An Empirical Criterion to Classify T Tauri Stars and Substellar Analogs Using Low-Resolution Optical Spectroscopy

We have compiled and studied photometric and spectroscopic data published inthe literature of several star forming regions and young open clusters (Orion,Taurus, IC348, Sco-Cen Complex, Chamaeleon I, TW Hya association, sigma Orioniscluster, IC2391, alpha Per cluster and the Pleiades). Our goal was to seek thedefinition of a simple empirical criterion to classify stars or brown dwarfswhich are accreting matter from a disk on the sole basis of low-resolutionoptical spectroscopic data. We show that using Halpha equivalent widths andspectral types we can statistically classify very young stars and brown dwarfsas classical T Tauri stars and substellar analogs. As a boundary betweenaccreting and non accreting objects, we use the saturation limit ofchromospheric activity at Log L(Halpha)/L(bol)=-3.3 (determined in the openclusters). We discuss the uncertainties in the classification scheme due to theoccurrence of flares. We have used this spectroscopic empirical criterion toclassify objects found in the literature, and we compute the fraction ofaccreting objects in several star forming regions. The fraction of accretingobjects appears to decrease from about 50% to about 5% from 1 Myr to 10 Myr forboth stars and brown dwarfs.Comment: Astronomical Journal, accepte