Boron Benchmarks for the Galactic Disk

Sixteen Population I solar-type dwarfs have been selected to ascertain the baseline B abundance in the Galactic disk for a range of a factor of 4 in metallicity: from [Fe/H] of -0.5 to +0.1. All the stars selected are undepleted in Be, which ensures that they have also retained their full initial abundance of B. Evaluation of the trend of B with Fe provides a means to study the evolution of B in the Galactic disk. We observed 16 bright stars around the B I 2497 Å line, using the STIS echelle spectrograph on HST . New observations of Li and Be in some stars were made, and previous abundance studies of Li and Be in these stars were reevaluated using revised parameters and a modified spectral synthesis code for consistency with the B measurements. Abundances of B were calculated by spectrum synthesis with the revised MOOG code, which accounts for the increased opacity in the UV due to metals; the LTE B abundances were then corrected for non-LTE effects. Four additional stars with undepleted Be have HST B observations, which increase our sample to 20. For these disk stars there is a shallow slope for B versus Fe and Be versus Fe, such that as Fe increases by a factor of 4, B and Be increase by 1.7 times. The slope for B LTE versus Fe is 0.31 ± 0.09, for B NLTE versus Fe 0.40 ± 0.12, and for Be versus Fe 0.38 ± 0.14. We have estimated the effect of additional UV opacity from Mg and find that an increase of 0.3 dex in Mg results in a higher B abundances by 0.1 dex for all the disk stars. Individual stars are not consistently above (or below) the mean in both B and Be, implying that the star-to-star differences are not due to variations in the elemental content of the "natal" clouds. We find that the trend of B abundance with [Fe/H] is consistent with the general trend observed in halo stars. If we connect the halo and disk stars, then an increase in the Fe abundance by 10 3 is accompanied by increases of 100 times in B and 550 times in Be. However, fitting two separate relations for the disk and the halo stars results in a somewhat steeper slope for Be for the halo stars (1.08 ± 0.07) relative to the disk stars (0.38 ± 0.14). This is the case for B also in LTE, with B halo (0.90 ± 0.07) versus B disk (0.32 ± 0.12). However, the NLTE B abundance increases more slowly for halo stars than the Be abundance does; since this is not predicted by light-element synthesis or depletion, we suggest that a full NLTE analysis would be preferable to making the (small) corrections to the LTE abundances. Some of the lowest metallicity stars are thought to have only upper limits on the B abundance; if that is the case, the NLTE B slope is steeper, nearing 1.0. The abundance of B in the disk stars is observed to be a factor of ~15 more than the abundance of Be in these stars, a result consistent with the predictions of Galactic cosmic-ray (GCR) spallation, B/Be = 15 ± 5. The upper envelope for Li versus Fe yields Li/B and Li/Be ratios that, when coupled with models and predictions, indicate that 20%-45% of Li might be produced by GCRs. While there is no evidence to support the production of B by neutrino spallation, we cannot rule it out.