ACCESS & LRG-BEASTS: a precise new optical transmission spectrum of the ultrahot Jupiter WASP-103b

TL;DR

This paper presents a high-precision optical transmission spectrum of the ultrahot Jupiter WASP-103b, combining multiple ground-based transits to analyze atmospheric composition and temperature inversion, highlighting the importance of stellar activity effects.

Contribution

It provides the most precise ground-based optical transmission spectrum of WASP-103b by combining multiple transits and reanalyzing archival data, revealing atmospheric features and the influence of stellar activity.

Findings

Strong evidence for unocculted bright regions on the star.

Weak evidence for water, HCN, and TiO in the atmosphere.

Optical spectrum matches features seen in WASP-121b, suggesting similar atmospheric phenomena.

Abstract

We present a new ground-based optical transmission spectrum of the ultrahot Jupiter WASP-103b ($T_{eq} = 2484$K). Our transmission spectrum is the result of combining five new transits from the ACCESS survey and two new transits from the LRG-BEASTS survey with a reanalysis of three archival Gemini/GMOS transits and one VLT/FORS2 transit. Our combined 11-transit transmission spectrum covers a wavelength range of 3900--9450A with a median uncertainty in the transit depth of 148 parts-per-million, which is less than one atmospheric scale height of the planet. In our retrieval analysis of WASP-103b's combined optical and infrared transmission spectrum, we find strong evidence for unocculted bright regions ($4.3\sigma$) and weak evidence for H$_2$O ($1.9\sigma$), HCN ($1.7\sigma$), and TiO ($2.1\sigma$), which could be responsible for WASP-103b's observed temperature inversion. Our optical transmission spectrum shows significant structure that is in excellent agreement with the extensively studied ultrahot Jupiter WASP-121b, for which the presence of VO has been inferred. For WASP-103b, we find that VO can only provide a reasonable fit to the data if its abundance is implausibly high and we do not account for stellar activity. Our results highlight the precision that can be achieved by ground-based observations and the impacts that stellar activity from F-type stars can have on the interpretation of exoplanet transmission spectra.