c2d Spitzer IRS Spectra of Disks around T Tauri Stars. III. [Ne II], [Fe I], and H2 Gas-Phase Lines

We present a survey of mid-infrared gas-phase lines toward a sample of 76 circumstellar disks around low-mass pre-main-sequence stars from the Spitzer ``Cores to Disks'' legacy program. We report the first detections of [Ne II] and [Fe I] toward classical T Tauri stars in ~20% and ~9% of our sources, respectively. The observed [Ne II] line fluxes and upper limits are consistent with [Ne II] excitation in an X-ray irradiated disk around stars with X-ray luminosities L_X=10²⁹-10³¹ erg s^-1. [Fe I] is detected at ~10^-5 to 10^-4 L_solar, but no [S I] or [Fe II] is detected down to ~10^-6 L_solar. The [Fe I] detections indicate the presence of gas-rich disks with masses of >~0.1 M_J. No H₂ 0-0 S(0) and S(1) disk emission is detected, except for S(1) toward one source. These data give upper limits on the warm (T~100-200 K) gas mass of a few Jovian masses, consistent with recent T Tauri disk models that include gas heating by stellar radiation. Compact disk emission of hot (T>~500 K) gas is observed through the H₂ 0-0 S(2) and/or S(3) lines toward ~8% of our sources. The line fluxes are, however, higher by more than an order of magnitude than those predicted by recent disk models, even when X-ray and excess UV radiation are included. The [Ne II]/H₂ 0-0 S(2) ratios for these sources are similarly lower than predicted, consistent with the presence of an additional hot molecular gas component not included in current disk models. Oblique shocks of stellar winds interacting with the disk can explain many aspects of the hot gas emission but are inconsistent with the nondetection of [S I] and [Fe II] lines.