Secret of Longevity: Protoplanetary Disks as a Source of Gas in Debris Disks

TL;DR

This study uses disk evolution simulations to demonstrate that primordial gas can survive in debris disks for over 10 million years, supporting the idea that some debris disk gas originates from long-lived protoplanetary disks.

Contribution

The paper provides the first detailed modeling showing long-lived primordial gas in debris disks, explaining observed gas and accretion signatures in old disks.

Findings

Gas can survive beyond 10 Myr in massive, low-turbulence disks.

Survival is most prolonged around 2 solar mass stars.

Predicted CO masses match observed values in gas-rich debris disks.

Abstract

While protoplanetary disks (PPDs) are generally thought to disperse within several million years, recent observations have revealed gas in their older counterparts, debris disks. The origin of this gas remains uncertain, with one possibility being the unexpectedly long survival of PPDs (the primordial-origin scenario). To explore the plausibility of this scenario, we conduct 1D disk evolution simulations, varying parameters like stellar mass, disk mass, turbulent stress, and the model of magnetohydrodynamic winds, while incorporating stellar evolution to account for time-varying photoevaporation rates. Our focus is on disks where small grains are depleted, as these are potentially long-lived due to reduced far-ultraviolet photoevaporation. Our results show that gas in these disks can survive beyond 10 Myr regardless of the stellar mass, provided they are initially massive ($M_{\mathrm{disk}}\approx 0.1M_*$) with relatively weak turbulent stress ($\alpha \ll 10^{-2}$). The longest lifetimes are consistently found for $M_* = 2 M_{\odot}$ across a wide parameter space, with gas typically persisting at $\sim 10$--$10^3$ au. Roughly estimated CO masses for these disks fall within the observed range for the most massive gas-rich debris disks around early A~stars. These alignments support the plausibility of the primordial-origin scenario. Additionally, our model predicts that accretion persists for as long as the disk survives, which could explain the accretion signatures detected in old disks hosted by low-mass stars, including Peter Pan disks. Our finding also suggests that ongoing accretion may exist in gas-rich debris disks. Thus, searching for accretion signatures could be a key to determining the origins of gas in debris disks.