Spectral Type Mismatch

The main source of error in the computation of the near infrared veiling for a given T Tauri star is the choice of the template star used. The way in which this choice was made was described in Section 4.2 above.

The stars for which the veiling determination should be least affected by a mismatch in spectral type are the K7V to M2V stars in the sample. Why? On the one hand inspecting the spectra of the T Tauri stars with spectral types K7V to M2V one sees that they are as expected for a star of those spectral types, while the same is not true for some stars of earlier spectral type (eg. GM Aur, HP Tau and T Tau). Also, determining the 'veiling' of the K7V template star using the M2V template as a template star gives r_J and r_K consistent with zero, meaning that an uncertainty in the spectral class about K7V to M2V does not introduce a large error in the veiling determinations.

For comparison, the histograms of the veiling measurements for the stars in the sample with spectral types between K7V and M2V only are also shown in Figure 4.6.

What is the error introduced in the J veiling determination by a mismatch between the spectral type of the T Tauri star and that of the template star used? The top panel of Figure 4.12 shows the distributions obtained for the J veiling when the K7V template star is used for all stars but DI Cep (G8V), FP Tau (M4) and Haro 6-13 (C) and when the M2V template is used for the same stars. The two distributions are very similar, with the number of stars with higher veiling increasing slightly when the M2V template is used. This increase in the veiling is within the uncertainties in the veiling determination, as can be seen by the lower plot of Figure 4.12, although using the M2V template tends to give slightly higher values for the J veiling. This result is expected since despite being very similar, the photospheric lines of a M2V star are slightly deeper than those of a K7V star. In actual fact, the 'J veiling' of the K7V template determined with the M2V template is , which is consistent with zero but positive. A spectral type mismatch between the T Tauri star and the template stars of 2 to 3 subclasses does not seem to influence the J veiling determination beyond the quoted uncertainties. Larger mismatches however can lead to an erroneous determination of the J veiling. As an example, if the K7V template is used to determine the 'J veiling' of the K0V template, the obtained value is . This is due to the fact that the photospheric lines used here to measure the veiling are not as deep in a K0V as in a K7V star.

In the K band the veiling of Gl380 (K7V) determined with Gl410 (M2V) results in . Therefore, the veiling of the T Tauri stars determined with either template is the same. Since early and late K spectral types have basically the opposite ratio of the photospheric lines Si 4590.17 cm^-1 and Ti 4589.50 cm^-1, it is almost impossible to misclassify a T Tauri star by so many spectral classes based on spectra in this wavelength range.