Using the helium triplet as a tracer of the physics of giant planet outflows
arXiv:2412.05258 · doi:10.1093/mnras/staf775
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 HeS state is unavailable as a potential reservoir for HeS 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 ~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.
arXiv commentAccepted for publication in MNRAS, 19 PDF pages
References in corpus (61)
- The California-Kepler Survey. III. A Gap in the Radius Distribution of Small Planets
- The evaporation valley in the Kepler planets
- Atmospheric Escape from Hot Jupiters
- Detection of oxygen and carbon in the hydrodynamically escaping atmosphere of the extrasolar planet HD209458b
- A giant comet-like cloud of hydrogen escaping the warm Neptune-mass exoplanet GJ 436b
- Roche lobe effects on the atmospheric loss of "Hot Jupiters"
- Planetary evaporation by UV & X-ray radiation: basic hydrodynamics
- Sculpting the Valley in the Radius Distribution of Small Exoplanets as a by-product of Planet Formation: The Core-Powered Mass-Loss Mechanism
- Helium in the eroding atmosphere of an exoplanet
- Atmospheric Escape and the Evolution of Close-in Exoplanets
- Temporal variations in the evaporating atmosphere of the exoplanet HD 189733b
- Solar UV and X-Ray Spectral Diagnostics
- The escape of heavy atoms from the ionosphere of HD209458b. I. A photochemical-dynamical model of the thermosphere
- A New Window into Escaping Exoplanet Atmospheres: 10830 Å Line of Helium
- CHIANTI - an atomic database for emission lines - Paper XV: Version 9, improvements for the X-ray satellite lines
- VULCAN: an Open-Source, Validated Chemical Kinetics Python Code for Exoplanetary Atmospheres
- 3D model of hydrogen atmospheric escape from HD209458b and HD189733b: radiative blow-out and stellar wind interactions
- The HST PanCET Program: Exospheric Mg II and Fe II in the Near-UV transmission spectrum of WASP-121b using Jitter Decorrelation
- Detection of He I Å absorption on HD 189733 b with CARMENES high-resolution transmission spectroscopy
- Magnetic moment and plasma environment of HD 209458b as determined from Ly observations
- Energy-limited escape revised
- The California-Kepler Survey. X. The Radius Gap as a Function of Stellar Mass, Metallicity, and Age
- UV driven evaporation of close-in planets: energy-limited; recombination-limited and photon-limited flows
- Photoevaporation vs. core-powered mass-loss: model comparison with the 3D radius gap
- Helium Absorption at 1083 nm from Extended Exoplanet Atmospheres: Dependence on Stellar Radiation
- New views of the solar wind with the Lambert W function
- He I 10830 Å in the transmission spectrum of HD 209458 b
- The Upper Atmospheres of Terrestrial Planets: Carbon Dioxide Cooling and the Earth's Thermospheric Evolution
- Modelling the He I triplet absorption at 10830 Angstroms in the atmosphere of HD 209458 b
- Simulated Photoevaporative Mass Loss from Hot Jupiters in 3D
- The Upper Edge of the Neptune Desert Is Stable Against Photoevaporation
- p-winds: an open-source Python code to model planetary outflows and upper atmospheres
- The sub-Neptune desert and its dependence on stellar type: Controlled by lifetime X-ray irradiation
- Global 3D hydrodynamic modeling of in-transit Lyα absorption of GJ436b
- Creating the Radius Gap without Mass Loss
- A fading radius valley towards M-dwarfs, a persistent density valley across stellar types
- Stellar Wind Confinement of Evaporating Exoplanet Atmospheres and its Signatures in 1083 nm Observations
- Escaping particle fluxes in the atmospheres of close-in exoplanets. ii. reduced mass loss rates and anisotropic winds
- The post-transit tail of WASP-107b observed at 10830A
- Irradiation-driven escape of primordial planetary atmospheres I. The ATES photoionization hydrodynamics code
- Modelling the He I triplet absorption at 10830 Angstroms in the atmospheres of HD 189733 b and GJ 3470 b
- Metastable Helium Absorptions with 3D Hydrodynamics and Self-Consistent Photochemistry I: WASP-69b, Dimensionality, XUV Flux Level, Spectral Types, and Flares
- Metastable Helium Absorptions with 3D Hydrodynamics and Self-Consistent Photochemistry II: WASP-107b, Stellar Wind, Radiation Pressure, and Shear Instability
- The fundamentals of Lyman-alpha exoplanet transits
- Formation of Venus, Earth and Mars: Constrained by isotopes
- Homogeneous search for helium in the atmosphere of 11 gas giant exoplanets with SPIRou
- MOVES III. Simultaneous X-ray and ultraviolet observations unveiling the variable environment of the hot Jupiter HD 189733b
- A second order positivity preserving well-balanced finite volume scheme for Euler equations with gravity for arbitrary hydrostatic equilibria
- Effects of Magnetic Fields on the Location of the Evaporation Valley for Low-Mass Exoplanets
- Disk-fed giant planet formation
- Helium Enhanced Planets Along the Upper Edge of the Radius Valley
- Modeling H and He 10830 transmission spectrum of WASP-52b
- Aiolos- A multi-purpose 1-D hydrodynamics code for planetary atmospheres
- The open-source sunbather code: modeling escaping planetary atmospheres and their transit spectra
- A high performance and portable all-Mach regime flow solver code with well-balanced gravity. Application to compressible convection
- Expanding the inventory of spectral lines used to trace atmospheric escape in exoplanets
- Hydrodynamic atmospheric escape in HD 189733 b: Signatures of carbon and hydrogen measured with the Hubble Space Telescope
- An efficient Monte Carlo model for the slowing down of photoelectrons. Application to H- in exoplanet atmospheres
- Observing atmospheric escape in sub-Jovian worlds with JWST
- Evolution of helium triplet transits of close-in gas giants orbiting K-dwarfs
- Mysterious non-detection of HeI (23S) transit absorption of GJ436b