Massive star formation in the outer Galaxy: S284

Triggered star formation by high-mass stars has been widely explored from the theoretical point of view (Elmegreen 1998), while observations do not seem to convincingly confirm it. The difficulty arises when trying to discriminate between the inherent properties of these star forming regions and the effects induced by nearby systems. In order to study this process in isolation, the candidate HII regions must present simple morphologies. This condition is most likely fulfilled in the areas slightly off the Galactic plane (Deharveng et al. 2005) and in the outer Galaxy. S284 is a giant HII region located at the Galactic anti-centre. This object is situated further out the Perseus arm (Rgal~13.5 kpc) where star formation significantly drops. S284 harbours a widely studied OB cluster in its centre: Dolidze-25. In this conference, we will present Spitzer/IRAC and H-alpha(INT) observations of this region and surroundings. The observations show a symmetical distribution of the surrounding dust and the ionised gas with respect to the central OB cluster. Moreover, the data show substructures indicative of ongoing high-mass star formation at the rim of the ionised nebula on various scales. With these data we have characterised the ionising sources, constrained the ages of some of the substructres/second generation clusters and determine the luminositiy of the youngest embedded stars and protostars. The isolated nature of S284 is a very interesting feature, also favoured by its proximity (5.5 kpc). However, what makes S284 a particularly intriguing object is the fact that it has low metallicity. This allows us to determine whether there are any fundamental differences between this low-metallicity environment and better studied high-metallicity sites of star formation. Furthermore, this site may well be a much better suited template for massive star formation in the more distant Universe.