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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.

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Presentation on theme: "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."— Presentation transcript:

1 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

2 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

3 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

4 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

5 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)!

6 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

7 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

8 8 No surface effects

9 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

10 10 17 Calibrations

11 11 17 Calibrations Convection treatment: MLT and FST; overshooting

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

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

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

15 15 Results of the calibrations HR Diagram A B

16 16 if radii fitted if radii fitted too high Seismic Observables

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

18 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)

19 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 

20 20 Frequencies

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

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

23 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

24 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

25 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

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

27 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

28 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

29 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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