Using the helium triplet as a tracer of the physics of giant planet outflows
File(s)
Published version
Author(s)
Schulik, Matthäus
Owen, James E
Type
Journal Article
Abstract
Hydrodynamic outflows, such as those observed escaping close-in gas giant planets, are not isothermal in structure. Their highly ionized nature allows them to cool adiabatically at distances beyond several planetary radii. The contrast between the hottest gas temperatures at around 10,000K and the coldest at around 1,000K triggers an excess population of the observable helium triplet. This excess is caused by the suppression of collisional de-excitation from the triplet state at cool temperatures. Using radiation-hydrodynamic simulations, we show that this helium triplet excess may explain the excess broadening seen in HD 189733b’s observed transmission spectrum, demonstrating adiabatic cooling of its outflow, confirming its hydrodynamic nature on scales of several planetary radii. However, further observations are required to confirm this conclusion. Furthermore, we explore a range of electron transitions for neutral helium which were not considered in the previous literature. We find that the He21S state is unavailable as a potential reservoir for He23S electrons. Additionally, the de-excitation to the ground state must be considered for stellar spectra later than K2 in predicting the correct helium triplet population. Importantly, since triplet helium inherits momentum from ionized helium as it is generated by recombination, it is significantly less prone to fractionation than ground-state neutral helium. However at separations of ≳ 0.05 au, ionization at the flow base and drag on helium weaken, leading to significant fractionation of the then mostly neutral helium. This in turn, can cause a suppression of the Helium transit depth, even though the helium line width remains large.
Date Issued
2025-09-01
Date Acceptance
2025-05-01
Citation
Monthly Notices of the Royal Astronomical Society, 2025, 542 (2), pp.927-946
ISSN
0035-8711
Publisher
Oxford University Press (OUP)
Start Page
927
End Page
946
Journal / Book Title
Monthly Notices of the Royal Astronomical Society
Volume
542
Issue
2
Copyright Statement
© The Author(s) 2025. Published by Oxford University Press on behalf of Royal Astronomical Society. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.
License URL
Identifier
10.1093/mnras/staf775/8128267
Subjects
Planets and satellites
atmospheres -Planets and satellites
detection
Publication Status
Published
Date Publish Online
2025-05-10