The t tauri phase down to nearly planetary masses : Echelle spectra of 82 very low mass stars and brown dwarfs

摘要: Using the largest high-resolution spectroscopic sample to date of young, very low mass stars and brown dwarfs, we investigate disk accretion in objects ranging from just above hydrogen-burning limit all way nearly planetary masses. Our 82 targets span spectral types M5 M9.5, or masses 0.15 M? down about 15 jupiters. They are confirmed members ? Ophiuchus, Taurus, Chamaeleon I, IC 348, R Coronae Australis, Upper Scorpius, TW Hydrae star-forming regions young clusters, with ages <1 ~10 Myr. The contains 41 dwarfs (spectral ?M6.5). We have previously presented optical spectra for roughly half sample; rest new. This is a close complete survey known so far examined, except Oph 348 (where limited by combination extinction distance). find that (1) classical T Tauri-like persists substellar domain deuterium-burning limit; (2) while an H? 10% width 200 km s-1 our prime diagnostic (following previous work), permitted emission lines Ca II, O He I also good indicators, as Tauri (we caution against blind use alone, since inclination rotation effects on line especially important at rates objects); (3) II ?8662 flux excellent quantitative measure rate (as higher Stars), correlating remarkably well obtained veiling modeling; (4) diminishes rapidly mass?our measurements support suggestions (albeit considerable scatter) extend this correlation entire range masses; (5) fraction stellar accretors decreases substantially age, stars; (6) any given comparable accretor (7) number sources infrared excesses arising dusty disks do not evince measurable signatures, incidence such mismatch increasing age: implies low-mass regime can persist beyond main phase parallels transition post-T stage more massive stars. These strong similarities ages, between one hand bodies below other, consistent common formation mechanism two regimes.