Multicolour photometry for exoplanet candidate validation

Abstract

Context. The TESS and PLATO missions are expected to find vast numbers of new transiting planet candidates. However, only a fraction of these candidates will be legitimate planets, and the candidate validation will require a significant amount of follow-up resources. Radial velocity (RV) follow-up can be carried out only for the most promising candidates around bright, slowly rotating, stars. Thus, before devoting RV resources to candidates, they need to be vetted using cheaper methods, and, in the cases for which an RV confirmation is not feasible, the candidate's true nature needs to be determined based on these alternative methods alone. Aims. We study the applicability of multicolour transit photometry in the validation of transiting planet candidates when the candidate signal arises from a real astrophysical source (transiting planet, eclipsing binary, etc.), and not from an instrumental artefact. Particularly, we aim to answer how securely can we estimate the true uncontaminated star-planet radius ratio when the light curve may contain contamination from unresolved light sources inside the photometry aperture when combining multicolour transit observations with a physics-based contamination model in a Bayesian parameter estimation setting. More generally, we study how the contamination level, colour differences between the planet host and contaminant stars, transit signal-to-noise ratio, and available prior information affect the contamination and true radius ratio estimates. Methods. The study is based on simulations and ground-based multicolour transit observations. The contamination analyses are carried out with a contamination model integrated into the PyTransit v2 transit modelling package, and the observations are carried out with the MuSCAT2 multicolour imager installed in the 1.5 m Telescopio Carlos Sanchez in the Teide Observatory, Tenerife. Results. We show that multicolour transit photometry can be used to estimate the amount of flux contamination and the true radius ratio. Combining the true radius ratio with an estimate for the stellar radius yields the true absolute radius of the transiting object, which is a valuable quantity in statistical candidate validation, and enough in itself to validate a candidate whose radius falls below the theoretical lower limit for a brown dwarf.

MuSCAT2 multicolour validation of TESS candidates: an ultra-short-period substellar object around an M dwarf

“…The MuS-CAT2 pipeline reduction also yields posterior estimates for the transit model, including a contamination estimate based on the approach described in Sect. 4.2 and detailed further in Parviainen et al (2019). While the final analysis presented in Sect.…”

“…The analyses were carried out with a custom Python code based on PyTransit v2 5 (Parviainen 2015;Parviainen et al 2019), which includes a physics-based contamination model based on the PHOENIX-calculated stellar spectrum library by Husser et al (2013). The limb darkening computations were carried out with LDTk 6 (Parviainen & Aigrain 2015), and MCMC sampling was carried out with emcee All the code for the analyses presented in this paper is available from GitHub as Python code and Jupyter notebooks from https://github.com/hpparvi/parviainen_2019b_ toi_263.…”

“…The candidate validation is based on the true planetary radius estimate obtained modelling the multicolour transit photometry with a transit model that includes a physics-based light contamination component. The contamination is parametrised by the effective temperatures of the host and the contaminant(s) (T Eff,H and T Eff,C , respectively), and the amount of contamination in some reference passband (c) (Parviainen et al 2019).…”

“…The contamination factor depends on the wavelength if the host and contaminant(s) are of different spectral type (that is, if T Eff,H T Eff,C ), which leads to passband dependent variations in the transit depth (k 2 app ). Besides the transit depth variations, a subtle colour dependent signature exists that is directly related to the true radius ratio, and constrains the contamination even if T Eff,H = T Eff,C (Drake 2003;Tingley 2004;Tingley et al 2014;Parviainen et al 2019).…”

TOI 564 b and TOI 905 b: Grazing and Fully Transiting Hot Jupiters Discovered by TESS

“…MuSCAT2 can take high-precision simultaneous multicolor photometry in the g′, r′, i′, and z s bands. Data were reduced with a dedicated MuSCAT2 pipeline (Parviainen et al 2019). The first night's observation only covered an ingress, as the observation was interrupted by high humidity.…”