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Reporting period: 2016-09-01 to 2018-08-31

Understanding the nature of the first stars (Pop III stars) is a fundamental problem in Cosmology and Galaxy Formation. Numerical simulations suggest that primordial stars were more massive than present-day stars, and so rapidly disappeared. Heavy elements newly produced by these stars enriched the surrounding gas, out of which long-lived, low-mass stars, formed. These “second-generation” stars survive until present-day, preserving in their photospheres the chemical imprint of the first stars. In the Milky Way and nearby dwarf galaxies, i.e. in the Local Group, high-resolution spectroscopic studies offer the unique opportunity to reveal this fossil signature.
But second-generation stars are extremely rare, making their detection challenging.
In the current era of wide and deep spectroscopic surveys, such as Gaia-ESO, SEGUE, and APOGEE, we will have the chance to catch many of these second-generation stars if we know where to look for. The PRIMORDIAL project aimed at characterizing the first stars by hunting their living fossils, hidden in the Local Group. To this end, we adopted a novel strategy, which combined theory and observations to:
1) determine the chemical evolution of the Local Group and understand what different chemical species can tell us about the Initial Mass Function (IMF) of the first stars;
2) define a theoretically-driven observational strategy to look for second-generation stars in ongoing and planned stellar surveys.
Thus, to achieve these two objectives we progressed from both a theoretical and observational point of view.

By including the sparse sampling of the PopIII IMF in my merger tree chemical evolution models (e.g. Salvadori et al. 2015, MNRAS, 454, 1320), we studied the implications of the observed Metallicity Distribution Function (MDF) and fraction of Carbon-enhanced metal-poor stars for the properties of the first stars. Our findings show that a flat PopIII IMF with M* = (10-300) Msun is strongly disfavored by these data. Further, by studying the typical [Fe/H] range of second-generation stars we found that those imprinted by Pop III stars with M* = (10-40) Msun exploding as faint supernovae appear
at [Fe/H] < -4.5 and represent the 100% of the stars at these low [Fe/H]. Thus, these stellar fossils can be identified by looking for the most iron-poor stars. On the other hand, the descendants of Pair Instability Supernovae (PISN), M* = (140-260) Msun, typically have higher [Fe/H] and thus they only represent a few percent of the total number of stars at that [Fe/H]. In particular, second-generation stars imprinted by PISN at a > 50% level cover a broad metallicity range, -5.0 < [Fe/H] < -1, with a peak at [Fe/H]~ -1.8 (Fig. 1 from de Bennassuti, Salvadori et al. 2017, MNRAS, 465, 2540)

These results point toward the quest for a novel, theoretical-strategy to identify these rare stellar relics among all the stars with the same [Fe/H].
To this end I studied how does the peculiar chemical abundance pattern of an Inter Stellar Medium (ISM) polluted by PISN vary when the gaseous environment is also imprinted by subsequent generations of normal stars. My results show that, independent of the choice of the free parameters,
an ISM imprinted by the heavy elements from PISN at a >50% level is most likely deficient of some key chemical species: Nitrogen, Copper, and Zinc. Thus, looking for the absence of these “killing elements” in large samples of ancient metal-poor stars, will enable us to identify the direct descendants
of very massive first stars. This is a perfect science case for both the ongoing Australian GALAH Survey and the forthcoming European WEAVE Galactic Archaeology Survey (Salvadori, Bonifacio and Caffau, in prep).

Yet, there are some complications. Our observational results for the evolution of [Zn/Fe] vs [Fe/H] for Local Group stars show that at [Fe/H] > -2 a low value of [Zn/Fe] < -0.5 can be a results of the contribution from supernova Ia to the chemical enrichment. This is true for both stars in the Sculptor dwarf galaxy (Skuladottir, Tolstoy, Salvadori et al. 2017, A&A, 606, 71) and those observed in the disk of the Milky Way by the Gaia-ESO survey (Duffau et al. including Salvadori 2017, A&A, 604,128). Our results underline the complicated nucleo-synthetic origin of Zinc, which does not trace Iron at all metallicities and in all environments (different dwarf galaxies, stellar halo, disk, bulge) making it difficult to be used as a proxy for Iron in studies of high-z Damped Lyman Alpha systems (Fig.2 from Skuladottir, Salvadori et al. 2018, A&A in press).
Thus, from a theoretical point of view it is extremely important to understand if there is a way to understand a low [Zn/Fe] driven by SNIa or the imprint from very massive first stars. This is the subject of my present-day calculations (Salvadori, Bonifacio and Caffau, in prep).
In conclusion, our results have progressed beyond the state-of-the-art from both a theoretical and on observational point of view.
First, Local Group stellar data were never used so far to constrain the shape/mass range of the Pop III IMF. Further since our findings are based only on the interpretation of [C/Fe] and [Fe/H] for halo stars it is very likely that stronger limits will arise by the simultaneous use of different chemical species and in different environment. Second, it has been the first time that [Zn/Fe] abundances were derived for a large sample of stars (100) outside the Milky Way, and that deviation from a solar value were observed for -2 < [Fe/H] < -1. Finally, to our knowledge a theoretical investigation to determine the chemical species that result largely under- (over-) produced in ISM enriched by both very massive first stars and normal stars have never been performed so far. This is the key to catch such rare second-generation stars that have been searched for decades.
From de Bennassuti, Salvadori et al. 2017, MNRAS, 465, 2540
From Skuladottir, Salvadori et al. 2018, A&A in press