Limits on the Boron Isotopic Ratio in HD 76932

Abstract

Data in the 2090 A B region of HD 76932 have been obtained at high S/N using the HST GHRS echelle at a resolution of 90,000. This wavelength region has been previously identified as a likely candidate for observing the B11/B10 isotopic splitting. The observations do not match a calculated line profile extremely well at any abundance for any isotopic ratio. If the B abundance previously determined from observations at 2500 A is assumed, the calculated line profile is too weak, indicating a possible blending line. Assuming that the absorption at 2090 A is entirely due to boron, the best-fit total B abundance is higher than but consistent with that obtained at 2500 A, and the best-fit isotopic ratio (B11/B10) is in the range ~10:1 to ~4:1. If the absorption is not entirely due to B and there is an unknown blend, the best-fit isotopic ratio may be closer to 1:1. Future observations of a similar metal-poor star known to have unusually low B should allow us to distinguish between these two possibilities. The constraints that can be placed on the isotopic ratio based on comparisons with similar observations of HD 102870 and HD 61421 (Procyon) are also discussed.