validation of transiting planet candidates with blender · validation of transiting planet...

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



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



Sample light curves from OGLE



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” 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



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



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



• 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



(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)



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)



Background

eclipsing binary

scenario

Changes in light

curve shape

Best blend

model



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



• High-resolution imaging (sensitivity curves)

– Adaptive optics imaging

– Speckle imaging



• 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



• 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



Kepler-62e

Borucki et al. 2013



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



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



• 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)



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



• 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



• 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



Five-planet system Kepler-62

Planet priors

require

extrapolations

Kepler-62c

Kepler-62f

Borucki et al. 2013



Summary of BLENDER validations

validation of transiting planet candidates with blender · validation of transiting planet...

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