As noted in the previous section, the magnetospheric accretion model brings the solution for a number of problems in the understanding of T Tauri stars. Given its importance for the development of an even deeper knowledge of these stars, the model must stand on a solid base. The work presented and discussed here aims at testing the magnetospheric accretion model in T Tauri stars.
Hydrogen emission lines tend to reflect the dynamics of the region where they are formed. Clear examples of these are normal or inverse P Cygni profiles found in the Balmer lines of T Tauri stars. Hydrogen Balmer lines correspond to transitions to/from level n=2. H Alpha corresponds to a n=3-2 transition and is the most observed hydrogen line in T Tauri stars. H Beta, corresponding to n=4-2, is also commonly observed in T Tauri stars. The strongest hydrogen lines in the near infrared part of the electromagnetic spectrum are lines from the Paschen and Brackett series, i.e. corresponding to transitions to/from levels =3 and n=4 respectively. Given that these lines arise in higher energy levels when compared to the lower Balmer lines (e.g. H Alpha and H Beta) and also that they are generally optically thinner, one expects them to form at different depths into a cloud of hydrogen.
Near Infrared hydrogen lines should form in the denser parts of the circumstellar envelope of T Tauri stars and should therefore trace infalling material in a magnetospheric accretion scenario. The development of near infrared high spectral resolution high sensitivity spectroscopy allows the study of profiles of near infrared hydrogen lines. Such a study constitutes one of the few observational tests that can be made of the magnetospheric accretion model.
In order to carry out the proposed task a sample of T Tauri stars in the Taurus-Auriga Complex was observed at near infrared wavelengths. Pa Beta and/or Br Gamma line profiles were obtained for the stars in the sample. These lines correspond to transitions from n=5-3 (Pa Beta) and from n=7-4 (Br Gamma), therefore requiring different excitation conditions than H Alpha and H Beta, and they are strong enough to be observed efficiently with current instrument sensitivities.
The observations are described in Chapter 2 and the data reduction procedures in Chapter 3. Many of the observed stars turned out to display near infrared veiling. The results on near infrared veiling and their discussion is done in Chapter 4. The results on the Pa Alpha and Br Gamma line profiles are presented and discussed in Chapter 5. Chapter 6 deals with results from radiative transfer modeling of Pa Beta and Br Gamma line profiles and their comparison with the observations. Finally, conclusions and future directions are presented in Chapter 7.
Before proceeding on to Chapter 2 an overview of the existing models for the formation of hydrogen emission lines in T Tauri stars is presented in the next section.