Evidence for high accretion rates in weak-line T Tauri stars?

ABSTRACT We have discovered T Tauri stars which show startling spectral variabil-ity between observations seperated by 20 years. In spectra published byBouvier & Appenzeller (1992) these objects showed very weak Hα emission, broadCaII absorption and so called “composite spectra”, where the spectral type inferredfrom the blue region is earlier than that inferred from the red. We present here newspectroscopy which shows that all four stars now exhibit strong Hα emission, narrowCaII emission and a spectral type which is consistent at all wavelengths.We propose a scheme to understand these changes whereby the composite spectraof these stars can be explained by a period of active accretion onto the central, youngstar. In this scheme the composite spectrum consists of a contribution from the stellarphotosphere and a contribution from a hot, optically thick, accretion component. Theoptically thick nature of the accretion flow explains the weakness of the Hα emissionduring this phase. Within this scheme, the change to a single spectral type at allwavelengths and emergence of strong Hα emission are consistent with the accretioncolumns becoming optically thin, as the accretion rate drops. There is a strong analogyhere with the dwarf novae class of interacting binaries, which show similar behaviourduring the decline from outbursts of high mass-transfer rate.The most important consequence of this interpretation is that these objects bringinto question the association of Weak-Line T Tauri stars (WTTs) with non-accretingor discless objects. In light of this result we consider the justification for this paradigm.Key words: accretion, accretion disks, stars:pre-main-sequence, instabilities,stars:dwarf novae, planetary systems: protoplanetary discs