[Paper Review] Dispersal of protoplanetary discs: How stellar properties and the local environment determine the pathway of evolution

Gavin A. L. Coleman, Thomas J. Haworth|arXiv (Cornell University)|Apr 5, 2022

Astrophysics and Star Formation StudiesPhysics and Astronomy90 references53 citations

TL;DR

This study models the combined effects of internal and external photoevaporation on viscously evolving protoplanetary discs, identifying five distinct dispersal pathways—ranging from inside-out and outside-in clearing to intermediate regimes with lingering inner material. The key finding is that high UV environments suppress transition disc formation due to rapid outer disc dispersal, explaining their under-abundance in regions like the Orion Nebula Cluster.

ABSTRACT

We study the evolution and final dispersal of protoplanetary discs that evolve under the action of internal and external photoevaporation, and different degrees of viscous transport. We identify five distinct dispersal pathways, which are i) very long lived discs ($>20\,$Myr), ii) inside-out dispersal where internal photoevaporation dominates and opens inner holes, iii) outside-in dispersal where external photoevaporation dominates through disc truncation and two intermediate regimes characterised by lingering material in the inner disc with the outer disc dispersed predominantly by either internal or external photoevaporation. We determine how the lifetime, relative impact of internal and external winds and clearing pathway varies over a wide, plausible, parameter space of stellar/disc/radiation properties. There are a number of implications, for example in high UV environments because the outer disc lifetime is shorter than the time-scale for clearing the inner disc we do not expect transition discs to be common, which appears to be reflected in the location of transition disc populations towards the Orion Nebular Cluster. Irrespective of environment, we find that ongoing star formation is required to reproduce observed disc fractions as a function of stellar cluster age. This work demonstrates the importance of including both internal and external winds for understanding protoplanetary disc evolution.

Motivation & Objective

• To understand how stellar properties and local environments jointly determine the evolution and dispersal of protoplanetary discs.
• To resolve the challenge of estimating disc lifetimes due to observational snapshots and ongoing star formation in clusters.
• To investigate the interplay between internal photoevaporation (X-ray/EUV) and external photoevaporation (FUV from nearby massive stars) in shaping disc evolution.
• To determine whether observed disc fractions and the prevalence of transition discs can be reproduced without fine-tuning disc lifetimes.
• To assess the role of viscous transport and its impact on dispersal pathways across a wide parameter space.

Proposed method

• Uses a 1D viscous disc model with time-dependent surface density evolution governed by the diffusion equation.
• Incorporates mass loss via internal photoevaporation (X-ray and EUV from central star) and external photoevaporation (FUV from nearby massive stars).
• Applies the maximum of internal and external photoevaporation rates at each radius: ẟΣPE(r) = max(ẟΣI,EUV, ẟΣI,X, ẟΣE,FUV).
• Varying stellar mass, internal X-ray luminosity, external FUV flux, and viscosity (α) across a wide, observationally motivated parameter space.
• Tracks disc evolution over time to identify distinct dispersal pathways based on clearing sequence (inside-out vs. outside-in).
• Empirically defines a 'clearing radius' to distinguish between inside-out and outside-in clearing regimes and identify intermediate states.

Experimental results

Research questions

• RQ1 What are the dominant dispersal pathways of protoplanetary discs when both internal and external photoevaporation are active?
• RQ2 How do stellar mass, internal X-ray luminosity, and external FUV flux jointly influence disc dispersal timescales and pathways?
• RQ3 Why are transition discs rare in high-UV environments like the Orion Nebula Cluster, and can this be explained by external photoevaporation?
• RQ4 To what extent does ongoing star formation affect the observed disc fraction as a function of cluster age?
• RQ5 Can the existence of very long-lived 'Peter Pan' discs be explained by low internal and external photoevaporation rates?

Key findings

• Five distinct dispersal pathways are identified: (i) long-lived discs (>20 Myr), (ii) inside-out clearing via internal photoevaporation, (iii) outside-in truncation via external photoevaporation, (iv) inside-out with continued accretion in a compact inner region, and (v) outside-in with transient gaps.
• In high UV environments, the outer disc disperses faster than the inner disc, making transition discs—characterized by inner holes—uncommon, consistent with observations in the Orion Nebula Cluster.
• Ongoing star formation is essential to reproduce observed disc fractions over time; without it, disc fractions decline too rapidly, especially in the first ~3 Myr.
• The model reproduces the rarity of 'Peter Pan' discs (>40 Myr) only when both internal X-ray luminosity and external FUV flux are low, indicating they require shielded, quiescent environments.
• The impact of environment on disc fractions is most significant in the first ~3 Myr, after which the decline is governed by weaker ongoing star formation.
• High-viscosity, high-internal-photoevaporation models may act as proxies for magnetohydrodynamic (MHD) wind-driven dispersal in low-viscosity discs, suggesting MHD winds are not required to reproduce key outcomes.