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 {\em 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 $\pm$ 0.27 and
-3.18 $\pm$ 0.19 km/s 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]$_{\rm LTE}$=-4.79 $\pm$ 0.14 for Pr 221 and [Fe/H]$_{\rm
LTE}$=-4.22 $\pm$ 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]$_{\rm NLTE}$=-4.40 $\pm$ 0.13 and
[Fe/H]$_{\rm NLTE}$=-3.93 $\pm$ 0.12, for Pr 221 and Pr 237, respectively.
Analysis of the G-band yields [C/Fe]$_{\rm 1D-LTE} \leq$ +2.3 and [C/Fe]$_{\rm
1D-LTE} \leq$ +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
NLTE abundances to theoretical yields of zero-metallicity supernovae. This
suggests that the supernovae progenitors had masses ranging from 10.6 to 14.4
M$_{\odot}$ and low-energy explosions with 0.3-1.2 $\times$ 10$^{51}$ erg.
