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AB

Models and Models and frequencies forfrequencies for αα CenCen

&Josefina Montalbán & Andrea MiglioInstitut d’Astrophysique et de Géophysique de LiègeBelgian Asteroseismology Group

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1. Cen: observarional dalta

M/M 0.934 ± 0.006

A B

1.105 ± 0.007

1.522 ± 0.030

5810 ± 50

1.224 ± 0.003 0.863 ± 0.005

0.503 ± 0.020

5260 ± 50

R/R

L/L

Teff

0.25 ± 0.02 0.24 ± 0.03[Fe/H] Kervella et al. 2003Bigot et al. 2005

Neuforge & Magain. 1997

Eggenberger et al. 2004

Pourbaix et al. 2002

Neuforge & Magain. 1997

747.1 ± 1.2 mas Söderhjelm 1999

mVProt(d)

vsin i

0.0 ± 0.003

23 +5/-2

2.7 ± 0.7

1.33 ± 0.003

36.9 ± 0.5

1.1 ± 0.8

Jay et al. 1996

Saar & Osten 1997

SpT G2V K1V

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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:

Aosc = 32 cm/s max = 2.24 mHz

Aosc = 12.5 cm/s max = 4.01 mHz

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1. Observarional daltaSpectroscopic detection of Solar-like

oscillations in both components

CORALIE, at 1.2m Telesc. Bouchy & Carrier (2002) A&A 390 13 nights; = 1.5 m/s, 0.96Hz, 1.3 Hz28 p-modes : = 1.8 – 2.9 mHz A = 12 – 44 cm/s amp= 4.3 cm/s <> = 105.5 Hz and <> = 5.6 Hz

Cen A:

Two-site observations, UVES (8m Telesc) and UCLES (4m Tele.) Bedding et al. (2004) ApJ 6144.6d ; 42 p-modes: = 2.02 – 2.97 mHz A = 6 – 40 cm/s amp= 2 cm/s <> = 106.2 Hz

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α Cen B

12 p-modes

= 3 - 4.6 mHz

Carrier & Bourban (2003)

A ~ 8 - 13 cm/s amp= 3.75 cm/s

11.57 Hz shifted freq.

BUT

<> = 161.1 Hz<> = 8.7 Hz.(ONLY two points)!

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α Cen BKjeldsen et al. (2005)

38 p-modes

<> = 161.3 Hz <> = 10.14 Hz.

amp= 1.39 cm/s

Freq. resolution: FWHM = 1.44 mHz

Modes lifetime:3.3d at 3.6 mHz1.9d at 4.6 mHz

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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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No surface effects

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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. 1994 ApJ 427

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17 Calibrations

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17 Calibrations• Convection treatment: MLT and FST; overshooting

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17 Calibrations• Convection treatment: MLT and FST

• Microscopic diffusion

• Convection treatment: MLT and FST; overshooting

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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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17 Calibrations• Convection treatment: MLT and FST

• Microscopic diffusion

• Equation of State: CEFF / OPAL96

• Convection treatment: MLT and FST; overshooting

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Results of the calibrations

HR Diagram

A

B

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if radii fittedif radii fitted too hightoo high

Seismic Observables

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Calibrations with

biased by low value of

Carrier & Bourban (2003)

Calibration with r02 A

Kjeldsen & Bedding (2005)

Observational value

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Results of the calibrations

MB

MA

RA

RB

δνA

δνB

ΔνA

ΔνB

Y0

Age

αA

Z0

αB

Kjeldsen et al,(2005)

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A

“perfect” agreement not to be sought*

- Freq shift- Inaccurate radii

Bigot et al. (2005)

Kjeldsen et al. (2005)

RB/R=0.863±0.003

*unless “surface effects” taken into account

νB 11.57μHz higher

General result

New observations!

B

Preliminary results:

A: B: R

M 1.1 σν 12 -> 20 μHz

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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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Much more precise seismic data needed!

eos

no diffusion

Different envelope He

A=B

Input physics

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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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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/sCenB12cm/s

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CenA

CenB

CenA

15 days with SIAMOIS

Formal frequency resolution:

A ~ 2.5d FWHM ~ 1.67 Hz

Mode lifetime:

B ~ 1.9-3.3d

FWHM ~ 1.12-1.94 Hz

Rotational splitting:

amp= 2.4 cm/s

1./Tobs~0.77 Hz

A ~ 0.5 Hz

B ~ 0.3 Hz

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90 days with SIAMOIS

CenA

CenB

CenA

Formal frequency resolution:

amp= 1 cm/s

1./Tobs~0.12 Hz

Rotational splitting

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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 observedDuring 90d.

Ballot et al. 2004

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Diffusion/EoS

• He in CZ

CenA with Diffusion

CenA NO Diffusion

Rcz/RA=0.708Ys=0.242Y0=0.284

Rcz/RA=0.725Ys=0.270Y0=0.270

Diffusion has two effects : 1. change He content in envelope2. depth of CZ

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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)