Transcript siamois

SIAMOIS : asteroseismic
observations after CoRoT:
the need for spectroscopic
measurements
Benoit Mosser - LESIA
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(presented by Jean-Pierre Maillard, IAP)
Spectroscopy at Dome C
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Outline
1. Asteroseismology
- Photometric observations with CoRoT
- Spectroscopic results from ground (HARPS, …)
2. Performance comparison
- Photometric measurements
- Doppler measurements
3. Doppler measurements
- Grating spectrometer
- Fourier tachometer
4. SIAMOIS
- Principle
- Scientific program
- Schedule
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Asteroseismology purpose
• Age determination
• Stellar radii (impact for exoplanet radii)
• Stellar composition
~ a few %
~ a few %
• Diagnostic of convective cores
• Depth of convection and of second helium ionization zones
• Mode excitation mechanisms (convection)
• Rotation and internal structure
Specification:


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eigenfrequency resolution
continuous observations
long duration (dν = 1/T)
Spectroscopy at Dome C
dν = 0.2 μHz
(h > 80 %)
(T > 2.5 months)
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CoRoT
• launched on December 27th , 2006
• by Soyuz 2, from Baikonour, Kazakhstan
• low Earth polar orbit, 896 km altitude
• orbital period 6184 s (~1h43mn, 162 mHz)
• high precision photometry
The CoRoT space mission was developped
and is operated by CNES, with the
contribution of Austria, Belgium, Brazil,
ESA, Germany and Spain
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CoRoT light curves
Typical CoRoT light curve
 Photon noise limited performance ~ 1 ppm
 150 days
 Duty cycle ~ 92%
Typically 10-4 in 30 s
Variability below the 10-3 level over 20 days of a 6th magnitude F star
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Photometry (1)
HD 49933, mV=5.7, F5V, observed during the initial run (60 days)
Mode amplitudes ~ 1 few ppm
 observation of p-mode oscillations in solar-like stars not
achievable by photometric ground-based measurements
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Photometry (2)
HD 181420, mV=6.7, F2V, first long run (150 days)
Stellar granulation: important contribution at low frequency
 limits the spectrum SNR for f < 2 mHz
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Ground-based observations
• solar-like oscillations in solar-like stars
- HARPS @ ESO 3.6-m
- UCLES @ AAT
- CORALIE @ Euler telescope
- SOPHIE @ OHP
+ instruments @ SARG, McD, Okoyama, Lick
Oscillation detection
Mode identification
2-sites observation
Network observation
Stellar structure
modelling
Rotation, fine
structure…
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٧
٧
٧
٧
٧
٧
~ 20 targets
for ~ 12 targets
5 targets
1 target (Procyon)
~ 2 targets
Observations limited
to a few days
insufficient
precision
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Spectroscopic result (1)
Procyon, 10-day network
observation
(11 observatories, Jan. 2007)
 Identification of mixed modes
 Definitely a post-MS star
Mosser et al 2008, A&A 478, 197
Bedding et al 2008, in preparation
Day aliases (11.57 Hz) still present;
too short duration compared to
stellar rotation period
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Spectroscopic result (2)
HD 203608 ; F6V ; mV = 4.8
Old star of the thick galactic disk
5 days observation with HARPS
duty cycle 40%
Stellar modelling
L/Lo
M/Mo
R/Ro
T (K)
Fe/H
Age (Gyr)
before
with asteroseismic
constraints
1.40 ± 0.13
0.88 ± 0.07
1.04 ± 0.12
6070 ±150
-0.60 ± 0.10
10.5 ± 4
1.38 ± 0.045
0.928 ± 0.028
1.06 ± 0.02
6051 ± 45
-0.55 ± 0.05
7.2 ± 0.3
Mosser et al 2008, submitted to A&A
Precision still hampered by poor frequency resolution and
duty cycle
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Doppler asteroseismometry
• Principle : photon noise limited performances
- Q quality factor of the spectrum
- Ne number of photoelectrons collected
• Q depends on:
- the spectral type and the v.sini (rotation) of the star
- the type of instrument
GS: grating spectrometer
FS: Fourier Transform spectrometer
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Quality factor
The quality factor Q gives
a measure of the:
- number
- depth
- width
of the lines in the stellar
spectrum
Q # dln A /dln l
Better Q factor for cooler stars
Better performances in the blue part of the visible spectrum
Supposes a high resolving power (~ 100 000) of the grating spectrometer
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Comparison: Photometry/Spectrometry
Photometry
Spectrometry
Q = stellar oscillation quality factor
Oscillation amplitudes
Target
1 ppm

10 cm/s
Dm
Quality
factor
Photometry
hyp: Ne,p ~ 1012
mV ~ 6
Tachometry
Type K
low vsini
1500
1 ppm
0.36 m/s
3
Type F
vsini = 12 km/s
500
1 ppm
1.1 m/s
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with Ne,v ~ Ne,p / 3
Photometric observations: dimmer targets, or smaller telescope
1 ppm sensitivity require space-borne observations
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Doppler / photometry on the Sun
Solar granulation noise: photometric observations 50 times noisier
at low frequency than Doppler measurements
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Granulation noise
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l=3 modes
Small
separation
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l=3 modes
have
higher
visibility in
spectroscop
y
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Doppler / photometry on the Sun
Core size determination
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
Gabriel et al 1998
low frequency noise
+
l=3 modes
Inversion 4
times more
precise with
Doppler data
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Space / Ground
space
ground
Observation
photometry
spectrometry
Max. degree
2
Spectral type
T > Tsun
v sin i
--
3
Bright
Any
< 15 km/s
Inversion
1
4 time more precise
Targets magnitude Dim
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Fourier Transform Seismometry
Fourier transform
Seismometry:
The Doppler signal is
retrieved from the
interferogram of the
stellar spectrum
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Fourier Transform Seismometry
FT seismometry
successfully tested
with the FTS at CFHT
Procyon
Mosser et al. 1998, A&A
340, 457
Jupiter
Mosser et al. 2000, Icarus
144, 104
• FTS at CFHT: repeated scan of one selected fringe of the interferogram
• shift of the fringe signal with time  Doppler signal
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FS: quality factor
with
(Mosser, Maillard, Bouchy 2003, PASP 115, 990)
•Q increases with
- wavenumber s0
- working path difference dopt
- fringe contrast C
• A high fringe contrast C requires a narrow bandwidth
• To be compatible with a high Ne factor requires a dispersion of the
fringes (post-disperser) = many adjacent narrow bandwiths
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FS: Q with post-dispersion
Fourier transform
seismometry with
post-dispersion
The Doppler signal is
searched in the
interferogram of each
spectral element
defined by the postdisperser
Q factor as a function of the post-dispersion resolution and the spectral type for 3 vsini
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GS / FS
GS: HARPS (ref = ThAr lamp) R ~ 115000
FS: post-dispersion resolution R~ 1000
δv(GS) / δv(FS) as a function of v sini and T of the star
GS > FS if reference = ThAr lamp (Mosser et al. 2003)
GS ~ FS if reference = iodine cell
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GS / FS
Input Fiber
Quality factor
Resolution
GS
FS
double scrambler ( /400)
1"~ 6 m/s
simple scrambler ( /100)
1" ~ 1 cm/s
QGS = Q(Q* , R)
QFS = Q(Q* , Rpd)
R ~ 105
Path difference ~1 cm
Rpd ~1000
Grating
CCD
~ 10 x 40 cm
Two ~5x5 cm
4k x 2k
1k x 256
FS: smaller and simpler instrument than a GS
monolithic interferometer = no moving parts (SIAMOIS concept)

possible installation and setup at Dome C
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SIAMOIS = Système Interférentiel A Mesurer les OscIllations Stellaires
• A Fourier Spectrometer dedicated to
asteroseismology with no moving parts
• to be installed at Dome C behind a 40-cm telescope
• phase A completed
• P.I. B. Mosser
• Scientific Committee
Th. Appourchaux (France, pdt), C. Catala (inst. scientist), S.
Charpinet (France), D. Kurz (UK), Ph. Mathias (France), A. Noels
(Belgium), E. Poretti (Italy),
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SIAMOIS performances at Dome C
Photon noise limited performances
SIAMOIS, at Dome C, 40-cm telescope,
120 hours with 95% duty cycle, mV = 4
‘‘SNR’’ on circumpolar targets
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SIAMOIS performances at Dome C
SIAMOIS with post-disperser R = 1000 at Dome C for 3 solar-like stars
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Targets
1) K, G, F, class IV & V targets
2) Red giants
3) Delta-Scuti, gamma Dor, PMS…
Since long-duration observations are required, a 40-cm telescope provides already
a scientific program on p-mode oscillation in solar-like targets as large as the
CoRoT program
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Targets with a 40-cm telescope
COROT
Observable solar-like
stars with p-mode
oscillations for a
dedicated 40-cm
telescope
• 40-cm telescope:
- 7 bright targets, type: F, G, K class: IV & V
- many red giants; d Scuti (v sin i < 20 km/s)
 Scientific program for more than 6 winterings
 Program complementary to CoRoT
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Clear sky fraction at Dome C
Clear sky fraction measured by Eric Aristidi (2006 winter)
Clear sky fraction > 90% during 84% of the time
Average number of consecutive clear days: 6.8 days
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Duty cycle
Better performance at Dome C compared to a 6-site network
(Mosser & Aristidi 2007, PASP)
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SIAMOIS
• 40-cm telescope
• Interferometer
•
Data
small size, low cost, easy ‘antarctization’, dedicated to the project
fiber fed Mach Zehnder interferometer, operated at room temperature,
monolithic no moving parts, photon noise limited performance
automatic pipeline reduction, telemetry: limited flow < 100 kb/day
Phase A completed, April 2007
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Simulations
l= 2
0
3
1
F6V star, mV = 4.5, vsini = 5 km/s, 90-day long run
Modelling: stochastic excitation + intrinsic damping
 Lorentzian profiles (Anderson et al 1990)
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Simulations
Longer lifetimes at low frequency
 clear multiplets
l= 2
0
3
1
F6V star, mV = 4.5, vsini = 5 km/s, 90-day long run
Precision on the eigenfrequency measurement: 0.10 –
0.25 mHz (Libbrecht 1992)
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Fourier tachometer
• Another advantage:
multi-object advantage
 simultaneous observations of several targets
First step: small telescope + FT
Then: multi-targets observation = small telescopes + 1 FT
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Planning & budget
< 2006
principle: monolithic Fourier
Tachometer
• 2007
thermo-mechanical analysis
phase A
• 2009-2011
PDR
FDR
integration
• 2011-2012
tests
summer campaign: Dome C
• 2013
First winterover at Dome C
LESIA (Obs. Paris),
IAS (Orsay), LUAN (Nice),
OMP (Toulouse) + SESO
Budget ~ 860 k€ << budget for an equivalent 6-site network
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Perspectives
Asteroseismology requires uninterrupted long-duration time series !
1 dedicated 40-cm telescope:
- first season observation
- fiber FOV = 5’’ (>> seeing)
 stellar magnitude
< 5 for solar-like oscillations
< 7 for classical pulsators
2 or 3 dedicated small telescopes
- next step
 simultaneous observations of 2 or 3 stars
2-m class telescope?
-stellar magnitude < 8.5 for solar-like oscillations
- increase of the number of reachable targets
 possibility to achieve specific observations in selected targets
However, a dedicated telescope would be required
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Other projects: KEPLER
• NASA; launch = nov 2008
• High precision photometry
• a few fields reserved
for asteroseismology
CoRoT  Kepler :
tel.
27 cm
 95 cm
orbit
polar
 L2
+
duty cycle in L2
-
sensitivety (mV > 9), radiations in L2
?
exact scientific case for asteroseismology?
29-31 October 2007: First KASC workshop, Paris. The Kepler Asteroseismic
Science Consortium (KASC) is an international consortium of researchers
dedicated to the asteroseismic analysis of Kepler data.
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SONG
• Project currently in phase 0
• Danish asteroseismology centre, Aarhus University
• Network of 6 to 8 small telescopes (6080 cm)
• Echelle spectrometer + iodine cell
• Expected schedule: 1 prototype for 2012-2013
>> 2012
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Comparison
CoRoT
Kepler
SONG
SIAMOIS
2 eyes
diam = 12°
10° x 10°
(Cygnus-Lyra)
|d| < 30°
d < - 45°
Duty cycle
92 %
~ CoRoT
~ 85 %
~ 90 %
5-day perf.
0.6 ppm
> 1.2 ppm
Max obs.
5 months
 4 years
Magnitude
>6
>9
<7
# targets
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Up to 40 : 4 yr
Up to 160 : 1 yr
Up to 1000 : 90 d
> 30
# solar-like
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Status
Instrument
cost
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In operation
2-20 cm/s
3 months
3 months
7
Launch
= 11/ 2008
65 M€
Spectroscopy at Dome C
Phase 0
Prototype > 2012
Phase A is OK
2013 at Dome C
> 6 M€ (6 tel)
0.86 M€ (1 tel)
1.02 M€ (2 tel)
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Conclusions
Space-borne observations = photometric observations
CoRot
Kepler
unique results
not primarily specified for asteroseismology
sensitivity for p-mode oscillations under question
very dim targets  uncertainty on fundamental parameters
Ground-based observations = Doppler observations
measurement of modes up to degree l = 3
much less low frequency noise
 much better inversion and modelling
observation of low mass stars
Network very late schedule, complex organization
Dome C = unique site for asteroseismology
3-month continuous observation with duty cycle ~ 90%
High performance with a 40-cm collector
Better performance than a 6-site network
http://siamois.obspm.fr
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