Dust Evolution Can Produce Scattered Light Gaps in Protoplanetary Disks

TL;DR

This paper demonstrates that dust evolution alone, without planetary influence, can create observable scattered light gaps in protoplanetary disks, challenging the assumption that such gaps necessarily indicate planet presence.

Contribution

It introduces a simple dust evolution model showing how dust fragmentation and related processes can produce disk gaps observable in scattered light, independent of planets.

Findings

Dust evolution can produce ring-like gaps in scattered light images.

The position and occurrence of gaps depend on dust-to-gas ratio, fragmentation velocity, turbulence, and disk age.

Gaps in thermal emission at millimeter wavelengths are less straightforward and may not coincide with scattered light gaps.

Abstract

Recent imaging of protoplanetary disks with high resolution and contrast have revealed a striking variety of substructure. Of particular interest are cases where near-infrared scattered light images show evidence for low-intensity annular "gaps." The origins of such structures are still uncertain, but the interaction of the gas disk with planets is a common interpretation. We study the impact that the evolution of the solid material can have on the observable properties of disks in a simple scenario without any gravitational or hydrodynamical disturbances to the gas disk structure. Even with a smooth and continuous gas density profile, we find that the scattered light emission produced by small dust grains can exhibit ring-like depressions similar to those presented in recent observations. The physical mechanisms responsible for these features rely on the inefficient fragmentation of dust particles. The occurrence and position of the proposed "gap" features depend most strongly on the dust-to-gas ratio, the fragmentation threshold velocity, the strength of the turbulence, and the age of the disk, and should be generic (at some radius) for typically adopted disk parameters. The same physical processes can affect the thermal emission at optically thin wavelengths ($\sim$1 mm), although the behavior can be more complex; unlike for disk-planet interactions, a "gap" should not be present at these longer wavelengths.