The Ba II hfs components were calculated in a manner

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BARIUM ABUNDANCES IN EXTREMELY METAL-POOR STARS ANDREWMCWILLIAM

Observatories of the Carnegie Institution of Washington, 813 Santa Barbara Street, Pasadena, CA 91101 ; andy=marmite.ociw.edu

Received 1997 September 15 ; revised 1997 December 18

ABSTRACT

New, improved, barium abundances for 33 extremely metal-poor halo stars from the 1995 sample of McWilliam et al. have been computed. The mean [Ba/Eu] ratio for stars with [Fe/H]¹[2.4 is [0.69^0.06 dex, consistent with pure r-process nucleosynthesis within the measurement uncertainties. Although the [Sr/Fe] and [Ba/Fe] abundance ratios span a range of 2.6 dex, the mean values are approximately constant with [Fe/H]. This is consistent with a model of chemical evolution in which the parent clouds were enriched by small numbers of supernova events. In this model, the decreasing heavy-element dispersion with increasing [Fe/H] is simply due to the averaging of heavy-element yields from many supernovae at higher [Fe/H] ; however, it is necessary to increase the number of extremely metal-poor stars known in order to conÐrm this picture. In addition to the random Sr component from the

r-process, the [Sr/Ba] ratios indicate that there is a second, also random, source of Sr from an as yet unidentiÐed nucleosynthesis site.

Key words :Galaxy : abundances È Galaxy : evolution È Galaxy : halo È stars : abundances È stars : Population II

1.

INTRODUCTION

This work was motivated by the need to reÐne the barium abundance scale for the McWilliam et al. (1995, hereafterMPSS95)sample of 33 extremely metal-poor stars, by including hyperÐne splitting (hfs) e†ects in the abun-dance analysis. In the original analysis, MPSS95 were not aware of the hfs splitting constants ofRutten (1978)and so could not compute the wavelengths of the hfs components. Since the size of the Ba hfs e†ect was unknown, MPSS95 adopted a single-line treatment to compute the abundances. This approximation is always good for weak, unsaturated lines but breaks down for strong lines if the hyperÐne com-ponents are widely separated.

Previous abundance studies of halo stars have used either a single-line treatment for barium (e.g.,Gratton & Sneden or an hfs treatment for the 4554 line using

1988 ; MPSS95) A

the highly incomplete hfs list ofSte†en (1985 ;e.g.,Gratton & Sneden1994). As a result, the barium abundances from essentially all the early works have been overestimated. For halo stars, at least, the Ðrst study to include a full hfs treat-ment of the Ba II lines was the detailed analysis of the

r-processÈrich star CS 22892-052 bySnedenet al.(1996). The barium abundance in halo stars can in principle be accurately measured, because of the presence of numer-ous strong Ba II lines. In the solar system, 85% of the barium is thought to have been produced by s-process nucleosynthesis and 15% by the r-process, in contrast to 3%s- and 97%r-process fractions for europium (Kappeler, Beer, & Wisshak1989).Thus, the [Ba/Eu] abundance ratio provides a useful diagnostic of the neutron-capture pro-cesses that formed the heavy elements. The nucleosynthetic origin of the heavy elements in low-metallicity stars is a contentious issue ; for example, Truran (1981), Sneden & Parthasarathy (1983), Gilroy et al. (1988), MPSS95, and Cowan et al.(1995, 1996)claimed that the elemental abun-dance patterns are consistent with r-processÈdominated nucleosynthesis. Contrary to this Ðnding, however, Magain and & Zhao found that the BaII4554 (1995) Magain (1993)

line proÐle in the metal-poor star HD 140283 is con-A

sistent with a nearly pure s-process isotopic composition,

and not with a pure, or near-pure,r-process isotopic abun-dance ratio.

Besides being a useful diagnostic of the ofs- andr-process fractions, improved Ba abundances are also important for understanding the source of the large heavy-element disper-sion below [Fe/H]\ [2.5. In this paper, revised barium abundances are presented for 33 extremely metal-poor stars from the sample ofMPSS95, computed using the Kurucz (1992, private communication) grid of model atmospheres.

2.

HFS LINE LIST

The Ba II hfs components were calculated in a manner identical to that outlined for other elements by MPSS95. The hyperÐne A-constants were taken from Rutten (1978). Since observed and calculated constants were in excellent agreement, I adopted the computed values for terms without a measured splitting constant ; the second-order

B-constants were assumed to be zero. The components for Ðve Ba II lines are given in Table 1 ; note that these are identical to the hfs splittings used bySneden et al.(1996), although they did not publish their line list. The hfs line list inTable 1is consistent with the scale of the splittings given byMagain (1995) ;however, MagainÏs list contained fewer hfs components.

3.

BARIUM ISOTOPIC COMPOSITION

In order to correctly model the BaIIline formation, and thus derive total Ba abundances, it is necessary to know the relative fraction of each Ba isotope. The odd-numbered Ba isotopes exhibit hyperÐne splitting and so desaturate strong lines, and the even isotopes do not show hyperÐne splitting (apart from a small isotopic shift) ; thus, an increased frac-tion of odd Ba isotopes will result in lower derived abun-dances for strong lines. As pointed out by Magain (1995) andSnedenet al.(1996),the computed abundance depends upon whether Ba was produced bys- orr-process neutron capture, because the two neutron-capture processes result in di†erent fractions of odd-numbered Ba isotopes.

LikeSneden et al.(1996), ther-process and solar system Ba isotopic composition adopted here was based on the 1640

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II HFSLINELIST Ba

Wavelength Wavelength

(A) Strength (A) Strength

BaIIj4130.70 :s\2.722 eV, loggf\ ]0.56 BaIIj5853.70 :s\0.604 eV, loggf\ [1.01

134Ba : 134Ba : 4130.700 . . . 1.0000 5853.700 . . . 1.0000 135Ba : 135Ba : 4130.698 . . . 0.0625 5853.697 . . . 0.0875 4130.699 . . . 0.0563 5853.698 . . . 0.1000 4130.699 . . . 0.1312 5853.699 . . . 0.0625 4130.700 . . . 0.0063 5853.700 . . . 0.0250 4130.700 . . . 0.0729 5853.700 . . . 0.1250 4130.700 . . . 0.2333 5853.700 . . . 0.3500 4130.701 . . . 0.3750 5853.701 . . . 0.0625 4130.702 . . . 0.0042 5853.703 . . . 0.1000 4130.702 . . . 0.0583 5853.704 . . . 0.0875 136Ba : 136Ba : 4130.700 . . . 1.0000 5853.700 . . . 1.0000 137Ba : 137Ba : 4130.698 . . . 0.0625 5853.696 . . . 0.0875 4130.699 . . . 0.0563 5853.697 . . . 0.1000 4130.699 . . . 0.1312 5853.699 . . . 0.0625 4130.700 . . . 0.0063 5853.700 . . . 0.0250 4130.700 . . . 0.0729 5853.700 . . . 0.1250 4130.700 . . . 0.2333 5853.700 . . . 0.3500 4130.701 . . . 0.3750 5853.702 . . . 0.0625 4130.702 . . . 0.0042 5853.703 . . . 0.1000 4130.702 . . . 0.0583 5853.704 . . . 0.0875 138Ba : 138Ba : 4130.700 . . . 1.0000 5853.700 . . . 1.0000 BaIIj4554.00 :s\0.000 eV, loggf\ ]0.17 BaIIj6141.70 :s\0.704 eV, loggf\ [0.07

134Ba : 134Ba : 4554.000 . . . 1.0000 6141.700 . . . 1.0000 135Ba : 135Ba : 4553.969 . . . 0.1562 6141.695 . . . 0.0042 4553.970 . . . 0.1562 6141.697 . . . 0.0583 4553.971 . . . 0.0625 6141.698 . . . 0.0063 4554.017 . . . 0.4375 6141.699 . . . 0.3750 4554.020 . . . 0.1562 6141.699 . . . 0.0729 4554.021 . . . 0.0313 6141.701 . . . 0.0563 136Ba : 6141.701 . . . 0.2333 4554.000 . . . 1.0000 6141.702 . . . 0.0625 137Ba : 6141.702 . . . 0.1312 4553.965 . . . 0.1562 136Ba : 4553.967 . . . 0.1562 6141.700 . . . 1.0000 4553.968 . . . 0.0625 137Ba : 4554.020 . . . 0.4375 6141.703 . . . 0.0625 4554.022 . . . 0.1562 6141.701 . . . 0.0563 4554.023 . . . 0.0313 6141.698 . . . 0.0063 138Ba : 6141.702 . . . 0.1312 4554.000 . . . 1.0000 6141.699 . . . 0.0729 Ba_IIj4934.10 :s\0.000 eV, loggf\ [0.15 6141.695 . . . 0.0042 134Ba : 6141.701 . . . 0.2333 4934.100 . . . 1.0000 6141.697 . . . 0.0583 135Ba : 6141.699 . . . 0.3750 4934.059 . . . 0.3125 138Ba : 4934.070 . . . 0.0625 6141.700 . . . 1.0000 4934.118 . . . 0.3125 4934.129 . . . 0.3125 136Ba : 4934.100 . . . 1.0000 137Ba : 4934.054 . . . 0.3125 4934.066 . . . 0.0625 4934.121 . . . 0.3125 4934.132 . . . 0.3125 138Ba : 4934.100 . . . 1.0000

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1642 MCWILLIAM Vol. 115 TABLE 2

LINEABUNDANCES

4130 A 4554 A 4934 A 5853 A 6141 A

STAR EW v(Ba) EW v(Ba) EW v(Ba) EW v(Ba) EW v(Ba)

HD 2796 . . . 141 [0.88 133 [0.94 42 [0.54 83 [0.79 HD 4306 . . . 58 [1.80 34 [1.89 . . . 19 [1.51 HD 5426 . . . 135 [0.57 148 [0.31 52 [0.06 70 [0.62 HD 6268 . . . 190 [0.33 167 [0.74 59 [0.54 120 [0.57 HD 13979 . . . 95 [1.50 86 [1.42 . . . 63 [0.96 HD 128279 . . . 86 [0.59 85 [0.39 . . . 33 [0.43 HD 126587 . . . 118 [0.77 105 [0.99 . . . . HD 178443 . . . 31 0.63 152 [0.33 171 0.03 69 0.26 . . . . HD 186478 . . . 19 0.09 165 [0.75 157 [0.86 54 [0.57 97 [0.84 HD 189009 . . . . HD 200654 . . . 52 [1.47 28 [1.61 . . . 21 [1.08 BD[18¡5550 . . . 74 [1.89 62 [1.83 . . . 33 [1.53 CD[38¡245 . . . 11 [2.90 10 [2.67 . . . 13 [1.87 CS 22873-055 . . . 139 [1.49 118 [1.68 26 [1.22 . . . . CS 22873-128 . . . 35 [2.04 16 [2.17 . . . . CS 22873-166 . . . 110 [1.81 101 [1.81 . . . 63 [1.48 CS 22877-011 . . . 23 [2.06 . . . . CS 22878-101 . . . 81 [1.84 63 [1.84 . . . 26 [1.68 CS 22885-096 . . . . CS 22891-200 . . . 49 [2.35 33 [2.36 . . . . CS 22891-209 . . . 100 [1.82 70 [1.99 . . . 32 [1.81 CS 22892-052 . . . 21 0.24 173 [0.06 194 0.26 78 0.03 131 0.16 CS 22896-154 . . . 98 [0.72 107 [0.47 28 [0.09 57 [0.36 CS 22897-008 . . . 27 [2.50 17 [2.47 . . . . CS 22898-027a. . . 71 2.68 224 2.45 193 2.41 81 2.21 129 2.35 CS 22947-187 . . . 48 0.86 192 0.67 211 0.94 96 0.67 140 0.68 CS 22948-066 . . . 41 [2.10 40 [1.86 . . . 26 [1.42 CS 22949-037 . . . 26 [2.07 . . . . CS 22949-048 . . . . CS 22950-046 . . . 41 [2.54 28 [2.51 . . . . CS 22952-015 . . . 25 [2.74 . . . . CS 22953-003 . . . 128 [0.55 121 [0.68 . . . . CS 22968-014 . . . .

aSolar system Ba isotopic composition adopted for abundance calculation. results of Kappeler et al. (1989) ; however, in this work I

have employed a somewhat di†erentr-process composition than Sneden et al. (1996). For the r-process Ba isotopic composition, Sneden et al. used the values reported by

FIG. 1.ÈPlot of barium abundances computed byMPSS95 against values found in the present work. On average, the present results are 0.17 dex lower than MPSS95, if the systematic error for CD [38¡245 is omitted.

et al. but the only reliably measured Kappeler (1989),

r-process isotopic abundance was for135Ba. Unfortunately, the uncertainty in the137Ba isotopic abundance was 110% of the value, and the quoted138Ba isotopic abundance was an upper limit only, resulting from the subtraction of the larges-process component. Because of the large abundance uncertainties for these two isotopes, ther-process isotopic abundances were chosen using a function relating abun-dance to atomic weight, which is purportedly a smooth relation for ther-process (see, e.g.,Kappeleret al.1989). In this paper, I used a hand-drawn curve to approximate the trend ofr-process isotopic abundances (fromKappeleret al. between masses 130 and 150. Accordingly, the solar 1989)

system r-process isotopic fractions for 135Ba, 137Ba, and 138Ba are 40%, 32%, and 28%, respectively ; the estimated uncertainty on these values is about 12%. This composition implies an abundance for the137Ba isotope 1.5pabove the value measured by Kappeler et al., which is reasonable. The implied solar system 138Ba abundance is approximately half the upper limit given by Kappeler et al. Ther-process isotopic composition is in excellent agreement with the 37 : 35 : 28 mix adopted byBurriset al.(1998),based on the cross section measurements ofWisshak,Voss, & Kappeler Note that the 134Ba and 136Ba isotopes cannot be (1996).

made by ther-process, because of shielding from the stable nuclei134Xe and136Xe ; thus ther-process fraction for these two isotopes is zero. By comparison, Anders & Grevesse suggest proportions for the 134Ba, 135Ba, 136Ba, (1989)

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TABLE 3 AVERAGEABUNDANCES

Star v(Ba) p(Ba) [Ba/Fe] [Ba/Eu] [Sr/Ba]

HD 2796 . . . [0.72 0.10 [0.42 [0.51 0.49 HD 4306 . . . [1.75 0.08 [1.08 . . . 0.90 HD 5426 . . . [0.35 0.10 [0.15 [0.64 0.35 HD 6268 . . . [0.53 0.10 [0.16 [0.81 [0.01 HD 13979 . . . [1.18 0.12 [0.74 [0.62 [0.21 HD 126587 . . . [0.91 0.22 [0.27 [0.64 0.41 HD 128279 . . . [0.46 0.11 [0.59 . . . [0.14 HD 178443 . . . 0.14 0.09 0.00 [0.34 0.22 HD 186478 . . . [0.70 0.09 [0.33 [0.79 0.79 HD 200654 . . . [1.45 0.09 [0.84 . . . 0.36 BD[18¡5550 . . . [1.72 0.09 [1.02 [1.16 [0.16 CD[38¡245 . . . [2.78 0.11 [0.97 . . . 0.34 CS 22189-009 . . . \[2.50 . . . \[1.27 . . . [0.39 CS 22873-055 . . . [1.34 0.13 [0.67 [0.52 [0.11 CS 22873-128 . . . [2.09 0.11 [1.42 . . . 0.95 CS 22873-166 . . . [1.64 0.15 [0.95 . . . 0.63 CS 22877-011 . . . [2.06 0.14 [1.34 . . . 0.10 CS 22878-101 . . . [1.79 0.10 [0.87 . . . 0.34 CS 22885-096 . . . \[2.40 . . . \[0.82 . . . [[0.57 CS 22891-200 . . . [2.36 0.12 [1.08 . . . [0.28 CS 22891-209 . . . [1.89 0.09 [0.91 . . . 0.80 CS 22892-052 . . . 0.10 0.11 0.93 [0.51 [0.28 CS 22896-154 . . . [0.31 0.11 0.21 [0.73 0.41 CS 22897-008 . . . [2.49 0.12 [1.35 . . . 1.96 CS 22898-027 . . . 2.33 0.11 2.48 0.47 [1.54 CS 22947-187 . . . 0.77 0.11 1.05 0.35 [0.51 CS 22948-066 . . . [1.84 0.09 [1.02 . . . 0.44 CS 22949-037 . . . [2.07 0.17 [0.28 . . . 0.69 CS 22949-048 . . . \[2.80 . . . \[1.84 . . . [0.33 CS 22950-046 . . . [2.53 0.11 [1.34 . . . 0.68 CS 22952-015 . . . [2.74 0.15 [1.57 . . . 0.60 CS 22953-003 . . . [0.61 0.24 0.00 [0.70 0.28 CS 22968-014 . . . \[2.60 . . . \[1.40 . . . [[0.43

71.8%, respectively, for the solar system mixture of barium isotopes.

The r-process isotopic composition employed here and the measurements ofKappeler et al.(1989) suggest a best estimate for the solar systemr-process barium abundance of dex, and an acceptable range of v(Ba)

r\1.33^0.05_{dex. In contrast, the recent}

measure-v(Ba)

r\1.39È0.92

ments by Wisshak et al. (1996) indicate a solar system

r-process Ba abundance of v(Ba) dex. The

r\1.46^0.15

uncertainty associated with this value is almost entirely due to the poor measurement of the138Ba abundance. Inspec-tion of a plot of abundance versus atomic weight for the et al. data indicates that a smaller error bar Wisshak (1996)

should be assigned, suggesting a best estimate ofv(Ba)

r\

1.46 with acceptable values in the interval 1.55È1.39 dex. 4.

CALCULATION OF THE ABUNDANCES

The barium abundances were computed by the method of LTE model atmosphere spectral synthesis using the Kurucz (1992, private communication) grid of atmospheres, and the atmosphere parameters of MPSS95. The same weights assigned to each line by MPSS95 were used here. In this paper it will not be necessary to compute new abun-dances for thes-process, or solar system, mixture of barium isotopes. The single-line treatment of MPSS95 yielded upper limits to the Ba abundances and [Ba/Eu] abundance ratios that cannot be explained with a signiÐcants-process component or solar system mixture. Abundances in the present work were computed with ther-process isotopic Ba composition discussed above ; the results for all measured lines appear inTable 2. The small number of abundances

based on the BaII4130A line were found to be systemati-cally higher than for other lines ; since this is likely due to blending of the 4130 A line, it has been excluded from the average. A note about BaII4934 and 4554A lines : The 4934 results followed the 4554 results very closely, thus

indi-A A

cating no signiÐcant e†ect from an FeIline blend suggested byMoore,Minnaert, & Houtgast(1966).In the case of CD [38¡245, the Ba abundance derived from the weak line at 6141A was rejected because the computed abundance was inconsistent, at the 1 dex level, with results from the 4554 and 4934 A lines ; it appears that in this star the 6141 A measurement was dominated by noise. The Ðnal weighted-average barium abundances are given in Table 3, on the meteoritic scale ofAnders& Grevesse(1989).

The new barium abundances are on average 0.17 dex lower than found by MPSS95, slightly in excess of the typical uncertainty estimate of 0.15 dex quoted by MPSS95 ; this closely parallels the Ðndings of Burris et al. (1998). shows a comparison of barium abundances given Figure 1

by MPSS95 and the values found here ; the new abundances show the largest deviation for stronger lines, as expected. Note that the new barium abundance for CS 22892-052 agrees remarkably well with the result fromSneden et al. based on a higher resolution, higher signal-to-noise (1996),

ratio (S/N) spectrum and hfs analysis. 5.

DISCUSSION

In Figure 2, I show a plot of [Ba/Fe] with [Fe/H], including results from Gratton & Sneden(1988, 1994) for comparison ; it can be seen that the results from two samples agree reasonably well in the region of overlap.

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1644 MCWILLIAM Vol. 115

FIG. 2.È(a) [Ba/Fe] vs. [Fe/H] for the 33 extremely metal-poor stars. Triangles represent upper limits to [Ba/Fe] based on a detection limit of 20 mA. Open circles indicate CS 22898-027 and CS 22947-187, which are believed to be contaminated by AGBs-process nucleosynthesis. (b) The new [Ba/Fe] values (Ðlled circles) compared with the results of Gratton & Sneden(1988, 1994 ;squares). Crosses indicate the average [Ba/Fe] and [Fe/H] ratios of the present sample for bins 0.5 dex wide in [Fe/H], and are consistent with a constant [Ba/Fe].

As noted by MPSS95, the star with [Ba/Fe]\ ]2.5 is CS 22898-027, a CH subgiant ; its envelope has probably been contaminated bys-process material accreted from an evolved companion, so the composition does not reÑect the composition of the gas from which the star formed. Another star, CS 22947-187, also appears to be contaminated with large carbon and s-process element enhancements. Although this star seems to be on the asymptotic giant branch (AGB), it is too hot to have polluted its own envelope ; the most likely explanation is that the CS 22947-187s-process and carbon enhancements occurred as a result of accretion from the envelope of an evolved com-panion. Since the chemical composition of these objects is probably the result of red giant evolution, CS 22898-027 and CS 22947-187 will not be considered further for the purpose of understanding the chemistry of the halo.

Figures 2 and 3 show, as noted by MPSS95, that the [Ba/Fe] and [Sr/Fe] abundance ratios possess a spread of

D2.6 dex, which is much larger than the measurement uncertainty (typically 0.15 dex). Most of the stars lie at low [Ba/Fe] and [Sr/Fe], with a few points at or above the solar ratio. I emphasize that typical metal-poor stars exhibit deÐ-ciencies in [Ba/Fe] and [Sr/Fe] ofD1 dex ; however, the decline is accompanied by a roughly 300-fold range in these

ratios. The e†ect of a few stars with high ratios is to increase average to a value higher than the median ratio.

The large crosses in Figures2band3b indicate the mean [Ba/Fe] and [Sr/Fe] ratios for stars falling in 0.5 dex bins of [Fe/H] ; note that these crosses represent the mean number of Ba, or Sr, atoms divided by the mean number of Fe atoms, and not the mean of the logarithm of the number.

The average [Sr/Fe] number ratio for the sample is approximately solar, independent of [Fe/H](Fig. 3), indi-cating no required change in the average Sr/Fe yield ratio for extremely metal-poor stars. This conclusion does not depend on the result for any individual star. Like [Sr/Fe], the average [Ba/Fe] number ratios are consistent with a constant average [Ba/Fe] value (Fig. 2b) ; however, the result for Ba/Fe depends critically on a small number of Ba-rich stars, in particular CS 22892-052, and so is not an independent conclusion. Thus, it is possible that the average yield for Ba may not be constant with [Fe/H] ; more data are required in order to resolve this issue.

and et al. suggested that the MPSS95 McWilliam (1996)

large dispersion in [Ba/Fe] and [Sr/Fe] sets a minimum range for the ratio of the yields of Sr and Ba to Fe from Type II supernovae (SNe), and that the MPSS95 sample included stars born from ejecta dominated by individual SN

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events. In this model, the decreasing [Sr/Fe] and [Ba/Fe] dispersion with increasing [Fe/H] can be understood as being due to the averaging of ejecta from many SN events at high [Fe/H]. If this simple stochastic model is correct, and if the SN heavy-element yields did not vary signiÐcantly from [Fe/H]\ [4 to [Fe/H]\ [2.5, then the mean [Sr/Fe] and [Ba/Fe] ratios should not vary with [Fe/H] ; this is demonstrated to be approximately true in Figures2b and Another observation that favors the stochastic model is 3b.

the presence of individual stars with highly unusual elemen-tal ratios ; these stars are CS 22892-052, with r-process enhancements 40 times the solar value ; CS 22949-037, in which the iron-peak elements, from Ti to Co, are deÐcient byD1 dex ; and CS 22897-008, which has an enhancement of Sr 40 times the value expected from the Ba abundance.

The intrinsic heavy-element dispersion from SNe might

be much larger than the present observations ; if this is so, then the composition of the current sample of stars, in the interval [Fe/H]\ [4 to [2.5, was produced by ejecta from many SN events, and stars with metallicities well below [Fe/H]\ [4 should exist. It is, therefore, important to discover the lower limit to metallicity ; Audouze & Silk (1995) predicted a lowest possible metallicity of [Fe/H]\ [4, based on the physics of SN ejecta. The com-position of the stars with the lowest metallicity possible will be very useful, as they will most likely reÑect the element yields from individual SN events. However, at the present time the number of extremely metal-poor stars known (e.g., from the study ofBeers,Preston, & Shectman1992)is still rather small.

The revised [Ba/Eu] abundance plot for the MPSS95 sample is shown inFigure 4.Included in the Ðgure is a line indicating the [Ba/Eu] ratio for an extremes-process calcu-lation by Malaney (1987). The solar system r-process [Ba/Eu] ratio is also shown, with the value based on et al. indicated by a dotted line, while the Kappeler (1989)

FIG. 4.È[Ba/Eu] vs. [Fe/H] for stars in the sample with measured Eu and Ba abundances. Open circles indicate the CH stars CS 22898-027 and CS 22947-187. The solar system r-process value based on the data of Voss, & Kappeler at [Ba/Eu]\ [0.72, is indicated by a

Wisshak, (1996),

solid line ; the r-process ratio of [Ba/Eu]\ [0.85 from the data of Beer, & Wisshak is shown as a dotted line. The dashed Kappeler, (1989)

line indicates an extremes-process value, from the theoretical calculations ofMalaney (1987).The mean value for non-CH stars, with [Fe/H] below [2.4 dex, is[0.69 dex, consistent with the purer-process ratio within the measurement uncertainties.

solid line indicates the value inferred from the data of et al. Note that the [Ba/Eu] plot in Wisshak (1996).

MPSS95 showed an incorrect solar system r-process [Ba/Eu] ratio, due to an error in interpreting the Kappeler et al.(1989)tables.

Since the revised Ba abundances are typically 0.2 dex lower than found byMPSS95,the e†ect of including hfs is to move the points inFigure 4 closer to the solar system

r-process line. The average [Ba/Eu] value for the 10 stars with [Fe/H]¹[2.4 is[0.69^0.06 dex.

Comparison of the observed mean [Ba/Eu] ratio with the

r-process value based on the work ofWisshak etal. (1996 ; at [0.72 dex) shows that, to the level of accuracy of the abundance measurements, the composition of the MPSS95 sample below [Fe/H]\ [2.4 is consistent with a purer -process composition. The acceptable solar v(Ba) interval

r

from Wisshak et al. (1996), 1.55È1.39 dex, indicates an acceptable value for the solar [Ba/Eu]r-process ratio in the range[0.62 to[0.78 dex. Although the best estimate is a purer-process composition, theWisshak et al.(1996)data and the measurement uncertainties of the abundances in the present work permit up to 20% of the barium to have been produced by thes-process. If one adopts the [Ba/Eu] ratio indicated by theKappeleret al.(1989)results, at[0.85 dex, then it is necessary to conclude that the mean [Ba/Eu] ratio di†ers from the purer-process value by slightly more than twice the rms abundance measurement uncertainty, and up to 30% bariums-process fraction is allowed. Thus, there is no clear-cut detection of an s-process component in the sample. At present the main source of uncertainty MPSS95

is due to the e†ects of the poorly measured solarr-process abundance for138Ba. Improved measurement limits on the barium s-process fraction may be obtained by measuring the135Ba and137Ba abundances from proÐle Ðtting (similar to Magain 1995), thus avoiding the difficulty associated with138Ba.

This almost purer-process heavy-element composition is approximately supported by the published data for [La/Eu] ratios in metal-poor stars (see, e.g., McWilliam computed Eu abundance limits for stars in 1997). MPSS95

the sample without detected Eu lines ; the resulting [Ba/Eu] limits do not a†ect the conclusion ofr-processÈdominated heavy-element nucleosynthesis.

The dominant r-process composition reÑected by the [Ba/Eu] ratios in the present sample is consistent with the earlier conclusions of Truran (1981), Sneden & Partha-sarathy(1983), Gilroyet al.(1988),and Cowan et al.(1995, but is inconsistent with the results of & Zhao

1996) Magain

and who claimed that the Ba isotopic (1993) Magain (1995),

composition in the very metal-poor halo dwarf, HD 140283, reÑects a nearly pures-process origin.

The star HD 178443, with [Fe/H]\ [2.07, possesses a [Ba/Eu] ratio at [0.34 dex, which is signiÐcantly higher than the remaining non-CH stars, indicating that the barium in this star is 60% s-process material. The high [Ba/Eu] ratio for HD 178443 is close to the value found by & Sneden for poor stars with metal-Gratton (1994)

licities in excess of [Fe/H]D[2.0. This is consistent with the result ofCowanet al.(1996),who found it necessary to include ans-process component in the composition of HD 126238, with [Fe/H]\ [1.7. Thus it may be that stars more metal-rich than [Fe/H]D[2 possess a heavy-element component that reÑects early Galactic s-process nucleosynthesis. If this is correct, then the nearly pure

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1646 MCWILLIAM Vol. 115

r-process composition is yet another characteristic unique to stars with metallicities below [Fe/H]\ [2.4, in addition to the Co, Cr, Mn, and heavy-element abundances dis-cussed by MPSS95.

InFigure 5,I present the revised plot of [Sr/Ba] for the sample of stars. The mean [Sr/Ba] ratio appears MPSS95

to be approximately constant, at]0.33 dex, but an intrinsic scatter in the ratio is required to explain the dispersion about the mean value. If the measurement uncertainties are taken at face value, then a 1pintrinsic dispersion of 0.3 dex is required to account for the observed scatter. The signiÐ-cantly smaller scatter in the [Sr/Ba] plot than seen in the [Sr/Fe] and [Ba/Fe] plots is evidence that Sr and Ba syn-thesis are well correlated.

If the adopted measurement uncertainties are approx-imately correct, then it is signiÐcant that ther-processÈrich star CS 22892-052 has the lowest [Sr/Ba] ratio of all non-CH stars in the sample ; at [Sr/Ba]\ [0.28 dex, it is within 1 p measurement uncertainty of the solar system

r-process [Sr/Ba] value ([0.13 dex ;Kappeleret al. 1989). The value found here is in excellent agreement with that of et al. who found [Sr/Ba]\ [0.21 for this Sneden (1996),

star. In fact, the uncertainty in the solar systemr-process Sr abundance is larger than the Sr abundance measurement uncertainty for CS 22892-052 ; thus, it may be that the [Sr/Ba] ratio in this star provides a more precise estimate of ther-process Sr component than the solar abundance dis-tribution.

Stars in the sample with [Sr/Ba] greater than the

r-process value of CS 22892-052 presumably contain a sig-niÐcant fraction of Sr from a nucleosynthetic source other than the normalr-process. The star CS 22897-008, with an unusually high [Sr/Ba] ratio of]1.96 dex, provides strong evidence for this second source of Sr. The apparent intrinsic dispersion in [Sr/Ba] for the remaining non-CH stars also favors a second source of strontium ; in particular, no reasonable measurement error can explain the D0.6 dex di†erence between the well-determined [Sr/Ba] ratio for CS 22892-052 and the average ratio for the sample.

It is necessary to establish whether the Sr enhancements are primordial or due to internal evolution of the program stars. In this regard it will be useful to determine whether

FIG. 5.È[Sr/Ba] vs. [Fe/H]. Open circles indicate the two CH stars, and triangles indicate lower limits (assuming a 20mA detection threshold). The solar systemr-process value fromKappeleret al.(1989)is indicated by the dashed line. The data show that ther-process value forms a lower envelope to the Sr abundance and suggest a second source for Sr synthesis in addition to the ““ normal ÏÏr-process value.

halo dwarfs exhibit the same [Sr/Ba] characteristics as the present sample of giants. It seems, however, that the second Sr component is most likely primordial in origin because there is no di†erence between the mean [Sr/Ba] ratio for the AGB and red giant branch stars. An internal evolution mechanism would also be made difficult by the necessity to a†ect most of the stars in the sample.

One candidate for a second source of Sr is the weak

s-process, which is predicted to occur in the cores of massive stars (e.g., Raiteri et al. 1993 ; Prantzos, Hashimoto, & Nomoto1990).However, if it is assumed that the progenitor SN was of low metallicity, then the amount of Sr produced by the weaks-process is expected to be very low (see, e.g., et al. this is simply due to the secondary Prantzos 1990) ;

nature of the weak s-process. An alternative possibility is that the second Sr source is thea-rich freeze-out, as detailed byWoosley& Ho†man(1992) ;this mechanism is predicted to synthesize elements up to the Sr region, and as a primary process should not be extinguished at low metallicity.

In the case of ther-processÈrich star CS 22892-052, the huge overabundance of r-process elements completely dominated the [Sr/Ba] ratio, which is why its [Sr/Ba] value lies on the lower envelope of the distribution inFigure 5.If the stated measurement errors are correct, then the second Sr component is unusually low or absent in stars like CS 22891-200 and HD 13979, which have low barium abun-dances yet possess ther-process [Sr/Ba] ratio.

CS 22897-008 has one of the lowest [Ba/Fe] values and one of the highest [Sr/Fe] ratios in the sample. The com-bined e†ect of a very small r-process component and an unusually large second source component makes CS 22897-008 very conspicuous in Figure 5. Since the heavy elements are probably dominated by the second nucleo-synthetic source in this star, a more detailed study of its composition and comparison with the theoretical predic-tions of Raiteri et al. (1993), Prantzos et al. (1990), and & Ho†man is warranted ; the abundances of Woosley (1992)

Rb and Zr may provide especially useful comparisons with theoretical predictions of thes-process and thea-rich freeze-out.

Although it seems unlikely that the measurement uncer-tainties are large enough to account for the dispersion in it would be very useful to have much higher S/N Figure 5,

spectra than obtained by MPSS95, in order to reduce the observational scatter ; this task is being pursued byBurriset al.(1998).A more precise measurement of the solar system

r-process Sr abundance would also provide a useful test of the ideas just discussed.

6.

SUMMARY

The revised Ba abundances for the MPSS95 sample of metal-poor stars are typically 0.2 dex smaller than orig-inally thought and bring the [Ba/Eu] ratios closer to the purer-process value. To the level of accuracy of the abun-dance measurements, the mean [Ba/Eu] ratio of the present sample of halo stars, with [Fe/H] below[2.4, is consistent with the purer-process ratio fromWisshak et al.(1996). A major source of uncertainty lies in the measurement of the solar systemr-process barium abundance.

Although there is a large (300-fold) range in [Sr/Fe] and [Ba/Fe] at low metallicity, the average [Sr/Fe] and [Ba/Fe] ratios suggest no change in the mean ratios with [Fe/H] ; this is consistent with stochastic chemical evolu-tion, in which the composition from ejecta of individual SN

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events are detected. In this paradigm, the decreasing disper-sion in [Sr/Fe] and [Ba/Fe] with increasing [Fe/H] is due to the averaging of ejecta from larger numbers of SN events at higher metallicity. This idea needs to be tested by measuring the heavy-element abundances of a larger sample of extremely metal-poor stars ; at present this is limited by a paucity of stars known with metallicity below [Fe/H]D[3.0.

The [Sr/Ba] ratios show that there is a lower limit to the Sr abundances in these stars, which scales with ther-process Ba abundances. In addition to the stochastic base level of

r-process Sr, there is a second, also random, source of Sr. The second source is of uncertain origin, but because of the low metallicity of these stars, it seems doubtful that an

s-process component can account for the Sr abundances ; a

more probable mechanism is the a-rich freeze-out of & Ho†man Detailed abundance analysis of Woosley (1992).

stars like CS 22897-008 and a comparison with nucleo-synthesis predictions is required in order to identify the source of the extra Sr. Since these conclusions depend on the intrinsic dispersion in [Sr/Ba] ratios, a detailed study of metal-poor stars with high S/N would be particularly useful ; also required is an improvement in the accuracy of the solar systemr-process Sr abundance.

I gratefully acknowledge NSF support through grant AST 96-18623. Thanks also go to John Cowan and Chris Sneden for useful discussions, and to Andrew Carnegie for gainful employment.

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