Escaping Helium from TOI 560.01, a Young Mini Neptune

TL;DR

This study detects helium absorption from the escaping atmosphere of the young mini Neptune TOI 560.01, analyzes its transit characteristics, and models the outflow, revealing complex interactions influenced by stellar wind, metallicity, and stellar EUV flux.

Contribution

First detection of helium escape in TOI 560.01, combined with detailed transit analysis and hydrodynamic modeling to understand atmospheric escape mechanisms.

Findings

Helium absorption detected with 0.68% depth during transit.

Absorption shows asymmetry and slight redshift, indicating complex outflow dynamics.

Models overpredict absorption magnitude, affected by metallicity and stellar EUV flux.

Abstract

We report helium absorption from the escaping atmosphere of TOI 560.01 (HD 73583b), a $R=2.8 R_\oplus$, $P=6.4$ d mini Neptune orbiting a young ($\sim$600 Myr) K dwarf. Using Keck/NIRSPEC, we detect a signal with an average depth of $0.68 \pm 0.08$% in the line core. The absorption signal repeats during a partial transit obtained a month later, but is marginally stronger and bluer, perhaps reflecting changes in the stellar wind environment. Ingress occurs on time, and egress occurs within 12 minutes of the white light egress, although absorption rises more gradually than it declines. This suggests that the outflow is slightly asymmetric and confined to regions close to the planet. The absorption signal also exhibits a slight 4 km/s redshift rather than the expected blueshift; this might be explained if the planet has a modest orbital eccentricity, although the radial velocity data disfavors such an explanation. We use XMM-Newton observations to reconstruct the high energy stellar spectrum and model the planet's outflow with 1D and 3D hydrodynamic simulations. We find that our models generally overpredict the measured magnitude of the absorption during transit, the size of the blueshift, or both. Increasing the metallicity to 100$\times$ solar suppresses the signal, but the dependence of the predicted signal strength on metallicity is non-monotonic. Decreasing the assumed stellar EUV flux by a factor of 3 likewise suppresses the signal substantially.