A

1
Models and frequencies for a Cen
A

B
Josefina Montalbán Andrea Miglio
Institut dAstrophysique et de Géophysique de
Liège Belgian Asteroseismology Group
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1. aCen observarional dalta
A
B
p
747.1 1.2 mas
Söderhjelm 1999
M/M?
0.934 0.006
1.105 0.007
Pourbaix et al. 2002
L/L?
1.522 0.030
0.503 0.020
Eggenberger et al. 2004
Teff
5810 50
5260 50
Neuforge Magain. 1997
Fe/H
0.25 0.02
0.24 0.03
Neuforge Magain. 1997
Kervella et al. 2003 Bigot et al. 2005
R/R?
1.224 0.003
0.863 0.005
mV
1.33 0.003
0.0 0.003
Prot(d)
36.9 0.5
23 5/-2
Jay et al. 1996
1.1 0.8
vsin i
Saar Osten 1997
2.7 0.7
SpT
K1V
G2V
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(No Transcript)
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1. Observarional dalta
Spectroscopic detection of Solar-like
oscillations in both components
aCen A
CORALIE, at 1.2m Telesc. Bouchy Carrier (2002)
AA 390 13 nights s 1.5 m/s, 0.96mHz, 1.3
mHz 28 p-modes n 1.8 2.9 mHz A
12 44 cm/s samp 4.3 cm/s ltDngt
105.5 mHz and ltdngt 5.6 mHz
?
?
Two-site observations, UVES (8m Telesc) and
UCLES (4m Tele.) Bedding et al. (2004) ApJ
614 4.6d 42 p-modes n 2.02 2.97 mHz
A 6 40 cm/s s amp 2 cm/s
ltDngt 106.2 mHz
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a Cen B
Carrier Bourban (2003)
12 p-modes
n 3 - 4.6 mHz
A 8 - 13 cm/s samp 3.75 cm/s
BUT
11.57 mHz shifted freq.
ltDngt 161.1 mHz ltdngt 8.7 mHz. (ONLY two
points)!
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a Cen B
Kjeldsen et al. (2005)
38 p-modes
samp 1.39 cm/s
Freq. resolution FWHM 1.44 mHz
ltDngt 161.3 mHz ltdngt 10.14 mHz.
Modes lifetime 3.3d at 3.6 mHz 1.9d at 4.6 mHz
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Properties of high-order p-modes
In the asymptotic approximation Tassoul (1980)
ApJS 43, Smeyers et al (1996) AA 301
p-mode frequencies
constant frequency spacing
Information from stellar center
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No surface effects
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Modelling aCen AB
Miglio Montalbán (2005)
physics included in the stellar evolution code
Dependence of best model on
Parameters considered in the modelling
Constraints included in fitting procedure
Brown et al. 1994 ApJ 427
Levenberg-Marquardt minimization algorithm
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17 Calibrations
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17 Calibrations

• Convection treatment MLT and FST overshooting

12
17 Calibrations

• Convection treatment MLT and FST

• Convection treatment MLT and FST overshooting

• Microscopic diffusion

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17 Calibrations

• Convection treatment MLT and FST

• Convection treatment MLT and FST overshooting

• Microscopic diffusion

• Equation of State

• Equation of State CEFF / OPAL96

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17 Calibrations

• Convection treatment MLT and FST

• Convection treatment MLT and FST overshooting

• Microscopic diffusion

• Equation of State CEFF / OPAL96

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Results of the calibrations
A
HR Diagram
B
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Seismic Observables
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Observational value
Carrier Bourban (2003)
Calibration with r02 A
Calibrations with
biased by low value of
Kjeldsen Bedding (2005)
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Results of the calibrations
Y0
d?A
MA
Kjeldsen et al,(2005)
MB
Age
d?B
aB
??A
aA
RA
RB
Z0
??B
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General result
New observations!
B
A
Bigot et al. (2005)
?
RB/R?0.8630.003
Kjeldsen et al. (2005)
?B 11.57µHz higher
A ?
B R ? M 1.1 s ? 12 -gt 20 µHz
Preliminary results
perfect agreement not to be sought
- Freq shift - Inaccurate radii
unless surface effects taken into account
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Frequencies
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Models with overshooting
!
Clearer indicator
model A4 rejected
Current data not in favor of a c.core in a Cen A
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Input physics
eos
no diffusion
Different envelope He
AB
Much more precise seismic data needed!
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Partial conclusions

• Fundamental stellar parameters do not depend on
  the treatment of convection FST MLT. Frequencies
  slightly better with FST
• The age of the system slightly depends on the
  inclusion of gravitational settling and is biased
  by the small frequency separation of component B
• Internal structure is better constrained by
  Roxburgh Vorontsovs separation ratios
• BUT more precise frequencies are needed.
• Present error bars are too large
• 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
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15 days with SIAMOIS
samp 2.4 cm/s
Formal frequency resolution
aCenA
1./Tobs0.77 mHz
aCenB
Montgomery Donoghue 1999
s (n) 0.045 mHz at A18cm/s
Mode lifetime
aCenA
tA 2.5d
FWHM 1.67 mHz
tB 1.9-3.3d
FWHM 1.12-1.94 mHz
Rotational splitting
WA 0.5 mHz
WB 0.3 mHz
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90 days with SIAMOIS
aCenA
samp 1 cm/s
Formal frequency resolution
aCenB
1./Tobs0.12 mHz
aCenA
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)
Ballot et al. 2004
HeII 1 mHz
70 for aCen A observed During 90d.
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Diffusion/EoS

• change He content in envelope
• k depth of CZ

Diffusion has two effects
aCenA NO Diffusion
aCenA with Diffusion

• He in CZ ?

Rcz/RA0.725 Ys0.270 Y00.270
Rcz/RA0.708 Ys0.242 Y00.284
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Conclusions
15d observations

• Huge number of detections with high S/N

• Resolve lorentzian profile of modes ??

90d observations

• inversion c(r)

• 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)