The explosive yields produced by the first generation of core collapse supernovae and the chemical composition of extremely metal poor stars

We present a detailed comparison of an extended set of elemental abundances observed in some of the most metal poor stars presently known and the ejecta produced by a generation of primordial core collapse supernovae. At variance with most of the analysis performed up to now (in which mainly the global trends with the overall metallicity are discussed), we think that it is important (and complementary to the other approach) to fit the (available) chemical composition of specific stars. In particular, we first discuss the differences among the five stars that form our initial database and define a "template" ultra-metal-poor star that is then compared to the theoretical predictions. Our main findings are the following: (1) The fit to [Si/Mg] and [Ca/Mg] of these very metal-poor stars seems to favor the presence of a rather large C abundance at the end of the central He burning; in a classical scenario in which the border of the convective core is strictly determined by the Schwarzschild criterion, such a large C abundance would imply a rather low C-12(alpha, gamma)O-16 reaction rate. (2) A low C abundance left by the central He burning would imply a low [Al/Mg] (<1.2 dex) independently on the initial mass of the exploding star, while a rather large C abundance would produce such a low [Al/Mg] only for the most massive stellar model. (3) At variance with current beliefs that it is difficult to interpret the observed overabundance of [Co/Fe], we find that a mildly large C abundance in the He exhausted core (well within the present range of uncertainty) easily and naturally allows a very good fit to [Co/Fe]. (4) Our yields allow a reasonable fit to eight of the 11 available elemental abundances. (5) Within the present grid of models it is not possible to find a good match of the remaining three elements, Ti, Cr, and Ni (even for an arbitrary choice of the mass cut). (6) The adoption of other yields available in the literature does not improve the fit. (7) Since no mass in our grid provides a satisfactory fit to these three elements, even an arbitrary choice of the initial mass function would not improve their fit.