Embedded Young Stellar Object Candidates in the Active Star Forming
  Complex W51: Mass Function and Spatial Distribution
  We present 737 candidate Young Stellar Objects (YSOs) near the W51 Giant
Molecular Cloud (GMC) over an area of 1.25 deg x 1.00 deg selected from Spitzer
Space Telescope data. We use spectral energy distribution (SED) fits to
identify YSOs and distinguish them from main-sequence or red giant stars,
asymptotic giant branch stars, and background galaxies. Based on extinction of
each YSO, we separate a total of 437 YSOs associated with the W51 region from
the possible foreground sources. We identify 69 highly embedded Stage 0/I
candidate YSOs in our field with masses > 5 Msun (corresponding to mid-to
early-B main-sequence spectral types) 46 of which are located in the central
active star forming regions of W51A and W51B. From the YSOs associated with
W51, we find evidence for mass segregation showing that the most massive YSOs
are concentrated on the W51 HII region complex. We find a variation in the
spatial distribution of the mass function (MF) of YSOs in the mass range
between 5 Msun and 18 Msun. The derived slopes of the MF are -1.26 and -2.36 in
the active star-forming region and the outer region, respectively. The
variation of the MF for YSOs embedded in the molecular cloud implies that the
distribution of stellar masses in clusters depends on the local conditions in
the parent molecular cloud.
