A Deep Objective Prism Survey for Classical T Tauri Stars in the σ Orionis Region

A very deep 1976 CTIO Curtis‐Schmidt objective prism plate of the σ Orionis association taken under excellent conditions has permitted us to search for fainter classical T Tauri stars associated with this cluster. We have discovered 63 Hα emission stars in the area to a limiting magnitude of about 18. Most of these stars are new additions to the growing list of pre–main‐sequence objects identified in this very dense, very young cluster. Since T Tauri stars vary in light and emission‐line strength on all timescales, these data also provide an historical record of the association. By comparing these with later observations, we are able to estimate the average space density of the Hα emission‐line stars in the surveyed area as between 0.52 and 0.86 pc −3 down to a spectral class of dM0. This is about a third of the number of dwarf or subgiant stars in the region that could have been detected as emission‐line stars in these surveys.