A Spitzer Census of Transitional Protoplanetary Disks with AU-Scale Inner Holes

TL;DR

This study uses Spitzer infrared data to identify and analyze transitional protoplanetary disks with AU-scale inner holes, revealing their properties, accretion activity, and how their frequency varies with age and stellar mass.

Contribution

It provides a comprehensive classification and analysis of transition disks across multiple young stellar regions, highlighting their evolutionary stages and accretion behaviors.

Findings

Approximately 20 classical transition disks identified in nearby young regions.

Most classical and warm excess transition disks are still accreting gas.

The frequency of classical transition disks increases with stellar age, especially after 3 Myr.

Abstract

[abridged] Protoplanetary disks with AU-scale inner clearings, often referred to as transitional disks, provide a unique sample for understanding disk dissipation mechanisms and possible connections to planet formation. Observations of young stellar clusters with the Spitzer Space Telescope have amassed mid-infrared spectral energy distributions for thousands of star-disk systems from which transition disks can be identified. From a sample of 8 relatively nearby young regions (d <= 400 pc), we have identified about 20 such objects, which we term "classical" transition disks, spanning a wide range of stellar age and mass. We also identified two additional categories representing more ambiguous cases: "warm excess" objects with transition-like spectral energy distributions but moderate excess at 5.8 microns, and "weak excess" objects with smaller 24 micron excess that may be optically thin or exhibit advanced dust grain growth and settling. From existing Halpha emission measurements, we find evidence for different accretion activity among the three categories, with a majority of the classical and warm excess transition objects still accreting gas through their inner holes and onto the central stars, while a smaller fraction of the weak transition objects are accreting at detectable rates. We find a possible age dependence to the frequency of classical transition objects, with fractions relative to the total population of disks in a given region of a few percent at 1-2 Myr rising to 10-20% at 3-10 Myr. The trend is even stronger if the weak and warm excess objects are included. Classical transition disks appear to be less common, and weak transition disks more common, around lower-mass stars (M <= 0.3 Msun).