Keck/NIRSPEC Studies of He i in the Atmospheres of Two Inflated Hot Gas Giants Orbiting K Dwarfs: WASP-52b and WASP-177b

Abstract

We present the detection of neutral helium at 10833 Å in the atmosphere of WASP-52b and tentative evidence of helium in the atmosphere of the grazing WASP-177b, using high-resolution observations acquired with the NIRSPEC instrument on the Keck II telescope. We detect excess absorption by helium in WASP-52b’s atmosphere of 3.44% ± 0.31% (11σ), or equivalently 66 ± 5 atmospheric scale heights. This absorption is centered on the planet’s rest frame (Δv = 0.00 ± 1.19 km s−1). We model the planet’s escape using a 1D Parker wind model and calculate its mass-loss rate to be ∼1.4 × 1011 g s−1, or equivalently 0.5% of its mass per gigayear. For WASP-177b, we see evidence for redshifted (Δv = 6.02 ± 1.88 km s−1) helium-like absorption of 1.28% ± 0.29% (equal to 23 ± 5 atmospheric scale heights). However, due to residual systematics in the transmission spectrum of similar amplitude, we do not interpret this as significant evidence for He absorption in the planet’s atmosphere. Using a 1D Parker wind model, we set a 3σ upper limit on WASP-177b’s escape rate of 7.9 × 1010 g s−1. Our results, taken together with recent literature detections, suggest the tentative relation between XUV irradiation and He i absorption amplitude may be shallower than previously suggested. Our results highlight how metastable helium can advance our understanding of atmospheric loss and its role in shaping the exoplanet population.

Modeling the Hα and He 10830 Transmission Spectrum of WASP-52b

“…In the work of Oklopčić & Hirata (2018), q 13a = 4.5 × 10 −20 cm 3 s −1 , q 31a = 2.6 × 10 −8 cm 3 s −1 , and q 31b = 4.0 × 10 −9 cm 3 s −1 , evaluated at an "isothermal" temperature of 9000 K, were used. In a similar way, Kirk et al (2022) reproduced the He 10830 line of WASP-52b by assuming an "isothermal" Figure 29. (a) Source and sink terms related to the population of He(2 3 S) for the model with H/He = 98/2, F XUV = 0.5F 0 , and m = 0.3.…”

“…Assuming the same spectral shape as is observed for WASP-69b, WASP-52b was found to have an amplitude of 1.31% ± 0.94% in the deepest line center of the He 10830 triplet in the work of Vissapragada et al (2020). Using high-resolution spectroscopy, Kirk et al (2022) recently detected the neutral helium line at 10830 Å in the atmosphere of WASP-52b. They measured the spectral shape of the absorption line and found the excess absorption depth of He 10830 to be about 3.44% ± 0.31% (at 11σ significance).…”

“…We now compare the absorption depth at the He 10830 line center C(He 10830). According to Kirk et al (2022), the absorption depth of 3.44% ± 0.31% (11σ) was detected at the He 10830 line center. In comparing the absorption depth, we define the models with C(He 10830) located in the 3σ confidence ranges to be acceptable.…”

“…This assumption, however, can give a sense of what one might expect from high-resolution spectroscopy. Recently, Kirk et al (2022) measured the spectral shape of the absorption line of neutral helium at 10830 Å, using high-resolution spectroscopy, in the atmosphere of WASP-52b and estimated an excess absorption of He 10830 of ∼3.44% ± 0.31%. Our work is based on this spectroscopic observation.…”

“…They obtained that  M 1.25 10 10 < ´g s −1 (  M 1.25 < 10 11 g s −1 ) for an isothermal atmosphere with a temperature of 7000 K (12,000 K). In Kirk et al (2022), they used an isothermal Parker wind to model the He 10830 transmission spectrum of WASP-52b and found a mass-loss rate of about 1.4 × 10 11 g s −1 . In our hydrodynamic simulations, the mass-loss rate increases with F XUV and is slightly affected by β m and the H/He ratio.…”