The existence of very massive metal-free stars, M* = (140-260) M, has never been proved. Yet, these elusive stars should exist according to state-of-the-art numerical simulation, and they are expected to be the key sources of early metal-enrichment, ionizing photons, and primordial stellar black holes. Probing the existence of these stars is thus fundamental not only for Cosmology but also for galaxy formation. A possible way to make this major step-forward is by catching the chemical signatures that these elusive stars left in their descendants, i.e. in long-lived low-mass stars formed out of their ``ashes”. Indeed, very massive first stars are predicted to end their life as energetic Pair Instability Supernovae (PISNe), which spread out in the surrounding Interstellar Medium (ISM) their peculiar chemical products.
By investigating the free parameter space of the problem, we studied how does the peculiar chemical abundance pattern of an ISM polluted by PISNe vary when subsequent generations of “normal” Pop II stars contaminate it. In fact, PopII stars are predicted to form early on after the first supernovae (SNe) explosions, and thus they can washed-out their key chemical signatures. Still, independent of the choice of the free parameters, we find that an ISM imprinted by the heavy elements from PISNe at a >50% level it is most likely deficient in Copper and Zinc. Further, these stars predominantly have [Fe/H] ~ 2 (Salvadori et al. 2019).
By further developing this model, we investigate the imprint of PopIII SNe with lower masses (10-100) Msun, which can explode with a variety of energy (Skuladottir, SS et al. 2021/23). With this simple and general tool, we interpret the properties of halo stars, showing that those with [C/Fe] > +2.5 are truly second-generation objects, solely imprinted by low-energy PopIII SNe. These results were confirmed by our chemical-evolution model for ultra-faint dwarf galaxies (Rossi, SS et al. 2021/23) and for the Milky Way assembly (Koutsouridou, SS et al. 2023). Still, the degeneracy between the mass distribution of PopIII stars and the energy distribution of the first SNe make C-enhanced stars not useful (Koutsouridou, SS et al. 2023). To overcome this issue and break the degeneracy, we need to find the descendants of very massive first stars exploding as PISNe (Koutsouridou, SS, Skuladottir 2024). But where are them?
Our predictions show that the descendants of PISNe should be predominantly found in the bulge (Pagnini, SS et al. 2023), or in relatively massive ancient dwarf galaxy, such as Fornax, which can retain the chemical products of such energetic explosions (Rossi, SS et al. submitted). The ESO/4MOST large observational program 4DWARFS (512 000 fibre hours) was awarded in 2021 to our Team (PI: Skuladottir, 33 co-Is including SS, Gelli, Rossi) and it will allow us to search for these objects by exploiting the tools developed by the NEFERTITI project: the PISN-explorer to pin-point PISN descendants in large stellar surveys (Aguado, SS et al. 2023) and the NEFERTITI model, which can drive observations towards the best Local Group environments (Koutsouridou, SS et al. 2023). Alternatively, we can look for the chemical signatures of massive PISNe in distant (z > 3) and diffuse gas clouds, where the PISN fingerprints can be identified exploiting our novel chemical diagnostics (Vanni, SS et al. 2024). Indeed, the signature of PopIII SNe can be found in diffuse gaseous absorbers, which can preserve their signature because they do not easily form stars (Saccardi, SS et al. 2023).
