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A STEP Expected Yield of Planets …. Understanding transit survey results. Survey strategy. The CoRoTlux Code. Fressin, Guillot, Morello, Pont. The future of transit searches.

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A step expected yield of planets


Expected Yield of Planets …

Understanding transit survey results

Survey strategy

The CoRoTlux Code

Fressin, Guillot, Morello, Pont

The future of transit searches
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

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


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

Stellar formation model constraints

(Sato et al 2005)

A step expected yield of planets

Continuous observations

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

  • 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


For the same telescope with a permanent phase coverage

A step expected yield of planets

Understanding transit

survey results

- 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

A step expected yield of planets

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 band

  • spectral type : F0 to M9

A step expected yield of planets

A STEP - 1

target field

Target stellar field for first campaign

A step expected yield of planets

Possibility to alternate

different fields for

following observation campaigns

Target stellar field for first campaign

A step expected yield of planets



Stellar Field Generation


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

A step expected yield of planets


Stellar field generation

with astrophysical noise sources

Blends simulation

Light curves generation

and transit search algorithms coupling

A step expected yield of planets

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

A step expected yield of planets

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

A step expected yield of planets

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)

A step expected yield of planets

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

A step expected yield of planets

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 : sr= 3 mmag

  • Sr = 9 as detection threshold

A step expected yield of planets

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 reproduce

  • OGLE results:

  • metallicity bellow or over - 0.07

  • OGLE results indicate that low

    metallicity stars are unlikely to

    have close-in planets

A step expected yield of planets

Simulations of OGLE survey

to validate CoRoTlux and its hypotheses

  • 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)

Simulation of 20 x OGLE combined campaigns

A step expected yield of planets

… and A STEP expectations

  • First 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

A step expected yield of planets

… and A STEP expectations

Average 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 campaign

40 cm telescope

~ 0.9

~ 0.7

~ 4.2

~ 2.2



A step expected yield of planets


  • 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 based

  • surveys … comparable with space ?

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

A step expected yield of planets


synthetic population of targets (Besancon model, real targets)

stellar companion, triple systems, planets

expected noise + stellar variability

influence zone of background stars

simulated light curves

transit detection algorithm and/or detection criteria

list of transit candidates

estimate of amount and type of ground-based observations needed

from OGLE follow-up and Blind Test 2

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