[Paper Review] Mapping out the parameter space for photoevaporation and core-powered mass-loss

James E. Owen, Hilke E. Schlichting|arXiv (Cornell University)|Jul 31, 2023

Stellar, planetary, and galactic studiesPhysics and Astronomy3 citations

TL;DR

This paper maps the parameter space where core-powered mass-loss and photoevaporation dominate atmospheric escape in close-in exoplanets. It shows that the transition between regimes is determined by the ratio of planetary radius to Bondi radius, with core-powered mass-loss dominating for low-gravity, hot planets and photoevaporation for high-gravity, cooler ones, and that planets can evolve from core-powered to photoevaporative mass-loss but not vice versa.

ABSTRACT

Understanding atmospheric escape in close-in exoplanets is critical to interpreting their evolution. We map out the parameter space over which photoevaporation and core-powered mass loss dominate atmospheric escape. Generally, the transition between the two regimes is determined by the location of the Bondi radius (i.e. the sonic point of core-powered outflow) relative to the penetration depth of XUV photons. Photoevaporation dominates the loss when the XUV penetration depth lies inside the Bondi radius ($R_{XUV}

Motivation & Objective

• Understand the conditions under which core-powered mass-loss and photoevaporation dominate atmospheric escape in close-in exoplanets.
• Identify the physical parameters that determine the transition between the two mass-loss regimes.
• Investigate how planetary evolution affects the dominance of one mechanism over the other during a planet's lifetime.
• Assess the implications of these regimes for the observed exoplanet population, particularly the radius-valley feature.
• Determine whether core-powered mass-loss can enhance photoevaporation in certain regions of parameter space.

Proposed method

• Uses semi-analytic modeling to evaluate the penetration depth of XUV photons relative to the Bondi radius (sonic point of core-powered outflow).
• Defines the transition between regimes by comparing XUV penetration depth (R_XUV) to the Bondi radius (R_B): photoevaporation dominates when R_XUV < R_B, core-powered mass-loss when R_B < R_XUV.
• Derives the critical R_p/R_B ratio at which the transition occurs, showing it is approximately constant (~1/5–1/9) and only weakly dependent on stellar and planetary parameters.
• Applies the derived transition criterion to observed super-Earth populations to infer the relative contributions of each mechanism to atmospheric stripping.
• Considers the evolutionary sequence of planets, assessing whether transitions from one regime to another are possible.
• Evaluates the potential for core-powered mass-loss to enhance photoevaporation even when photoevaporation dominates.

Experimental results

Research questions

• RQ1 At what planetary and stellar parameter values does core-powered mass-loss transition to photoevaporation?
• RQ2 How does the R_p/R_B ratio determine the dominant mass-loss mechanism?
• RQ3 Can a planet transition from core-powered mass-loss to photoevaporation during its evolution, and if so, under what conditions?
• RQ4 To what extent can core-powered mass-loss enhance photoevaporation in the same parameter space?
• RQ5 Which mass-loss mechanism is primarily responsible for the observed radius-valley in exoplanet populations?

Key findings

• The transition between core-powered mass-loss and photoevaporation occurs at a roughly constant R_p/R_B ratio of approximately 1/5 to 1/9, with only logarithmic dependence on planetary and stellar properties.
• Core-powered mass-loss dominates for low-gravity, high-equilibrium-temperature planets, while photoevaporation dominates for high-gravity, low-equilibrium-temperature planets.
• Planets can transition from core-powered mass-loss to photoevaporation during evolution, but not vice versa, meaning a planet stripped by core-powered mass-loss has never experienced photoevaporation.
• A planet stripped by photoevaporation may have undergone an early phase of core-powered mass-loss, indicating that core-powered mass-loss can precede and enhance photoevaporation.
• In the observed super-Earth population, significant fractions of planets experienced either core-powered mass-loss or photoevaporation as the final stripping mechanism.
• Photoevaporation is likely responsible for the final carving of the exoplanet radius-valley, while core-powered mass-loss played a key role earlier in planetary evolution, particularly in shaping initial atmospheric loss.