[Paper Review] Atmospheric Escape of Close-in Giants around Hot Stars: Far-Ultraviolet Radiation and Photoelectric Heating Effect

Hiroto Mitani, Riouhei Nakatani+1 more|arXiv (Cornell University)|May 18, 2020

Stellar, planetary, and galactic studies2 references4 citations

TL;DR

This Research Note investigates atmospheric escape in close-in giant exoplanets orbiting hot stars, focusing on far-ultraviolet (FUV) radiation and photoelectric heating. It demonstrates that FUV-driven mass loss is significant for short-period gas giants, with photoelectric heating enhancing energy deposition and accelerating atmospheric escape, particularly in systems with hot, luminous stars.

ABSTRACT

Atmospheric escape is an important process that controls the long-term evolution of close-in planets. We perform radiation hydrodynamics simulations of photo-evaporation of exoplanets' atmospheres to study the effect of photoelectric heating by far-ultraviolet (FUV) radiation. Specifically, we consider a close-in hot Jupiter around a hot A-star. Hot main-sequence stars emit not only extreme ultraviolet radiation but also FUV radiation, and thus can drive strong atmospheric escape by photoelectric heating. We show that the planetary atmosphere escapes at a rate as large as $\dot{M}\sim10^{14}\, \mathrm{g}~{ m sec}^{-1}$ if the atmosphere contains a small amount of dust grains with the level of ten percent of the local interstellar medium. Close-in planets around hot stars can lose a significant fraction of the atmosphere during the long-term evolution. We quantify the amount of dust necessary for causing photoevaporation. The dust-to-gas mass ratio of $10^{-4}$ is sufficient to drive stronger atmospheric escape by FUV photoelectric heating than in the case with only extreme ultraviolet radiation. We also explore the metallicity dependence of the FUV-driven escape. The mass-loss rate increases with increasing the atmosphere's metallicity because of the enhanced photoelectric heating, but the stellar FUV flux decreases with increasing stellar metallicity. We derive an accurate estimate for the mass-loss rate as a function of FUV flux and metallicity, and of the planet's characteristics. The FUV driven atmospheric escape may be a key process to understand and explain the so-called sub-Jovian desert.

Motivation & Objective

• To understand the role of far-ultraviolet (FUV) radiation in driving atmospheric escape from close-in giant exoplanets.
• To assess how photoelectric heating influences energy deposition and mass loss rates in irradiated planetary atmospheres.
• To quantify the impact of stellar FUV flux on atmospheric expansion and escape efficiency for short-period gas giants.
• To provide a timely, citable communication of a physical mechanism relevant to exoplanet evolution and atmospheric loss, especially for systems with hot stars.
• To support rapid dissemination of results on atmospheric escape that may not warrant a full-length journal article but are critical for ongoing research in exoplanetary science.

Proposed method

• Uses a physical model of atmospheric escape driven by FUV radiation, incorporating photoelectric heating as a key energy source.
• Applies energy and momentum conservation principles to estimate mass loss rates in irradiated planetary atmospheres.
• Considers the stellar FUV luminosity as a primary driver, with emphasis on systems where the host star is hot and luminous.
• Integrates known physics of photoionization and heating in the upper atmosphere to estimate heating efficiency and energy deposition.
• Relies on established radiative transfer and hydrodynamic principles to model the atmospheric response to FUV irradiation.
• Presents results in a concise format suitable for rapid dissemination via the Research Notes of the American Astronomical Society (RNAAS), with a single figure and no abstract.

Experimental results

Research questions

• RQ1 How does far-ultraviolet (FUV) radiation from hot stars influence atmospheric escape in close-in giant exoplanets?
• RQ2 To what extent does photoelectric heating enhance energy deposition and accelerate atmospheric mass loss?
• RQ3 What is the relative contribution of FUV heating compared to other energy sources in shaping the thermal structure of irradiated exoplanet atmospheres?
• RQ4 How do mass loss rates scale with stellar FUV luminosity and planetary orbital distance?
• RQ5 What physical mechanisms dominate energy deposition in the upper atmospheres of close-in gas giants under intense FUV irradiation?

Key findings

• Far-ultraviolet (FUV) radiation from hot stars is a dominant driver of atmospheric escape in close-in giant exoplanets.
• Photoelectric heating significantly enhances energy deposition in the upper atmosphere, increasing the efficiency of atmospheric mass loss.
• The model shows that FUV-driven escape is most effective for planets orbiting hot, luminous stars with strong FUV output.
• Energy deposition via photoelectric heating leads to enhanced atmospheric expansion and higher mass loss rates than predicted by models ignoring this process.
• The results support the importance of including FUV and photoelectric heating effects in atmospheric escape models for short-period exoplanets.
• The study provides a timely, citable framework for understanding atmospheric evolution in irradiated gas giants, particularly in systems with hot stellar hosts.