Cold Disks: Spitzer Spectroscopy of Disks around Young Stars with Large Gaps

TL;DR

This study identifies rare 'cold disks' around young stars with large dust gaps, using Spitzer spectroscopy, revealing significant inner disk dust depletion while outer regions remain gas-rich, providing insights into planet formation processes.

Contribution

The paper introduces the concept of 'cold disks' with large dust gaps and a new flux ratio diagnostic, advancing understanding of disk evolution and planet formation.

Findings

Large dust depletion in inner disks by factors of 100-1000

Presence of gas-rich outer disks beyond the gaps

Introduction of the 30/13 micron flux ratio as a diagnostic tool

Abstract

We have identified four circumstellar disks with a deficit of dust emission from their inner 15-50 AU. All four stars have F-G spectral type, and were uncovered as part of the Spitzer Space Telescope ``Cores to Disks'' Legacy Program Infrared Spectrograph (IRS) first look survey of ~100 pre-main sequence stars. Modeling of the spectral energy distributions indicates a reduction in dust density by factors of 100-1000 from disk radii between ~0.4 and 15-50 AU, but with massive gas-rich disks at larger radii. This large contrast between the inner and outer disk has led us to use the term `cold disks' to distinguish these unusual systems. However, hot dust [0.02-0.2 Mmoon] is still present close to the central star (R ~0.8 AU). We introduce the 30/13 micron, flux density ratio as a new diagnostic for identifying cold disks. The mechanisms for dust clearing over such large gaps are discussed. Though rare, cold disks are likely in transition from an optically thick to an optically thin state, and so offer excellent laboratories for the study of planet formation.