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Session 97 - Galactic Structure, Galactic Center.
Oral session, Friday, January 09
Georgetown,
I present a new measure of the diffusion of stellar orbits of low-mass stars in the old Milky Way disk near t= -5 to-6 Gyr and galactocentric radii near 4 to 6 kpc. The presolar cloud arrested and stored dust particles that were being grown within the winds of AGB carbon stars near 2-3 solar masses that, owing to their peculiar velocities, intersected it. The SiC particles deposited by those stellar winds from C-stars are now recovered by routine chemistry from primitive meteorites and are identified by their abnormal isotopic compositions. The silicon isotopes within individual SiC grains align in a three-isotope plot that records the chemical evolution of the ISM near the stars' birthplaces (Timmes amp; Clayton, 1996 ApJ, 472, 723). As metallicity increases, so too do the 29/28 and the 30/28 isotopic ratios. The decade-old puzzle, now solved (Clayton, 1997 ApJ, 484, L67), is the richness of both their 29/28 and 30/28 isotopic ratios in comparison with the Earth's. Diffusion radially from more central regions having higher metallicity allows these metal-richer stars to end their lives near the sun's birthplace in gas of lower metallicity. The opportunities for astronomy lie in why the low-mass stars move outward and in what they scatter from. Although stellar orbits are scattered to positions both inside and outside of their circular-orbit birthplaces, from the vantage point of solar birth, outside the molecular-cloud and black-hole scattering centers, almost all interlopers arose from birthplaces inside that of the sun. This accounts for the roughly 10:1 predominance of metal-rich AGB stars over metal-poor ones in their frequency of intersection with the presolar cloud. A new astronomy derives from the relative frequency of metallicities in this integral measure of past orbital diffusion in the middle-aged Milky Way.
