New observational frontiers in the multiplicity of young stars

It has now been known for over a decade that low-mass stars located in star-forming regions are very frequently members of binary and multiple systems, even more so than main sequence stars in the solar neighborhood. This high multiplicity rate has been interpreted as the consequence of the fragmentation of small molecular cores into a few seed objects that accrete to their final mass from the remaining material and dynamically evolve into stable multiple systems, possibly producing a few ejecta in the process. Analyzing the statistical properties of young multiple systems in a variety of environments therefore represents a powerful approach to place stringent constraints on star formation theories. In this contribution, we first review a number of recent results related to the multiplicity of T Tauri stars. We then present a series of studies focusing on the multiplicity and properties of optically-undetected, heavily embedded protostars. These objects are much younger than the previously studied pre-main sequence stars, and they therefore offer a closer look at the primordial population of multiple systems. In addition to these observational avenues, we present new results of a series of numerical simulations that attempt to reproduce the fragmentation of small molecular cores into multiple systems, and compare these results to the observations.