Relative photometry of HAT-P-1b occultations

TL;DR

This study uses HST observations of HAT-P-1b occultations to measure its geometric albedo, leveraging a binary companion for improved photometric accuracy, resulting in an upper limit of 0.64 for the albedo.

Contribution

The paper demonstrates the advantage of using a binary companion as a reference star to reduce bias in photometric measurements of exoplanet albedo.

Findings

Achieved a 2 sigma upper limit of 0.64 for HAT-P-1b's geometric albedo.

Showed that using the binary companion as a reference star simplifies data detrending.

Reduced bias in albedo estimation by minimizing the detrending parameters.

Abstract

We present HST STIS observations of two occultations of the transiting exoplanet HAT-P-1b. By measuring the planet to star flux ratio near opposition, we constrain the geometric albedo of the planet, which is strongly linked to its atmospheric temperature gradient. An advantage of HAT-P-1 as a target is its binary companion ADS 16402 A, which provides an excellent photometric reference, simplifying the usual steps in removing instrumental artifacts from HST time-series photometry. We find that without this reference star, we would need to detrend the lightcurve with the time of the exposures as well as the first three powers of HST orbital phase, and this would introduce a strong bias in the results for the albedo. However, with this reference star, we only need to detrend the data with the time of the exposures to achieve the same per-point scatter, therefore we can avoid most of the bias associated with detrending. Our final result is a 2 sigma upper limit of 0.64 for the geometric albedo of HAT-P-1b between 577 and 947 nm.