Interpreting Near Infrared Hydrogen Line Ratios in T Tauri Stars
  In accreting young stars one of the prominent spectral features in the near
infrared is the Paschen and Brackett series in emission. We examine hydrogen
line ratios for 16 classical T Tauri stars from SpeX spectra and and assess the
trends with veiling and accretion. The observed line ratios are compared to two
theoretical models for line formation: (1) Baker and Menzel's (1938) Case B for
radiative ionization and recombination and (2) a set of local line excitation
calculations designed to replicate the conditions in T Tauri winds and magnetic
accretion columns (Kwan & Fischer 2011). While the comparison between Case B
and observed line ratios implies a wide range in electron density and
temperature among the hydrogen line formation regions in T Tauri stars, the
predictions of the local line excitation models give consistent results across
multiple diagnostics. Under the assumptions of the local line excitation
calculations, we find that n_H in the hydrogen line formation region is
constrained to 2x10^10 -- 2x10^11 cm^-3, where stars with higher accretion
rates have densities at the higher end of this range. Because of uncertainties
in extinction, temperature is not well delineated but falls within the range
expected for collisional excitation to produce the line photons. We introduce
new diagnostics for assessing extinction based on near infrared hydrogen line
ratios from the local line excitation calculations.
