Abundance observations indicate the presence of often surprisingly large
amounts of neutron capture (i.e., s- and r-process) elements in old Galactic
halo and globular cluster stars. These observations provide insight into the
nature of the earliest generations of stars in the Galaxy -- the progenitors of
the halo stars -- responsible for neutron-capture synthesis. Comparisons of
abundance trends can be used to understand the chemical evolution of the Galaxy
and the nature of heavy element nucleosynthesis. In addition age
determinations, based upon long-lived radioactive nuclei abundances, can now be
obtained. These stellar abundance determinations depend critically upon atomic
data. Improved laboratory transition probabilities have been recently obtained
for a number of elements. These new gf values have been used to greatly refine
the abundances of neutron-capture elemental abundances in the solar photosphere
and in very metal-poor Galactic halo stars. The newly determined stellar
abundances are surprisingly consistent with a (relative) Solar System r-process
pattern, and are also consistent with abundance predictions expected from such
neutron-capture nucleosynthesis.Comment: 8 pages, 2 figures, 1 table. To appear in the Proceedings of the NASA
  Laboratory Astrophysics Workshop in Las Vegas, NV (February 2006

Collier, Jason

Cowan, John J.

DenHartog, E. A.

Dodge, Homer L.

Lawler, James E.

Sneden, Christopher

English

arXiv

Abundance observations indicate the presence of often surprisingly large amounts of neutron capture (i.e., s- and r-process) elements in old Galactic halo and globular cluster stars. These observations provide insight into the nature of the earliest generations of stars in the Galaxy the progenitors of the halo stars responsible for neutron-capture synthesis. Comparisons of abundance trends can be used to understand the chemical evolution of the Galaxy and the nature of heavy element nucleosynthesis. In addition age determinations, based upon long-lived radioactive nuclei abundances, can now be obtained. These stellar abundance determinations depend critically upon atomic data. Improved laboratory transition probabilities have been recently obtained for a number of elements. These new gf values have been used to greatly refine the abundances of neutron-capture elemental abundances in the solar photosphere and in very metal-poor Galactic halo stars. The newly determined stellar abundances are surprisingly consistent with a (relative) Solar System r-process pattern, and are also consistent with abundance predictions expected from such neutron-capture nucleosynthesis

NASA Technical Reports Server

INVITED TALK NASA LAW, February 14-16, 2006, UNLV, Las Vegas
Nucleosynthesis: Stellar and Solar Abundances and Atomic Data
John J. Cowan,1 James E. Lawler,2 Christopher Sneden,3 E. A. Den Hartog,2
and Jason Collier1
ABSTRACT
Abundance observations indicate the presence of often surprisingly large
amounts of neutron capture (i.e., s- and r-process) elements in old Galactic halo
and globular cluster stars. These observations provide insight into the nature
of the earliest generations of stars in the Galaxy – the progenitors of the halo
stars – responsible for neutron-capture synthesis. Comparisons of abundance
trends can be used to understand the chemical evolution of the Galaxy and the
nature of heavy element nucleosynthesis. In addition age determinations, based
upon long-lived radioactive nuclei abundances, can now be obtained. These stel-
lar abundance determinations depend critically upon atomic data. Improved
laboratory transition probabilities have been recently obtained for a number of
elements. These new gf values have been used to greatly refine the abundances
of neutron-capture elemental abundances in the solar photosphere and in very
metal-poor Galactic halo stars. The newly determined stellar abundances are
surprisingly consistent with a (relative) Solar System r-process pattern, and are
also consistent with abundance predictions expected from such neutron-capture
nucleosynthesis.
1. Stellar Abundances and New Atomic Data
Abundance studies of heavy elements and isotopes in Galactic halo stars are providing
important clues and insights into the nature of, and sites for, nucleosynthesis early in the
history of the Galaxy (Sneden & Cowan 2003; Cowan & Thielemann 2004; Cowan & Sneden
1Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, Norman, OK 73019;
cowan@nhn.ou.edu, collier@nhn.ou.edu
2Department of Physics, University of Wisconsin, Madison, Madison, WI 53706; jelawler@wisc.edu, ead-
enhar@wisc.edu
3Department of Astronomy and McDonald Observatory, University of Texas, Austin, Texas 78712;
chris@verdi.as.utexas.edu
https://ntrs.nasa.gov/search.jsp?R=20060052432 2019-08-29T18:46:45+00:00Z
– 83 –
2006). Observations of heavy neutron-capture elements (i.e., those produced in the slow
(s) or rapid (r)-process) in both old (low iron, or metallicity) stars and younger (more
metal-rich) stars are also providing clear indications of the nature of the Galactic chemical
evolution. In addition the detection of the long-lived radioactive n-capture elements, such
as thorium and uranium, are allowing direct age determinations for the oldest stars in the
Galaxy - thus, providing lower limits on both the age of the Galaxy and the Universe.
All of these abundance determinations and studies depend critically upon atomic data,
particularly transition probabilities. During the past few years there have been dramatic im-
provements in the experimental determinations of these transition probabilities for a number
of rare earth elements: these include improved laboratory values for the elements Ce (Palmeri
et al. 2000), Nd (Den Hartog et al. 2003), Ho (Lawler et al. 2004), Pt (Den Hartog et al.
2005), and Sm (Lawler et al. 2006). New values for the element Gd have also recently been
obtained (Den Hartog et al. 2006), but Gd abundances in the current paper do not reflect
these new data. These improved data sets have been employed to determine elemental abun-
dances in three metal-poor Galactic halo stars. In addition new (refined) solar photospheric
abundances have been obtained for Nd, Ho and Sm - this was not possible, however, for Pt.
We show in Figure 1 the abundances from Ba-Er (normalized at the r-process element Eu) in
CS 22892-052 (Sneden et al. 2003), BD +17◦3248 (Cowan et al. 2002), HD 115444 (Westin
et al. 2000) and the Sun (Lodders 2003). We focus in the left panel on abundance deter-
minations for Nd, Sm and Ho in these four stars based upon published atomic data. The
abundances for those same three elements, employing the newer laboratory atomic data,
is shown in the right panel of this figure. It is clear, that as a result of employing the
new atomic data, the star-to-star scatter among the three metal-poor Galactic halo stars is
markedly reduced, and there is good agreement between the elemental values in these stars
and the solar system r-process value (indicated by the horizontal line).
2. Discussion
Employing the new atomic data, we have updated several of the abundance values from
Sneden et al. (2003) for CS 22892-052. We show those values in Figure 2 (top panel) com-
pared to an s-only (dashed line) and an r-only (solid line) solar system elemental abundance
curve. These solar system elemental abundance curves from Simmerer et al. (2004) are the
sums of individual isotopic contributions from the s and r-process and are tabulated in ta-
ble 1. Using the so called “classical model” approximation in conjunction with measured
neutron capture cross-sections, the individual s-process contributions are first determined.
Subtracting these s-process isotopic contributions from the total solar abundances predicts
the residual r-process contributions. These individual s- and r-process isotopic solar system
abundances (based upon the Si = 106 scale) that are listed in table 1, are derived from the
work of Ka¨ppeler et al. 1989, Wisshak et al. 1998 and O’Brien et al. 2003, and can be used
for future isotopic studies.
– 84 –
Fig. 1.— (left) Abundance values (scaled at the element Eu) for selected elements in the
stars CS 22892-052 (open circles), HD 115444 (squares), BD +17◦3248 (triangles) and the
Sun (filled circles), based upon previously published atomic data. (right) Newly derived
abundance values based upon recent experimental lab data (after Lawler et al. 2006).
It is clear in both the top and bottom panels of Figure 2 that the abundances of the
stable elements Ba and above are consistent with the scaled solar system elemental r-process
distribution. This agreement has been seen in other r-process-rich stars and strongly suggests
that the r-process is robust over the history of the Galaxy. It further demonstrates that early
in the history of the Galaxy, all of the n-capture elements were synthesized in the r-process,
and not the s-process. Additional comparisons in the figure, however, suggest the lighter
elements do not fall on this same curve that fits the heavier n-capture elements. The upper
limit on Ge, for example, falls far below the scaled solar system r-process curve. Recent
analyses of this element indicates its abundance is tied to the iron level in metal-poor halo
stars (Cowan et al. 2005), and thus suggests Ge synthesis in some type of charged-particle
reactions, or other primary process, in massive stars and supernovae in the early Galaxy
(see discussion in Cowan & Sneden 2006). We also note that the abundances of the n-
capture elements from Z = 40–50 in CS 22892-052, in general, fall below the solar r-process
curve. This has been interpreted as suggesting perhaps a second r-process synthesis site
in nature (see discussion in Kratz et al. 2006 and Cowan & Sneden 2006). Interestingly,
new abundances studies of the star HD 221170 show better agreement with the scaled solar
system curve for the elements from Z = 40-50 than CS 22892-052 does (Ivans et al. 2006).
In spite of the good overall match between the scaled-solar r-process abundances and
– 85 –
Fig. 2.— (top) Abundance values for CS 22892-052 compared to a scaled solar system s-
process (dashed line) and r-process distribution (solid line, Simmerer et al. 2004). (bottom)
Differences between observed abundances in CS 22892-052 and the scaled solar system r-
process abundances (after Cowan & Sneden 2006).
those of very metal-poor stars, some small deviations are becoming apparent, as shown in
Figures 1 and 2. The increasingly accurate stellar abundance determinations, resulting in
large part from the more accurate laboratory atomic data, are helping to constrain, and
ultimately could predict, the actual values of the solar system r-process abundances. Thus,
the current differences might suggest that some of those solar system r-process elemental
predictions need to be reassessed. Recall that the elemental r-process curve is based upon the
isotopic r-process residuals, resulting from the subtraction of the s-process isotopic contri-
butions from the total solar system abundances. Thus, a reanalysis of some of the s-process
contributions, and new neutron-capture cross sections measurements, might be in order.
3. Conclusions
Abundance observations of metal-poor (very old) Galactic halo stars indicate the pres-
ence of n-capture elements. New laboratory atomic data is dramatically reducing the stellar
abundance uncertainties in these stars and increasingly improving the Solar System abun-
– 86 –
dances. Recently determined abundances, based upon the new laboratory data, indicate that
the elemental patterns in the old halo stars are consistent with each other and with a scaled
solar system r-process abundance distribution – demonstrating that all of these elements
(even s-process ones like Ba) were synthesized in the r-process early in the history of the
Galaxy. These more accurate abundance determinations, based upon more precise labora-
tory atomic data, might be employed to constrain predictions for the solar system r-process
abundances. In the future additional (and improved) lab data will have implications for a
number of synthesis studies, including providing evidence regarding the possibility of two
astrophysical r-process sites. Such new experimental data will also help in understanding
the Galactic chemical evolution of the elements, and could have an impact on chronometric
age estimates of the Galaxy and the Universe. New stellar and atomic data in both the UV
and IR wavelength regimes may also allow the detection of never before seen elements in
these stars, and will aid in new determinations of isotopic abundance mixtures for elements
such as Sm.
We thank our colleagues for valuable insights and contributions. This work has been
supported in part by NSF grants AST 03-07279 (J.J.C.), AST 05-06324 (J.E.L.), AST 03-
07495 (C.S.), and by STScI.
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– 88 –
Table 1. s- AND r-PROCESS ISOTOPIC SOLAR SYSTEM ABUNDANCES
Element Z Isotope N[s] N[r] Element Z Isotope N[s] N[r]
Ga 31 69 11.137 11.600 Pd 46 104 0.165 0.000
71 10.413 4.700 105 0.040 0.269
Ge 32 70 15.000 0.000 106 0.186 0.193
72 18.323 14.000 108 0.226 0.145
73 3.531 5.670 110 0.000 0.163
74 15.733 27.300 Ag 47 107 0.058 0.239
76 0.000 9.200 109 0.059 0.196
As 33 75 1.456 5.330 Cd 48 110 0.178 0.000
Se 34 76 4.656 0.000 111 0.042 0.165
77 1.679 3.040 112 0.198 0.196
78 7.402 7.210 113 0.060 0.139
80 7.446 24.300 114 0.287 0.140
82 0.000 5.710 116 0.000 0.121
Br 35 79 0.450 0.000 In 49 115 0.057 0.121
81 0.479 4.640 Sn 50 116 0.511 0.000
Kr 36 80 1.021 0.000 117 0.146 0.128
82 6.207 0.000 118 0.706 0.137
83 1.989 3.750 119 0.155 0.065
84 10.575 18.000 120 1.099 0.078
86 9.481 0.930 122 0.000 0.154
Rb 37 85 0.690 2.790 124 0.000 0.199
87 2.214 0.100 Sb 51 121 0.047 0.113
Sr 38 86 2.111 0.000 123 0.000 0.132
87 1.443 0.000 Te 52 122 0.133 0.000
88 16.986 2.550 123 0.047 0.000
Y 39 89 3.344 1.310 124 0.248 0.000
Zr 40 90 4.529 0.990 125 0.088 0.267
91 1.158 0.040 126 0.450 0.525
92 1.289 0.540 128 0.000 1.526
94 1.687 0.170 130 0.000 1.634
96 0.000 0.300 I 53 127 0.050 0.851
Nb 41 95 0.229 0.110 Xe 54 128 0.126 0.000
Mo 42 95 0.189 0.213 129 0.066 1.240
96 0.475 0.000 130 0.199 0.000
97 0.156 0.087 131 0.087 0.954
98 0.514 0.093 132 0.498 0.800
100 0.000 0.242 134 0.000 0.449
Tc 43 99 0.006 0.172 136 0.000 0.373
Ru 44 99 0.049 0.000 Cs 55 133 0.056 0.315
100 0.242 0.000 Ba 56 134 0.178 0.000
101 0.050 0.266 135 0.068 0.298
102 0.261 0.327 136 0.500 0.000
104 0.000 0.348 137 0.372 0.283
Rh 45 103 0.055 0.289 138 3.546 0.225
La 57 139 0.337 0.110 Lu 71 175 0.006 0.031
Ce 58 140 0.894 0.089 176 0.002 0.000
142 0.000 0.115 Hf 72 176 0.008 0.000
Pr 59 141 0.079 0.082 177 0.005 0.024
Nd 60 142 0.227 0.000 178 0.021 0.022
143 0.037 0.065 179 0.007 0.015
144 0.105 0.094 180 0.035 0.020
145 0.020 0.049 Ta 73 181 0.009 0.013
– 89 –
Table 1—Continued
Element Z Isotope N[s] N[r] Element Z Isotope N[s] N[r]
146 0.091 0.053 W 74 182 0.024 0.012
148 0.004 0.044 183 0.013 0.007
150 0.000 0.047 184 0.029 0.013
Sm 62 147 0.003 0.031 186 0.006 0.031
148 0.038 0.000 Re 75 185 0.004 0.014
149 0.005 0.031 187 0.001 0.033
150 0.022 0.000 Os 76 186 0.012 0.000
152 0.018 0.053 187 0.006 0.000
154 0.000 0.059 188 0.016 0.079
Eu 63 151 0.000 0.042 189 0.004 0.111
153 0.002 0.048 190 0.021 0.168
Gd 64 152 0.001 0.000 192 0.001 0.293
154 0.009 0.000 Ir 77 191 0.005 0.241
155 0.003 0.045 193 0.003 0.408
156 0.015 0.055 Pt 78 192 0.010 0.000
157 0.007 0.046 194 0.020 0.431
158 0.027 0.058 195 0.006 0.457
160 0.000 0.072 196 0.035 0.312
Tb 65 159 0.004 0.060 198 0.000 0.099
Dy 66 160 0.009 0.000 Au 79 197 0.010 0.176
161 0.004 0.075 Hg 80 198 0.035 0.000
162 0.016 0.101 199 0.016 0.043
163 0.002 0.093 200 0.051 0.030
164 0.018 0.091 201 0.020 0.027
Ho 67 165 0.006 0.083 202 0.079 0.026
Er 68 164 0.004 0.000 204 0.000 0.020
166 0.012 0.072 Tl 81 203 0.042 0.012
167 0.005 0.053 205 0.060 0.041
168 0.020 0.047 Pb 82 204 0.057 0.000
170 0.001 0.037 206 0.326 0.223
Tm 69 169 0.006 0.031 207 0.313 0.280
Yb 70 170 0.006 0.000 208 1.587 0.118
171 0.004 0.029 Bi 83 209 0.051 0.093
172 0.018 0.036 Th 90 232 0.000 0.042
173 0.008 0.031 U 92 235 0.000 0.006
174 0.040 0.037 238 0.000 0.020
176 0.000 0.030


Nucleosynthesis: Stellar and Solar Abundances and Atomic Data

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