J. Plasma Fusion Res. SERIES, Vol.7 (2006) 154-156

Abundance Analyses of r-Process Elements in Very Metal-Poor Stars
HONDA Satoshi, AOKI Wako, KAJINO Toshitaka, ANDO Hiroyasu and BEERS Timothy C.1
National Astronomical Observatory, Mitaka 181-8588, Japan
1 Department of Physics and Astronomy, Michigan State University, USA（Received: 15 November 2004 / Accepted: 21 December 2005）

Abstract
We obtained high resolution, high S/N spectra for two very metal-poor stars with the Subaru TelescopeHigh Dispersion Spectrograph. One object shows large overabundances of neutron-capture elements relative
to iron in general, and the other shows excesses only in light neutron-capture elements. By using the recent
atomic line data including hyperfine splitting eﬀects, the chemical abundance patterns were investigated indetail the range of a wide atomic number. These results will provide strong constraints on the models of ther-porcess nucleosynthesis, and give hints for the astrophysical site of r-process, which are still unknown todate.
Keywords:
metal-poor star, abundance analysis, r-process element, spectroscopy, nucleosynthesis
tios of light neutron-capture elements (38 ≤ Z ≤ 46) toheavier ones (56 ≤ Z ≤ 70) show a large dispersion [2],suggesting another process that largely contributes only
to the light neutron-capture elements.
In order to investigate the processes that are corresponding to heavy and light neutron-capture (r-process)elements, we selected two bright stars from our previous sample for more detailed chemical abundance
study: one star (HD 6268) shows large overabundancesof neutron-capture elements relative to iron in general,and the other (HD 122563) shows excesses only in lightneutron-capture elements.

1．Introduction
The chemical composition of extremely metal-poorstars are expected to reflect the yields from very small
number of nucleosynthesis processes. Recent abundance analyses for extremely metal-poor stars have provided very valuable information on the origin of the elements, and the individual nucleosynthesis processes involved [1].
The rapid neutron capture process (r-process) is
known to be responsible, in solar system material, forabout half of the abundances of elements beyond theiron-peak. However, the site where r-process is occurring is still unclear. In order to clarify the origin of
r-process elements, it is important to obtain the abundance pattern of many neutron-capture elements covering a wide range of atomic number in metal-poor stars.
We previously obtained high resolution, high quality spectra of 40 metal-poor stars for abundance studies, and inspected the elemental abundance patterns ofheavy elements in detail for seven of them that show
excesses of neutron-capture elements relative to lighter
metals like Fe [2]. We confirmed the fact that theabundance patterns of the heavy neutron-capture elements (56 ≤ Z ≤ 70) in metal-poor stars are similar
to that of the solar system r-process component, which
has already been found by previous studies. This result proves that heavy neutron-capture elements in thesevery metal-poor stars are primarily synthesized by the rprocess, and the elemental abundance pattern producedby the r-process is universal at least in this atomic number range [3]. By way of contrast, the abundance ra-

2．UV observations and measurement of elemental abundances
Though most of transitions of neutron-capture elements exist in the UV region, observations using
ground-based telescopes for this wavelength region are
diﬃcult due to the low atmospheric transmission. We
selected the bright metal-poor stars HD 6268 and HD
122563 to obtain very high quality spectra (S/N > 500
at 3800 Å) with high spectral resolution (R = 90, 000)
using the High Dispersion Spectrograph (HDS) for theSubaru Telescope. Figure 1 shows examples of the observed spectra. Very weak lines of neutron-capture elements are successfully detected in these high-quality
spectra.
Since the atmospheric parameters (e.g., eﬀectivetemperature, metallicity) of those two objects are alC 2006 by The Japan Society of Plasma

Science and Nuclear Fusion ResearchCorresponding author’s e-mail: honda@optik.mtk.nao.ac.jp154Abundance Analyses of r-Process Elements in Very Metal-Poor Stars

1HD 6268
Fe |
Ce

0.8

0.6
4070

|
|
Nd Ce

Fe + Ce

Fe4074FeLaFeFe + Gd

HD 122563
HD 6268

Fe4072

0Fe Mn|Dy |Celog εNormalized Flux

1DyFe4076-1

Sr4078-24080

Wavelength (Angstrom)-3

Fig. 1 Comparison of spectra between HD 6268 (dashedline) and HD 122563 (solid line).

4050607080

90Atomic number
HD 1225630most the same, the absorption features of Fe and Mn
are very similar in both stars (Fig. 1). However, theabsorption features of heavy neutron capture elements(e.g., La, Ce, Dy) are much stronger in HD 6268 thanin HD 122563, clearly indicating a large diﬀerence inabundances of heavy neutron-capture elements. Theabsorption of the light neutron-capture element Sr in
HD 122563 has similar strength of that in HD 6268,
suggesting that both objects have similar abundances oflight neutron-capture elements.
Accurate atomic data of neutron-capture elements
are indispensable in the study of these extremely metalpoor stars. Recent progress in measurements of thetransition probabilities, including hyperfine splitting effect, for neutron-capture elements [4] enables us todetermine accurate abundances of these elements inmetal-poor stars.

log ε-1-2-340

5060708090

Atomic numberFig. 2 The abundances in HD 6268 and HD 122563(dots) compared with the solar system r-process
abundance. The present results are shown byﬁlled circles, and previous studies [2] are also
shown by open circles. The arrow shows thevalue of upper limit. Th (Z = 90) is radioactive; thedashed lines indicate the solar-system Th abundance at present, while the solid lines mean theestimate of the initial Th abundance of the solarsystem. Log  means log10 (X/H) + 12.0 for elements X. Error bars indicates the 1 – σ uncertainty
in the abundance determination.

3．New detections of light and heavy neutroncapture elements:
Figure 2 shows the abundance patterns of neutroncapture elements for the two stars. Compared to theprevious studies that determined the elemental abundances near the abundance peaks at Z ∼ 40, 56 and 76,
our new measurements cover the elements between theabundance peaks (Z ∼ 45, 70) for both objects. These
detailed abundance patterns are crucial to constrain themodeling of the r-process.
Figure 2 compares the elemental abundances ofour objects with that of r-process component in solarsystem material. The elemental abundances of HD
6268 are basically in agreement with the solar-systemr-process pattern. However, some disagreement in thelight neutron-capture elements is found, as has been reported by previous works[2, 5]. We also found a small
departure of the abundances of heavy elements with odd
atomic number (Tb and Tm) from the solar-system rprocess abundance pattern. This point will be studiedin more detail in our future work. We note that we usedreliable atomic line data of Tb that have been quite recently provided by Lawler et al. [6].
HD 122563 has a significantly diﬀerent abundance
patterns from that of HD 6268: the abundance of lightneutron-capture elements (38 ≤ Z ≤ 47) are muchhigher than that of heavy ones. However, our newmeasurements clearly demonstrate that the elemental
abundances gradually decreases with increasing atomic
number. Especially, the tendency that the abundancecontinuously decreases in the region of 38 ≤ Z ≤47 does not agree with any neutron-capture processes
known (i.e., main s-process, weak s-process, or mainr-process). Since the abundance pattern of such very
metal-poor stars, in contrast to that of solar system material, is expected to be formed by a small number of nu-155

Abundance Analyses of r-Process Elements in Very Metal-Poor Stars

cleosynthesis events another process yielding elementswith 38 ≤ Z ≤ 47 would be required. The abundance pattern of HD 122563 will provide a very new
constraint on modeling of the process that contributed
to the production of light neutron-capture elements inthe early galaxy [7], which is sometimes referred to as“weak r-process”.

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