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Stars of extragalactic origin in the solar neighborhood
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STARS OF EXTRAGALACTIC
ORIGIN IN THE SOLAR
NEIGHBORHOOD
T.V. Borkova, V.A. Marsakov
Institute of Physics, Rostov State University,
194, Stachki street, Rostov-on-Don, Russia, 344090
e-mail: borkova@ip.rsu.ru, marsakov@ip.rsu.ru
accepted 2004, Astronomy Letters, Vol. 30, No. 3, P.148-158
Abstract
We computed the spatial velocities and the galactic orbital elements
using Hipparcos data for 77 nearest main-sequence F–G stars with pub-
lished the iron, magnesium, and europium abundances determined from
high dispersion spectra and with the ages estimated from theoretical
isochrones. A comparison with the orbital elements of the globular clus-
ters that are known was accreted by our Galaxy in the past reveals stars
of extragalactic origin. We show that the relative elemental abundance
ratios of r- and α-elements in all the accreted stars differ sharply from
those in the stars that are genetically associated with the Galaxy. Ac-
cording to current theoretical models, europium is produced mainly in
low-mass Type II supernovae (SNe II), while magnesium is synthesized in
larger amounts in high-mass SN II progenitors. Since all the old accreted
stars of our sample exhibit a significant Eu overabundance relative to Mg,
we conclude that the maximum masses of the SN II progenitors outside
the Galaxy were much lower than those inside it are. On the other hand,
only a small number of young accreted stars exhibit low negative ratios
[Eu/Mg] 0. The delay of primordial star formation burst and the
explosions of high-mass SNe II in a relatively small part of extragalactic
space can explain this situation. We provide evidence that the interstellar
medium was weakly mixed at the early evolutionary stages of the Galaxy
formed from a single proto-galactic cloud and that the maximum mass of
the SN II progenitors increased in it with time simultaneously with the
increase in mean metallicity.
Keywords: chemical composition
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