Protostars and Planets VI, Heidelberg, July 15-20, 2013

Poster 2S013

Interpreting Near Infrared Hydrogen Line Ratios in T Tauri Stars

Edwards, Suzan (Smith College)
Kwan, John (University of Massachusetts)
Fischer, William (University of Toledo)
Hillenbrand, Lynne (California Institute of Technology)
Finn, Kimberley (Mount Holyoke College)
Fedorenko, Krystyna (Smith College)
Feng, Wanda (Smith College)

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 find differences between stars with low and high mass accretion rates. The observed line ratios are compared to two theoretical models for line formation: (1) Baker and Menzel\'s Case B for radiative ionization and recombination and (2) a set of local line excitation calculations by Kwan and Fischer designed to replicate the conditions in T Tauri winds and magnetic accretion columns. The predictions of the local line excitation models give consistent results across multiple diagnostics, in contrast to Case B. Under the assumptions of the local line excitation, we find that the number density in the hydrogen line formation region is constrained to 2x10^{10} - 2x10^{11} per cubic centimeter, where stars with higher accretion rates have densities at the higher end of this range. Because of uncertainties in extinction, temperature is not well constrained, but falls within the range expected for collisional excitation to produce the line photons. The local line excitation ratios offer a new approach to determining extinction to embedded sources as well as densities in the region where hydrogen lines are formed.

Click here to view poster PDF