Young Stellar Objects in the Massive Star-Forming Regions W51 and W43
  We present the results of our investigation of the star-forming complexes W51
and W43, two of the brightest in the first Galactic quadrant. In order to
determine the young stellar object (YSO) populations in W51 and W43 we used
color-magnitude relations based on Spitzer mid-infrared and 2MASS/UKIDSS
near-infrared data. We identified 302 Class I YSOs and 1178 Class II/transition
disk candidates in W51, and 917 Class I YSOs and 5187 Class II/transition disk
candidates in W43. We also identified tens of groups of YSOs in both regions
using the Minimal Spanning Tree (MST) method. We found similar cluster
densities in both regions even though Spitzer was not able to probe the densest
part of W43. By using the Class II/I ratios, we traced the relative ages within
the regions and based on the morphology of the clusters we argue that several
sites of star formation are independent of one another in terms of their ages
and physical conditions. We used spectral energy distribution (SED)-fitting to
identify the massive YSO (MYSO) candidates since they play a vital role in the
star formation process and then examined them to see if they are related to any
massive star formation tracers such as UCH II regions, masers and dense
fragments. We identified 17 MYSO candidates in W51, and 14 in W43, respectively
and found that groups of YSOs hosting MYSO candidates are positionally
associated with H II regions in W51, though we do not see any MYSO candidates
associated with previously identified massive dense fragments in W43.
