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

Abstract

We have discovered T Tauri stars which show startling spectral variability between observations separated by 20 yr. In spectra published approximately 20 years ago these objects showed very weak Hα emission, broad Ca II 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 Hα emission, narrow Ca II 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 on to 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 Hα emission during this phase. Within this scheme, the change to a single spectral type at all wavelengths and the emergence of strong Hα 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 with non-accreting or discless objects. In light of this result we consider the justification for this paradigm.