Optical spectroscopy of close companions to nearby Herbig Ae / Be and T Tauri stars ⋆

We present VLT-FORS2 optical (5700-9400 Å) spectroscopy of close (r < 1.5 ") companions to three nearby (d < 200 pc) Herbig Ae/Be stars (HD 144432, HD 150193, KK Oph) and one T Tauri star (S CrA). We report the detection of Li I (6707 Å) in absorption and emission lines (H α , Ca II triplet) in the spectra of the companions. Our observations strongly suggest that the companions are physically associated pre-main-sequence stars. The spectral type derived for the companions is K5Ve for HD 144432 B, F9Ve for HD 150193 B, and G6Ve for KK Oph B. S CrA A and B were observed simultaneously. The spatially resolved spectra indicate that S CrA A (primary, north) is a G star and that S CrA B (secondary, south) is a K star. Using photometry from the literature and estimations of the R and I magnitude derived from the spectra, we localized primaries and companions in the HR diagram, derived their masses and assuming coevality constrained the age of the systems. KK Oph B (7 Myr) and S CrA B (2 Myr) are actively accreting T Tauri stars and are very likely surrounded by disks. HD 150193 B (10 Myr) and HD 144432 B (8 Myr) are weak-line T Tauri stars. Three of the four systems studied (HD 144432, HD 150193, KK Oph) have ages > 7 Myr. These systems retained their disks for a longer time than typical of a young star. Our results suggest that binarity may be a key issue in understanding the lifetime of disks.