Spitzer/IRAC view of Sh 2-284: Searching for evidence of triggered star
  formation in an isolated region in the outer Milky Way
  Using Spitzer/IRAC observations of a region to be observed by the CoRoT
satellite, we have unraveled a new complex star-forming region at low
metallicity in the outer Galaxy. We perform a study of S284 in order to outline
the chain of events in this star-forming region. We used four-band Spitzer/IRAC
photometry as well as Halpha imaging obtained with INT/WFC. Combining these
data with the optical photometry obtained in the frame of CoRoTs preparation
and the 2MASS catalog we analysed the properties and distribution of young
stellar objects (YSOs) associated with point-like sources. We also studied the
SEDs of regions of extended emission, complementing our dataset with IRAS and
MSX data. We find that S284 is unique in several ways: it is very isolated at
the end of a spiral arm and both the diffuse dust and ionized emission are
remarkably symmetric. We have partially resolved the central clusters of the
three bubbles present in this region. Despite the different scales present in
its multiple-bubble morphology, our study points to a very narrow spread of
ages among the powering high-mass clusters. In contrast, the particular
sawtooth structure of the extended emission at the rim of each ionized bubble
harbours either small lower-mass clusters with a younger stellar population or
individual young reddened protostars. In particular, triggered star formation
is considered to be at work in these regions.
