We have observed the \ion B1 \lambda 2497 Å line to derive the boron abundances of two very metal-poor stars selected to help in tracing the origin and evolution of this element in the early Galaxy : BD + 23 \arcdeg 3130 and HD 84937 .
The observations were conducted using the Goddard High Resolution Spectrograph on board the Hubble Space Telescope .
A very detailed abundance analysis via spectral synthesis has been carried out for these two stars , as well as for two other metal-poor objects with published spectra , using both Kurucz and OSMARCS model photospheres , and taking into account consistently the NLTE effects on the line formation .
We have also re-assessed all published boron abundances of old disk and halo unevolved stars .
Our analysis shows that the combination of high effective temperature ( T _ { eff } \gtrsim 6000 K , for which boron is mainly ionized ) and low metallicity ( [ Fe/H ] \lesssim - 1 ) makes it difficult to obtain accurate estimates of boron abundances from the \ion B1 \lambda 2497 Å line .
This is the case of HD 84937 and three other published objects ( including two stars with [ Fe/H ] \sim - 3 ) , for which only upper limits can be established .
BD + 23 \arcdeg 3130 , with [ Fe/H ] \sim - 2.9 and log N ( B ) _ { NLTE } = 0.05 \pm 0.30 , appears then as the most metal-poor star for which a firm measurement of the boron abundance presently exists .
The evolution of the boron abundance with metallicity that emerges from the seven remaining stars with T _ { eff } < 6000 K and [ Fe/H ] < -1 , for which beryllium abundances were derived using the same stellar parameters , shows a linear increase with a slope \sim 1 .
Furthermore , the B/Be ratio found is constant at a value \sim 20 for stars in the range -3 < [ { Fe / H } ] < -1 .
These results point to spallation reactions of ambient protons and \alpha particles with energetic particles enriched in CNO as the origin of boron and beryllium in halo stars .