A Six Year, Low-resolution, Multibroadband Transit Photometry Study of HD 189733b

Transmission spectroscopy offers an invaluable opportunity to characterize the atmospheres of exoplanets. We present new ground-based optical transmission spectra of the hot Jupiter HD 189733b, derived from nine transits observed over a six year time span (2016-2021) using near-simultaneous $u^{\prime} g^{\prime} r^{\prime} i^{\prime} $ broadband observations. We achieve an average (best) precision of 435 (280) ppm by implementing an optical diffuser on the prime focus spectrograph from the 2.3 m Wyoming Infrared Observatory telescope. The data provide new measurements of the apparent planetary radius with respect to the stellar radius, the spectral index of atmospheric opacity, and the time variability of the two quantities. Our results indicate an enhanced spectral slope in the optical regime ≈2.4 times steeper than would be expected from canonical Rayleigh scattering and that is consistent with earlier measurements of a super-Rayleigh slope (SRS). While the effect of stellar activity on the transmission spectrum complicates the measurement of the spectral slope, our multiepoch data set over six years can measure and average over stellar variations, yielding a mean spectral index of -9.9 ± 4.4. The 1200 K equilibrium temperature of HD 189733b places it in a sweet spot for the formation of SRSs and is consistent with vigorously mixing hazes in the atmosphere. Additionally, we find variations in the depth of the lightcurve during two of the transits, explainable as an increase in occulted star spots during June 2021. Although the star is active, the mean level of stellar activity does not seem to vary dramatically over our six years of observations, leading us to conclude that the variability in stellar activity is modest at most.