Physical properties and radius variations in the HAT-P-5 planetary system from simultaneous four-colour photometry

TL;DR

This study uses simultaneous multi-wavelength photometry to investigate radius variations in the HAT-P-5 planetary system, revealing larger apparent radii at shorter wavelengths likely due to observational systematics rather than atmospheric effects.

Contribution

It provides the first simultaneous multi-band transit observations of HAT-P-5 and refines its physical parameters, highlighting potential systematic errors in u-band measurements.

Findings

Detected larger planetary radius in u-band compared to other wavelengths.

Refined the orbital ephemeris and physical properties of HAT-P-5b.

Identified systematic errors likely affecting short-wavelength photometry.

Abstract

The radii of giant planets, as measured from transit observations, may vary with wavelength due to Rayleigh scattering or variations in opacity. Such an effect is predicted to be large enough to detect using ground-based observations at multiple wavelengths. We present defocussed photometry of a transit in the HAT-P-5 system, obtained simultaneously through Stromgren u, Gunn g and r, and Johnson I filters. Two more transit events were observed through a Gunn r filter. We detect a substantially larger planetary radius in u, but the effect is greater than predicted using theoretical model atmospheres of gaseous planets. This phenomenon is most likely to be due to systematic errors present in the u-band photometry, stemming from variations in the transparency of Earth's atmosphere at these short wavelengths. We use our data to calculate an improved orbital ephemeris and to refine the measured physical properties of the system. The planet HAT-P-5b has a mass of 1.06 +/- 0.11 +/- 0.01 Mjup and a radius of 1.252 +/- 0.042 +/- 0.008 Rjup (statistical and systematic errors respectively), making it slightly larger than expected according to standard models of coreless gas-giant planets. Its equilibrium temperature of 1517 +/- 29 K is within 60K of that of the extensively-studied planet HD 209458b.