Keck/NIRSPEC studies of He I in the atmospheres of two inflated hot gas giants orbiting K dwarfs: WASP-52b and WASP-177b

TL;DR

This study detects helium in the atmospheres of two hot gas giants, WASP-52b and WASP-177b, using high-resolution spectroscopy, providing insights into atmospheric escape and mass-loss rates.

Contribution

First detection of helium in WASP-52b's atmosphere and tentative evidence in WASP-177b, employing high-resolution Keck/NIRSPEC observations and modeling atmospheric escape.

Findings

WASP-52b shows 3.44% helium absorption with 11σ significance.

WASP-177b shows tentative helium absorption, but not statistically significant.

Mass-loss rate for WASP-52b estimated at 1.4×10^{11} g/s.

Abstract

We present the detection of neutral helium at 10833A 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 $3.44 \pm 0.31$% ($11\sigma$), or equivalently $66 \pm 5$ atmospheric scale heights. This absorption is centered on the planet's rest frame ($\Delta v = 0.00 \pm 1.19$km s$^{-1}$). We model the planet's escape using a 1D Parker wind model and calculate its mass-loss rate to be $\sim 1.4 \times 10^{11}$g s$^{-1}$, or equivalently 0.5% of its mass per Gyr. For WASP-177b, we see evidence for red-shifted ($\Delta v = 6.02 +/- 1.88$km s$^{-1}$) helium-like absorption of $1.28 \pm 0.29$% (equal to $23 \pm 5$ 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 $3\sigma$ upper limit on WASP-177b's escape rate of $7.9 \times 10^{10}$ 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.