Transcript A STEP Expected Yield of Planets

A STEP
Expected Yield of Planets …
Understanding transit
survey results
Survey strategy
The CoRoTlux Code
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
We foresee that exoplanetology
will have as its core the study of
transiting exoplanets
examples:
A correlation between the metallicity of stars
and planets (Guillot et al. A&A 2006)
Stellar formation model constraints
(Sato et al 2005)
Continuous observations
A good phase coverage is
determinant to detect
the large majority of
transits from ground
With a “classical” survey, only
the “stroboscopic” planets are
detectable !
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
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 followup of planetary candidates
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
ATarget
STEPstellar
- 1 field for first campaign
target field
Possibility
alternate
Target stellar
fieldtofor
first campaign
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
CoRoTlux
Stellar field generation
with astrophysical noise sources
Light curves generation
and 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 :
sr = 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 reproduce
OGLE results:
metallicity bellow or over - 0.07
OGLE results indicate that low
metallicity stars are unlikely to
have close-in 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
… 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
… and A STEP expectations
Average number of planets found for :
1 month single-field coverage
~ 0.9
3 months 8hours in a row/24
~ 0.7
3 months with 3 alternate
fields (15 minutes on each
field in a row) – if technically
mastered
~ 4.2
3 months single-field with red
noise lowered to 2 mmag
~ 2.2
A STEP 3 years campaign
30 cm telescope
~10.1
A STEP 3 years campaign
40 cm telescope
~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 based
surveys … comparable with space ?
What will be the future of transit search – cornerstone of
exoplanetology ? – Which combination of telescope(s) at
Dome C ?
CoRoTlux
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
from OGLE follow-up
and Blind Test 2
type of follow-up needed,
object-by-object
estimate of amount and
type of ground-based
observations needed