On the origin of transition disk cavities: Pebble-accreting protoplanets vs Super-Jupiters

TL;DR

This study explores how multiple low-mass, pebble-accreting planets can create dust cavities in transition disks, explaining observations of gas-rich cavities and high accretion rates without requiring massive companions.

Contribution

It demonstrates that low-mass planets can produce observed transition disk features through pebble accretion, offering an alternative to models with massive companions.

Findings

Multiple pebble-accreting planets can form dust cavities consistent with observations.

The number of planets needed decreases with lower turbulence and specific dust properties.

Transition disks with extended rings and high gas content may host multiple Neptunes rather than gas giants.

Abstract

Protoplanetary disks surrounding young stars are the birth places of planets. Among them, transition disks with inner dust cavities of tens of au are sometimes suggested to host massive companions. Yet, such companions are often not detected. Some transition disks exhibit a large amount of gas inside the dust cavity and relatively high stellar accretion rates, which contradicts typical models of gas-giant-hosting systems. Therefore, we investigate whether a sequence of low-mass planets can produce cavities in the dust disk. We evolve the disks with low-mass accreting embryos in combination with 1D dust transport and 3D pebble accretion, to investigate the reduction of the pebble flux at the embryos' orbits. We vary the planet and disk properties. We find that multiple pebble-accreting planets can efficiently decrease the dust surface density, resulting in dust cavities consistent with transition disks. The number of low-mass planets necessary to sweep up all pebbles decreases with decreasing turbulent strength and is preferred when the dust Stokes number is $10^{-2}-10^{-4}$. Compared to dust rings caused by pressure bumps, those by efficient pebble accretion exhibit more extended outer edges. We also highlight the observational reflections: the transition disks with rings featuring extended outer edges tend to have a large gas content in the dust cavities and rather high stellar accretion rates. We propose that planet-hosting transition disks consist of two groups. In Group A disks, planets have evolved into gas giants, opening deep gaps in the gas disk. Pebbles concentrate in pressure maxima, forming dust rings. In Group B, multiple Neptunes (unable to open deep gas gaps) accrete incoming pebbles, causing the appearance of inner dust cavities. The morphological discrepancy of these rings may aid in distinguishing between the two groups using high-resolution ALMA observations.