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Asteroseismology with A-STEP
The sun from the South Pole
Grec, Fossat & Pomerantz, 1980, Nature, 288, 541
Asteroseismology
Angular structure of the modes
• n = number of radial nodes
•  = total number of surface nodes
• m = number of surface nodes that are lines of
longitude
•  – m = number of surface nodes that are lines of
latitude
Dipole modes
m
1
Y
 ,   P1m cos  eim
l=1, m=0
l=1, m=+1
Quadrupole modes
Y2m  ,    P2m cos   eim
l=2, m=0
l=2, m=-1
l=2, m=-2
p modes and g modes
p modes
(n,) = (8,100), (8,2)
g mode
(n,) = (10,5)
Gough et al., 1996, Science, 272, 1281
p modes and g modes
J. P. Cox, 1980, Theory of Stellar Pulsation, Princeton University Press.
The sun as a star - BiSON
The sun as a star - GOLF
 n   0  n  2     
large separation
small separation
An asteroseismic HR diagram
Solar-like Oscillations in  Centauri
Bedding, T., et al. 2004, ApJ, 614, 380
• UVES & UCLES
• 42 oscillation frequencies
• ℓ = 1-3
• Mode lifetimes only
1-2 days
• Noise level = 2 cm s-1!
From G. Houdek
Amplitudes in velocity between 10 to 250 cm/s
Amplitudes in intensity are of the order of 1-10 ppm
roAp
HR 1217 WET Xcov20
Kurtz et al., 2005, MNRAS, 358, 651
HR 1217 WET Xcov20
Kurtz et al., 2005, MNRAS, 358, 651
p modes:  Cephei stars
HD 129929 = V836 Cen
20-yr multicolour photometry
Core
overshooting
with aOV = 0.1
Non-rigid
rotation: 4
times faster
near core
Aerts et al., 2003, Science, 300, 926
Asteroseismology of HD129929: Core overshooting and nonrigid rotation
p modes: EC 14026 stars - sdBV
PG 1336 + 018
g Dor – mixed-mode pulsators
HD 49434
White dwarfs – g-mode pulsators
BPM 37093
A giant solar-like oscillator
http://www.lcse.umn.edu/
COROT Field of view
Adaptation of COROTLUX software for variables
Input sample: Besançon model ~ 25000 stars
Pulsation in EXO FoV
 Cep : 2
SPB : 29
 Sct : 2500
g Dor : 3200
Hyb : 2200
Ceph : 1
LP : 46
Fraction of variables:
30%?
Stellar limitation
• Photon noise
• Stellar noise
• Activity
< 10-4 up to M=15
low frequencies
see S. Aigrain
Instrumental limitation
• Read-out noise
• Thermal noise
• Guiding noise
• Cosmic rays
• Gain variation
• Shutter noise
Lower than photon noise for M<16
negligible
2 p
3



5
.
10

PRNU~1%,
I
3
4
S
TBE
no data. Global/pixel
1- 2 ms -> Ti > 30 s
Atmospheric limitations of photometric observations
• Transparency fluctuations
• Interruptions (clouds)
• Scintillation
•Diffused light
No dust, low humidity, snow
< 15 %
Better than anywhere else,
but still limiting
Few auroras, moon
These values are still uncertain:
A-STEP will contribute to the site qualification
A few words about atmospheric turbulence
and scintillation
  2  2

2
r0  0.42  sec Z Cn (h)dh h 
   


3
5
sec Z  h 6 Cn 2(h)dh h
7
2
3
2
 I  19.12 D 3 sec Z  h 2Cn (h)dh h
7
 I  19.12
2

5 / 6
0
6
11
6
5
 K cos z
I
4 / 3
D 
D 

h 
h
From Dravins et al, 1997
Optical/interferometric
parameters
Integrated from h=8m
Balloons
(10)
Seeing (arcs)
1.6
t0 (ms)
7.0
0 (arcs)
5.3
Dimms
(March- May
05)
1.2
GSM h=3.5m
L0 = 10 m
h>0m
opd
3
h > 30 m 1 
3.6
Interferometric coherence times
t0 = 0.31 r0/ v ~7 ms
Integrated from h =30m
Balloons
AASTINO
2004 data
Seeing (arcs)
0.4
0.27 ‘’
t0 (ms)
11.2
7.9
0 (arcs)
5.3
5.7 ‘’
s
T0 = 0.31
L0/ v ~775 ms
Optical/interferometric
parameters
Integrated from h=8m
Balloons
(10)
Seeing (arcs)
1.6
t0 (ms)
7.0
0 (arcs)
5.3
Dimms
(March- May
05)
1.2
GSM h=3.5m
L0 = 10 m
h>0m
opd
3
h > 30 m 1 
3.6
Interferometric coherence times
t0 = 0.31 r0/ v ~7 ms
Integrated from h =30m
Balloons
AASTINO
2004 data
Seeing (arcs)
0.4
0.27 ‘’
t0 (ms)
11.2
7.9
0 (arcs)
5.3
5.7 ‘’
s
T0 = 0.31
L0/ v ~775 ms
Conclusion
• Asteroseismology benefits a lot from
continuity
• A-STEP very similar to COROT EXO Field
• Unlike COROT, the full dataset can be
recovered
• roAp, gDor are top-priority programs
• PMS Scuti, Red Giants to be investigated
• Solar-type stars ?
Requirements
•
•
•
•
•
•
30 – 60 s integration time
90 days continuous observations
Good guiding
Precise timing
Best telescope height still unknown
Color photometry allow mode identification