The Pristine survey - XIV. Chemical analysis of two ultra-metal-poor stars

Elemental abundances of the most metal-poor stars reflect the conditions in the early Galaxy and the properties of the first stars. We present a spectroscopic follow-up of two ultra-metal-poor stars ([Fe/H] < -4.0) identified by the survey Pristine: Pristine 221.8781+9.7844 and Pristine 237.8588+12.5660 (hereafter Pr 221 and Pr 237, respectively). Combining data with earlier observations, we find a radial velocity of -149.25 +/- 0.27 and -3.18 +/- 0.19 km s(-1) for Pr 221 and Pr 237, respectively, with no evidence of variability between 2018 and 2020. From a one-dimensional (1D) local thermodynamic equilibrium (LTE) analysis, we measure [Fe/H](LTE) = -4.79 +/- 0.14 for Pr 221 and -4.22 +/- 0.12 for Pr 237, in good agreement with previous studies. Abundances of Li, Na, Mg, Al, Si, Ca, Ti, Fe, and Sr were derived based on the non-LTE (NLTE) line formation calculations. When NLTE effects are included, we measure slightly higher metallicities: [Fe/H](NLTE) = -4.40 +/- 0.13 and -3.93 +/- 0.12, for Pr 221 and Pr 237, respectively. Analysis of the G band yields [C/Fe](1D-LTE) <= +2.3 and [C/Fe](1D-LTE) <= +2.0 for Pr 221 and Pr 237. Both stars belong to the low-carbon band. Upper limits on nitrogen abundances are also derived. Abundances for other elements exhibit good agreement with those of stars with similar parameters. Finally, to get insight into the properties of their progenitors, we compare NINE abundances to theoretical yields of zero-metallicity supernovae (SNe). This suggests that the SNe progenitors had masses ranging from 10.6 to 14.4 M-circle dot and low-energy explosions with (0.3-1.2) x 10(51) erg.

Automated summary

The study presented a spectroscopic follow-up of two ultra-metal-poor stars, revealing their elemental abundances and radial velocities. The results show slight differences in metallicities compared to previous studies when non-LTE effects are considered. Both stars belong to the low-carbon band, with upper limits on nitrogen abundances derived. Abundances of other elements show good agreement with stars with similar parameters. Further analysis compares elemental abundances to theoretical yields of zero-metallicity supernovae, suggesting characteristics of the supernova progenitors.