The Type I Line Profiles

The majority of both Pa Beta and Br Gamma line profiles are classified as Type I profiles. These profiles lack absorption features and therefore they have no clear signatures for formation in either a wind or an accretion scenario.

The spectral type of the stars displaying Type I profiles at either Pa Beta and/or Br Gamma ranges from G5V to M3V. The various line parameters are not seen to be different between stars with earlier spectral types and stars with later spectral types. For IPC Pa Beta line profiles, as pointed out in the previous section, there is a hint for the emission component of the line to be stronger in stars with earlier spectral types (recall Figure 5.24). For Type I profiles such dependence does not seem to exist.

Despite lacking redshifted absorption features, some characteristics of Type I line profiles, such as being centrally peaked, slightly blueshifted and having Asymmetry Factors that agree with accretion model results, as discussed in Section 5.4.1, indicate that infalling material in a magnetospheric accretion scenario might be in the origin of these emission lines. Similarly to IPC line profiles, in such a scenario one would expect a correlation between the emission strength and the mass accretion rate. However, as shown in Figures 5.29 and 5.30, where the mass accretion rate is plotted versus the EW of the Pa Beta and Br Gamma Type I lines such a correlation, or even a trend in the expected sense, is not observed (similar plots result if EW is substituted by L/C at the line peak).

The range of accretion rates in the stars displaying IPC and in the stars displaying Type I line profiles are similar. That is, stars with a given type of line shape do not show any preference for a given range of accretion rates (compare Figures 5.26 and 5.29). Therefore, the lack/presence of redshifted absorption in some line profiles does not seem to be related with stars accreting matter at different rates or, at least, the accretion rate is not the most important factor determining the shape of the line profiles.

The line shapes produced by the magnetic accretion models of Hartmann, Calvet, Muzerolle and co-workers for high inclinations are, with the exception of the presence of a redshifted absorption, qualitatively similar to the observed Type I profiles. As discussed before, the presence of a redshifted absorption dipping below the continuum (the clear indication of infall) is very sensitive to the temperature of the accretion shock and to the geometry of the accretion flow. The fact that one sees so many Type I profiles, which is not predicted by the magnetospheric models, does not necessarily rule out the hypothesis that these line profiles are produced in an accretion flow. In fact, they might just be an indication that such flows are not as simple as those modeled by Hartmann, Calvet, Muzerolle and co-workers. A different geometry and/or different physical conditions in the accretion flow might make the presence of the redshifted absorption less likely without altering substantially the emission profile. The observation of line profile variability implying variable accretion (eg. Johns & Basri 1995b) and of localized accretion spots (eg. Unruh, Collier Cameron & Guenther 1998) indeed confirm that axially symmetric accretion models, such as those considered by Hartmann and co-workers, are not truly applicable in T Tauri stars. The virtue of these models is that they do explain, at least in a qualitative fashion, the general shape of Pa Beta and Br Gamma line profiles.

The line profile modeling work in Hartmann, Hewett & Calvet (1994) shows that the profiles of lines arising in magnetospheric accretion flows display a distinctive asymmetry due both to geometrical and radiative transfer effects. As pointed out by Edwards et al. (1994), the contribution of geometrical effects for the line profile asymmetry can be judged from the optically thin calculations in Hartmann, Hewett & Calvet (1994). These calculations show that emission in the red wing is suppressed due to occultation by the opaque disk, with lower inclinations leading to more asymmetric profiles. For higher inclinations it is also apparent that the blue wing is more extended than the red wing, as observed for the Pa Beta and Br Gamma lines (recall results from Section 5.3.5). This last property of the line profiles occurs for isothermal models only. For non-isothermal models the situation is inverted, i.e. the red wing is more extended than the blue wing, unlike the observations.

Despite lacking redshifted absorption(s) that dip below the continuum the Type I line profiles can plausibly be formed in accretion flows. The asymmetry observed would then be the result of the geometry of the flow with a contribution from radiative transfer effects that tend to remove emission from the red wing, therefore skewing the line profile.

An alternative to infall models for the origin of Type I emission line profiles are wind models. If emission in Type I line profiles results from material in a wind then a correlation between line strength and wind mass loss rate is expected. Plotting the wind mass loss rate versus the EW of Type I lines (Figure 5.31) results in no clear correlation between these two quantities.

None of the characteristics displayed by Type I line profiles, from its actual shape (see Section 5.4.1) to the lack of correlation with mass loss rate, seem to be consistent with wind models for line formation. As far as the line shape is concerned, stochastic wind models might have a word to say (please refer again to Section 5.4.1) but more detailed calculations are needed.