Infrared observations of metastable 2 ^ { 3 } S 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 2 ^ { 3 } S 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 \pm 0.045 % ( 11.1 \sigma ) , 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 } \times 10 ^ { -4 } ~ { } M _ { \mathrm { J } } / \mathrm { Gyr } ( 3.32 ^ { +0.67 } _ { -0.56 } \times 10 ^ { -3 } ~ { } M _ { \mathrm { J } } / \mathrm { 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 \times 10 ^ { -4 } ~ { } M _ { \mathrm { J } } / \mathrm { Gyr } ( 2.1 \times 10 ^ { -3 } ~ { } M _ { \mathrm { J } } / \mathrm { Gyr } ) .
These results show that ultra-narrowband photometry can reliably quantify absorption in the metastable helium feature .