AAS 205th Meeting, 9-13 January 2005
Session 21 Stars Throughout the Milky Way
Poster, Monday, January 10, 2005, 9:20am-6:30pm, Exhibit Hall

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[21.03] Galactic Abundance Gradient in Boron

D. Semler, K.A. Venn (Macalester College), D.L. Lambert (University of Texas at Austin)

We present B III stellar abundances in five main sequence B-stars with different galactocentric distances from HST UV STIS spectroscopy. The boron gradient is of interest because the present-day interstellar boron abundance is dominated by spallation reactions between galactic cosmic rays and interstellar oxygen atoms. Knowing the present-day oxygen gradient is near -0.07 dex/kpc, then a determination of a boron gradient allows us to examine spallation reactions and the galactic cosmic ray flux with distance. Based on the low nitrogen abundances, the program stars (LS 5130, NGC 6231-295, NGC 884-2361, S 285-6, and S 289-2) are thought to be unmixed with CNO-cycled gas, and thus their stellar boron abundances should be representative of the present-day boron in the interstellar medium at each galactocentric location. However, while enriched nitrogen is an indicator that boron depletion has occurred, unenriched nitrogen levels do not guarantee undepleted boron levels for a star. Two stars have well determined boron abundances, which yield a boron abundance gradient of -0.06 dex/kpc. Due to low S/N or a lack of a boron feature, upper limits are set for three additional stars. The most simple interpretation of this nearly linear B/O gradient is that the formation of boron and the galactic cosmic ray flux is similar throughtout the Galactic disk.

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Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.