Implicit biases in transit models using stellar pseudo-density

Abstract

The transit technique is responsible for the majority of exoplanet discoveries to date. Characterizing these planets involves careful modeling of their transit profiles. A common technique involves expressing the transit duration using a density-like parameter, $\tilde{\rho }$, often called the "circular density." Most notably, the Kepler project -- the largest analysis of transit lightcurves to date -- adopted a linear prior on $\tilde{\rho }$. Here, we show that such a prior biases measurements of impact parameter, $b$, due to the non-linear relationship between $\tilde{\rho }$ and transit duration. This bias slightly favors low values ($b\lesssim 0.3$) and strongly disfavors high values ($b\gtrsim 0.7$) unless transit signal-to-noise ratio is sufficient to provide an independent constraint on $b$, a criterion that is not satisfied for the majority of Kepler planets. Planet-to-star radius ratio, $r$, is also biased due to $r-b$ covariance. Consequently, the median Kepler DR25 target suffers a $1.6\%$ systematic underestimate of $r$. We present a techniques for correcting these biases and for avoiding them in the first place.

12 pages, 7 figures, submitted to AAS journals

References in corpus (11)

• Array Programming with NumPy
• The Transiting Exoplanet Survey Satellite
• EXOFAST: A fast exoplanetary fitting suite in IDL
• Accurate, Empirical Radii and Masses of Planets and their Host Stars with Gaia Parallaxes
• Planetary Candidates Observed by Kepler VI: Planet Sample from Q1-Q16 (47 Months)
• exoplanet: Gradient-based probabilistic inference for exoplanet data & other astronomical time series
• Planetary Candidates Observed by Kepler V: Planet Sample from Q1-Q12 (36 Months)
• Analytic Approximations for Transit Light Curve Observables, Uncertainties, and Covariances
• A nearby M star with three transiting super-Earths discovered by K2
• Effects of Orbital Eccentricity on Extrasolar Planet Transit Detection and Lightcurves
• Accurate modeling of grazing transits using umbrella sampling

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