The Evolution of Dusty Disks Around Low-Mass Pre-Main Sequence Stars.

Abstract

The evolution of protoplanetary disks is intricately tied to the origin of planets. The details of how these disks evolve from initially well-mixed distributions of gas and dust into systems composed mostly of rocky planets and gas giants like our own solar system is not well understood and is a fundamental question in astronomy. It is widely accepted that dust grain growth and settling to the disk midplane play an integral part in creating the planetesimals that amalgamate into planets. Newly formed planets will then interact with the disk, clearing the material around themselves and creating gaps and holes.

Several disks which have nearly photospheric near-infrared emission but substantial excesses above the stellar photosphere at wavelengths beyond ~20 μm have been observed and are referred to as “transitional disks.” This deficit of flux can be explained by the presence of an inner disk hole that is mostly devoid of small dust. Here we model the transitional disks of CS Cha and CVSO 224. We also present evidence for a new class of “pre-transitional disks” around UX Tau A and LkCa 15. These objects have a deficit of flux in the mid-infrared (5–20 μm) and significant emission at longer wavelengths, as is seen in transitional disks. However, pre-transitional disks have significant near-infrared excesses (2–5 μm) relative to their stellar photospheres, indicative of an optically thick inner disk. This points to a gap within the disk rather than an inner disk hole.

We also present simulated spectral energy distributions of ~240 disks around low-mass classical T Tauri stars and find that the majority of observed disks within Taurus, Chamaeleon, and Ophiuchus lie within the parameter space probed by the models and are therefore “full disks.” However, we find that some disks cannot be explained by full disk models and are not known to be transitional or pre-transitional disks. We propose that these objects are pre-transitional disks with smaller gaps than previously observed, emphasizing that much still remains to be understood regarding the dust component of disks.