Accretion in Young Stellar/Substellar Objects

We present a study of accretion in a sample of 45 young, low mass objects in a variety of star forming regions and young associations, about half of which are likely substellar. Based primarily on the presence of broad, asymmetric Hα emission, we have identified 13 objects (∼30% of our sample) which are strong candidates for ongoing accretion. At least 3 of these are substellar. We do not detect significant continuum veiling in most of the accretors with late spectral types (M5-M7). Accretion shock models show that lack of measurable veiling allows us to place an upper limit to the mass accretion rates of . 10 M⊙ yr . Using magnetospheric accretion models with appropriate (sub)stellar parameters, we can successfully explain the accretor Hα emission line profiles, and derive quantitative estimates of accretion rates in the range 10 < Ṁ < 10 M⊙ yr . There is a clear trend of decreasing accretion rate with stellar mass, with mean accretion rates declining by 3-4 orders of magnitude over ∼ 1− 0.05 M⊙. Subject headings: accretion disks, brown dwarfs, stars: emission-line, pre-main sequence, circumstellar matter,