The extra continuum emission that veils the photospheric spectrum of the T Tauri stars at optical wavelengths is thought to arise in the accretion shock that results as matter hits the stellar surface. Hartigan et al. (1991) find that the spectral energy distribution of the optical veiling imply a temperature of 9000 K for the veiling source. Such temperature imply that the veiling in the J band due to the accretion shock should be one tenth that found in the V band, i.e. the J veiling should be nearly zero.
The data set presented in this work allows for the first time the measurement of the veiling in the J band (rJ) for T Tauri stars. The K band veiling (rJ) was also measured.
The average rJ and rJ are < rJ > = 0.56 and < rK > = 1.31. These results show that there is considerable near infrared (NIR) veiling which cannot be due to the accretion shock.
A possible source for NIR veiling is continuum emission from the
accretion disk. However, models for emission from accretion disks
[Calvet et al. 1997, Calvet,
private comunication,] predict that for accretion rates typical of those
expected in T Tauri stars, i.e.
--
yr-1, a disk with a inner
hole of a few stellar radii in size, as expected for the magnetospheric
accretion scenario, produces a negligible amount of J and K veiling.
The veiling measurements presented here were done with data taken for different purposes, namely the study of Pa Beta and Br Gamma line profiles. The photospheric lines used were not therefore optimal to perform a veiling study. The observed set of template stars was not very large and not all spectral types were exactly matched. These led to large uncertainties in the veiling measurements presented. Despite of that, the high values found for the veiling seem genuine. Although confirmation requires a careful study using more suitable photospheric lines and a better grid of template stars, the high frequency of stars with low veiling at V seen as 'continuum' stars in the near infrared support the results presented. Such results cannot be explained by the current theoretical models.