[Paper Review] The dispersal of protoplanetary discs -- III: Influence of stellar mass on disc photoevaporation

Giovanni Picogna, Barbara Ercolano+1 more|arXiv (Cornell University)|Oct 4, 2021

Astrophysics and Star Formation Studies47 references62 citations

TL;DR

This study models X-ray photoevaporation in protoplanetary discs around low-mass stars (0.1–1 M⊙), showing that mass-loss rates scale linearly with stellar mass due to changes in disc aspect ratio and X-ray penetration. The models successfully reproduce the observed inside-out dispersal trend, with shorter inner-disc lifetimes for lower-mass stars, confirming X-ray photoevaporation as a dominant dispersal mechanism in late-stage disc evolution.

ABSTRACT

The strong X-ray irradiation from young solar-type stars may play a crucial role in the thermodynamics and chemistry of circumstellar discs, driving their evolution in the last stages of disc dispersal as well as shaping the atmospheres of newborn planets. In this paper we study the influence of stellar mass on circumstellar disc mass-loss rates due to X-ray irradiation, extending our previous study of the mass-loss rate's dependence on the X-ray luminosity and spectrum hardness. We focus on stars with masses between 0.1 and 1 Solar mass, which are the main target of current and future missions to find potentially habitable planets. We find a linear relationship between the mass-loss rates and the stellar masses when changing the X-ray luminosity accordingly with the stellar mass. This linear increase is observed also when the X-ray luminosity is kept fixed because of the lower disc aspect ratio which allows the X-ray irradiation to reach larger radii. We provide new analytical relations for the mass-loss rates and profiles of photoevaporative winds as a function of the stellar mass that can be used in disc and planet population synthesis models. Our photoevaporative models correctly predict the observed trend of inner-disc lifetime as a function of stellar mass with an increased steepness for stars smaller than 0.3 Solar mass, indicating that X-ray photoevaporation is a good candidate to explain the observed disc dispersal process.

Motivation & Objective

• To investigate how stellar mass influences X-ray-driven photoevaporative mass-loss in protoplanetary discs.
• To determine whether X-ray photoevaporation can explain the observed trend of shorter inner-disc lifetimes for lower-mass stars.
• To derive analytical relations for mass-loss rates and wind profiles as functions of stellar mass for use in population synthesis models.
• To test the robustness of photoevaporation models against observational constraints from young clusters like λ Ori.
• To improve the physical fidelity of disc evolution models by incorporating stellar mass-dependent disc structure and irradiation effects.

Proposed method

• Hydrodynamical simulations using a modified PLUTO code in spherical coordinates with high radial resolution (500 cells) and polar refinement near the wind-launching region.
• Initial disc structure derived from DIAD radiative transfer models, assuming 1 Myr old stars with Z = 0.02 and no overshooting.
• X-ray irradiation modeled using stellar X-ray luminosity scaling with mass via log10(L_X) = 1.54 log10(M★) + 30.31, based on Güdel et al. (2007).
• Temperature profiles in disc surfaces computed using the MOCCaSS code, accounting for X-ray heating and ionization parameter ξ = L_X / (n r²).
• Steady-state wind solutions reached after evolving the system for several hundred orbits, with mass-loss rates measured from cumulative outflow.
• Analytical fitting functions for surface mass-loss rate profiles derived across stellar masses (0.1–1 M⊙), enabling use in population synthesis codes.

Experimental results

Research questions

• RQ1 How does stellar mass affect the X-ray-driven photoevaporation rate in protoplanetary discs?
• RQ2 What role does disc aspect ratio play in modulating X-ray penetration and wind efficiency across different stellar masses?
• RQ3 Can X-ray photoevaporation alone explain the observed trend of decreasing inner-disc lifetime with decreasing stellar mass?
• RQ4 How do the resulting mass-loss rates and wind profiles scale with stellar mass, and can they be described by analytical relations?
• RQ5 To what extent do the model predictions match observed disc fractions in young clusters like λ Ori?

Key findings

• The cumulative mass-loss rate due to X-ray photoevaporation scales linearly with stellar mass when X-ray luminosity is scaled according to the observed L_X ∝ M★^1.54 relation.
• For fixed X-ray luminosity, mass-loss rates still increase with decreasing stellar mass due to flatter disc aspect ratios allowing deeper X-ray penetration and enhanced wind formation.
• The temperature at the sonic surface of the wind scales linearly with stellar mass, indicating a direct thermal coupling between stellar output and disc wind energetics.
• The models reproduce the observed trend of inner-disc lifetimes increasing with stellar mass, with a steeper decline for stars below 0.3 M⊙, matching observational data from λ Ori.
• Analytical fitting functions for surface mass-loss rate profiles are provided in Table 2, enabling direct implementation in disc and planet population synthesis models.
• The model with initial disc mass scaling linearly as 0.14 M★ best matches the observed disc fraction in λ Ori, outperforming previous models with fixed or sub-linear scaling.