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On the Frequency of Gas Giant Planets in the Metal-Poor Regime

On the Frequency of Gas Giant Planets in the Metal-Poor Regime. Alessandro Sozzetti 1 , D.W. Latham 2 , G. Torres 2 , R.P. Stefanik 2 , S.G. Korzennik 2 , A.P. Boss 3 , B.W. Carney 4 , J.B. Laird 5 (1) INAF/OATo - (2) CfA - (3) CIW - (4) UNC - (5) BGSU.

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On the Frequency of Gas Giant Planets in the Metal-Poor Regime

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  1. On the Frequency of Gas Giant Planets in the Metal-Poor Regime Alessandro Sozzetti1, D.W. Latham2, G. Torres2, R.P. Stefanik2, S.G. Korzennik2, A.P. Boss3, B.W. Carney4, J.B. Laird5 (1) INAF/OATo - (2) CfA - (3) CIW - (4) UNC - (5) BGSU

  2. 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)

  3. Do giant planets form by Core Accretion, Disk Instability, or both? Ida & Lin (ApJ, 2004), Kornet et al. (A&A, 2006): “The frequency of giant planet formation by core accretion is roughly a linear function of Z” Boss (ApJL, 2002): “The frequency of giant planet formation by disk instability is remarkably insensitive to Z” N/A

  4. Globular Cluster 47 Tucanae HST (WFPC2) observed about 34,000 stars in 47 Tuc, obtaining time series photometry over a period of 8.3 days 11 Gyr, 106 stars [Fe/H] ~ - 0.7 HST/WFPC2 No planet eclipses were seen. Gilliland et. al. (ApJ 2000), Weldrake et al. (ApJ 2005) DSS

  5. However… • Crowding can impact giant planet formation, migration, and survival • The absence of Hot Jupiters in a metal-poor environment does not imply they don’t exist at larger radii GCs are not optimal Go to the field

  6. Fp vs [Fe/H] Linear Dependence? Flat tail for [Fe/H] < 0.0? Low statistics for [Fe/H] < -0.5 Quadratic dependence? Flat tail for [Fe/H]<0.0? Low statistics for [Fe/H] < -0.5 Fischer & Valenti (ApJ, 2005): K > 30 m/s, P < 4 yr, -0.5<[Fe/H]<0.5: Santos et al. (A&A, 2004): No P, K, [Fe/H] thresholds: Fp ~ Z, for Z > 0.02) Fp ~ const, for Z < 0.02

  7. Small-number statistics for [Fe/H] < -0.5 prevents one from drawing conclusions: Is Fp([Fe/H]) bimodal or monotonic? What is the dominant mode of giant planet formation?

  8. Keck/HIRES Metal-Poor Planet Search • 200 stars from the Carney-Latham and Ryan samples • No close stellar companions • Cut-offs: -2.0 < [Fe/H] < -0.6, Teff < 6000 K, V < 12 • Reconnaissance for gas giant planets within 2 AU • Campaign duration: 3 years Sozzetti et al. (ApJ, 2006)

  9. The RV dispersion of the full sample peaks at 9 m/s Sozzetti et al. (ApJ, 2006)

  10. No clear RV trends are seen as a function of Teff, [Fe/H], and ΔT

  11. Analysis: Methodology • Statistical analysis: testing for excess variability (F-test, 2-test, Kuiper test) • Analysis of long-term (linear and curved) trends • Limits on companion mass and period from detailed simulations • Upper limits on fp and new powerful constraints on fp([Fe/H]) in the metal-poor regime

  12. RV Variables Follow-up Follow-up with direct infrared imaging (MMT/Clio) to determine their nature (low-mass stars or brown dwarfs) About 6% of the stars in the sample have long-period companions

  13. MMT/Clio Imaging @ 5 μm ~1” ~0.5” ΔM ~ 2.5 mag ΔM ~ 6.5 mag

  14. COMPLETENESS: • 6 observations, 3-yr baseline; • sRV = 9 m/s • 99.5% confidence level • Sensitivity to companions with • 1MJ<Mpsin(i)<6MJ (K > 100 m/s), • with orbital periods between • a few days and 3 years • - Strong dependence of detection • thresholds on eccentricity WE FIND NONE…

  15. Frequency of Close-in Companions(-2.0<[Fe/H]<-0.6, K > 100 m/s, P < 3 yr, e < 0.3) For n=0, N=160: For n=1, N=160:

  16. b=0.99 Reliability of the Atmospheric Parameters Compare with the SPOCS database: b=1.05 (σ = 134 K) (σ = 0.12 dex) b=0.89 (σ = 0.06 MSUN)

  17. Sozzetti et al. 2009 (ApJ, in press): K > 100 m/s, P < 3 yr, -1.0<[Fe/H]<0.5:

  18. Summary • We observe a dearth of gas giant planets (K > 100 m/s) within 2 AU of metal-poor stars (-2.0 < [Fe/H] < -0.6), confirming and extending previous findings • The resulting average planet frequency is Fp< 0.67% (1σ) • Fp(-1.0<[Fe/H]<-0.5) appears to be a factor of several lower than Fp([Fe/H]>0.0), but it’s indistinguishable from Fp(-0.5<[Fe/H]<0.0). • Is Fp([Fe/H]) bimodal or not? It is consistent with being so. However, need larger and better statistics to really discriminate… • 1) Expand the sample size; 2) lower the mass sensitivity threshold; 3) search at longer periods. Next generation RV surveys and future high-precision space-borne astrometric observatories (Gaia, SIM-Lite) will help…

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