A STEP Expected Yield of Planets

…

Survey strategy

The CoRoTlux Code

Understanding transit survey results

Fressin, Guillot, Morello, Pont

The future of transit searches

Combined to radial-velocimetry, it is the only way to determine the density, hence the global composition of a planet

Transit spectroscopy offers additional possibilities not accessible for “normal” planets

examples:A correlation between the metallicity of stars and planets (Guillot et al. A&A 2006)

Stellar formation model constraints (Sato et al 2005)

We foresee that exoplanetology will have as its core the study of transiting exoplanets

A good phase coverage is determinant to detect the large majority of transits from ground

OGLE: transits discovered•really short periods P ~ 1 day (rare !)•stroboscopic periods

Hot Jupiters: periods around 3 days, depth ~1%

Probability of detection of a transit for a survey of 60 days

With OGLE

For the same telescope with a permanent phase coverage

Continuous observationsWith a “classical” survey, only the “stroboscopic” planets are detectable !

- Real number of “transitable” stars

- Star crowding and spatial sampling effects on differential photometry

- Time correlated noise sources or Red Noise

- Magnitude-limited and time consuming follow-up of planetary candidates

Understanding transit survey results

Observation strategy

Fields of view scheduled- Single field constant following for first campaign

(90 days – polar winter 2008)- Alternate fields for 2009-2010 campaigns

Target stars : all main-sequences stars with

magnitude-range : 11 - 16.5 in R bandspectral type : F0 to M9

Target stellar field for first campaignA STEP - 1 target field

Target stellar field for first campaignPossibility to alternate different fields for

following observation campaigns

CoRoTlux:from

Stellar Field Generation to

Transit Search Simulation and Analysis

T. Guillot, F. Fressin, V. Morello, A. Garnier (OCA)F. Pont, M. Marmier (Geneva)

Thanks to C. Moutou, S. Aigrain, N. Santos

CoRoTluxStellar field generation

with astrophysical noise sources

Light curves generationand transit search algorithms

coupling

Blends simulation

The 3 goals of CoRoTlux

• Survey strategy / Estimation of Transit search efficiency

• Estimation of different contamination sources and blends

-> Characterization of follow up needs

• Understanding of real light curves / survey analysis

Stellar field generation :• Combination of

- real stellar counts (as a function of mag and stellar type) when available-Besancon model of the galaxy for stellar characteristics- Geneva-Copenhagen distribution for metallicity (Nordström et al)

• Double and triple systems

• Background stars generated up to magnitude = (faintest targets mag) + 5

Planetary distribution/characteristics:

• Considering only giant planets (mass over 0.3 MJ)

• Based on planets discovered by radial velocimetry

• Metallicity-linked distribution(Fischer-Valenti 2003., Santos 2006)

Planetary radius …• Use of Tristan’s model of planetary evolution

(linked to stellar irradiation, mass of the planet, and mass of its core – function of stellar metallicity Guillot

2006)

Anti correlation between radius and host star metallicity

Event detectability• CoRoTlux takes into account the different astrophysical noise sources (contamination, blends)

• But it does not compute environmental, instrumental, atmospheric noise sources.

• We consider a level of white noise and a level of correlated noise for a given survey – Pont 2006

• In this simulation : r = 3 mmag

• Sr = 9 as detection threshold

Free parameters and hypotheses

• 2 free parameters:- planetary distribution as a function of stellar type (unknown from G-stars biased RV surveys)

- distribution of “Very Hot Jupiter” planets, undiscovered by RV up to date

• 2 subsets for planetary distribution to reproduceOGLE results:metallicity bellow or over - 0.07

OGLE results indicate that low metallicity stars are unlikely to have close-in planets

• average of 4.1 planets on 50 OGLE campaigns in good agreement with - stellar metallicity

- stellar type - period (Very Hot Jupiter – Stroboscopic planets) - transit depth (directly linked to,planet radius)

Simulations of OGLE surveyto validate CoRoTlux and its hypotheses

Simulation of 20 x OGLE combined campaigns

… and A STEP expectationsFirst goals of A STEP are:- To know how precise a wide-field differential-photometry survey could be at Dome C- To qualify the site for this kind of survey with a simple instrument

We thus focus on following a single stellar field during all winter for first campaign ~1.5 planets for a 90 days survey

Results of 60 single field continuous campaigns

… and A STEP expectationsAverage number of planets found for :

1 month single-field coverage

3 months 8hours in a row/24

3 months with 3 alternate fields (15 minutes on each field in a row) – if technically mastered

3 months single-field with red noise lowered to 2 mmag

A STEP 3 years campaign 30 cm telescope

A STEP 3 years campaign40 cm telescope

~ 0.9

~ 0.7

~ 4.2

~ 2.2

~10.1

~14.8

Conclusions

CoRoTlux is a useful device :- to prepair incoming transit campaigns

- to qualify follow-up needs - to analyse the survey’s results

A STEP should have higher returns than other ground basedsurveys … comparable with space ?

What will be the future of transit search – cornerstone of exoplanetology ? – Which combination of telescope(s) at Dome C ?

synthetic population of targets (Besancon model, real targets)

expected noise + stellar variability

influence zone of background stars

simulated light curves

transit detection algorithm and/or detection criteria

list of transit candidates

type of follow-up needed, object-by-object

estimate of amount and type of ground-based observations needed

stellar companion, triple systems, planets

from OGLE follow-up and Blind Test 2

CoRoTlux