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Astron. Astrophys. 321, 497-512 (1997)

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1. Introduction

Classical T Tauri stars (hereafter cTTS) are now recognized as pre-main sequence stars accreting material from an extended circumstellar disk (Appenzeller & Mundt 1989). Although the stars appear to accrete a substantial amount of mass from the disks during this phase of evolution, their rotation rates are surprisingly low (Bouvier et al. 1993). T Tauri stars (hereafter TTS) without disks (so-called "weak-line" TTS: wTTS) rotate more rapidly than T Tauri stars with disks (the cTTS: Bouvier et al. 1995). Although wTTS may have the same age as cTTS, they represent a more evolved population of low-mass pre-main sequence stars in the sense that all matter has been accreted, and the disks - if they were ever present in the forms found in cTTS - have largely been dispersed. Therefore, it is natural to assume that the rotation rate is kept low in classical TTS by processes involving the disk, since more evolved young stars could spin up considerably due to their contraction towards the main sequence after the disks have been dispersed (Bouvier 1994). This interaction could take place either via a viscous accretion disk (Lynden-Bell & Pringle 1974) or via magnetic field lines coupling the disk with the rapidly rotating - and hence presumedly highly magnetic - pre-main sequence star (Cameron, Campbell & Quaintrell 1995; Armitage & Clarke 1996).

One of the best empirical probes of the mode of accretion is optical spectroscopy: the veiling continuum and the emission lines seen in the spectra of cTTS are interpreted in completely different manners in various models. In the accretion disk model, the emission lines are assumed to be closely correlated with the boundary layer, which is interpreted as the source of the veiling continuum (Basri & Batalha 1990). In the magnetic model, the emission lines are interpreted as being formed by the matter falling freely along the field lines to the star (Hartmann et al. 1994), and the veiling continuum is the emission from material which is heated by the soft X-ray and UV emission from the accretion shock (Königl 1991; Gullbring 1994).

The aim of this paper is to place better phenomenological constraints on where the emission lines and the associated veiling continuua are formed by measuring the spectral behavior in a few systems with considerable veiling over many contiguous nights. First, we extract the veiling continua by carefully determining the spectral type of the underlying star. Next, we study the correlation between the different emission lines and the veiling continuum in each system in order to study how closely the two spectral components are related. Finally, variations in the line profiles obtained from higher spectral resolution data are studied.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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