PHYSICAL CONDITIONS OF ACCRETING GAS IN T TAURI STAR SYSTEMS

We present results from a low-resolution (R similar or equal to 300) near-infrared spectroscopic variability survey of actively accreting T Tauri stars (TTSs) in the Taurus-Auriga star-forming region. Paschen and Brackett series H I recombination lines were detected in 73 spectra of 15 classical T Tauri systems. The values of the Pan(up)/Pa beta, Brn(up)/Br gamma, and Br gamma/Pan(up) H I line ratios for all observations exhibit a scatter of less than or similar to 20% about the weighted mean, not only from source to source, but also for epoch-to-epoch variations in the same source. A representative or global'' value was determined for each ratio in both the Paschen and Brackett series, as well as the Br gamma/Panup line ratios. A comparison of observed line ratio values was made to those predicted by the temperature-and electron density-dependent models of case B hydrogen recombination line theory. The measured line ratios are statistically well fit by a tightly constrained range of temperatures (T less than or similar to 2000 K) and electron densities (10(9) cm(-3) < n(e) less than or similar to 10(10) cm(-3)). A comparison of the observed line ratio values to the values predicted by the optically thick and thin local thermodynamic equilibrium cases rules out these conditions for the emitting H I gas. Therefore, the emission is consistent with having an origin in a non-LTE recombining gas. While the range of electron densities is consistent with the gas densities predicted by existing magnetospheric accretion models, the temperature range constrained by the case B comparison is considerably lower than that expected for accreting gas. The cooler gas temperatures will require a nonthermal excitation process (e. g., coronal/accretion-related X-rays and UV photons) to power the observed line emission.