Comparison with Accretion/Wind Models

The accretion and wind models for T Tauri stars were described in the introductory chapter. The results obtained above are now discussed in the light of those models.

A clear signature for winds in T Tauri stars is, as discussed before, the presence of blueshifted absorption features in the hydrogen lines. The only star in the sample presented in this work that shows such a clear indication for outflow is CW Tau, for which a blueshifted absorption is present in the Pa Beta line profile. Model predictions for the line profile of NIR hydrogen lines are not usually found in the literature. Hartmann et al. (1990) present results for some near infrared lines, namely for Pa Alpha, Br Alpha and Br Gamma. The only Br Gamma line profile shown by Hartmann et al. (1990) is for a non-isothermal wind model with a spherically symmetric steady flow (model 12 of Hartmann et al (1990), reproduced here in Figure 5.18).

A comparison between that line profile and our high resolution observations show that they do not agree. The profile that results from the model peaks at a redshifted velocity larger than 50 km/s, which is not a typical feature in the observed profiles. In fact, as discussed above, the latter tend to peak at slightly blueshifted velocities.

Also, the Af in the model line profile is clearly smaller than 1, again in contradiction with the observations (please refer back to Section 5.3.4). The Pa Beta and Br Gamma line profiles computed by Hartmann et al. (1990) tend to have either a flat top with the line peak being redshifted or to display a very prominent and broad blueshifted absorption (P Cygni profile). Neither of these features are identified in the NIR line profiles presented in this work.

Pedrosa (1996) computes model H Alpha line profiles only. Given that a hydrogen atom with 15 principal levels is used in that work, the calculation of Pa Beta and Br Gamma line profiles is possible and desirable. Surely, this wind model will not be able to explain the IPC line profiles observed. However, it is not clear whether it can generate Type I profiles with the observed characteristics (refer to Section 5.3.5 for these), namely with the right FWHM and peak intensities, which, as referred to in Section 1.3 is a problem faced by the model H Alpha line profiles, and with the line peak occuring at slight blueshifted velocities.

The stochastic wind models do not attempt to calculate line profiles for any NIR hydrogen lines. Mitskevich, Natta & Grinin (1993) present in their Figures 5 and 7 profiles for lines referred by the authors as ``of high optical depth'' and ``of intermediate optical depth'' respectively. These figures are reproduced here in Figures 5.19 and 5.20.

In Mitskevich, Natta & Grinin (1993), the ``high optical depth'' lines are compared to H Alpha observations and the ``intermediate optical depth'' lines are compared to the CaII infrared triplet. The line profiles in panel c) of their Figure 5 (Figure 5.19 here) are similar in shape to the Pa Beta line profile of CW Tau, however the line peak in the model profiles are far too high. The NIR lines observed to be double peaked are classified as Type II R and Type III R whereas the double peaked line profiles that result form their models are all of Types II B and III B. Also, the observed line peaks are never as intense as those produced by the models. Figure 7 in Mitskevich, Natta & Grinin (1993) (Figure 5.20 here) shows that, with the appropriate choice of parameters, ``lines of intermediate optical depth'' are nearly symmetric and centrally peaked. As discussed above, T Tauri stars have a high percentage of NIR lines with this sort of line profile. The model line profiles can have roughly the same peak intensities as those observed for Pa Beta and for Br Gamma. The model line wings extend to about 200 km/s both in the blue and in the red. While the red wing in the observed Pa Beta and Br Gamma line profiles extends typically to km/s, the blue wing is more extended, reaching typically km/s. This is a feature of the observations that the model line profiles presented in Mitskevich, Natta & Grinin (1993) fail to reproduce. Finally, the model line profiles also tend to be too centrally peaked and their Af's tend to be too near unity.

Calvet & Hartmann (1992), Hartmann, Hewett & Calvet (1994) and Muzerolle, Calvet & Hartmann (1998) compute hydrogen line profiles for a magnetospheric accretion scenario. Their work concentrates on lines from the Balmer series but Br Gamma profiles are also computed in the last of these papers. As pointed out by Hartmann, Hewett & Calvet (1994), their results for the model line profiles near the line centre should be regarded with some caution since the Sobolev approximation breaks down for low velocities and also because rotation is neglected. This should be borne in mind in the discussion that follows.

The characteristics of the model Br Gamma profiles are similar to those obtained for lines from the Balmer series, ie. they are double peaked at low inclinations with no redshifted absorptions and with the red wing extending to higher velocities than the blue wing. For higher inclinations the profiles are centrally peaked and slightly blueshifted, displaying also a redshifted absorption. Figure 5 from Muzerolle et al. showing the model results for the Br Gamma line is reproduced here in Figure 5.21.

Qualitatively there is a good agreement between model line profiles that show redshifted absorption and the observations. Besides the presence of a redshifted absorption, the model line profiles are, like the observations, centrally peaked with the line peak being slightly blueshifted. From a quantitative point of view, and judging from the model Br Gamma line profiles presented in Muzerolle, Calvet & Hartmann (1998), the model emission lines peak at higher intensities relative to the continuum than those typically seen in the observations and the line widths in the model profiles are considerably smaller than those observed (FWHM from model are between 75 and 110 km/s while the observed Br Gamma lines have FWHM mostly in the range 150 to 250 km/s, as discussed in Section 5.3.1). The Af's of the observed line profiles agree with the results from the model.

The ability of the magnetospheric accretion model to produce Type I profiles is more doubtful. None of the Br Gamma line profiles presented in Muzerolle, Calvet & Hartmann (1998) is of Type I. The model profile obtained for an inclination of 60^o (see Figure 5.21) has some of the characteristics of the Type I profiles, like being centrally peaked and slightly blueshifted, however, it has a shallow redshifted absorption. It should be noted here that, as mentioned by Hartmann, Hewett & Calvet (1994), the presence of such an absorption feature depends on the temperature of the accretion shock, as well as on the viewing angle. Models without the shock component do not produce an absorption at all. This is easily seen in Figure 5.22 where part of Figures 8 and 9 from Hartmann, Hewett & Calvet (1994) are reproduced. Both of these line profiles are H Gamma line profiles for magnetospheric accretion models with i=75^o. The difference in the line profiles (presence of redshifted absorption or not) results from taking into account, or not, the accretion shock, by introducing a hot accretion ring in the model.

As mentioned before, at low inclinations a double peak is produced in the model line profiles. The only stars in the sample presented in this work, for which their Br Gamma lines display this sort of shape are RY Tau and SU Aur. However, unlike the model line profiles, the central absorption seen in the Br Gamma line of these stars goes below the level of the continuum. Pa Beta lines classified as Type II R are similar to the Br Gamma model results for low inclinations. The line wings of the model profiles for low inclinations display different characteristics than those observed. The wings are not as extended as those of the observed profiles (wings in model line profiles never even reach 200 km/s) and the asymmetry observed, in the sense that the blue wing extends further than the red wing (recall results presented in Section 5.2, Figure 5.4), is exactly the opposite than the one produced by the model (red wing extending to higher velocities than the blue wing). As pointed out by Hartmann, Hewett & Calvet (1994), the results for the model line profiles near the line centre (for the magnetospheric accretion scenario) should be regarded with some caution since the Sobolev approximation breaks down for low velocities and also because rotation is neglected.

A general comparison of the line profile results from various models and the Pa Beta and Br Gamma observations show that wind models seem to have great difficulties in explaining the observed line profiles. They tend to produce blueshifted absorptions, which are not observed but for one line profile, and the model line peaks tend to be redshifted which again is not observed. ``Stochastic Wind'' models produce line profiles that resemble more the observations but are too symmetric at the level of the line wings and too centrally peaked, with peak velocity occurring at zero velocity rather than being slightly blueshifted. As expected, all wind models fail to produce IPC line profiles. Magnetospheric accretion models produce IPC line profiles with characteristics qualitatively similar to those displayed by the observations. Type I line profiles can occur in magnetospheric accretion models if the geometry of the system is favorable, that is if the accretion column is not observed against the accretion shock.