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

摘要

Oxygen abundances are derived in a sample of 13 field F and G dwarfs or subgiants with metallicities in the range -0.75 less than or equal to Fe/H less than or equal to +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 Angstrom and O I 6157 Angstrom and 6158 Angstrom 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 epsilon (B) = 1.39 +/- 0.08 log epsilon (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 epsilon (B) versus log epsilon (O) is identical within the uncertainties to that found by previous investigators for log epsilon (Be) versus log epsilon (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) proportional to N(O) in the halo to N(B) proportional to N(O)(1.4) in the disk. On the other hand, if O/Fe increases substantially in the halo (such that O/Fe proportional to -0.4Fe/ H), as suggested by some studies of the 3100-3200 Angstrom 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) proportional to N(O)(1.4). References: 33