Photometric detection of the starlight reflection by a

“Pegasi” planet

Martin Vannier(1), Tristan Guillot(2), Suzanne Aigrain(1)

(1) ESO, Chile(2) OCA, France(3) Institute of Astronomy, Cambridge, UK Corot Week 2005 Ubatuba, Brazil, 2-6 Oct.

A description of two proposals in the

Corot Additional Programme

2 proposals in the Corot Additional Programme(M. Vannier, T. Guillot, S. Aigrain)

● I: Observation of the starlight reflected by a “Pegasi” planet ➢Phase B

● II: Photometric detection of “Pegasi” planets in the seismo field➢Accepted

Photometric detection of the starlight reflection by a “Pegasi” planet

Corot Week 2005 Ubatuba, Brazil, 2-6 Oct.

Corot Week 2005 Ubatuba, Brazil, 2-6 Oct.

Photometry of Star+Planet varies with planetary phase

Principle

In a perfect simple world :

⇨ Periodical variations of the photometry

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Planetary signal

Corot Week 2005 Ubatuba, Brazil, 2-6 Oct.

Amplitude of the signal (homogeneous reflection, circular orbit):

S=1/2 A sin(i) (Rpl/a)2

A: Albedoi: orbital inclination Rpl: planetary radiusa: orbital distance

⇨Degeneracy between A, i, (R) ⇨constrains the parameters space

Signal / fundamental noise

Corot Week 2005 Ubatuba, Brazil, 2-6 Oct.

● Amplitude of the signal:

S=1/2 A sin(i) (Rpl/a)2

➢ “Pegasi” are much favored

E.g.: HD46375 (target for Prop. II), a=0.04 AU, Rpl=1.3 Rjup

⇨ S= a few 10-5

● Photon Noise: B=1/sqrt(Nph) with Nph photons per sample. E.g.: mV=7.9

Sample =3h ⇨ B=4 10-6, SNR=30

● Instrumental (white) noise also nulls out

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Stellar activity

Corot Week 2005 Ubatuba, Brazil, 2-6 Oct.

E.g.: simulations of HD46375 (S. Aigrain) : A fairly quiet K1 IV star, RMS=170 ppm

Planet (i=/6, A=0.5)

(Dotted: Star alone)

Planet + Star + photon noise

Spectral Analysis with known orbital period and phase (Prop. I)

Corot Week 2005 Ubatuba, Brazil, 2-6 Oct.

➢ Stellar activity exceeds the signal in amplitude, including at the (known) orbital frequency

➢ Fit with a sine, to best match both the amplitude and phase at the orbital frequency.

In the case of HD46375, the precision on the amplitude of the planetary reflection would be:⇨ 30% for a 20-days short run⇨ <10% for a 150-days run

Sine fit

Planetary signal

Measured signal,including stellar activity

Field of View and Target (Prop. I)

Corot Week 2005 Ubatuba, Brazil, 2-6 Oct.

HD46375:

- in FOV- K1 IV type-star- a=0.04 AU- mV=7.94

Favorable planet-host stars and the Corot “eyes”

Field of View and Target (Prop. I)

Corot Week 2005 Ubatuba, Brazil, 2-6 Oct.

HD46375:

- K1 IV type-star- a=0.04 AU- mv=7.94- in FOV

Together with short-run primary target HD46558 in seismo field

➢ Phase BPegasi-planet target HD46375(cross)

together with primary target HD46558

Proposal IIPotential for new detections

Corot Week 2005 Ubatuba, Brazil, 2-6 Oct.

Number of detection =

Number of objects observed 60 (10 per season)

× Proportion hosting a Pegasi planet1%

× Fraction of these for which the planetary reflection signal can be distinguished from the stellar activity. Largely unknown... 1/3 ??

0.2 ??⇨ Not much, but potentially high value result at a free cost !

Corot Week 2005 Ubatuba, Brazil, 2-6 Oct.

Orbital frequency

Sine fit (frequency, amplitude, phase) on a HD46375-type star: ⇨ Precision over 150 days: <10% on the amplitude of the planetary reflection

5% on its period

Spectral Analysis with unknown orbital elements

Coping with stellar activity for new detections

Corot Week 2005 Ubatuba, Brazil, 2-6 Oct.

● Depends on rotational velocity, colour index and age of the star

● Used simulations for MS stars with: type = F5 to K5, rotational period = 5 to 40 days

● Sine fit on a 150-days serie with two free parameters(*) yields:

➢ a precision on the amplitude ranging from 20% to a few % (depending on S.T) for slow-rotating stars (P=40 d)

➢ strongly degraded precision for fast rotators (prohibitive for P≤15 d)

➢ a number of local minima ⇨ fake alarms or dubious cases

(*) Orbital period and phase. A fixed ⇨ amplitude = f(period)

Corot Week 2005 Ubatuba, Brazil, 2-6 Oct.

⇨A potential for new detection of Pegasi planets around low-activity stars of the seismo field. But...

Further work to be done...Need for:

● A better simulation including:- eccentric orbit, albedo depending on the planetary

phase (⇨ peaked “Mercury-type” reflection)

- estimated stellar activity representative of the actually observed population

- a smarter fit algorithm

● RV follow-up to raise the ambiguity on the dubious cases

Corot Week 2005 Ubatuba, Brazil, 2-6 Oct.

For a circular orbit and a homogeneous albedo:

S=1/4 A (Rpl/a)2 (1-sin(i)*cos(2 pi t/P))

But the variations or not sine in case of :

⇨eccentric orbit

⇨surface albedo depends on the orbital configuration

Planetary signal as a function of time

Planetary signal + Fundamental Noise???

Corot Week 2005 Ubatuba, Brazil, 2-6 Oct.

Dominated by photon noise: B=1/sqrt(Nph) with Nph: stellar flux, time

E.g.: HD46375 SNR =