Survival of Protoplanetary Disks in Upper Scorpius from Population Synthesis Models with External Photoevaporation

Abstract

We present population synthesis models of viscous protoplanetary disks subject to mild external far-ultraviolet (FUV) radiation fields ($F_{\rm UV}=1\text{-}100\,$G$_0$). Our simulations focus on gas disk evolution, exploring stellar masses drawn from an Initial Mass Function and a range of initial disk conditions. We quantify the fraction of surviving disks across $10\,\mathrm{Myr}$ of evolution, track the evolution of gas disk mass and size, and compare 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 $3\text{-}7\,\mathrm{Myr}$, consistent with observed dispersal timescales, particularly for $10^{-4} \leq α\leq 10^{-2}$. Low-mass stars ($0.1\,$M$_\odot$) 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 $10\,\mathrm{Myr}$. The majority of the long-lived disks are those with low viscosity $α< 10^{-3.5}$ and initial characteristic radius $R_c < 125\,\mathrm{AU}$, while 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 $0.2\,\mathrm{Myr}$ of evolution, followed by a slight increase that reflects survivorship bias. We also explore correlations between gas disk mass and size vs. 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.

Figures (19)

• Figure 1: PDF of external FUV flux $F_{\mathrm{UV}}$, fitted with log normal distribution. Light, medium and dark blue lines represent 16th, median and 84th value of the FUV flux in the Upper Sco star forming region, respectively. The red line enclosing the blue lines is the distribution from which we draw FUV flux $F_{\mathrm{UV}}$ in our models.
• Figure 2: PDF of stellar mass $M_{\star}$ for the 169 disk-bearing stars in Upper Sco by carpenter2025. The stellar masses are presented in Pinilla2025.
• Figure 3: The decline in disk fraction $f_{\mathrm{disk}}$ with disk age. Blue lines are the disk fraction considering disk dispersal when losing 90% (light), 95% (medium) and 99% (dark) of the initial mass. The red points correspond to the observed disk fractions of nearby clusters within 200 pc, and the dashed line is the fitted exponential decay, $f(t)= \exp(-t/\tau)$, yielding $\tau =7$ Myr from Pfalzner2022. The yellow points are from Fedele2010, giving $\tau =3$ Myr. The top and bottom panel correspond to disks with viscosity ranges of $10^{-5} \leq \alpha \leq 10^{-2}$ and $10^{-4} \leq \alpha \leq 10^{-2}$, respectively. The $\chi^2$ values are shown in the bottom left of each figure.
• Figure 4: The percentage of surviving disks in each stellar mass bin from our simulations at 5 (red) and 10 (orange) Myr of disk evolution, compared to the 169 samples of carpenter2025 (blue). The stellar masses are presented in Pinilla2025.
• Figure 5: The distribution of disks with certain characteristic radius $R_{\mathrm{c}}$ and viscosity parameter $\alpha$ that survive after 5 Myr of evolution, assuming that gas disk is dispersed when losing 90% of the initial mass. The red and blue represent the distributions at birth and 5 Myr, respectively.