Studies of the distinct regions due to CO selective dissociation in the
  Aquila molecular cloud
  Aims. We investigate the role of selective dissociation in the process of
star formation by comparing the physical parameters of protostellar-prestellar
cores and the distinct regions with the CO isotope distributions in
photodissociation regions. We seek to understand whether there is a better
connection between the evolutionary age of star forming regions and the effect
of selective dissociation
  Methods. Wide-field observations of the $\rm ^{12}CO$, $\rm ^{13}CO$, and
$\rm C^{18}O$ ( J = 1 - 0) emission lines are used to study the ongoing star
formation activity in the Aquila molecular region, and the 70 $\mu$m and 250
$\mu$m data are used to describe the heating of the surrounding material and as
an indicator of the evolutionary age of the core.
  Results. The protostellar-prestellar cores are found at locations with the
highest $\rm C^{18}O$ column densities and their increasing evolutionary age
would relate to an increasing 70$\mu$m/250$\mu$m emission ratio at their
location. An evolutionary age of the cores may also follow from the $\rm
^{13}CO$ versus $\rm C^{18}O$ abundance ratio, which decreases with increasing
$\rm C^{18}O$ column densities. The original mass has been estimated for nine
representative star formation regions and the original mass of the region
correlated well with the integrated 70 $\mu$m flux density. Similarly, the $
X_{\rm ^{13}CO}$/$X_{\rm C^{18}O}$ implying the dissociation rate for these
regions correlates with the 70$\mu$m/250$\mu$m flux density ratio and reflects
the evolutionary age of the star formation activity.
