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Observation of the photocentre position variations with CoRoT
- Scientific motivations and expected performance
C. Moutou (LAM) – H. Deeg (IAC) – M. Ollivier (IAS)
M. Auvergne (LESIA) – R. Diaz (LAM) – F. Bouchy (IAP) - P. Bordé (IAS)
2CoRoT SC – 18 april 2012
Why measuring the photocentre position ?
3CoRoT SC – 18 april 2012
Why measuring the photocentre position ?
Fastidious photometric and spectroscopic follow-up
4CoRoT SC – 18 april 2012
Photocentre variations ?
• What to do ?• What is the amplitude of the phenomenon ?• Are we able to detect it ?
x
y
++
€
x ph =
x i .Fii
∑
Fii
∑y ph =
y i .Fii
∑
Fii
∑
5CoRoT SC – 18 april 2012
Accuracy of the photocentre position determination
• First approach : simulation “Best case” : – photon noise limited observation + no other noise sources– Measurement of photocentre position on a 32 s sampled LC– Rms over 256 points (136 min)
8 9 10 11 12 13 14 15 16 170.00E+00
2.00E-03
4.00E-03
6.00E-03
8.00E-03
1.00E-02
1.20E-02
1.40E-02
1.60E-02
axe x (line)
axe y (colomn)
mV
Ph
oto
ce
nte
r p
osit
ion
rm
s
(px)
6CoRoT SC – 18 april 2012
Accuracy of the photocentre position determination
• Second approach : “realistic case” : Estimation of the photocenter position using the imagettes– Determination of the optimal mask for the exo target– Estimation of the photocentre position using the pixel within the mask as it is
done for the sismo stars (white light)– Determination of the residual jitter thanks to the sismo stars– Correction of the jitter in the photocentre curve for the exo target– Estimation of the photocentre stability (rms of the photocentre position curve)– “clever” filtering : gain by a factor of 2 in the rms value ?
• Work done for 2 targets– Star 101062850 R=15.07– Star 102797300 R=13.68
7CoRoT SC – 18 april 2012
Accuracy of the photocentre position determination
Star 102797300 R=13.68 : rms=0.015 px -> 0.0075 px
8CoRoT SC – 18 april 2012
Accuracy of the photocentre position determination
– Star 101062850 R=15.07 rms = 0.022 px -> 0.011 px
9CoRoT SC – 18 april 2012
Photocentre position averaging
• If we average over the transit duration + several transits:
• If à 80 day run
€
G = NT . DT32s
Orbital period (d) 2 4 10 80
Transit duration (min)
136 170 230 460
Gain 100 80 60 40
Accuracy (px) 10-4 1,2 x 10-4 1,6 x 10-4 2,5 x 10-4
10CoRoT SC – 18 april 2012
Amplitude of the photocentre position variations
• First approach : simulation “Best case” : – Photon noise limited observation + no other noise sources– Optimized photometric mask– 1 contaminant (BBS) star at a varying distance– Several values of the extinction rate– Mean value of the photocentre over 136 min (256 frames every
32 s)– Determination of the rms with a 32 s sampling– Determination of the photocentre variation w/o eclipse of the
BBS.
11CoRoT SC – 18 april 2012
Amplitude of the photocentre position variations (ΔmV=3)
-15 -10 -5 0 5 10 15
-0.06
-0.04
-0.02
0
0.02
0.04
0.06
0.08
0.1
0.12
transit 10%
transit 30%
transit 50%
CEB distance (arsec)
Ph
oto
ce
ntr
e p
osit
ion
va
ria
tio
n (
px)
ΔF/F0.6 %
2 %
3 %
12CoRoT SC – 18 april 2012
Amplitude of the photocentre position variations (ΔmV=5)
-10 -8 -6 -4 -2 0 2 4 6 8 10
-2.00E-03
-1.00E-03
0.00E+00
1.00E-03
2.00E-03
3.00E-03
4.00E-03
CEB distance (arcsec)
Ph
oto
ce
ntr
e p
osit
ion
va
ria
tio
n
(arc
se
c)
Transit : 10 % (ΔF/F=10-3)
13CoRoT SC – 18 april 2012
Amplitude of the photocentre position variations
• Second approach : analytic calculation
Δphot : photocentre position variation (pix)
fraccont : fraction of the contaminant flux on the total flux in the aperture
Ft, Fc : target and contaminant flux
k : fraction of the contaminant flux in the target aperture
ΔCEB : relative depth of the CEB eclipse
dtc : distance target contaminant in arcsec
€
Δ phot = fraccont .ΔCEB .dt→c2.32
=ΔF
F
⎛
⎝ ⎜
⎞
⎠ ⎟alam
.dt→c2.32
€
fraccont =Fc .k
(Fc .k + Ft )= k.
Fc /Ft1+ k.Fc /Ft
14CoRoT SC – 18 april 2012
Amplitude of the photocentre position variations
• Second approach : analytic calculation (k≈1)
dtar-BBS = 5 arcsec
dtar-BBS = 1 arcsec
15CoRoT SC – 18 april 2012
Amplitude of the photocentre position variations
• Case of 21observed CEBs
16CoRoT SC – 18 april 2012
Other effect : simulation tool
17CoRoT SC – 18 april 2012
Other effect (1) : chromatic stellar flux variation
• Analysis algorithms comparable to stellar flux to disentangle from stellar activity
0.00E+00 5.00E-03 1.00E-02 1.50E-02 2.00E-02 2.50E-020.00E+00
2.00E-03
4.00E-03
6.00E-03
8.00E-03
1.00E-02
1.20E-02
1.40E-02
1.60E-02
1.80E-02Chromatic flux variation
Delta Xph (red)
Delta Xph (blue)
relative flux variationP
hoto
centr
e s
hif
t (p
x)
18CoRoT SC – 18 april 2012
Other effect (2) : main target eclipse in a crowed field
• Main target : mV=11• 2 contaminants : mV=14 @ 5 arcsec in x and y
0 0.02 0.04 0.06 0.08 0.1 0.120
0.002
0.004
0.006
0.008
0.01
0.012
Main target eclipse
Delta Xph
Relative flux variation
Photo
centr
e s
hif
t (p
x)
19CoRoT SC – 18 april 2012
Other effect (3) : effect of hot pixels
• 4 saturated pixels at extreme x positions
• Δx = 3 px
20CoRoT SC – 18 april 2012
Photocentre and follow-up
21CoRoT SC – 18 april 2012
Photocentre and follow-up
• Expected from 2013-2015 runs– 1000 new candidates– 200 planet-like candidates (no secondary
eclipse detected, shallow transit depth)– 45% of them are CEBs = 90– 60% of CEB lead to measurable
photocentre drift
22CoRoT SC – 18 april 2012
Conclusion• The photocentre position determination is
useful for CEB identification• The accuracy appears to be sufficient at least
for mR < 13-14• Other effects can be identified• The implementation strategy has to be
discussed:– Modification of on-board SW to compute the
photocentre position of oversampled stars– Ground base a posteriori evaluation using the
imagette after re-allocation of the imagettes
23CoRoT SC – 18 april 2012
Discussion
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