2015AJ....150..100K

We have conducted a sensitive search down to the hydrogen burning limit for unextincted stars over ∼200 square degrees around Lambda Orionis and 20 square degrees around Sigma Orionis using the methodology of Koenig & Leisawitz. From WISE and 2MASS data we identify 544 and 418 candidate young stellar objects (YSOs) in the vicinity of λ and σ respectively. Based on our followup spectroscopy for some candidates and the existing literature for others, we found that ∼80% of the K14-selected candidates are probable or likely members of the Orion star-forming region. The yield from the photometric selection criteria shows that WISE sources with K_s - w3 > 1.5 mag and K_S between 10 and 12 mag are most likely to show spectroscopic signs of youth, while WISE sources with K_s - w3 > 4 mag and K_s > 12 were often active galactic nuclei when followed up spectroscopically. The population of candidate YSOs traces known areas of active star formation, with a few new ''hot spots'' of activity near Lynds 1588 and 1589 and a more dispersed population of YSOs in the northern half of the H ii region bubble around σ and ∈ Ori. A minimal spanning tree analysis of the two regions to identify stellar groupings finds that roughly two-thirds of the YSO candidates in each region belong to groups of 5 or more members. The population of stars selected by WISE outside the MST groupings also contains spectroscopically verified YSOs, with a local stellar density as low as 0.5 stars per square degree.