Abstract

At the end of the Dark Ages gravity began to make matter collapse around over-densities, resulting in the formation of the first stars, the Pop III . These stars are very important in the cosmological context, the most massive ones contributed ionising photons that played a role in the reionisation of the Universe and synthetized the first metals. We know that the formation mode of these first stars was different than the one operating today. The lack of metals implies it is difficult to cool a contracting cloud, which would argue for a preferential formation of massive or very massive stars. However from the theoretical point of view it is not yet possible to determine the initial mass function of the first stars. Simulations show that, even in the absence of metals, a collapsing cloud may fragment into smaller clouds giving rise to stars that span a range of masses, even sub-solar. Once the first metals begin to be shed in the clouds cooling becomes possible, either through atomic lines or through dust. It is likely that there is a transition from Pop III to ‘normal’ Pop II star formation at some critical metallicity, however the precise value of this critical metallicity cannot be presently determined theoretically. In this seminar I will describe the observational efforts that our group is conducting in the local Universe in order to derive as many constraints as possible on the nature of the Pop III stars. These include searches of the most metal-poor stars, and their chemical characterisation. Both the metallicity distribution function and the chemical pattern of the most metal-poor stars contain precious information of the first stars.