Spitzer/IRAC view of Sh 2-284 ^*,**

Searching for evidence of triggered star formation in an isolated region in the outer Milky Way

¹ Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium e-mail: elena@ster.kuleuven.be
² Laboratoire AIM, CEA/DSM, CNRS, Université Paris Diderot, DAPNIA/SAp, 91191 Gif-sur-Yvette, France
³ GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, 5 place Jules Janssen, 92190 Meudon, France
⁴ Instituto Astrofísico de Canarias,C/ Vía Láctea s/n, 38200, La Laguna, Spain
⁵ Sterrenkunde Instituut, Universiteit van Amsterdam, Kruislaan 403, 1098SJ Amsterdam, The Netherlands
⁶ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
⁷ INAF – Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131 Napoli, Italy


Received: 24 July 2008
Accepted: 19 March 2009

Abstract

Aims. 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.

Methods. We used four-band Spitzer/IRAC photometry as well as Hα 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.

Results. 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 observed 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.