Probing the Dust and Gas in the Transitional Disk of CS Cha with Spitzer

TL;DR

This study uses Spitzer IRS data to analyze the dust and gas in the transitional disk of CS Cha, revealing a large inner hole, advanced dust evolution, and ongoing accretion, with implications for disk evolution and planet formation.

Contribution

First detailed Spitzer IRS spectrum analysis of CS Cha's transitional disk, revealing a large inner hole, dust settling, and gas accretion features.

Findings

Largest modeled inner hole at ~43 AU in a transitional disk.

Presence of optically thin gas indicated by [Ne II] emission.

Disk shows advanced dust evolution with larger grains and settling.

Abstract

Here we present the Spitzer IRS spectrum of CS Cha, a member of the ~2 Myr old Chamaeleon star-forming region, which reveals an optically thick circumstellar disk truncated at ~43 AU, the largest hole modeled in a transitional disk to date. Within this inner hole, ~5x10^-5 lunar masses of dust are located in a small optically thin inner region which extends from 0.1 to 1 AU. In addition, the disk of CS Cha has bigger grain sizes and more settling than the previously modeled transitional disks DM Tau, GM Aur, and CoKu Tau/4, suggesting that CS Cha is in a more advanced state of dust evolution. The Spitzer IRS spectrum also shows [Ne II] 12.81 micron fine-structure emission with a luminosity of 1.3x10^29 ergs s^-1, indicating that optically thin gas is present in this ~43 AU hole, in agreement with H_alpha measurements and a UV excess which indicate that CS Cha is still accreting 1.2x10^-8 M_sun yr^-1. We do not find a correlation of the [Ne II] flux with L_X, however, there is a possible correlation with mass accretion rate, which if confirmed would suggest that EUV fluxes due to accretion are the main agent for formation of the [Ne II] line.