Spitzer 3.6 micron and 4.5 micron full-orbit lightcurves of WASP-18

TL;DR

This study presents detailed Spitzer lightcurves of WASP-18b, analyzing its thermal phase effect and system geometry, revealing low heat redistribution, low albedo, and consistent system parameters with previous findings, with improved precision.

Contribution

First detailed full-orbit infrared lightcurves of WASP-18 covering thermal phase effect and system geometry, with improved accuracy and constraints on atmospheric heat redistribution and companion stars.

Findings

Thermal phase effect has a sinusoidal shape with peak-to-peak amplitude similar to secondary eclipse depth.

Maximum brightness occurs near mid-occultation within a few degrees.

Low heat redistribution and low planetary albedo inferred from the data.

Abstract

We present new lightcurves of the massive hot Jupiter system WASP-18 obtained with the Spitzer spacecraft covering the entire orbit at 3.6 micron and 4.5 micron. These lightcurves are used to measure the amplitude, shape and phase of the thermal phase effect for WASP-18b. We find that our results for the thermal phase effect are limited to an accuracy of about 0.01% by systematic noise sources of unknown origin. At this level of accuracy we find that the thermal phase effect has a peak-to-peak amplitude approximately equal to the secondary eclipse depth, has a sinusoidal shape and that the maximum brightness occurs at the same phase as mid-occultation to within about 5 degrees at 3.6 micron and to within about 10 degrees at 4.5 micron. The shape and amplitude of the thermal phase curve imply very low levels of heat redistribution within the atmosphere of the planet. We also perform a separate analysis to determine the system geometry by fitting a lightcurve model to the data covering the occultation and the transit. The secondary eclipse depths we measure at 3.6 micron and 4.5 micron are in good agreement with previous measurements and imply a very low albedo for WASP-18b. The parameters of the system (masses, radii, etc.) derived from our analysis are in also good agreement with those from previous studies, but with improved precision. We use new high-resolution imaging and published limits on the rate of change of the mean radial velocity to check for the presence of any faint companion stars that may affect our results. We find that there is unlikely to be any significant contribution to the flux at Spitzer wavelengths from a stellar companion to WASP-18. We find that there is no evidence for variations in the times of eclipse from a linear ephemeris greater than about 100 seconds over 3 years.