Observation of the photocentre position variations with CoRoT - Scientific motivations and expected performance C. Moutou (LAM) – H. Deeg (IAC) – M. Ollivier

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 ?


Why measuring the photocentre position ?

Fastidious photometric and spectroscopic follow-up


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

∑


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)



• 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



Star 102797300 R=13.68 : rms=0.015 px -> 0.0075 px



– Star 101062850 R=15.07 rms = 0.022 px -> 0.011 px


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


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.


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 %


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)



• 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



• Second approach : analytic calculation (k≈1)

dtar-BBS = 5 arcsec

dtar-BBS = 1 arcsec



• Case of 21observed CEBs


Other effect : simulation tool


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)


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)


Other effect (3) : effect of hot pixels

• 4 saturated pixels at extreme x positions

• Δx = 3 px


Photocentre and follow-up


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


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


Discussion

Documents

Observation of the photocentre position variations with CoRoT - Scientific motivations and expected performance C. Moutou (LAM) – H. Deeg (IAC) – M. Ollivier