Evidence from Boron for Massive Star Evolution

Physics

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Hst Proposal Id #7400 Hot Stars And Stellar Corpses

Scientific paper

We propose to observe the B III and B II resonance lines at 2065.8 and 1362.5 Angstroms, respectively, in a sample of main-sequence and slightly evolved blue stars using the HST STIS MAMA detectors and echelle gratings. The newest stellar evolution models that include rotation have significant differences for intermediate and massive stars that are vital for understanding the colour-magnitude diagrams of all young stellar systems, and, therefore, the evolution of galaxies, and these new models can be tested. Only they predict that light elements {Li, Be, B} are depleted on the main-sequence before significant nitrogen/helium enrichments occur. Boron is the only light element observable in B- stars, and only by the UV resonance lines. We wish to examine the relationship of boron with age and nitrogen abundance in 10-15 solar mass stars on and near the main-sequence as a unique and important test of the new stellar evolution models. Boron can also be used to distinguish blue post-AGB stars from normal, metal-poor B-stars in the Galactic halo, since boron cannot survive in highly evolved objects. One high latitude star, PG 0832+676, has been identified as the most distant, normal, Galactic halo B-dwarf, but this interpretation has been questioned and has implications for understanding the young, early-type stars found far out in the Galactic halo. We propose to observe this star, which has vsini 0 km/s, and hence also provides a unique template of the UV spectral lines in B-type stars.

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