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 in the literature of several star-forming regions and young open clusters (Orion, Taurus, IC 348, Sco-Cen complex, Chamaeleon I, TW Hydrae association, σ Orionis cluster, IC 2391, α Persei cluster, and the Pleiades). Our goal was to seek the definition of a simple empirical criterion to classify stars or brown dwarfs that are accreting matter from a disk on the sole basis of low-resolution optical spectroscopic data. We show that, using Hα equivalent widths and spectral types, we can statistically classify very young stars and brown dwarfs as classical T Tauri stars and substellar analogs. As a boundary between accreting and nonaccreting objects, we use the saturation limit of chromospheric activity at log [L(Hα)/L(bol)] = -3.3 (determined in the open clusters). We discuss the uncertainties in the classification scheme due to the occurrence of flares. We have used this spectroscopic empirical criterion to classify objects found in the literature, and we compute the fraction of accreting objects in several star-forming regions. The fraction of accreting objects appears to decrease from about 50% to about 5% from 1 to 10 Myr for both stars and brown dwarfs.