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.Comment: accepted to ApJ Letter