Optical spectra of T Tauri stars often reveal their photospheric spectrum. For those stars spectral types have been determined using that information. However, many T Tauri stars have spectra devoid of any photospheric features, or the photospheric lines are not as deep as expected for their spectral type. It has been found that this can be explained by the presence of an extra continuum source that veils the underlying photospheric spectrum [Basri & Batalha 1990]. In some cases this continuum is so strong that the photospheric spectrum disappears altogether. The amount of veiling can be quantified by comparing the spectrum of the T Tauri star with that of a template star, chosen such that its spectrum represents accurately the photospheric spectrum of a star of the appropriate spectral type. The definition of veiling (r) is
where 
is the excess flux and 
is the photospheric
flux.
The spectra presented in Figures 3.12 and 3.13 show that quite a number of these T Tauri stars display photospheric features in our spectral region. This can be easily seen in Figure 4.1 where the Pa Beta spectrum of AA Tau and the Br Gamma spectrum of GG Tau are shown together with the spectrum of the K7 V template star Gl380. The photospheric lines present at Pa Beta are, from left to right in Figure 4.1: Fe 7806.01 cm-1, Ti 7803.36 cm-1, Ca 7800.59 cm-1, Ti 7797.16 cm-1, Ti 7791.23 cm-1, Ti 7781.77 cm-1, Fe 7761.99 cm-1, Mn 7749.92 cm-1, and at Br Gamma Si 4625.52 cm-1, Si 4590.17 cm-1, Ti 4589.50 cm-1 (vacuum wavenumbers). The photospheric lines were identified using the Near Infrared Solar Atlas of Livingston & Wallace (1991).
Figure 4.1: Top panel - Pa Beta spectrum of AA Tau and
Gl380 (K7 V). Bottom panel - Br Gamma spectrum of GG Tau and Gl380 (K7 V).
See text for wavenumber of the photospheric lines.
Photospheric lines are present in the Pa Beta spectrum of 75% of the T Tauri stars observed at this wavelength region and in the Br Gamma spectrum of 70% of the T Tauri stars observed at Br Gamma. This fact allows one to determine the amount of veiling present in these spectra. The photospheric lines used for this determination were Ti 7791.23 cm-1, Ti 7781.77 cm-1 and Fe 7761.99 cm-1 for the Pa Beta wavelength range and Ti 4589.50 cm-1 and Si 4590.17 cm-1 for the Br Gamma wavelength range. The other photospheric lines are either not strong enough to determine the veiling or they fall on top of the emission line and therefore cannot be used.
