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

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, ~5×10-5 lunar masses of dust are located in a small optically thin inner region that 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 μm fine-structure emission with a luminosity of 1.3×1029 ergs s-1, indicating that optically thin gas is present in this ~43 AU hole, in agreement with Hα measurements and a UV excess that indicate that CS Cha is still accreting 1.2×10-8 Msolar yr-1. We do not find a correlation of the [Ne II] flux with LX; however, there is a possible correlation with M˙, which if confirmed would suggest that EUV fluxes due to accretion are the main agent for formation of the [Ne II] line.