Boron in Very Metal‐poor Stars

We have observed the B I 2497 A line to derive the boron abundances of twovery metal-poor stars selected to help in tracing the origin and evolution ofthis element in the early Galaxy: BD +23 3130 and HD 84937. The observationswere conducted using the Goddard High Resolution Spectrograph on board theHubble Space Telescope. A very detailed abundance analysis via spectralsynthesis has been carried out for these two stars, as well as for two othermetal-poor objects with published spectra, using both Kurucz and OSMARCS modelphotospheres, and taking into account consistently the NLTE effects on the lineformation. We have also re-assessed all published boron abundances of old diskand halo unevolved stars. Our analysis shows that the combination of higheffective temperature (Teff > 6000 K, for which boron is mainly ionized) andlow metallicity ([Fe/H]<-1) makes it difficult to obtain accurate estimates ofboron abundances from the B I 2497 A line. This is the case of HD 84937 andthree other published objects (including two stars with [Fe/H] ~ -3), for whichonly upper limits can be established. BD +23 3130, with [Fe/H] ~ -2.9 andlogN(B)_NLTE=0.05+/-0.30, appears then as the most metal-poor star for which afirm measurement of the boron abundance presently exists. The evolution of theboron abundance with metallicity that emerges from the seven remaining starswith Teff < 6000 K and [Fe/H]<-1, for which beryllium abundances were derivedusing the same stellar parameters, shows a linear increase with a slope ~ 1.Furthermore, the B/Be ratio found is constant at a value ~ 20 for stars in therange -3<[Fe/H]<-1. These results point to spallation reactions of ambientprotons and alpha particles with energetic particles enriched in CNO as theorigin of boron and beryllium in halo stars.