More Evidence for Variable Helium Absorption from HD 189733b

Abstract

We present a new Keck/NIRSPEC observation of metastable helium absorption from the upper atmosphere of HD 189733b, a hot Jupiter orbiting a nearby moderately active star. We measure an average helium transit depth of 0.420% ± 0.013% integrated over the [−20, 20] km s⁻¹ velocity range. Comparing this measurement to eight previously published transit observations with different instruments, we find that our depth is 32% (9σ) lower than the average of the three CARMENES transits, but only 16% (4.4σ) lower than the average of the five GIANO transits. We perform 1D hydrodynamical simulations of the outflow, and find that XUV variability on the order of 33%–common for this star–can change the helium absorption depth by up to 60%, although a more typical change is 15%. We conclude that changes in stellar XUV flux can explain the observational variability in helium absorption, but that variability in the stellar He line cannot be excluded. 3D models are necessary to explore other sources of variability, such as shear instability and changing stellar wind conditions.