The Remarkable Boron-depleted, Lithium-normal Population II Star HD 160617

Abstract

We present results of an analysis of the boron spectral region (2500 Å) in the metal-poor star HD 160617, observed with the Goddard High Resolution Spectrograph of the Hubble Space Telescope. HD 160617 has a metallicity of [Fe/H]~-1.8, a T_eff~5900 K, and a log g ~ 3.5. Its Li abundance is the normal, primordial ``Spite plateau'' value log ɛ(Li) ~ 2.2. The spectrum synthesis indicates that the boron abundance of this star is ~0.5 dex below that of other stars of similar metallicity (see, e.g., Duncan et al. and García-Lopez et al.). Differential comparison with slightly more and less metal-poor stars confirms the underabundance of boron; Be is also likely depleted. This is the only known case of a star with normal Li and depleted B. Since nuclear destruction occurs at lower temperatures for Li than for Be or B, no known destruction mechanism can account for this star. Rather, the abundance may be evidence that inhomogeneities were present in the gas from which stars of these metallicities formed. If so, it provides evidence of significant inhomogeneities in the halo at relatively late stages of its formation. If Be is also slightly depleted, then the star might have formed out of gas with a low initial amount of Be and B, which is consistent with the recent theories suggesting that (isolated) regions of massive star supernovae are the most important production sites of the light elements (Ramaty et al.). A less likely possibility is that this star might indicate the presence of a change of slope in the [B] versus [Fe] trend during this epoch of halo formation.

Based on spectroscopic observations obtained with the NASA/ESA Hubble Space Telescope at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.