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VIII. Stellar Abundance, Galactic Chemical Evolution and Nucleo-Cosmochronology

VIII. Stellar Abundance, Galactic Chemical Evolution and Nucleo-Cosmochronology

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MEASUREMENTS OF LI AND EU ISOTOPW

ABUNDANCES IN METAL-DEFICIENT STARS



WAKO AOKI

National Astronomical Observatory of Japan

2-21-1, Osawa, Mitaku, Tokyo, 181-8588 Japan

E-mail: aokiwakoQnao.ac.jp

Measurements of isotope abundances give quite strong constraints on nucleosynthesis models. High resolution spectrographs recently mounted on large telescopes

enable us t o measure isotope abundances for several elements in metal-deficient

stars. We report on the measurements of isotope abundances for Europium and

Lithium using the 8.2m Subaru Telescope.



1. Measurements of isotope abundances in stellar

photospheres

Measurements of chemical abundances in st.ellar photospheres have been

giving valuable information t,o understand the nucleosynthesis processes in

the universe. The measuremenk are made by the detailed analysis of stellar spectra obtained with high resolution spectrograph using model stellar

photospheres. The analyses are generally made for ekmental abundances

rather than isotopic ones? because the spectral lines are basically determined by the chemical nature rather than the mass of the nuclei.

For this reason! the prediction by nucleosynthesis models are usually

reduced to the elemental abundances to compare with observed chemical

abundances measured for stars. An exception is the isotope composit.ion

in solar systeni material, most of which can be measured by the analysis

of meteorites. Measurenients of isotope abnundacnes for stars give quite

strong constraints on nucleosynthesis models.

Though t.he measurements of isotope abundances in stellar photospheres

are difficult, there are some opportunities (Table 1). One is to use niolecular

spectra, which are sonietinies significantly affect,edby the difference of mass

of the nuclei'. For instance, carbon isotope ratios (12C/13C) has been measured for a number of cool stars in field stars in our Galaxy as well as those

in clusters from the analyses of spectra of carbon-bearing molecules (e.g.,



429



430

Table 1. Measurements of isotope fractions in stellar photospheres

features

Li I6708 A

CH, CN, CO, C2



objects

metal-deficient stars

cool stars



references



OH

MgH 5130 8,

SiO

Ba 114554 8,

1 5 1 E ~ / 1 5 3 E ~ E u 114205 A etc.



cool stars

metal-deficient stars

cool stars

metal-deficient stars

metal-deficient stars



12



isotopes

' ~ i / ~ ~ i

12c/13c

1 6 0 / 1 7 0/18 0

24Mg/25Mg/26blg

28Si/29SiJ3'Si

Ba (even/odd)



5s6,7



9,10,11

13,2

15



17,18919



CH, CN, CO). The carbon isotope ratio is a quite useful indicator of the

evolutionary stages of giant stars. Recent analysis of MgH lines have made

a rapid progress in the understanding of magnesium isotope abundances

(24Mg: 251\,Ig,and 26hi1g) in field and globular cluster stars (e.g., Yong et

al.'). These results have impact on the interpretation of the wavelength

shifts of Mg resonance lines found in some quasar absorption ~ y s t e r n ~ ' ~ ,

which is sonietimes interpreted as an evidence of the time variation of the

fine structure constant. Oxygen and silicon isotope abundances have been

measured for cool stars using molecular spectra in near infrared ranges (see

references in Table 1).

The atomic spectra of light elements are also affected by the difference of

the mass of the nuclei. The isotope shifts of hydrogen lines are well known?

and are measured in niany astronomical objects including quasar absorption

system (e.g.! Kirkman et a1.20). The third lightest, element lithium also

shows a rather large isotope shifts in the resonance line at 6708 A. The

measurements of Li isotope abundances are discussed in section 3.

The other possibility is to make use of the hyperfine splitting found in

heavy elements. The behavior of hyperfine splitting is different between

isotopes of an element in general, and the difference sometimes enables us

to estimate the isotope ratios by the detailed profile analysis of absorption

lines in stellar specbra. Magain & Zhao14 analyzed the absorption profile

of the Ba I1 4554 A resonance line to estimate the isotope comp0nent.s in

the metal-deficient ([F~/H]N-2.5) sOar HD 140283. Since Ba has 7 stable

isotopes, it is very difficult to determine the isotope fractions. However,

production of 138Ba dominates in the s-process, while isotopes with odd

mass number (L35Baand 137Ba) as well as 138Ba are yielded by r-process.

For this reason, the ratio of the contribut.ion by r- and s-processes to the

Ba in a star can be estimated from the analysis of the Ba line profile.

Their result suggested t.hat a significant part of the Ba in HD140283 is

produced by the s-process. This is a surprising result because only a small

contribut.ion of s-process is expected for stars with such low metallicity.



431



The Ba isotope fract'ions in this object is still in controversy (e.g., Lanibert

et d.16).

Recently, analyses of Eu lines were made for some metal-deficient,stars.

Our recent studies are reported in section 2.

Iv1easurement.sof the isotope abundances require very high quality spectra, because the analysis is usually based on t,he detailed profile fitting of

spectra calculated using niodel stellar phot.osphere to observed ones. The

spectral resolution of R 100,000 (3km s-') or higher is desirable to fully

resolve the stellar spectra which are intrinsically broadened by thernial motion and turbu1enc.e in the photosphere with several kni s-'. In order to

obtain high signal-to-noise spectra with such high spectral resolution, large

telescopes (e.g., the ESO Very Large Telescopes, the Subaru Telescope)

and high resolution spect,rographs are required. In this paper, we report

the recent isotope measurenients based on the high resolution spect,ra obtained with the High Dispersion Spectrograph (HDS2') of t.he 8.2m Subaru

Telescope.

N



2. Eu isotopes in very metal-deficient stars



Eu has two isotopes with odd niass number (15'Eu and 153E11). The effect

of hyperfine splitting is significant in both isotopes, but the degree of the

splitting is quite different. Hence, this element is an ideal case to nieasure isotope ratios. An accurate line list including hyperfine splitting was

recently provided by Lawler et a1.22,and, using this line list, Eu isotope

ratios were measured for three metal-deficient stars by Sneden et al.I7.



2.1. Eu isotopes produced by r-process

Figure 1 shows observed spectra of the Eu 11 4205 A line for three starslg.

HD 6268 (top panel) is known to have moderate enhancements of neutroncapture elements whose elemental abundance pattern is well explained by

the r-process nucleo~ynthesis~~.

The wavelengths and relative strength of

the hyperfine components for I5'Eu and 1 5 3 E are

~ shown in the top panel.

Since the hyperfine splitting of 151Eu is much more significant than that

of the other isotope, in particular in the bluer part of the absorption profile, the isotope ratios can be estimated from the profile analysis. The

dotted. solid, and dashed lines show the synthetic spectra calculated using

model photospheres for three different isotope fractions (fraction of 151Eu

(fr(15'Eu)) is set to be 0.38, 0.48, and 0.58). The x 2 fitting of these synthetic spectra to observed one derives fr(151Eu) to be 0.48 f 0.04. The



432

1



1



1



I.



HD6268



I.[



I



1



151Eu

153Eu



0.8



0.6



fr(151Eu)=0.48



0.4

1



1



1



LP625-44



1



kr(151Eu) 0.60



.--



0.8

0.6

0.4

1



1



1



CS31Od2-050



1



kr(151Eu)10.55



--_



0.8



0.6

0.4

4204.6



4204.8



4205



Wavelength



4



i?Aq5.2



4205.4



Figure 1. Comparison of the observed spectra (dots) and synthetic ones (lines) for the

Eu I1 4205 A line. The name of the object and the adopted fr(15'Eu) value are presented

in each panel. The solid line shows the synthetic spectra for the adopted fr(15'Eu); the

dotted and dashed lines show those for ratios which are smaller and larger by 0.10 in

fr(151Eu),respectively. The dot-dashed lines show the synthetic spectra for no Eu. The

~

wavelengths and relative strength of the hyperfine components for 151Eu and 1 5 3 E are

shown in the top panel.



uncertainty includes bhe 30 confidence level of the fitting and errors caused

by uncertainties of continuum level, line position, and Eu total abundance.



433

The result perfectly agrees with that of solar-system material24. Since

95% of the Eu in solar-system material is expected to originate from t.he

r - p r o c e ~,st,his

~ ~ ratio well represents that. of the r-process component in

the solar system. Similar analysis were also made for other three r-processelement-enhanced stars'' including CS 31082-001 in which uranium was

detected by Cayrel et a1.26 (see also Honda 27). These results and those

by Sneden et al.I7 show that the Eu isotope ratios in st,ars with excesses of

r-process elements are consistent u7it.h that in solar system material within

the errors. The remarkable agreement of elemental abundance patterns of

these objects with that of solar-system r-process component. was found by

previous studies (see references of H ~ n d a ~The

~ ) . analysis of Eu isot.opes

confirni for the first time this agreement in isotope level.



2.2. Eu isotopes produced by s-process: a new probe of



I5'Sm branching

The other two stars shown in Figure 1 (LP 625-44 and CS 31062-050) are

very nietal-deficient ([Fe/H]N -2.5) , but have large excesses of s-process

e l e n ~ e n t s ~ ' ; These

~ ~ . excesses are explained by the mass t.ransfer across

binary syst.ems containing AGB stars which have already evolved to white

dwarfs. Indeed, variat.ionof radial velocity has been confirnied for these two

stars, indicating they have unseen companions which are presumably white

dwarEs. Even though about, 95 % of Eu in solar system material is believed

to originate from r-process, and this element is sonietiiiies refereed to as 'rprocess element', the majority of the Eu in these two nietal-deficient stars

are estimated to originate from the s-process from the abundance patterns

of elements between Ba and Eu 28,29.

The I5'Eu fractions derived for LP 625-44 and CS 31062450 are 0.60

and 0.55, respectively, with uncert,ainties of about *0.05. This is the first.

estimate of the Eu isot.opefractions produced by s-process, because t.hat can

not be estimated from solar system material in which r-process contribut.ion

is donlinant. These values are higher than found in solar-system material

(fr(151Eu)=0.478). However, they agree well with the predictions of recent

s-process models by Arlandini et al.25>who deduced fr(151Eu)=0.541 and

0.585 from their best-fit stellar and classical models, respectively.

The Eu isotope fractions are quite useful as a probe of 151Smbranching

of s-process nucleosynthesis (Figure 2). For 151Sm, whose half-life is about

90 years: the $decay rate is strongly dependent on temperature, while t.he

neutron capture rate is not. This niakes the 151Sm branching an excellent,



434



... p-process

A



4



.........

........



151



.........



Smi



Sm



..........



..........

b.



k.



'..



r-process



Figure 2. The s-process reaction path around the ls1Srn branching. Unstabb nuclei

are shown by boxes with dotted lines.



thermo~iieter~~.

Previously; this branching has been analyzed using the

152Gdand 154Gdisotope ratios in solar-syst,emmat.erial, which are believed

to be significantly affected by this branching (e.g., Beer et al.31; Wisshak et

al.30). One dificu1t.yin this approach is that these Gd isotopes are affected

by a small amount of contamination from the pprocess: t.hough Obey are

shielded from the r-process.

We have made an analysis using t.he thermally pulsed s-process models, using updated reaction rates (see Aoki et al.lg). Figure 3 shows the

fr(151Eu) values calculated by our model. They are plotted as a function of

neutron density (Nn)

for four teniperatures (kT = 10, 15, 20, and 30 keV).

Also shown for coniparison by t,he hatched area is the fr(151Eu) range deduced for the s-process-element-enhanced st.ar LP 625-44 (the upper panel)

and CS 31062-050 (the lower panel). As can be seen in this figure, the

fr(151Eu) value is rather sensitive to t,he ambient temperature and neutron

density during the s-process.

The fr(151Eu) value is maximized in the range of neutxon density from

ATn = 5x lo7 to lo9 cmP3. For Arn > lo7 c ~ i i - ~the

? branching factor

at 151Sn1is higher than 0.9; and the nuclear flow bypasses 15'Eu. In this



435



6



7



a



9



10



a



9



10



Log N,



0.7



0.65



0.6

3



w2



v



& 0.55

0.5



0.45

6



7



Log N, (cm-9)



Figure 3. The fr(151Eu) values calculated for four temperatures (squares: kT = 30 keV,

triangles: 20 keV, circles: 15 keV, and asterisks: 10 keV) as a function of neutron density

(Nn). The upper and lower panels compare the calculations with the observed results

for LP 625-44 and CS 31062-050, respectively.



436



case; the fr(151Eu) value decreases wit.h increasing neutron density by the

effect of branching at 153Sm. In contrast.; in the range of low neutron

density (IV, < lo7 c111-~)~

the nuclear flow passes 151Euand 15’Gd, and the

elect.ron-capture on 153Gd is coniparable with, or faster than, the neutron

capture, which contributes to bhe production of 153Eu. This results in the

decrease of fr(151Eu)with decreasing neutron density in the low (Nn 5 lo7

c111r3) range (see Aoki et a1.l’ for more details).

The comparison of the calculations with the observational results indicates that s-processes wibh high neutron density (logN, 2 9) and low

temperat.ure (kT 5 20 keV), or those with quite low neut.ron density

(AT, < iO7c1r3), are not allowed for LP 625-44 (the upper panel of Figure

3). For CS 31062-050 (the lower panel), the process with high t.einperature

(kT 2 30 keV) and niediuni neutron density (10’ 5 ATn5 l O ’ ~ n - ~ is

) not

allowed. These are new constraints on s-process nucleosynthesis provided

by the Eu isotope analysis.

Recent niodels of AGB stars show that t.he abundance patterns of nuclei

in branchings are affected by the s-process both during the t,hermal pulses

and between pulses (e.g., Arlandini et al.25). The reaction which provides

neut.ron in the former phase is 22Ne(a,n)25Mg, which produces a neutron

density as high as 108-101* c111r3. In the int,erpulse phase? I3C(ck,n)160 is

assumed to be t.he neutron source reaction, which leads to a lower neutron

density (A:,

lo7 C I ~ I - ~ ) .

Though the uncertaint,ies in measurements are still large, the comparison of observational results with model calculations in Figure 3 suggests

that the contribution of s-process with low temperature between the thermal pulses is large in CS 31062-050, while the process during thermal pulse

plays an iniport,ant role in LP 625-44. This suggests the s-processes contributed to these two stars occurred in quite different conditions. It should

be noted that their Pb/Ba abundance ratios are significantly ~lifferent~~>~’.

The low Pb/Ba ratio of LP 625-44 cannot be explained by standard AGB

~ another

) ,

model for

models with low rnetallicity (e.g. Busso et a ~ ~and

a possible s-process during the t.herina1pulse was proposed by Iwanioto et

al.33. Further studies for Eu isotopes as well as elemental abundances with

higher accuracy are clearly desirable to understand the s-process nucleosynthesis in very metal-deficient AGB stars.

It should also be not.ed that, before deriving a clear conclusion, more

accurate reaction rates for isotopes around t.he branching point are required. Indeed, our first analysis using previous neutron-capt,ure rate of

151Srn which is about 50% higher than that used in the present work reN



437



sults in lower 151Eu fractions by 0.05-0.08. New experiments to determine

the neutron capture cross section of 151Sm are highly desirable to fix ratios

of 1 5 1 E ~ / 1 5 3 and

E ~ also 152Gd/154Gd.



3. Li isotopes

A number of studies have been made t.o accurately determine lithium abundances in nietal-deficient main-sequence stars to constrain the big-bang nucleosynthesis (see Coc et al.34). There are two stable isotopes of t’hiselement,

(‘Li and 7Li), and 7Li is believed t.o be dominant in most stars (t.he isotope ratio in solar system material is GLi/7Li= 0.08). The plateau found

in the abundance of this element in most metal-deficient stars has been

interpreted as the result of 7Li production in the big bang nucleosynthesis.

However, the value of the 7Li abundance plateau (log6 (7Li)

2.2)

is by 0.4-0.5 dex lower than that. expected from the standard bin-bang

nucleosynthesis model constrained by the recent nieasureriients of cosmic

microwave background radiation by WMAP and the cosmic D/H ratios

34). An important uncertainty in observational data is a possible depletion

of Li in the stellar photosphere from its original value. Since ‘Li is more

easily destroyed during stellar evolution than 7Li>the depletion of 7Li will

be excluded if 6Li is det.ected in the same object. This is the cosmological

interest to search for ‘Li in metal-deficient main-sequence stars.

In addition, the origin of 6Li in very metal-deficient stars is still unclear:

while the major producer of their 7Li would be the big bang nucleosynthesis. The spallat.ion of heavier nuclei by cosniic-rays provided by supernova

explosions only explains the ‘Li in higher metallicity. Recent,ly; Suzuki

8c 1 n 0 u e ~proposed

~

shocks produced by t,he formation of the large scale

structure of the Galaxy as a new source of cosniic-ray. The nietallicity

dependence of Li isotope ratios is expecbed to be a key to understand the

origin of ‘Li (see references in Table 1).

‘Li has been first detected by Smith et aL5 for the met.al-deficient

([Fe/H]= -2.2) st,ar HD84937. Subsequent.studies determined the 6Li/7Li

isotope ratios to be about. 0.05 for a few objecbs wit.h similar nietallicity.

We have started an observat,ional program using the Subaru Telescope

to measure 6Li/7Li ratios to ext’end the study to lower nietallicity. As t.he

first sample of this program, we obt.ained a very high quality spectmni

( R = 95,000 and S / N = 1000) of the bright subgiant HD140283, whic.h

has the lowest metallicity ([Fe/H]= -2.5) among the objects for which Li

isotopes have been t,ried to measure to date. The detailed profile analysis

p?



438



A



for the Li I 6708

line showed no evidence of 'Li in this object, and

determined a quite low upper liniit on the Li isotope ratio (6Li/7Li< 0.026).

The low upper-limit of 6Li/7Li may indicate a decrease of 6Li at the lowest

metallicity. However, unfortunately> this object may be sufficiently cool

t h a t 6Li was affected by internal processes of the star itself. even though

7Li and Be abundance5 show no depletion compared with other stars with

s i n d a r inetallicity.

In order to derive a clear conclusion, we also have obtained high quality spectra for several main-sequence stars with lower nietallicity. hIeasurenients of Li isotopes for larger sample of metal-deficient main-sequence

stars are ongoing using the ESO VLT. The behavior of Li isotopes in lowest

inetallicity will be soon revealed by these intensive studies.



Acknowledgments

Figure 3 was provided by N. Iwanioto who carried out the s-process model

calculations.



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3. Ashenfelter, T. P.? Mathews, G. J.: & Olive, K. A., ApJL? in press

4. Otsuki, K. et al. 2004, this volume

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