As pointed out in the introductory chapter, hydrogen Balmer line profiles in T Tauri stars are known to be variable. A few T Tauri stars for which spectra are presented in this work were observed more than once and some considerations regarding variations in the observed NIR line profiles are due. The stars for which the NIR lines were observed more than once are shown in Table 5.7 where the dates of the observations are also indicated.
The Pa Beta and Br Gamma spectra of these stars are presented in Figures 5.32 to 5.34. The colour coding is as follows: Pa Beta - spectra in black were obtained in October 1994 (UT9410), spectra in red are from UT951215 and spectra in green from UT951216; Br Gamma - spectra in black are from UT9410 and in red from UT951217.
Figure 5.32: Pa Beta spectra of DG Tau, DL Tau, DR Tau
and GI Tau. Black - UT9410 data; Red - UT951215 data and Green - UT951216
data.
Figure 5.33: Pa Beta spectra of GK Tau, RW Aur, RY Tau
and SU Aur. Black - UT9410 data; Red - UT951215 data and Green - UT951216
data.
Figure 5.34: Br Gamma spectra of DR Tau, GG Tau, GI
Tau, GK Tau and RW Aur. Black - UT9410 data and Red - UT951217
data.
Observations in consecutive nights of the same line for the same star were only carried out during the December 1995 run (red and green lines on Figures 5.32 and 5.33) and only for the Pa Beta line. The night to night variation observed is not very dramatic, in the sense that, for these stars, the type of line profile did not change. The changes were such that the overall line shape is maintained with relatively small variation in the line equivalent width. The most significant of the night to night variations is that of RY Tau, for which the line FWHM increases together with the line strength; the redshifted absorption also moves more into the red. More drastic changes are observed to occur in the spectra taken roughly one year apart, for some of the stars. While the Pa Beta lines of DG Tau and DL Tau and the Br Gamma line of RW Aur do not change very much (note also that nearly no emission is detected in the Br Gamma spectra of GI Tau and GK Tau for both dates), the Pa Beta lines of DR Tau and GK Tau fall in a different type of profile for the different observing dates. DR Tau changes from Type I into Type II R and GK Tau changes from Type I into an IPC line profile. The Br Gamma lines of DR Tau and GG Tau and the Pa Beta line of GI Tau change significantly in terms of equivalent width (due to line profile variations) but preserve the type of profile.
One therefore sees that the type of variations that can occur in these lines are diverse and surely reflect the dynamical activity and/or changes in the physical characteristics of the emitting region(s). The change of the Pa Beta profile of GK Tau from Type I to IPC surely represents that either an accretion region came into view or that accretion activity in the star changed between the first and second observing runs. Trying to decide which of these is responsible for the changes implies monitoring the line for some time, never inferior to one rotation period of the star. If accreting regions are localized they should come into view and there should be some rotational modulation. To try to understand such changes imply monitoring these lines in T Tauri stars.
The data presented here does not allow further studies in terms of variability other than establishing that the profiles of Pa Beta and Br Gamma lines do vary. Variability studies similar to the ones carried out using the H Alpha line [ eg. Gameiro et al. 1993,Johns & Basri 1995b,Johns & Basri 1995a] should be pursued with these two lines, allowing for a better understanding of the region where they are formed.
The NIR Pa Beta and Br Gamma lines with IPC profiles definitely trace infalling matter. Monitoring these lines over at least one rotation period of the star would tell us how the infalling matter behaves for those objects since any variability displayed by these lines (especially in the redshifted absorption component) should be associated with the accretion flow. If the magnetospheric accretion model is correct and if the rotation axis of the star is tilted relative to the magnetic axis one expects to find a correlation between the line profiles and the phase of the rotation period of the star [Johns & Basri 1995b].
