Survival of protoplanetary disks in Upper Scorpius from population synthesis models with external photoevaporation

¹ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
² Dipartimento di Fisica, Università degli Studi di Milano, Via Celoria 16, 20133 Milano, Italy
³ Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK
⁴ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 21 November 2025
Accepted: 23 February 2026

Abstract

We present population synthesis models of viscous protoplanetary disks subject to mild external far-ultraviolet (FUV) radiation fields (F_UV = 1–100 G₀). Our simulations focus on gas-disk evolution, exploring stellar masses drawn from an initial mass function and a range of initial disk conditions. We quantified the fraction of surviving disks across 10 Myr of evolution, tracked the evolution of gas-disk mass and size, and compared our results with observations of protoplanetary disks in the Upper Scorpius region, including the ten targets studied by the AGE-PRO ALMA Large Program. We find that models combining viscous evolution with external photoevaporation yield disk lifetimes of 3–7 Myr, which is consistent with observed dispersal timescales, particularly for 10⁻⁴ ≤ α ≤ 10⁻². Low-mass stars (0.1 M_⊙) are more susceptible to disk dispersal due to their weaker gravitational binding, with their fraction among all surviving disks dropping from 76% at birth to 51% by 10 Myr. The majority of the long-lived disks are those with low viscosity, α < 10^−3.5, and an initial characteristic radius of R_c < 125 AU; the initial disk-to-star mass ratio does not play an important role. The median gas-disk mass and radius of the surviving disks exhibit a sharp decline in the first 0.2 Myr of evolution, followed by a slight increase that reflects survivorship bias. We also explored correlations between gas-disk mass and size versus stellar mass and FUV strength. Our findings highlight the critical role of external photoevaporation in shaping disk populations even at moderate levels of FUV radiation fields.