Blueshifted absorptions are extremely rare in the Pa Beta and Br Gamma line profiles. This result is in complete contrast with what is obtained from studies of Balmer line profiles, especially H Alpha. Furthermore, redshifted absorption features are present in about 40% of the Pa Beta profiles and in 20% of the Br Gamma profiles, making them inverse P Cygni (IPC) line profiles. These are indicators of infall and such a high frequency of IPC profiles is not observed at H Alpha. A study of less bright higher members of the Balmer series (eg. H Gamma, H Delta) for a sample of 15 T Tauri stars (cf. with the 41 and 29 T Tauri stars studied here at Pa Beta and Br Gamma respectively) by Edwards et al. (1994) show that redshifted absorptions are seen to occur frequently in those lines. Most of the stars showing redshifted absorptions in H Gamma and H Delta also show IPC structure at either Pa Beta or Br Gamma. However, some of the Balmer series lines with redshifted absorptions display blueshifted absorptions in the same line profile indicating that these lines are affected by both infalling and outflowing gas. Such differences hint that the near infrared lines and the Balmer lines arise in different regions or, at least, that they are affected by radiative transfer effects differently. Certainly Balmer lines and the near infrared lines are conveying different information.
Typical line profiles of the near infrared lines studied here have full widths at half maximum of about 200 km/s, they are slightly blueshifted and the line wings extend to about 300 km/s to the blue and 200 km/s to the red. This asymmetry at the level of the line wings is also observed for line profiles showing no signs for redshifted absorptions and it is not the result of an overall blueshift of the line since line peaks are not shifted into the blue by as much as 100 km/s.
Velocities of the redshifted absorption features in the near infrared lines studied here are around 200 km/s. They are compatible with free fall velocities from a few stellar radii out as expected to occur for T Tauri stars in the magnetospheric accretion scenario.
While there is not much doubt that the high velocity redshifted absorption features should arise in infalling gas, it is harder to decide where emission in the Pa Beta and Br Gamma lines originates. In the IPC line profiles, the EW of the emission component seems to correlate with the mass accretion rate. This result may indicate that, at least in the IPC line profiles, emission has its origin in infalling material. One should be very careful with this interpretation though since Hartigan, Edwards & Ghandour (1995) show that the mass accretion rate and the mass loss rate are correlated so the EW of the emission component in IPC profiles and the mass loss rate should be correlated as well. No correlation or trend is seen between the EW of Type I line profiles and mass accretion rate or mass loss rate.
Comparing the observed line profiles with line profile modeling allows one to try to establish the origin of the hydrogen line emission. Qualitative comparisons with models found in the literature favour those suggesting that the Hydrogen lines are formed in infalling material. While wind models predict blueshifted absorptions, which are not observed at Br Gamma and only for CW Tau at Pa Beta, infalling models predict IPC profiles for high enough inclinations and when the accretion columns are seen against a hot spot. Infalling models also predict centrally peaked slightly blueshifted line profiles, as observed. Some discrepancies arise when quantitative comparisons are done though. Judging from the model Br Gamma line profiles presented in Muzerolle, Calvet & Hartmann (1998) (accretion scenario): emission line peaks at significantly higher intensities in model profiles; the width of model profiles is considerably smaller than the observed line widths (by about 100 km/s); the line wings in model profiles are not as extended as those observed; when no redshifted absorption feature is present in the model profiles the red wing is more extended than the blue wing, which is the opposite behaviour of what the observations show.
The paucity of Pa Beta and Br Gamma line modeling led to the computation of
line profiles in simple accretion and wind models. All models considered
used radial spherically symmetric flows. Wind models produce mostly
normal P Cygni line profiles unlike the observations show. The accretion
models explored produce IPC profiles with line widths and peak
intensities that, for a range of mass accretion rates (
M
M
yr-1) and for a range
of radii from which matter freely falls (
), can
explain those observed. The mass accretion rates needed for the
model line profiles are in good agreement with estimates from the
observations. The depth of the redshifted absorption
feature in the model line profiles is in general too deep when compared
to the observations, though. The observed range of emission peak
strength to IPC depth is, in fact, not generally reproduced, indicating
that spherically symmetric accretion is not occurring in T Tauri
stars.
The line profile modeling carried out using CLOUDY allows one to match simultaneously the peak intensities of both Pa Beta and Br Gamma. The Br Gamma line profiles resulting from the magnetospheric accretion modeling by Muzerolle, Calvet & Hartmann (1998) produce peak intensities which are significantly higher than what the observations show.
The wind and accretion models explored here are able to produce generally symmetric line profiles (Type I profiles) only for high flow densities/temperatures. In those circumstances the intensity of the model line profiles is far too high when compared to the observations. One concludes that the explored accretion and wind models fail to produced Type I line profiles with the observed intensities.
While the Pa Beta and Br Gamma IPC line profiles are most likely produced in infalling gas, the same origin for the Type I profiles is more doubtful. With the exception of the lack of redshifted absorption feature, the latter display similar characteristics to the IPC profiles. In particular, they are also centrally peaked and slightly blueshifted. These characteristics are very difficult to obtain in wind models [Calvet & Hartmann 1992] but arise naturally in inflow models due to absorption of infalling redshifted material.
The velocities of the redshifted absorption features in IPC profiles are consistent with matter falling freely from a few stellar radii out in a magnetospheric accretion model. The high frequency of IPC profiles in the Pa Beta and Br Gamma lines for the observed sample of stars indicate that a magnetospheric accretion scenario is likely for many T Tauri stars. The quantitative discrepancies found between observations and model line profiles hint that the axi-symmetric models considered thus far, in the context of a magnetospheric accretion scenario, are just a rough approximation to the real accretion flows in T Tauri stars.