On the Transitional Disk Class: Linking Observations of T Tauri Stars & Physical Disk Models

TL;DR

This paper refines the classification of transitional disks around T Tauri stars by linking their spectral energy distributions to physical disk structures, highlighting the importance of disk morphology and accretion rates in understanding planet formation.

Contribution

It introduces a clear definition for transitional and pre-transitional disks based on their SED features and physical structure, supported by detailed modeling of observed disks.

Findings

Transitional disks have inner holes or gaps at intermediate radii.

Pre-transitional disks can be modeled with full disks and require millimeter data for accurate characterization.

Transitional and pre-transitional disks show lower accretion rates than full disks, indicating possible planet formation processes.

Abstract

Two decades ago "transitional disks" described spectral energy distributions (SEDs) of T Tauri stars with small near-IR excesses, but significant mid- and far-IR excesses. Many inferred this indicated dust-free holes in disks, possibly cleared by planets. Recently, this term has been applied disparately to objects whose Spitzer SEDs diverge from the expectations for a typical full disk. Here we use irradiated accretion disk models to fit the SEDs of 15 such disks in NGC 2068 and IC 348. One group has a "dip" in infrared emission while the others' continuum emission decreases steadily at all wavelengths. We find that the former have an inner disk hole or gap at intermediate radii in the disk and we call these objects "transitional" and pre-transitional" disks, respectively. For the latter group, we can fit these SEDs with full disk models and find that millimeter data are necessary to break the degeneracy between dust settling and disk mass. We suggest the term "transitional" only be applied to objects that display evidence for a radical change in the disk's radial structure. Using this definition, we find that transitional and pre-transitional disks tend to have lower mass accretion rates than full disks and that transitional disks have lower accretion rates than pre-transitional disks. These reduced accretion rates onto the star could be linked to forming planets. Future observations of transitional and pre-transitional disks will allow us to better quantify the signatures of planet formation in young disks.