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Validation of Transiting Planet Candidates with BLENDER
2013 May 14 Planet Validation Workshop, Marseille 1
Validation of Transiting Planet
Candidates with BLENDER
Willie Torres
Harvard-Smithsonian Center for Astrophysics
Planet Validation Workshop, Marseille,
14 May 2013
Validation of Transiting Planet Candidates with BLENDER
2013 May 14 Planet Validation Workshop, Marseille 2
Historical Perspective
• First transiting planet candidates released
by the OGLE survey (Udalski et al. 2001)
– Great excitement: several teams struggled to
produce Doppler confirmations
– Much telescope time was invested
• Many candidates from other wide-field
surveys (e.g., TrES)
• Some false starts
Validation of Transiting Planet Candidates with BLENDER
2013 May 14 Planet Validation Workshop, Marseille 3
Sample light curves from OGLE
Validation of Transiting Planet Candidates with BLENDER
2013 May 14 Planet Validation Workshop, Marseille 4
General Philosophy of BLENDER
• Back-of-the-envelope assessments of blend likelihood are not good enough
• Use detailed shape information contained in the light curves
– Fit simulated (realistic) blend models to original photometry: background EBs, stars+planets, etc.
– If fit is unacceptably poor, blend can be rejected
• Predict properties of a blend that can be compared against observations
– Use of isochrones to simulate blends
Validation of Transiting Planet Candidates with BLENDER
2013 May 14 Planet Validation Workshop, Marseille 5
• “Validation” becomes necessary when dynamical
confirmation is not possible, by the detection of the
effect of a planet on the star or on other planets
– Reflex Doppler motion (+ bisector analysis)
– Transit timing variations in multiple systems
• General approach of BLENDER
– Estimate likelihood of a false positive
– Estimate likelihood of a true planet (planet ‘prior’)
– Compute the odds ratio: must be such that a true planet
is much more likely than a false positive (greater than
the 3σ confidence level) → VALIDATION
• References: Torres et al. 2004, 2011; Fressin et al. 2011;
and many Kepler papers; refinements still in progress
Validation of Transiting Planet Candidates with BLENDER
2013 May 14 Planet Validation Workshop, Marseille 6
Types of False Positive Configurations
Considered in BLENDER
• Background or foreground EB
• Background or foreground star transited by a planet
• Physically associated EB (hierarchical triples) – Rarely works when light curves are of high quality
• Physically associated companion transited by a (larger) planet – Valid type of blend when searching for planets of
specific sizes
• Additional stars in the photometric aperture can cause extra dilution that must be account for
Validation of Transiting Planet Candidates with BLENDER
2013 May 14 Planet Validation Workshop, Marseille 7
Exploring Blend Parameter Space
• EBs and star+planet light curves generated within
BLENDER with EBOP (binary light-curve program)
• Relevant blend properties
– Secondary / tertiary mass for EBs (M2,M3)
– Tertiary radius R3 (if blend is star+planet)
– Impact parameter b (inclination angle)
– Transit duration relative to circular orbit D/Dcirc(e,)
– Relative distance between target and background
or foreground object (distance modulus difference, )
– Absolute distance scale set by total apparent magnitude
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Validation of Transiting Planet Candidates with BLENDER
2013 May 14 Planet Validation Workshop, Marseille 8
• Properties for primary taken from isochrone (based on spectroscopic Teff, [Fe/H], log g, when available)
• Secondary and tertiary properties taken from same or different isochrone, depending on configuration
• Differential extinction accounted for in BLENDER
• Free parameters for the various scenarios: – Physically associated EB: M2, M3, b, D/Dcirc
– Companion star + planet: M2, R3, b, D/Dcirc
– Background / foreground EB: M2, M3, b, D/Dcirc ,
– Background / foreground star + planet: M2, R3, b, D/Dcirc ,
• Parameter space very large: BLENDER explores up to ~109 false positive configurations in a fine grid over wide ranges in each parameter, to establish constraints on blend properties
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Validation of Transiting Planet Candidates with BLENDER
2013 May 14 Planet Validation Workshop, Marseille 9
(2.1σ)
(10σ)
Kepler-10c
Fressin et al. 2011
Background EB blend models
Obtaining Constraints on the
Parameters of Blends
• Use 2 as a measure of the
goodness of fit of a blend model
• Compute the 2 of the fit for
each blend scenario
• Compute the 2 for a planet
model, to use as a reference
• In most cases the best blend fit
is visually as good as a planet fit
• A blend fit with a 2 much larger
than that of a planet fit is
considered to be rejected (e.g.,
at the “3σ” level)
Validation of Transiting Planet Candidates with BLENDER
2013 May 14 Planet Validation Workshop, Marseille 10
Visualization of BLENDER
constraints for Kepler-66b
1
2
3
(Meibom et al. 2013)
Background EBs
Viable blends
Background/foreground
transiting planets
Viable blends Physical triples
(star+planet)
Validation of Transiting Planet Candidates with BLENDER
2013 May 14 Planet Validation Workshop, Marseille 11
Background
eclipsing binary
scenario
Changes in light
curve shape
Best blend
model
Validation of Transiting Planet Candidates with BLENDER
2013 May 14 Planet Validation Workshop, Marseille 12
Incorporating Observational Constraints
• Centroid motion analysis from Kepler images – Centroid motion angular separation and flux decrement
– 3σ exclusion limit CM
• Color information (griz+JHKs, from the KIC) – Blend can be too blue or too red compared to measured
color index
Validation of Transiting Planet Candidates with BLENDER
2013 May 14 Planet Validation Workshop, Marseille 13
• High-resolution imaging (sensitivity curves)
– Adaptive optics imaging
– Speckle imaging
Validation of Transiting Planet Candidates with BLENDER
2013 May 14 Planet Validation Workshop, Marseille 14
• High-resolution spectroscopy: limits on the
brightness of companions that may fall
within the slit
– Simulations
Simulated companion temperature (K)
Sensitivity as a function of RV and
the temperature of the companion
Validation of Transiting Planet Candidates with BLENDER
2013 May 14 Planet Validation Workshop, Marseille 15
• Spitzer observations – Transits should be achromatic
– Constraints on SpT (or mass)
of intruding star #2
CoRoT-7b
Kepler-18c and 18d
Kepler
Spitzer
Fressin et al. 2012
Cochran et al. 2011
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2
3
Validation of Transiting Planet Candidates with BLENDER
2013 May 14 Planet Validation Workshop, Marseille 16
Kepler-62e
Borucki et al. 2013
Validation of Transiting Planet Candidates with BLENDER
2013 May 14 Planet Validation Workshop, Marseille 17
Kepler-62e
Borucki et al. 2013
Validation of Transiting Planet Candidates with BLENDER
2013 May 14 Planet Validation Workshop, Marseille 18
Kepler-62e
Borucki et al. 2013
Validation of Transiting Planet Candidates with BLENDER
2013 May 14 Planet Validation Workshop, Marseille 19
Computing Blend Frequencies
From Monte Carlo Simulations
• Use constraints from BLENDER, and any follow-
up observations available
• Main assumptions
– Binary and planet frequencies, from previous work
– Period, eccentricity, and mass ratio distributions for
binary companions from multiplicity surveys
– Stellar properties from isochrones
• Example of a blend configuration consisting of a
physically associated star transited by a planet
Validation of Transiting Planet Candidates with BLENDER
2013 May 14 Planet Validation Workshop, Marseille 20
Simulations for Kepler candidates • Draw a random stellar companion using binary mass ratio distribution,
and check against allowed BLENDER range of M2
• Compute blend color using isochrones, and check
against measured color of target
• Assign random binary orbital period, eccentricity,
orientation, and phase, and compute
• Check and brightness against centroid limit
CM , and against high-resolution imaging
• Compute orbital RV and apply spectroscopic
criterion on brightness if < slit half-width
• Check RV drift against RV observations, if any
• Assign a random planet to the companion from KOI list, and random e
• Check if {Rp,e} are allowed by BLENDER
• Apply dynamical stability criterion (Holman & Weigert 1999)
• Repeat many times, and count viable blends
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3
Validation of Transiting Planet Candidates with BLENDER
2013 May 14 Planet Validation Workshop, Marseille 21
• Account for binary and planet frequencies
• Perform similar Monte Carlo simulations for other blend configurations – Background EBs
– Background stars transited by a planet
• For background scenarios, draw stars from Besançon Galactic population model near the location of the target, and apply appropriate BLENDER constraints in the same way as before
• Add up all blend frequencies for the three cases
• Odds ratio planet ‘prior’ / total blend frequency (> 370, or 3σ confidence level)
Validation of Transiting Planet Candidates with BLENDER
2013 May 14 Planet Validation Workshop, Marseille 22
Estimating the Planet Prior
• For easy cases, use information available from KOI list
– Count number of actual planets detected in the appropriate radius (and period) range (Rp ± 3σ), using KOI list, and divide by total number of Kepler targets
– KOI list is neither complete nor pure; need to correct for biases (MC simulations: Fressin et al. 2013)
• Correct for incompleteness: around what fraction of Kepler targets would such planets be detected?
• Correct for contamination from false positives
Validation of Transiting Planet Candidates with BLENDER
2013 May 14 Planet Validation Workshop, Marseille 23
• Numerical example from the 3-planet system
Kepler-68 (Gilliland et al. 2013)
Background EBs 2.8 10-6
Background star+planet 7.0 10-8
Physical companion+planet 6.7 10-7
Total blend frequency 3.5 10-6
Planet prior = (719.4) / (9.7% 138,253) = 4.6 10-3
Kepler-68b Kepler-68c
53 ppm
Rp = 0.95 R
P = 9.6 days
False positive contamination Completeness
Odds ratio = 4.6 10-3 / 3.5 10-6 1300
Kepler-68c
Blend
freq.
CANDIDATE
VALIDATED
Validation of Transiting Planet Candidates with BLENDER
2013 May 14 Planet Validation Workshop, Marseille 24
• Determining the planet prior is more difficult in some cases because the statistics from Kepler are not yet robust enough
– Very small candidates (Rp much less than 1 R)
– Candidates with very long orbital periods (~200 days or more)
– Small candidates with long periods (the most interesting, potentially habitable!)
• In these cases reasonable extrapolations of planet frequencies are required to establish the planet prior
Validation of Transiting Planet Candidates with BLENDER
2013 May 14 Planet Validation Workshop, Marseille 25
Five-planet system Kepler-62
Planet priors
require
extrapolations
Kepler-62c
Kepler-62f
Borucki et al. 2013
Validation of Transiting Planet Candidates with BLENDER
2013 May 14 Planet Validation Workshop, Marseille 26
Summary of BLENDER validations
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