Hubble Space Telescope observations of the NUV transit of WASP-12b

TL;DR

This study uses HST/COS to observe four NUV transits of exoplanet WASP-12b, revealing significant excess absorption, asymmetric transit shapes, and variability, providing new insights into the planet's atmospheric properties.

Contribution

It presents high-resolution NUV transit observations of WASP-12b, showing variability and asymmetry, and refines understanding of its atmospheric absorption features.

Findings

Significant excess NUV absorption during transit

Asymmetric transit shape with post-conjunction flux higher

No evidence for early ingress in NUV transit

Abstract

We present new observations of four closely-spaced NUV transits of the hot Jupiter-like exoplanet WASP-12b using HST/COS, significantly increasing the phase resolution of the observed NUV light curve relative to previous observations, while minimising the temporal variation of the system. We observe significant excess NUV absorption during the transit, with mean normalised in-transit fluxes of $F_\mathrm{norm}\simeq0.97$, i.e. $\simeq$2-5 $\sigma$ deeper than the optical transit level of $\simeq0.986$ for a uniform stellar disk (the exact confidence level depending on the normalisation method used). We further observe an asymmetric transit shape, such that the post-conjunction fluxes are overall $\simeq$2-3 $\sigma$ higher than pre-conjunction values, and characterised by rapid variations in count rate between the pre-conjunction and out of transit levels. We do not find evidence for an early ingress to the NUV transit as suggested by earlier HST observations. However, we show that the NUV count rate observed prior to the optical transit is highly variable, but overall $\simeq$2.2-3.0 $\sigma$ below the post-transit values and comparable in depth to the optical transit, possibly forming a variable region of NUV absorption from at least phase $\phi\simeq$0.83, limited by the data coverage.