Spectroscopic Assessment of WISE-based Young Stellar Object Selection
  We have conducted a sensitive search down to the hydrogen burning limit for
unextincted stars over $\sim$200 square degrees around Lambda Orionis and 20
square degrees around Sigma Orionis using the methodology of Koenig & Leisawitz
(2014). From WISE and 2MASS data we identify 544 and 418 candidate YSOs in the
vicinity of Lambda and Sigma respectively. Based on our followup spectroscopy
for some candidates and the existing literature for others, we found that
$\sim$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--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 AGNs 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
HII region bubble around $\sigma$ and $\epsilon$ 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.
