Weak-line T Tauri stars : circumstellar disks and companions. I. Spectral energy distributions and infrared excesses

We have analysed new infrared photometric data taken by the Infrared Space Observatory, in combination with 2MASS data where available, of 12 weak-line T Tauri stars (WTTS), belonging to different molecular clouds. A control study of 6 classical T Tauri stars (CTTS) for comparison with IRAS data has also been made. WTTS traditionally are not expected to have any circumstellar material, but we have found that 11 of the WTTS studied, have a spectral energy distribution that suggests the presence of reprocessing circumstellar disks, albeit half of them with apparent inner "dust holes". The spectral energy distribution curves of all the stars in our sample show infrared excess in all cases except one. We have also searched for companions to these stars and studied their possible contributions to the infrared emission. We find that 10 of the WTTS with excess infrared emission have companions that may contribute measurably and, in some cases significantly, at infrared wavelengths. In the case of the binary systems, the amount of excess due to the companion is still unclear but it is unlikely they are the only source of all the observed excess. We give estimations of the excesses and masses of the stars and also find their ages and radii to be in agreement with those expected for T Tauri stars. Our results suggest there is a continuum in infrared excess properties from CTTS to WTTS rather than a clear differentiation based on a somewhat arbitrary EW(Ha) criterion.