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 = -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 = -4.40 +/- 0.13 and -3.93 +/- 0.12, for Pr 221 and Pr 237, respectively. Analysis of the G band yields C/Fe