The Observed Trend of Boron and Oxygen in Field Stars of the Disk

Abstract

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 A and O I 6157 A and 6158 A 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 ± 0.08log(O) - 9.62 ± 1.38 is obtained. The slope of mBO = 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 mBO = 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 A 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].