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Models of star formation in the early universe depend on the details ofaccretion, fragmentation and radiative feedback. Different simulations predictdifferent initial mass functions of the first stars, ranging from predominantlylow mass (0.1-10 Msol), to massive (10-100 Msol), or even supermassive(100-1000 Msol). The mass distribution of the first stars should lead to uniquechemical imprints on the low-mass second and later generation metal-poor starsstill in existence. The chemical composition of SMSS0313-6708, which has thelowest abundances of Ca and Fe of any star known, indicates it was enriched bya single massive supernova. However, even weak spectral lines may be affectedby strong 3D and NLTE effects in metal-poor stars. If these effects are ignoredor treated incorrectly, errors in the inferred abundances may significantlybias the inferred properties of the polluting supernovae. We redetermine thechemical composition of SMSS0313-6708 using 3D NLTE radiative transfer toobtain accurate abundances for Li, Na, Mg, Al, Ca and Fe. The model atomsemploy realistic collisional rates, with no calibrated free parameters. We findsignificantly higher abundances in 3D NLTE than 1D LTE by 0.8 dex for Fe, and0.5 dex for Mg, Al and Ca, while Li and Na are unaffected to within 0.03 dex.In particular, our upper limit for [Fe/H] is now a factor ten larger, at [Fe/H]< -6.53 (3 sigma), than previous estimates based on <3D> NLTE (i.e. usingaveraged 3D models). This higher estimate is due to a conservative upper limitestimation, updated NLTE data, and 3D-<3D> NLTE differences, all of which leadto a higher abundance determination. We find that the revised chemicalcomposition of SMSS0313-6708 matches supernova yields for massive progenitorsof 20-60 Msol exploding with low energies (1-2 x 10^51 erg), as well asprogenitors of 10 Msol with very low explosion energies (< 10^51 erg).

3D NLTE analysis of the most iron-deficient star, SMSS0313-6708