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1 A B Models and frequencies for frequencies for α Cen α Cen & Josefina Montalbán & Andrea Miglio Institut d’Astrophysique et de Géophysique de Liège Belgian Asteroseismology Group

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2 1. Cen: observarional dalta M/M ± A B ± ± ± ± ± ± ± 50 R/R L/L T eff 0.25 ± ± 0.03 [Fe/H] Kervella et al Bigot et al Neuforge & Magain Eggenberger et al Pourbaix et al Neuforge & Magain ± 1.2 mas Söderhjelm 1999 mV P rot (d) vsin i 0.0 ± / ± ± ± ± 0.8 Jay et al Saar & Osten 1997 SpT G2V K1V

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3 α Cen A α Cen B Courtesy of J.Christensen-Dalsgaard α Centauri AB Theory predicts solar-like oscillations (high order p-modes excited by convection) for Kjeldsen & Bedding 1995 A: B: A osc = 32 cm/s max = 2.24 mHz A osc = 12.5 cm/s max = 4.01 mHz

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4 1. Observarional dalta Spectroscopic detection of Solar-like oscillations in both components CORALIE, at 1.2m Telesc. Bouchy & Carrier (2002) A&A nights; = 1.5 m/s, 0.96 Hz, 1.3 Hz 28 p-modes : = 1.8 – 2.9 mHz A = 12 – 44 cm/s amp = 4.3 cm/s = Hz and = 5.6 Hz Cen A: Two-site observations, UVES (8m Telesc) and UCLES (4m Tele.) Bedding et al. (2004) ApJ d ; 42 p-modes: = 2.02 – 2.97 mHz A = 6 – 40 cm/s amp = 2 cm/s = Hz

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5 α Cen B 12 p-modes = mHz Carrier & Bourban (2003) A ~ cm/s amp = 3.75 cm/s Hz shifted freq. BUT = Hz = 8.7 Hz. (ONLY two points)!

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6 α Cen B Kjeldsen et al. (2005) 38 p-modes = Hz = Hz. amp = 1.39 cm/s Freq. resolution: FWHM = 1.44 mHz Modes lifetime: 3.3d at 3.6 mHz 1.9d at 4.6 mHz

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7 Properties of high-order p-modes p-mode frequencies In the asymptotic approximation: Tassoul (1980) ApJS 43, Smeyers et al (1996) A&A 301 constant frequency spacing Information from stellar center

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8 No surface effects

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9 Modelling Cen AB Dependence of “best model” on Constraints included in fitting procedure Parameters considered in the modelling Fitting procedure Efficient & objective reliable confidence intervals Levenberg-Marquardt minimization algorithm “physics” included in the stellar evolution code Miglio & Montalbán (2005) Brown et al ApJ 427

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10 17 Calibrations

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11 17 Calibrations Convection treatment: MLT and FST; overshooting

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12 17 Calibrations Convection treatment: MLT and FST Microscopic diffusion Convection treatment: MLT and FST; overshooting

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13 17 Calibrations Convection treatment: MLT and FST Microscopic diffusion Equation of State Convection treatment: MLT and FST; overshooting Equation of State: CEFF / OPAL96

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14 17 Calibrations Convection treatment: MLT and FST Microscopic diffusion Equation of State: CEFF / OPAL96 Convection treatment: MLT and FST; overshooting

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15 Results of the calibrations HR Diagram A B

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16 if radii fitted if radii fitted too high Seismic Observables

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17 Calibrations with biased by low value of Carrier & Bourban (2003) Calibration with r 02 A Kjeldsen & Bedding (2005) Observational value

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18 Results of the calibrations MBMB MAMA RARA RBRB δν A δν B Δν A Δν B Y0Y0 Age αAαA Z0Z0 αBαB Kjeldsen et al,(2005)

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19 A “perfect” agreement not to be sought* - Freq shift - Inaccurate radii Bigot et al. (2005) Kjeldsen et al. (2005) R B /R = 0.863±0.003 *unless “surface effects” taken into account ν B 11.57μHz higher General result New observations! B Preliminary results: A: B:R M 1.1 σ ν 12 -> 20 μHz

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20 Frequencies

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21 Clearer indicator ! model A4 rejected Current data not in favor of a c.core in α Cen A Models with overshooting

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22 Much more precise seismic data needed! eos no diffusion Different envelope He A=B Input physics

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23 Partial conclusions 1.Fundamental stellar parameters do not depend on the treatment of convection: FST MLT. Frequencies slightly better with FST 2.The age of the system slightly depends on the inclusion of gravitational settling and is biased by the small frequency separation of component B 3.Internal structure is better constrained by Roxburgh & Vorontsov’s separation ratios BUT more precise frequencies are needed. Present error bars are too large 4.The effects of EoS, Diffusion, solar mixture cannot be detected with present seismic data

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24 Siamois: performances at Dôme C Performances photon noise limited : SIAMOIS, at Dôme C, 40-cm telescope, 120 hours with duty cycle of 95%, mV = 4 ‘‘SNR’’ for observable circumpolar targets CenA 37cm/s CenB 12cm/s

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25 CenA CenB CenA 15 days with SIAMOIS Formal frequency resolution: A ~ 2.5dFWHM ~ 1.67 Hz Mode lifetime: B ~ d FWHM ~ Hz Rotational splitting: amp = 2.4 cm/s 1./T obs ~0.77 Hz A ~ 0.5 Hz B ~ 0.3 Hz

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26 90 days with SIAMOIS CenA CenB CenA Formal frequency resolution: amp = 1 cm/s 1./T obs ~0.12 Hz Rotational splitting

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27 HeII and BCZ location A step variation of sound speed (c) leads to oscillations in seismic observable parameters (e.g. Gough 1990): HeII ionization zone or at the bottom of convective envelope (BCZ) 70% for aCen A observed During 90d. Ballot et al. 2004

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28 Diffusion/EoS He in CZ CenA with Diffusion CenA NO Diffusion R cz /R A =0.708 Y s =0.242 Y 0 =0.284 R cz /R A =0.725 Y s =0.270 Y 0 =0.270 Diffusion has two effects : 1.change He content in envelope depth of CZ

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29 Conclusions 90d observations : Resolve rotational splitting Resolve lorentzian profile of modes Extract reliable information on the HeII ionization zone : observations of ~ 85d may be sufficient (Verner et al. 2006) BUT More than 150d are needed to locate the bottom of the convective (Ballot et al. 2004,Verner et al. 2006) 15d observations : Huge number of detections with high S/N Resolve lorentzian profile of modes ?? inversion c(r)

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