The observed trend of boron and oxygen in field stars of the disk

Oxygen abundances are derived in a sample of 13 field F and G dwarfs or subgiants with metallicities in the range -0.75 ≤ [Fe/H] ≤ +0.15. This is the same sample of stars for which boron abundances have been derived earlier from archived spectra obtained with the Hubble Space Telescope. Only the weak [O I] 6300 Å and O I 6157 Å and 6158 Å lines have been used to determine O abundances. It is argued that, over the range of temperature and metallicity spanned by the program stars, these [O I] and O I lines provide accurate oxygen abundances, largely free from non-LTE or one-dimensional model atmosphere effects. The results for oxygen are combined with the boron abundances published previously to define a boron versus oxygen abundance for field disk stars: the relation log (B/ H) + 12 = log ∈(B) = 1.39 ± 0.08 log ∈(O) - 9.62 ± 1.38 is obtained. The slope of m_BO = 1.39 (in log-log abundance by number coordinates) indicates that in the disk the abundance of B relative to O is intermediate between primary and secondary production (hybrid behavior). The slope found here for log ∈(B) versus log ∈(O) is identical within the uncertainties to that found by previous investigators for log ∈(Be) versus log ∈(O), where m_BO = 1.45. The two relations of B and Be versus O result in essentially solar B/Be ratios for field disk stars. A comparison of the results here for B-O in the disk to B-O in the halo (with B abundances taken from the literature) reveals that if [O/Fe] in the halo is nearly constant or undergoes only a gentle increase with decreasing [Fe/H], then boron behaves as a primary element relative to oxygen. In such a case, there is a transition from N(B) ∝ N(O) in the halo to N(B) ∝ N(O)^1.4 in the disk. On the other hand, if [O/Fe] increases substantially in the halo (such that [O/Fe] ∝ -0.4[Fe/H]), as suggested by some studies of the 3100-3200 Å electronic OH lines, then there is no significant difference between the behavior of B-O in the halo compared with that in the disk [i.e., N(B) ∝ N(O)^1.4].