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Description
Infrared observations of metastable 23S helium absorption with ground- and space-based spectroscopy are rapidly maturing, as this species is a unique probe of exoplanet atmospheres. Specifically, the transit depth in the triplet feature (with vacuum wavelengths near 1083.3nm) can be used to constrain the temperature and mass-loss rate of an exoplanet's upper atmosphere. Here, we present a new photometric technique to measure metastable 23S helium absorption using an ultranarrowband filter (FWHM 0.635nm) coupled to a beam-shaping diffuser installed in the Wide-field Infrared Camera on the 200inch Hale Telescope at Palomar Observatory. We use telluric OH lines and a helium arc lamp to characterize refractive effects through the filter and to confirm our understanding of the filter transmission profile. We benchmark our new technique by observing a transit of WASP-69b and detect an excess absorption of 0.498%{+/-}0.045% (11.1{sigma}), consistent with previous measurements after considering our bandpass. We then use this method to study the inflated gas giant WASP-52b and place a 95th percentile upper limit on excess absorption in our helium bandpass of 0.47%. Using an atmospheric escape model, we constrain the mass-loss rate for WASP-69b to be 5.25_-0.46_^+0.65^x10^-4^M_J_/Gyr (3.32_-0.56_^+0.67^x10^-3^M_J_/Gyr) at 7000K (12000K). Additionally, we set an upper limit on the mass-loss rate of WASP-52b at these temperatures of 2.1x10^-4^M_J_/Gyr (2.1x10^-3^M_J_/Gyr). These results show that ultranarrowband photometry can reliably quantify absorption in the metastable helium feature.
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