PEPSI’s non-detection of escaping hydrogen and metal lines adds to the enigma of WASP-12 b

ABSTRACT WASP-12 b is an ultra-hot Jupiter of special interest for atmospheric studies since it is on an inspiraling orbit in an extreme environment of intense radiation and circumstellar gas. Previously claimed detections of active mass-loss from this planet are controversial across the literature. To address this controversy, we obtain two new transit observations of WASP-12 b with the optical high-resolution PEPSI spectrograph on the Large Binocular Telescope. Contrary to previous work, we do not observe planetary H$\alpha$ absorption and rule out the amplitude of previously reported detections. Our non-detection may be limited by the sensitivity of our data or could indicate weaker mass-loss than suggested by previous studies. We conduct injection-recovery experiments to place constraints on the radial extent of WASP-12 b’s escaping atmosphere as probed by Balmer lines, but find that our data do not have the sensitivity to probe down to the planet’s Roche lobe. Using physically motivated models of atmospheric escape, we explore upper limit constraints on the planet’s mass-loss rate and deem the data quality in the wavelength regime of Balmer lines insufficient to determine a physically meaningful constraint. We also conduct a spectral survey of other optical absorbers to trace atmospheric circulation but detect no additional absorption. We conclude that previous claims of H$\alpha$ absorption from the atmosphere of WASP-12 b should be reevaluated. Given the anticipated line strength of Balmer/optical features, observing the atmosphere of this faint target will require stacking more observations even with the largest telescope facilities available.