Tracing the Energetics and Evolution of Dust with Spitzer: a Chapter in
  the History of the Eagle Nebula
  The Spitzer GLIMPSE and MIPSGAL surveys have revealed a wealth of details of
the Galactic plane. We use them to study the energetics and dust properties of
M16, one of the best known SFR. We present MIPSGAL observations of M16 at 24
and 70 $\mu$m and combine them with previous IR data. The MIR image shows a
shell inside the molecular borders of the nebula. The morphologies at 24 and 70
$\mu$m are different, and its color ratio is unusually warm. The FIR image
resembles the one at 8 $\mu$m that enhances the molecular cloud. We measure IR
SEDs within the shell and the PDRs. We use the DUSTEM model to fit the SEDs and
constrain dust temperature, dust size distribution, and ISRF intensity relative
to that provided by the star cluster NGC6611. Within the PDRs, the dust
temperature, the dust size distribution, and the ISRF intensity are in
agreement with expectations. Within the shell, the dust is hotter and an ISRF
larger than that provided by NGC6611 is required. We quantify two solutions.
(1) The size distribution of the dust in the shell is not that of interstellar
dust. (2) The dust emission arises from a hot plasma where UV and collisions
with electrons contribute to the heating. We suggest two interpretations for
the shell. (1) The shell matter is supplied by photo-evaporative flows arising
from dense gas exposed to ionized radiation. The flows renew the shell matter
as it is pushed by the stellar winds. Within this scenario, we conclude that
massive SFR such as M16 have a major impact on the carbon dust size
distribution. The grinding of the carbon dust could result from shattering in
collisions within shocks driven by the interaction between the winds and the
shell. (2) We consider a scenario where the shell is a SNR. We would be
witnessing a specific time in the evolution of the SNR where the plasma
pressure and temperature would be such that the SNR cools through dust
emission.
