C. Sneden et al., HUBBLE-SPACE-TELESCOPE OBSERVATIONS OF NEUTRON-CAPTURE ELEMENTS IN VERY METAL-POOR STARS, The Astrophysical journal, 496(1), 1998, pp. 235-245
Using the Goddard High-Resolution Spectrograph (GHRS) of the Hubble Sp
ace Telescope (HST) we have detected the neutron-capture elements osmi
um, platinum, and lead in the very metal poor ([Fe/H] similar or equal
to -2.8) Galactic halo star HD 115444. This star joins the metal-poor
giant HD 126238 ([Fe/H] similar or equal to -1.7) as the second Galac
tic halo star for which ''third neutron-capture peak'' elements have b
een detected. We have also determined upper limits on these same eleme
nts for the neutron-capture deficient star HD 122563 ([Fe/H] similar o
r equal to -2.7). We have identified zirconium and germanium features
in the HST spectra of all three stars; this marks the initial identifi
cation of the latter element in halo stars. We compare and contrast th
e spectra and abundances of the neutron-capture elements in these thre
e stars. Combining the new HST observations of the third neutron-captu
re peak elements of HD 115444 with ground-based data, we find the stel
lar abundance distribution, over a wider atomic number range than ever
before possible, is consistent with the solar system r-process abunda
nce curve. Comparison of this star to the more metal-rich HD 126238 fi
nds that the ratio of the third-peak element abundances to that of the
pure r-process element europium is the same in both stars. Earlier gr
ound-based studies have found that abundances of the lighter elements,
such as barium and europium, in HD 115444 exceed those of HD 122563 b
y similar to 0.7 dex. Our osmium, platinum, and lead upper limits show
a similar difference and are not inconsistent with either the scaled
solar r-process or the solar total distribution. Thus, the second and
third neutron-capture peak element (Z greater than or equal to 56) abu
ndances in all three stars are consistent with scaled solar system r-p
rocess abundances. However, the zirconium abundance is approximately t
he same in HD 122563 and HD 115444, so this element (near the first ne
utron-capture peak) is overabundant (somewhat) in HD 122563 and (sligh
tly) in HD 115444 with respect to the solar r-process abundances. Germ
anium (synthesized in approximately equal amounts by the r- and the s-
process in solar material) is underabundant in all three stars, but do
es seem to scale with metallicity-it is identical in HD 115444 and HD
126238, but significantly greater in the higher metallicity HD 126238.
These new results support previous observations that demonstrate the
operation of the r-process, including the synthesis of the heaviest su
ch elements, early in the history of the Galaxy. Implications of these
results for early Galactic chemical evolution are discussed.