Issue |
A&A
Volume 387, Number 2, May IV 2002
|
|
---|---|---|
Page(s) | 560 - 579 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361:20020434 | |
Published online | 13 May 2002 |
First stars. I. The extreme r-element rich, iron-poor halo giant CS 31082-001 *
Implications for the r-process site(s) and radioactive cosmochronology
1
Observatoire de Paris-Meudon, GEPI, 2 pl. Jules Janssen, 92195 Meudon Cedex, France
2
GRAAL, Université de Montpellier II, 34095 Montpellier Cedex 05, France e-mail: Bertrand.Plez@graal.univ-montp2.fr
3
Observatoire de Paris, GEPI, 61 av. de l'Observatoire, 75014 Paris, France, e-mail: Roger.Cayrel@obspm.fr
4
Department of Physics & Astronomy, Michigan State University, East Lansing, MI 48824, USA
5
Lund Observatory, Box 43, 221 00 Lund, Sweden
6
Astronomical Observatory, NBIfAFG, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
7
IAG, Universidade de São Paolo, Departmento de Astronomia, CP 3386, São Paulo, Brazil
8
Istituto Nazionale per l'Astrofisica – Osservatorio Astronomico di Trieste, via G.B. Tielpolo 11, 34131 Trieste, Italy
9
European Southern Observatory (ESO), Karl-Schwarschild-Str. 2, 85749 Garching b. München, Germany
Corresponding author: V. Hill, Vanessa.Hill@obspm.fr
Received:
28
January
2002
Accepted:
21
March
2002
We present a high-resolution ( 75 000, S/N
) spectroscopic analysis
of the bright (
11.7), extreme halo giant CS 31082-001([Fe/H] = -2.9), obtained in
an ESO-VLT Large Programme dedicated to very metal-poor stars. We find
CS 31082-001 to be extremely rich in r-process elements, comparable in this respect
only to the similarly metal-poor, but carbon-enriched, giant
CS 22892-052. As a result of the extreme overabundance of the heaviest r-process
elements, and negligible blending from CH and CN molecular lines, a reliable
measurement is obtained of the U II line at 386 nm, for the first time in a
halo star, along with numerous lines of Th II, as well as lines of 25 other
r-process elements.
Abundance estimates for a total of 43 elements (44 counting Hydrogen) are
reported in CS 31082-001, almost half of the entire periodic table.
The main atmospheric parameters of CS 31082-001 are as follows:
K,
(cgs), [Fe/H] = -2.9
(in LTE), and
microturbulence 1.8 ± 0.2 km s-1. Carbon and nitrogen are not significantly
enhanced relative to iron. As usual in giant stars, Li is depleted by dilution
(
(Li/H) = 0.85). The α-elements show the usual enhancements with
respect to iron, with [O/Fe]
(from [O I] 6300
Å), [Mg/Fe]
, [Si/Fe]
, and [Ca/Fe]
, while [Al/Fe] is near -0.5. The r-process elements show
unusual patterns: among the lightest elements (
40), Sr and Zr follow the
Solar r-element distribution, but Ag is down by 0.8 dex.
All elements with 56 ≤
Z
≤ 72 follow the Solar r-element
pattern, reduced by about 1.25 dex. Accordingly, the [ r/Fe] enhancement is
about +1.7 dex (a factor of 50), very similar to that of CS 22892-052.
Pb, in contrast, seems to be below the shifted
Solar r-process distribution, possibly indicating an error in the latter,
while thorium is more enhanced than the lighter nuclides. In CS 31082-001, log(Th/Eu) is
, higher than in the Solar System (-0.46) or in CS 22892-052
(-0.66). If CS 31082-001 and CS 22892-052 have similar ages, as expected for two extreme
halo stars, this implies that the production ratios were different by about 0.4
dex for the two objects. Conversely, if the Th/Eu production ratio were
universal, an age of 15 Gyr for CS 22892-052 would imply a negative age for CS 31082-001.
Thus, while a universal production ratio for the r-process elements seems to
hold in the interval 56 ≤
Z
≤ 72, it breaks
down in the actinide region.
When available, the U/Th is thus preferable to Th/Eu for radioactive dating,
for two reasons:
because of its faster decay rate and smaller
sensitivity to observational errors, and
because
the inital production ratio of the
neighboring nuclides 238U and 232Th is more robustly predicted than
the 151Eu/232Th ratio. Our current best estimate for the age of
CS 31082-001 is
Gyr. However, the computed actinide production ratios
should be verified by observations of daughter elements such as Pb and Bi in
the same star, which are independent of the subsequent history of star
formation and nucelosynthesis in the Galaxy.
Key words: Galaxy: evolution / Galaxy: halo / stars: abundances / stars: individual: BPS CS 31082-001 / nuclear reactions, nucleosynthesis, abundances / cosmology: early Universe
© ESO, 2002
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