Physical Conditions of Accreting Gas in T Tauri Star Systems

We present results from a low-resolution (R≃ 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/Paβ, Brn_up/Brγ, and Brγ/Pan_up H I line ratios for all observations exhibit a scatter of ≲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 Brγ/Pan_up 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≲ 2000 K) and electron densities (10⁹ cm ⁻³ < n_e≲ 10¹⁰ 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.