Formation of proto-clusters and star formation within clusters: apparent
  universality of the initial mass function ?
  It is believed that the majority of stars form in clusters. Therefore it is
likely that the gas physical conditions that prevail in forming clusters,
largely determine the properties of stars that form and in particular the
initial mass function. We develop an analytical model to account for the
formation of low mass clusters and the formation of stars within clusters. The
formation of clusters is determined by an accretion rate, the virial
equilibrium and energy as well as thermal balance. For this latter both
molecular and dust cooling are considered using published rates. The star
distribution is computed within the cluster using the physical conditions
inferred from this model and the Hennebelle & Chabrier theory. Our model
reproduces well the mass-size relation of low mass clusters (up to few $\simeq
10^3$ M$_\odot$ of stars corresponding to about 5 times more gas) and an
initial mass function which is $i)$ very close to the Chabrier's IMF, $ii)$
weakly dependent on the mass of the clusters, $iii)$ relatively robust to (i.e.
not too steeply dependent on) variations of physical quantities as accretion
rate, radiation and cosmic rays abundances. The weak dependence of the mass
distribution of stars with the cluster mass results from the compensation
between varying clusters densities, velocity dispersions and temperatures all
inferred from first physical principles. This constitutes a possible
explanation for the apparent universality of the IMF within the Galaxy though
variations with the local conditions could certainly be observed.
