The detection of IPC line profiles can be used to deduce lower limits on the masses of T Tauri stars and therefore constrain models of pre-main sequence evolutionary tracks, as has been shown using some of the data presented in this work, along with other data [Bonnell et al. 1998]. This paper is included in Appendix B. In this regard the Pa Beta and Br Gamma lines are particularly important since the the frequency of IPC line profiles is higher in these lines than in the brightest Balmer lines where IPC profiles have been usually observed.
The lack of blueshifted absorptions observed in the Pa Beta and Br Gamma line profiles can be used together with other data to assess the physical conditions in winds from T Tauri stars [Calvet 1997]. The fact that blueshifted absorptions are not present mean that both Pa Beta and Br Gamma are optically thin in the outflowing region surrounding T Tauri stars.
The magnetospheric accretion model is an important step towards the understanding of how matter is finally accreted onto a T Tauri star and how angular momentum is shed in order to keep rotational velocities as low as observed. It is, however, still a simplified model (eg. it is axially symmetric) and it needs to be refined. A knowledge of the dynamics of the infalling gas is crucial for such purpose. It was shown in this work that redshifted absorption features in the Pa Beta and Br Gamma lines, and possibly the line emission as well, probe infalling material. Given that they do not seem as affected by outflowing matter as the Balmer lines do they are more suitable to study the infalling matter than the optical Balmer lines.
Analysis of time series of the near infrared lines are an excellent tool to investigate the infalling gas and its dynamics. Variability in the line profiles is observed (see Section 5.4.7). Monitoring the Pa Beta/Br Gamma line profiles, especially those that display an IPC signature, allows one to learn about the region where the line(s) are formed by looking at the level of variability present in those lines, the sampled timescales on which the variability occurs and the degree to which variation across the line profile are correlated. Similar studies have been fruitful for a better understanding of the region(s) giving rise to optical lines such as H Alpha, HeI and Na D [Johns & Basri 1995b,Johns & Basri 1995a,Gameiro et al. 1993,Lago & Gameiro 1998, for example,]. In the magnetospheric accretion scenario, if the magnetic axis is tilted relative to the rotation axis of the star, an hypothesis put forward by Johns & Basri (1995b), one expects, due to viewing angle effects, a correlation between line variability and the rotation period of the star, especially in any redshifted absorption features. If the rotation and magnetic axis are aligned, Pa Beta/Br Gamma line variability should be associated with variations in the accretion flow.
In close connection with line variability studies is simultaneous monitoring of the near infrared veiling at different wavebands. The two possible sources of veiling in the near infrared, accretion shock veiling and disk veiling, should leave different signatures at different wavebands. Accretion shock veiling would dominate at shorter wavelengths and therefore variability in the veiling should be closely associated with variations in the line profiles. If the near infrared veiling continuum arises in the disk it would be uncorrelated with variations in the IPC profiles. If, however, the NIR veiling arises in the accretion columns themselves, then correlations between line profiles and veiling should be found.
The results presented in Chapter 4 are extremely important since they seem to show that the veiling in the near infrared is much higher than models predict. The study presented in this work used data taken for different purposes and makes use of photospheric lines that happen to lie in the observed wavelengths regions. Those lines are not optimal for a veiling study. Furthermore, the observed template stars do not cover all relevant spectral types and poor matching between the spectral type of the T Tauri star and that of the template star occurred at times. These facts contribute to the large error bars present in the veiling measurements of Chapter 4. It is therefore crucial to verify the results on the near infrared veiling by carefully measuring the veiling at different near infrared wavebands using observations planned specifically for that purpose. The use of much more suitable photospheric lines and a complete grid of template stars would allow a much better determination of the veiling at these wavelengths.
A natural extension of the line studies presented here is their application to heavier and/or to younger objects.
Herbig Ae/Be stars are thought to be the intermediate mass counterparts of T Tauri stars. For these higher luminosity objects one can test whether the NIR lines mainly reflect infall, or (as one might guess) become more dominated by winds.
Class I objects (from Lada's classification scheme) are invisible at optical wavelengths due to their intrinsic very high extinction. In order to probe deep into the enshrounding cocoon one must use infrared observations. Low and medium resolution spectroscopy show that Class I objects display near infrared hydrogen lines in emission [Greene & Lada 1996]. High resolution observations allow to obtain hydrogen line profiles for these objects and therefore investigate the gas dynamics in their innermost regions. Important clues regarding the way in which accretion occurs in these objects can therefore be obtained.
Near infrared high resolution spectroscopy opens interesting, and at times unexpected, doors for a better understanding of low mass star formation. As it was shown here, results from near infrared hydrogen line profiles tell us a different story than the optical counterparts in the Balmer series do. Near infrared veiling seems to be higher in the infrared than envisioned by current models. Surely, continuing research using instruments like CGS4 will provide the answer for yet unsolved puzzles, as well as bringing to light conundrums one is still to dream about.