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Scientific paper
Jul 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000hst..prop.8621l&link_type=abstract
HST Proposal ID #8621
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Hst Proposal Id #8621 Hot Stars
Scientific paper
This proposal aims to provide the first measurement of the Galactic abundance gradients for Boron and Iron. Boron abundances will be obtained for 5 B-type stars {with Galactocentric distances between 4-15 kpc} from the BIII 2066 Angstrom line. Optical spectroscopy of these {and other B-type} stars has yielded abundance gradients of -0.07 dex/kpc for N & the Alpha-elements {O, Mg, Si}. Determination of Boron abundances require UV spectra and HST/STIS. It is significant that stellar nucleosynthesis which is the leading source of oxygen {via Type II SN} is unlikely to be a major contributor to B synthesis {spallation being the leading candidate}. Hence, a measurement of the B abundance gradient provides novel information on the gradient of cosmic ray flux in the Galaxy. The same STIS spectra will also provide FeIII lines that will be used to obtain the first estimate of the present abundance gradient for iron {thought to originate predominantly in Type I SN}. Recent advances in modelling the chemical evolution of the Galactic disk have led to definite predictions of the spatial variation of the Alpha/Fe ratio in competing Galaxy formation scenarios, viz. the biased infall and biased outflow models. This study should yield differential Fe abundances accurate to +/-0.1 dex - which will allow a Alpha/Fe gradient to be compared directly with theory, as a critical test of formation models.
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