The dawn of star formation: a local perspective

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 ionizing
photons that played a role in the reionization of the Universe
and synthetised 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 characterization.
Both the metallicity distribution function and the chemical pattern
of the most metal-poor stars contain precious information of the first stars.