Spectroscopic assessment of Wise-based young stellar object selection near λ and σ Orionis

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 and 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 ϵ 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.