Paper detail

Evidence for high accretion-rates in Weak-Line T Tauri stars?

We have discovered T Tauri stars which show startling spectral variability between observations seperated by 20 years. In spectra published by Bouvier & Appenzeller (1992) these objects showed very weak H-alpha emission, broad CaII absorption and so called ``composite spectra'', where the spectral type inferred from the blue region is earlier than that inferred from the red. We present here new spectroscopy which shows that all four stars now exhibit strong H-alpha emission, narrow CaII emission and a spectral type which is consistent at all wavelengths. We propose a scheme to understand these changes whereby the composite spectra of these stars can be explained by a period of active accretion onto the central, young star. In this scheme the composite spectrum consists of a contribution from the stellar photosphere and a contribution from a hot, optically thick, accretion component. The optically thick nature of the accretion flow explains the weakness of the H-alpha emission during this phase. Within this scheme, the change to a single spectral type at all wavelengths and emergence of strong H-alpha emission are consistent with the accretion columns becoming optically thin, as the accretion rate drops. There is a strong analogy here with the dwarf novae class of interacting binaries, which show similar behaviour during the decline from outbursts of high mass-transfer rate. The most important consequence of this interpretation is that these objects bring into question the association of Weak-Line T Tauri stars (WTTs) with non-accreting or discless objects. In light of this result we consider the justification for this paradigm.