Transiting exoplanets and magnetic spots characterized with optical interferometry

TL;DR

This study analyzes how stellar spots and transiting exoplanets affect optical interferometry measurements, emphasizing the need for improved instrument sensitivity to distinguish signals and better characterize stellar activity and exoplanets.

Contribution

The paper introduces a numerical tool and detailed analysis of interferometric observables to differentiate between exoplanet and stellar spot signals in the visible domain.

Findings

Instrument sensitivity must improve by a factor of 10 for 0.1 mas exoplanet detection.

Phase measurements of about 5° are needed beyond the first null for detection.

Phases from the 3rd lobe can distinguish spots from exoplanets with same radius.

Abstract

Stellar activity causes difficulties in the characterization of transiting exoplanets. Studies have been performed to quantify its impact on infrared interferometry, but not in the visible domain, which however allows reaching better angular resolution and is also the one mostly used for spectroscopic and photometric measurements. We use a standard case to completely analyse the impact of an exoplanet and a spot on interferometric observables and relate it to current instrument capabilities, taking into account realistic achievable precisions. We built a numerical code called COMETS using analytical formulae to perform a simple comparison of exoplanet and spot signals. We explore instrumental specificities needed to detect them, like the baseline length required, the accuracy and S/N. We also discuss the impact of exoplanet and spot parameters on squared visibility and phase. We find that the main improvement to bring is the sensitivity of instruments. The accuracy on squared visibilities has to be improved by a factor 10 to detect an exoplanet of 0.1 mas, leading to $<0.5%$ precision, along with phase measurements of ~$5^{\deg}$ accuracy beyond the first null of visibility. For a 0.05 mas exoplanet, accuracies of ~$0.1%$ and ~$1^{\deg}$ from the first null are required on squared visibilities and phases, respectively. Magnetic spots can mimic these signals, leading to false exoplanet characterization. Phases measurements from the 3rd lobe is needed to distinguish between a spot and an exoplanet if they have the same radius. Increasing interferometer sensitivity, more objects will become common between interferometric targets and photometric ones. Furthermore, new missions like PLATO, CHEOPS or TESS will provide bright exoplanet host stars. Measurements will thus overlap and provide a better characterization of stellar activity and exoplanet.