ACCESS: An optical transmission spectrum of the warm Neptune HAT-P-26b

We present a new optical transmission spectrum of the warm Neptune HAT-P-26b taken with the Magellan/IMACS multi-object spectrograph as part of the ACCESS survey. We obtained spectrophotometry of HAT-P-26 as well as three comparison stars during the planet’s transits in the 0.5 to 0.9 micron wavelength range. While previous ACCESS works have used Gaussian processes (GPs) and principal component analysis (PCA) for light curve detrending, here we use the new GP method proposed by Panwar et al. (2022) for spectrophotometric detrending. In this method, a comparison star light curve is used directly as a GP regressor itself. Our multiple comparison stars allow us to test this new method together with our PCA framework. We present a preliminary comparison of our results with the standard ACCESS GP+PCA detrending technique. Determining the atmospheric composition of close-in Neptune-sized exoplanets such as HAT-P-26b through transmission spectra is key for distinguishing between core-accretion, in situ formation, or other potential formation scenarios, as the formation of this class of planets is still not well understood.