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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