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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
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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
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Validation of Transiting Planet Candidates with BLENDER
2013 May 14 Planet Validation Workshop, Marseille 3
Sample light curves from OGLE
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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
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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
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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
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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
1
2
3
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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
1
2
3
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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)
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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)
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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
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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
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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
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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
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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
1
2
3
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Validation of Transiting Planet Candidates with BLENDER
2013 May 14 Planet Validation Workshop, Marseille 16
Kepler-62e
Borucki et al. 2013
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Validation of Transiting Planet Candidates with BLENDER
2013 May 14 Planet Validation Workshop, Marseille 17
Kepler-62e
Borucki et al. 2013
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Validation of Transiting Planet Candidates with BLENDER
2013 May 14 Planet Validation Workshop, Marseille 18
Kepler-62e
Borucki et al. 2013
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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
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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
1
2
3
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Validation of Transiting Planet Candidates with BLENDER
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• 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)
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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
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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
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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
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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
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Validation of Transiting Planet Candidates with BLENDER
2013 May 14 Planet Validation Workshop, Marseille 26
Summary of BLENDER validations