Boron Abundances Across the "Li-Be Dip" in the Hyades

TL;DR

This study investigates boron abundances in Hyades F stars to identify a potential B dip, revealing a small decrease in B across the Li-Be dip and supporting rotational mixing models for element depletion.

Contribution

It provides the first evidence of a B abundance dip in the Hyades and compares B depletion with Li and Be, confirming models of stellar mixing and evolution.

Findings

Detected a small B abundance drop across the Li-Be dip.

Confirmed B depletion in Hyades giants due to convection zone deepening.

Supported rotational mixing as a mechanism for element depletion.

Abstract

Dramatic deficiencies of Li in the mid-F dwarf stars of the Hyades cluster were discovered by Boesgaard & Tripicco. Boesgaard & King discovered corresponding, but smaller, deficiencies in Be in the same narrow temperature region in the Hyades. With the Space Telescope Imaging Spectrograph on the Hubble Space Telescope we investigate B abundances in the Hyades F stars to look for a potential B dip using the B I resonance line at 2496.8 A. The light elements, Li, Be, and B, are destroyed inside stars at increasingly hotter temperatures: 2.5, 3.5, and 5x10^6 K respectively. Consequently, these elements survive to increasingly greater depths in a star and their surface abundances indicate the depth and thoroughness of mixing in the star. We have (re)determined Li abundances/upper limits for 79 Hyades dwarfs, Be for 43 stars, and B in five stars. We find evidence for a small drop in the B abundance across the Li-Be dip. The B abundances for the four stars in the temperature range 6100-6730 K fit the B-Be correlation found previously by Boesgaard et al. Models of rotational mixing produce good agreement with the relative depletions of Be and B in the dip region. We have compared our nLTE B abundances for the three high B stars on either side of the Li-Be dip with those found by Duncan et al. for the two Hyades giants. This confirms the factor of ~10 decline in the B abundance in the Hyades giants as predicted by dilution due to the deepening of the surface convection zone.