Mid Infrared View of the High Mass Star Formation Region W51A
  In this paper we present the results of a mid infrared study of G49.5-0.4, or
W51A, part of the massive starbirth complex W51. Combining public data from the
$Spitzer$ IRAC camera, and Gemini mid infrared camera T-ReCS at 7.73, 9.69,
12.33 and 24.56 \micron, with spatial resolution of $\sim$0.5\arcsec, we have
identified the mid infrared counterparts of 8 ultracompact HII regions, showing
that two radio sources are deeply embedded in molecular clouds and another is a
cloud of ionized gas. From the T-ReCS data we have unveiled the central core of
W51 region, revealing massive young stellar candidates. We modeled the spectral
energy distribution of the detected sources suggesting the embedded objects are
sources with spectral types ranging from B3 to O5, but the majority of the fits
indicate stellar objects with B1 spectral types. We also present an extinction
map of IRS~2, showing that a region with lower extinction corresponds to the
region where a proposed jet of gas has impacted the foreground cloud. From this
map, we also derived the total extinction towards the enigmatic source IRS~2E,
which amounts to $\sim$60 magnitudes in the $V$ band. We calculated the color
temperature due to thermal emission of the circumstellar dust of the detected
sources; the temperatures are in the interval of $\sim$100 -- 150 K, which
corresponds to the emission of dust located at 0.1 pc from the central source.
Finally, we show a possible mid infrared counterpart of a detected source at mm
wavelengths that was found by \cite{zap08,zap09} to be a massive young stellar
object undergoing a high accretion rate.
