Constraints on Metastable Helium in the Atmospheres of WASP-69b and WASP-52b with Ultra-Narrowband Photometry

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 1083.3 nm) 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 ultra-narrowband filter (full-width at half-maximum of 0.635 nm) coupled to a beam-shaping diffuser installed in the Wide-field Infrared Camera (WIRC) on the 200-inch 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% (11.1σ), consistent with previous measurements after considering our bandpass. Then, we 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^(+0.65)_(−0.46) × 10⁻⁴ M_J/Gyr (3.32^(+0.67)_(−0.56) × 10⁻³ M_J/Gyr) at 7,000 K (12,000 K). Additionally, we set an upper limit on the mass loss rate of WASP-52b at these temperatures of 2.1 × 10⁻⁴ MJ/Gyr (2.1×10⁻³ M_J/Gyr). These results show that ultra-narrowband photometry can reliably quantify absorption in the metastable helium feature.