burrows et al. (ApJ, 2007):
“The core mass of transiting
planets scales linearly
(or more) with [Fe/H]”
?
Guillot et al. (A&A, 2006):
“The heavy element content
of transiting extrasolar planets
should be a steep function
of stellar metallicity”
SAIT 2008 – Teramo, 8/5/2008
?
Improving R*, M*, Rp , Mp
TrES-2
1<t<9 Gyr
[Fe/H] = -0.15
The uncertainty on R* is several times smaller if,
together with the spectroscopic Teff estimate, the
photometrically measured a/R* is used instead of
the spectroscopically determined log(g)
By
1)
2)
3)
combining:
stellar properties,
spectroscopic mass function,
light-curve parameters
One obtains improved values for:
1) planet radius,
2) planet mass,
3) planet gravity
Sozzetti et al. (ApJ, 2007)
SAIT 2008 – Teramo, 8/5/2008
Not So Simple a Picture…
TrES-3:
Sozzetti et al. (ApJ, 2008)
TrES-4:
Parent star has [Fe/H]= -0.19±0.08
Parent star has [Fe/H]= +0.14±0.09
Mc = 0-30 ME
Mc = 0 ME
For at least 4 metal-rich stars (HD 209458, OGLE-TR-56, OGLE-TR-132, and
WASP-1) very similar to TrES-4, their transiting planets have radii so large
that even coreless models underestimate their sizes.
Simply connecting the host star's characteristics to the structural
properties of transiting planets may in fact be an over-simplification
SAIT 2008 – Teramo, 8/5/2008
Space-based Searches for
Transiting Rocky Exoplanets
SAIT 2008 – Teramo, 8/5/2008
The M Dwarf Opportunity
SAIT 2008 – Teramo, 8/5/2008
Why M dwarfs?
M-dwarfs are very attractive targets for simple reasons:
–they are the most numerous
–the low primary masses imply that very low-mass secondaries may be detected by
Doppler surveys
–transits by rocky planets can be detected with relatively modest precision
–transiting objects will have short periods but will lie in the habitable zone
–the flux contrast ratio is much more favorable for direct spectroscopic
investigation of the secondary eclipse
•May require the development of precise IR-based Doppler-techniques
•The detection of the first habitable terrestrial planet may likely be for a shortorbital period planet orbiting a late M-dwarf
•Spectroscopic investigation would be a much more humble undertaking than that
envisioned by TPF-(C/I)/Darwin, and could be pursued through, e.g., NASA’s
James Webb Space Telescope
A targeted ground-based transit search around the nearest
2000 M-dwarfs is not unreasonable as a detection method
SAIT 2008 – Teramo, 8/5/2008