High veiling at near infrared wavelengths in classical T Tauri stars

D. F. M. Folha, J. P. Emerson

Resumo
The near infrared veiling (the ratio of any non-photospheric excess flux to the photospheric flux) is studied for a sample of 50 mainly Classical T Tauri stars (CTTS), mostly from the Taurus-Auriga complex, based on high resolution spectra ({R} ~ 20,500) of wavelength regions in the J and K wavebands (near Pa β - 1.28215 mu m - and near Br γ - 2.16611 mu m respectively). The method used to compute the veiling at these wavelengths is independent of any assumptions about reddening/extinction towards the studied objects. Photospheric absorption lines are identified in the spectra of 73% of the CTTS observed at J and in 71% at K. For these stars the veiling in the J (r_J) and K (r_K) wavebands was determined. Average values for all the stars are < r_J> = 0.57 and < r_K> = 1.29. Considering only stars with veilings determined to better than 3 sigma these means are < r_J> = 0.97 and < r_K> = 1.76. For the remaining 27% of the stars observed at J and 29% at K, for which no photospheric lines were identified, lower limits were obtained for r_J and r_K. Considering only those stars with lower limits the mean lower limits are < r_J> > 1.34 and < r_K> > 2.4. Our results indicate high veiling of these stars in the NIR. The NIR veilings deduced are considerably greater than expected from extrapolation of the veilings measured in the optical, assuming that they result from an accretion shock, and are also bigger than the expected veiling from an accretion disk. Although the origin of this large NIR veiling is not clear one possibility deserving further study is that it partially arises within shocks at the base of magnetospheric accretion columns which cover a large fraction of the stellar surface. The possibility that the near infrared veiling of photospheric lines is not caused by a continuum, but is rather due to non-photospheric line emission from the disk or from a shock at the base of the accretion columns, should also be investigated by detailed modelling including the Ti, Fe, Si & Mn lines as opacity sources.

Astronomy and Astrophysics
Volume 352, Página 517
dezembro 1999

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