1 A B Models and frequencies for frequencies for α Cen α Cen & Josefina Montalbán & Andrea Miglio...
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Transcript of 1 A B Models and frequencies for frequencies for α Cen α Cen & Josefina Montalbán & Andrea Miglio...
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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)