Here we present the Spitzer IRS spectrum of CS Cha , a member of the \sim 2 Myr old Chamaeleon star-forming region , which reveals an optically thick circumstellar disk truncated at \sim 43 AU , the largest hole modeled in a transitional disk to date .
Within this inner hole , \sim 5 \times 10 ^ { -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 \mu m fine-structure emission with a luminosity of 1.3 \times 10 ^ { 29 } ergs s ^ { -1 } , indicating that optically thin gas is present in this \sim 43 AU hole , in agreement with H _ { \alpha } measurements and a UV excess which indicate that CS Cha is still accreting 1.2 \times 10 ^ { -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 { \dot { M } } , which if confirmed would suggest that EUV fluxes due to accretion are the main agent for formation of the [ Ne II ] line .