several factors and not only by the classical paradigm of fast gas expulsion.]{}

Introduction {#sec:introduction}
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Stars form by gravitational collapse of high-density fluctuations in the interstellar molecular gas, which are generated by supersonic turbulent motions [e.g., @Klessen2011-lectures]. Following the nomenclature of @Williams2000, star formation takes place in dense ($n \gtrsim 10^4$ ) *clumps*, which are in turn fragmented into denser ($n \gtrsim 10^5$ ) *cores*, in which individual stars or small multiple systems are born. Given this nature of the star formation process, stars are born correlated in space and time, with typical scales of 1 pc and 1 Myr, respectively [see @Kroupa2011], constituting young stellar agglomerates known as *embedded clusters* (ECs). @Bressert2010 studied the spatial distribution of star formation within 500 pc from the Sun and found that, in fact, most of the young stellar objects (YSOs) in their sample are found in regions with number densities greater than $\sim 2\,{\rm pc}^{-3}$, which is more than an order of magnitude higher than the density of field stars in the Galactic disk, $0.13\,{\rm pc}^{-3}$ [@Chabrier2001].

Many of the ECs defined in this way, however, are not gravitationally bound and will not become classical open clusters (OCs), i.e., bound stellar 