Diversity of Abundance Patterns of Light Neutron-capture Elements in Very-metal-poor Stars

0.36 MB·

by  Misa Aoki

PreprinttypesetusingLATEXstyleemulateapjv.12/16/11
DIVERSITYOFABUNDANCEPATTERNSOFLIGHTNEUTRON-CAPTUREELEMENTSINVERY-METAL-POOR
STARS
MISAAOKI1
InternationalChristianUniversity,Mitaka,Tokyo181-8585,Japan
YUHRIISHIMARU
InternationalChristianUniversity,Mitaka,Tokyo181-8585,Japan
7
1
0 WAKOAOKI
2 NationalAstronomicalObservatoryofJapan,Mitaka,Tokyo181-8588,Japan

r
a
M SHINYAWANAJO
DepartmentofEngineeringandAppliedSciences,SophiaUniversity,Chiyodaku,Tokyo102-8554,JapanandiTHESResearchGroup,RIKEN,Wako,
Saitama351-0198,Japan
2
StudybasedondatacollectedwiththeSubaruTelescope,operatedbytheNationalAstronomicalObservatoryofJapan.
] Received2016June26;revised2017January27;accepted2017January27;published2017February27
R
S
ABSTRACT
.
h We determine the abundances of neutron-capture elements from Sr to Eu for five very-metal-poor stars
p (−3<[Fe/H]<−2) in the Milky Way halo to reveal the origin of light neutron-capture elements. Previous
- spectroscopicstudieshaveshownevidenceofatleasttwocomponentsinther-process;onereferredtoasthe
o
“mainr-process”andtheotherasthe“weakr-process,”whichismainlyresponsibleforproducingheavyand
r
t lightneutron-captureelements, respectively. Observationalstudiesofmetal-poorstars suggestthatthereis a
s
universalpatterninthemainr-process,similartotheabundancepatternofther-processcomponentofsolar-
a
[ systemmaterial. Still, itisuncertainwhethertheabundancepatternoftheweakr-processshowsuniversality
or diversity,dueto the sparsenessofmeasuredlightneutron-captureelements. We have detectedthe keyel-

2 ements, Mo, Ru, and Pd, in five target stars to give an answer to this question. The abundance patterns of
v
lightneutron-captureelementsfromSrtoPdsuggestadiversityintheweakr-process. Inparticular,scatterin
9
theabundanceratiobetweenRuandPdissignificantwhentheabundancepatternsarenormalizedatZr. Our
9
resultsarecomparedwiththeelementalabundancespredictedbynucleosynthesismodelsofsupernovaewith
5
parameterssuchaselectronfractionorproto-neutron-starmass,toinvestigatesourcesofsuchdiversityinthe
8
abundancepatternsoflightneutron-captureelements. Thispaperpresentsthatthevariationintheabundances
0
ofobservedstarscanbeexplainedwithasmallrangeofparameters,whichcanserveasconstraintsonfuture
.
1 modelingofsupernovamodels.
0
Subjectheadings:metal-poorstars: general—nuclearreactions,nucleosynthesis,abundances
7
1
v: 1. INTRODUCTION neutron-capture elements include Sr, Y, and Zr, which lo-
i One of the major questions in the current nuclear astro- cate near the first abundance peak associated with the neu-
X tron magic number 50, and those up to Ag in this study.
physics is the site of the rapid neutron-capture process (r-
r process). Very-metal-poorstarsare consideredto be thekey Heavy neutron-captureelements include those near the sec-
a ond abundance peak and heavier elements (e.g., Ba, Eu).
to constrain the site, since their chemical abundances are
The abundance ratios of light and heavy neutron-captureel-
expected to exhibit the yields of a single to a few nucle-
ementssuchas[Sr/Ba]and[Zr/Ba]showlargescatteratlow
osynthetic events (e.g., McWilliametal. 1995; Ryanetal.
metallicity, suggestingthe presenceof at least two r-process
1996; Beersetal. 2005). Indeed, a small fraction of very-
components(e.g.,Burrisetal. 2000;Johnson&Bolte 2002;
metal-poor stars show large excesses of neutron-capture el-
Hondaetal. 2004;Aokietal. 2005). Oneiswellknownas
ements that are suggested to have been processed by the r-
the “main r-process” (e.g. Truranetal. 2002), which yields
process in the early Galaxy. Previous spectroscopic stud-
bothlightandheavyneutron-captureelements. Aremarkable
ieshavemeasureddetailedabundanceofneutron-captureel-
featureofthisprocessisthatitappearstoproducea“univer-
ementsforaconsiderablenumberofvery-metal-poorstarsin
salpattern,”beingalmostidenticaltotheabundancepatternof
theMilkyWayhalotoidentifytheoriginofther-process(e.g.
thesolar-systemr-processcomponentatleastforthoseheav-
Hondaetal. 2004;Wuetal. 2015).
ierthanthesecondabundancepeak(e.g.,Snedenetal. 1996,
A distinctive feature was revealed from abundance ratios
2008; Barbuyetal. 2011; SiqueiraMelloetal. 2013, 2014).
of light and heavy neutron-captureelements of a large sam-
The other is frequently referred to as the “weak r-process”
ple of metal-poor stars measured by previousstudies. Light
(e.g. Wanajo&Ishimaru 2006), which yields light neutron-
[email protected] capture elements. The latter is not confirmed to be a type
1 of r-process and is sometimes referred to as the Lighter El-
2 Aokietal.
ements Primary Process (LEPP; Travaglioetal. 2004). We 2.5
note that other processes like the s-process in rapidly rotat- CS22892-052
HD122563
ingmassivestars(e.g.,Chiappinietal. 2011;Cescuttietal. 2.0
2013)arealsosuggestedassourcesofthescatterin[Sr/Ba].
1.5
In this paper, however, we refer to it as the weak r-process
according to theoretical predictions for such a process from a] 1.0
B
thestudiesofsupernovanucleosynthesis(Wanajoetal. 2011; Zr/ 0.5
Wanajo 2013). [
Previous spectroscopic studies have determined detailed 0.0
neutron-captureabundancesfor severalmetal-poorstars that
-0.5
haveexcessesoflight-to-heavyratios(e.g.,[Sr/Ba]). Oneof
suchstars,HD122563,isawell-studiedobjectthatshowsde- -1.0
viation of the abundances of neutron-captureelements from -4.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5
thesolarr-processabundancedistribution(e.g.,Hondaetal. [Fe/H]
2006). Theabundancepatternofthisstarhasfrequentlybeen FIG.1.— [Zr/Ba] abundance ratios as a function of [Fe/H]. The black
referred to as a template for the weak r-process. The un- crossesshowdataofhalostarstakenfromtheSAGAdatabase(Sudaetal.
2008). Redsquare: CS22892-052;bluediamond: HD122563;largeorange
derstanding of the weak r-process abundancesis yet insuffi-
circles:thetargetsofthepresentwork.TheFe,Z,randBaabundancesofour
cient, partially due to the lack of observational constraints. targetstarsaretakenfromtheresultsofthisstudy.
Thekeyelementsfortheunderstandingoftheweakr-process
(−3<[Fe/H]2<−2)redgiantsthathaveabundanceratiosof
arethosebetweenthefirstandsecondabundancepeaks,such
[Zr/Ba]higherthanthesolar(orstarswithuniversalpatterns
as Mo, Ru, Pd, andAg. However,these elementshaveonly
likeCS22892-052)value,butnotasextremeasHD122563
weakabsorptioninthenear-UVspectralrange,whichmakes
asshowninFigure1.
them difficult to measure. Abundances of such elements
The targets were observed on 2007 February 8th and 9th,
have been studied for r-process-enhanced stars with very
with the 8.2m Subaru Telescope HDS (Noguchi2002). The
low metallicity (e.g., Hilletal. 2002; Snedenetal. 2003).
Pd and Ag are more generally studied for metal-poor stars wavelengthcoverageisfrom3000to4630A˚ witharesolving
(e.g.,Johnson&Bolte 2002; Hansenetal. 2012). Peterson power of R=90,000. The signal-to-noise ratio (S/N) of the
(2011) and Hansenetal. (2014) determined abundances of spectrum is estimated from photoncounts at 4100A˚. In ad-
Moand/orRuabundances,discussingtheoriginoftheseele- dition,weusearchivedHDSspectratakenfromtheJapanese
mentsassumingtheabundancepatternofHD122563forthe VirtualObservatory(JVO)3 formeasuringtheBaabundance
weakr-process.Mostoftheirsample,however,aremetal-rich fromlinesatlongerwavelengths(suchas4554,4934,5854,
stars,ormetal-poorstarswithmoderatecontributionfromthe and6497A˚)thatarenotcoveredwithouroriginaldata. The
main r-process. In this study, we select metal-poor objects weakBa lines atlongerwavelengthshavelittle effectof hy-
that are not main r-processrich stars, and have some excess perfinesplittingandsaturationcomparedtothestrong4554A˚
oflight-to-heavyelementratios. Furtherstudiesofsuchstars line;therefore,weexpectamoreaccuratemeasurementfrom
arerequiredto constrainthe abundancepatternsofthe weak theselines.
r-process. Positions of objects, exposure time, S/N, observed dates,
Still, it is uncertain whether the abundance of the weak andheliocentricradialvelocitiesare summarizedin Table 1.
r-process shows a universal pattern as found in the main r- RadialvelocitiesaremeasuredusingFelines,andtheirerrors
process or shows a diversity. To obtain observational con- are estimated from standard deviation. The obtained veloci-
straints, we analyze the abundance patterns of five very- tiesagreewellwiththoseobtainedbypreviousstudiesusing
metal-poorstarsshowingexcessesoflight-to-heavyratiosof high dispersion data. Hence, no signature of radial velocity
neutron-capture elements. The spectroscopic data of stars variationisfoundfromourdata.Thereductionoftherawdata
were obtained with the 8.2m Subaru Telescope. Following is carried out in a standard process using the IRAF echelle
the methods in previous studies (e.g., Snedenetal. 1996; package4.
Hondaetal. 2006), we measure the stellar abundances of
neutron-captureelements and compare the results to the so- 3. CHEMICALABUNDANCEANALYSIS
lar r-process pattern. We also compare the abundances of
Abundance determinations are performed on the basis
light neutron-capture elements among these stars to inspect
of the model atmospheres and spectral line data, assum-
thevariationofthepatterns.
ing the local thermodynamic equilibrium (LTE). The model
In §2, we describe ourtargetstars and detailsof the spec-
atmospheres with the revised opacity distribution function
troscopicobservationobtainedbytheSubaruHighDispersion
(NEWODF) by Castelli&Kurucz (2003) are adopted. We
Spectrograph(HDS).§3,givesdetailsofthechemicalanaly-
employ a one-dimensional LTE spectral synthesis code for
sis of neutron-capture elements. We present the results and
our calculations of synthetic spectra. The code is based on
discuss the source of the weak r-processfrom derivedabun-
the same assumptions as the model atmosphere program of
dancesin§4. Finally,wesummarizeourstudyin§5.
Tsuji(1978),whichhavebeenusedinpreviousstudies(e.g.,
2 Theabundanceratiooftwoelements(AandB)aredefinedas[A/B]=
2. OBSERVATIONS logXXBA−logXXBA⊙⊙.Theabundanceratioisnormalizedwiththesolarabundance
ofthesameelements
Five bright stars (HD 107752, HD 110184, HD 85773,
3http://jvo.nao.ac.jp/portal/top-page.do
HD 23798, and BD+6◦648) are selected for our study 4 IRAFisdistributedbytheNationalOptical AstronomyObservatories,
based on high-resolutionspectra of the UV-blue range. Pre- whichareoperated bytheAssociation ofUniversities forResearch inAs-
vious studies show that these objects are very-metal-poor tronomy,Inc.,withcooperationoftheNationalScienceFoundation
Neutron-captureElementsinVery-Metal-poorStars 3
TABLE1
OBJECTDATA
ObjectName R.A. Decl. Exp.Time S/N Date VH
(J2000) (J2000) (s) (4100A˚) (UT) (kms−1)
BD+6◦648 04:13:12.771 +06:35:49.43 1800 150 2007February08 -143.5±0.6
HD23798 03:46:45.544 −30:51:25.48 15,600 480 2007February08 89.3±0.6
HD85773 09:53:39.708 −22:50:18.36 10,800 220 2007February08 147.7±0.8
HD107752 12:22:53.449 +11:36:18.85 10,800 340 2007February08 219.8±0.6
HD110184 12:40:14.375 +08:31:25.76 20,400 630 2007February08 139.5±0.6
Aokietal. 2009).Spectrallinedataforlineidentificationsin by a Gaussian profile. The Sr abundance determined from
ouranalysisareadoptedfrompreviousstudiesofIvansetal. stronglinesisnotascertainasabundancesofotherelements
(2006)andHondaetal. (2006).Thelinelistusedintheanal- due to the saturation effect. Hence, we do not include the
ysisisgiveninTable2. abundanceofthiselementinlatersections,wherewediscuss
Amongthemodelatmosphereparameters,weadopteffec- thelightneutron-captureelements.
tive temperatures (T ) from studies given in Table 3. The Spectrum synthesis technique is applied to determine the
eff
exceptionis HD 107752for which very differentT (about Mo, Ru, and Pd abundance. Examples of detected spectral
eff
400 K) has been obtained by differentstudies. We estimate lines in HD 110184 are shown in Figure 2. The Mo abun-
Tefffromthecolor(V−K),adoptingtheTwoMicronAllSky danceis determinedusingtheMoI 3864A˚ line. The lineis
Survey K magnitude andV magnitude by Oja (1985) taken almostfreefromcontaminationbyotherlines(Fig. 2). Two
fromthe SIMBADdatabase.5 We preferV−K to B−V for RuIlinesat3499and3728A˚ aredetected,andcontamination
theestimateofTeff, becausetheV−K colorofgiantstarsis byotherlinesisalsochecked.MoandRuaredetectedforall
lessdependentonmetallicityandmolecularabsorption.From theobjects. ThePdabundanceisalsomeasuredforthetarget
the empirical calibration between Teff and V −K color for stars,usingthe3405A˚ line.Wecheckedtheblendingofother
giant stars by Gonza´lezHerna´ndez&Bonifacio (2009), we atomiclinesusingtheatomiclinelistbyKurucz (1993). The
estimate the Teff of HD 107752 to be 4760 K which agrees spectra are analyzed using the line list around this PdI line
well with the Teff estimated from B−V by Ishigakietal. providedbyJohnson&Bolte (2002).
(2013). Surface gravity (log g), metallicity ([Fe/H]) and
micro-turbulence(ξ)aredeterminedfromtheanalysisofFeI 3.2. HeavyNeutron-captureElements(56≤Z≤63)
andFeIIlinesastheirabundancesmeasuredusingindividual
TheabundanceofBaisdeterminedincludingtheeffectof
linesdonotshowanytrendorsystematicdifferences.Thepa-
hyperfine splitting (McWilliametal. 1998). The quality of
rametersdeterminedforeachtargetaresummarizedinTable
HDSspectrafromJVOformeasuring4934,5854,and6497
3.
A˚ Ba lines is sufficient for our measurement. The isotope
For elements for which several absorption lines are avail-
ratios of Ba estimated for the r-process componentof solar-
ablein ourspectra, theaveragedabundancesmeasuredfrom
systemmaterial(Snedenetal. 1996)areassumed.Theheavy
individual lines are taken as the final results. On the other
neutron-captureelements including Ba exhibit enhancement
hand,forthosethathaveonlyoneorafewmeasurablelines,
inthreeoutoffivestars,suggestingthes-processcontribution
thespectrumsynthesistechniqueis appliedforthe measure-
to Ba in these stars, as discussed in §4.1. We also attempt
ment of their abundances. The effects of hyperfine splitting
to determine the abundance assuming the s-process isotope
aretakenintoaccountintheanalysisofBa,La,andEulines.
ratios(Snedenetal. 1996)toinvestigatetheeffectonderived
Atotalof11neutron-captureelementsfromSrtoEuaremea-
abundance. The abundance derived assuming the s-process
sured foreach star. We measure the abundancesof Mo, Ru,
isotope ratio is 0.1 dex greater at most than the abundance
andPdforthefirsttimeforthefivestarsinoursample. Sris
derived from those for the r-process case. In particular, the
alsoobtainedforHD85773,HD23798,andHD107752for
effectsare negligible in the analysis of weak Ba lines in the
thefirsttime.
red range. Hence, we employ the results obtainedassuming
ThePbabundanceissensitivetothecontributionsofthes-
thesolar-systemr-processisotoperatioasthefinalresults.
process,inparticular,atlowmetallicity.Inordertoverifythe
We update the line list of Hondaetal. (2006) for Ce, by
contributionof the s-process to the targets, the Pb 3683 and
adoptinggf-valuesdeterminedby Lawleretal. (2009). The
4058A˚ linesareinspectedforourobjects.
differenceofgf-valuesbetweenthenewandpreviouslinelists
Inthefollowingsubsections,detailsoftheanalysisandre-
isaround0.1 dex, andat most0.34dexfor the 4015A˚ line.
sultsaredescribed.Theabundancesdeterminedfromindivid-
This line is used for the determination of the Ce abundance
uallinesaregiveninTable2. Abundanceresultsaregivenin
ofHD 85773; theabundanceof Ce of thisstar decreasedby
Table4. Toderivethe[X/Fe]values,weusethesolar-system
0.05dexafteradoptingthegf-valuesbyLawleretal. (2009).
abundancesobtainedbyAsplundetal. (2009).
Overall,the updateof gf-valueshaslittle impacton thefinal
results.
3.1. LightNeutron-captureElements(38≤Z≤48)
WehavedetectedEuIIlinesat3820,4130,and4205A˚.Hy-
TheSrabundanceismeasuredusingtwostrongSrIIlinesat perfinesplitting is taken into account, adoptingthe line data
4078and4216A˚ fittingthesyntheticspectratoourobserved of Lawleretal. (2001). We assume the isotope fraction of
spectra. Inthefittingprocess,linebroadeningaffectedbythe 151Euand153Eutobe0.5,whichisclosetothesolarisotope
instrument, macro-turbulence, and rotation is approximated fraction.
PbabundanceismeasuredusingthePbI3684A˚ line. We
5SIMBADAstronomicalDatabase:http://simbad.u-strasbg.fr/simbad/ areabletomeasurethePbabundanceoftwostars,BD+6◦648
4 Aokietal.
TABLE2
LINEDATAANDEQUIVALENTWIDTHS
BD+6◦648 HD23798 HD85773 HD107752 HD110184
Element Wavelength L.E.P loggf logε EW logε EW logε EW logε EW logε EW
(A˚) (eV) (mA˚) (mA˚) (mA˚) (mA˚) (mA˚)
SrII 4077.71 0.00 0.17 ··· ··· ··· ··· −0.02 201.4 −0.13 144.0 ··· ···
SrII 4215.52 0.00 −0.17 0.95 273.2 0.86 266.6 0.02 174.1 −0.39 117.5 0.46 247.4
YII 3549.01 0.13 −0.28 0.02 85.3 −0.26 83.9 −0.98 63.8 −0.90 42.6 −0.91 80.6
YII 3600.74 0.18 0.28 ··· ··· ··· ··· ··· ··· ··· ··· −0.99 92.6
YII 3611.04 0.13 0.10 −0.16 90.2 −0.07 103.7 −1.17 68.9 −0.98 52.6 −0.85 93.3
YII 3950.36 0.10 −0.49 0.21 93.7 0.03 96.6 −0.73 70.5 −0.84 41.4 −0.53 89.7
YII 4398.01 0.13 −1.00 ··· ··· 0.13 82.6 ··· ··· −0.78 21.8 ··· ···
ZrII 3438.23 0.09 0.42 ··· ··· ··· ··· ··· ··· −0.32 74.8 ··· ···
ZrII 3457.56 0.56 −0.53 ··· ··· 0.63 67.9 −0.25 44.2 −0.12 23.5 −0.15 64.0
ZrII 3479.02 0.53 −0.69 ··· ··· ··· ··· ··· ··· −0.26 14.8 ··· ···
ZrII 3536.94 0.36 −1.31 ··· ··· 0.54 39.8 ··· ··· −0.22 7.2 −0.31 30.9
ZrII 3573.08 0.32 −1.04 0.62 54.3 0.50 54.2 −0.36 29.6 −0.22 13.6 −0.28 43.1
ZrII 3630.02 0.36 −1.11 ··· ··· 0.54 55.1 −0.25 28.8 −0.28 9.6 −0.09 46.2
ZrII 4050.33 0.71 −1.00 ··· ··· 0.75 47.2 ··· ··· ··· ··· ··· ···
ZrII 4071.09 1.00 −1.60 ··· ··· 0.99 18.7 ··· ··· ··· ··· ··· ···
ZrII 4208.99 0.71 −0.46 ··· ··· 0.62 73.9 −0.05 47.0 −0.16 21.7 0.17 66.8
ZrII 4317.32 0.71 −1.38 0.91 33.9 0.85 32.8 ··· ··· ··· ··· 0.23 24.2
ZrII 4613.92 0.97 −1.52 ··· ··· 0.98 23.2 ··· ··· ··· ··· ··· ···
MoI 3864.10 0.00 −0.01 0.03 41.9 −0.11 30.0 −0.97 11.6 −0.90 2.0 −0.70 23.0
RuI 3498.94 0.00 0.31 −0.33 30.9 −0.28 29.1 −1.17 12.2 −0.96 2.5 −0.91 21.4
RuI 3728.03 0.00 0.27 −0.28 31.7 −0.06 38.0 −0.84 18.9 −0.95 2.3 −0.70 23.0
PdI 3404.58 0.81 0.32 −0.78 35.9 −0.74 35.1 −1.30 26.7 −1.35 4.1 −1.22 32.1
BaII 4554.03 0.00 0.14 0.10 204.3 −0.23 196.4 −0.76 170.6 −0.99 123.7 −0.91 182.2
BaII 4934.10 0.00 −0.16 0.15 206.0 −0.23 197.4 −0.74 174.7 −0.92 126.3 −0.91 184.5
BaII 5853.69 0.60 −0.91 0.30 93.1 −0.06 84.1 −0.82 50.7 −1.30 10.6 −1.31 30.2
BaII 6141.73 0.70 −0.08 0.32 140.5 −0.01 135.5 −0.76 104.1 ··· ··· ··· ···
BaII 6496.91 0.60 −0.38 ··· ··· 0.04 134.2 ··· ··· −1.13 45.4 −1.12 92.5
LaII 3794.77 0.24 0.21 ··· ··· ··· ··· ··· ··· −1.88 11.0 ··· ···
LaII 3988.52 0.40 0.21 ··· ··· ··· ··· ··· ··· −1.85 8.7 ··· ···
LaII 3995.75 0.17 −0.06 ··· ··· ··· ··· ··· ··· −1.82 9.2 ··· ···
LaII 4086.71 0.00 −0.07 −0.64 71.7 −1.05 58.6 ··· ··· −1.79 14.3 −1.84 40.6
LaII 4123.22 0.32 0.13 −0.70 60.2 −0.97 55.1 −1.78 25.6 −1.80 9.8 −1.76 33.3
LaII 4322.51 0.17 −0.93 −0.81 17.2 −0.94 14.6 ··· ··· ··· ··· −1.67 8.9
LaII 4429.91 0.23 −0.35 ··· ··· ··· ··· −1.64 18.0 ··· ··· −1.65 23.4
CeII 4014.90 0.53 −0.20 ··· ··· ··· ··· −1.33 5.3 ··· ··· ··· ···
CeII 4053.50 0.00 −0.61 ··· ··· ··· ··· ··· ··· −1.23 3.1 ··· ···
CeII 4073.47 0.48 0.21 ··· ··· −0.72 22.0 −1.52 9.2 ··· ··· ··· ···
CeII 4115.37 0.92 0.10 ··· ··· ··· ··· ··· ··· −1.15 1.6 ··· ···
CeII 4165.60 0.91 0.52 ··· ··· ··· ··· ··· ··· −1.45 2.1 ··· ···
CeII 4222.60 0.12 −0.15 −0.50 35.3 −0.69 28.3 −1.46 12.7 −1.40 4.4 ··· ···
CeII 4418.78 0.86 0.27 ··· ··· −0.67 11.3 ··· ··· ··· ··· −1.34 7.1
CeII 4483.89 0.86 0.10 ··· ··· ··· ··· ··· ··· ··· ··· −1.14 7.6
CeII 4523.08 0.52 −0.08 ··· ··· ··· ··· ··· ··· −0.99 4.6 ··· ···
CeII 4539.74 0.33 −0.08 −0.37 33.4 −0.62 24.0 ··· ··· −1.30 3.9 ··· ···
CeII 4551.29 0.74 −0.42 ··· ··· ··· ··· ··· ··· ··· ··· −1.19 3.3
CeII 4562.36 0.48 0.21 −0.36 39.2 −0.58 29.8 −1.30 14.1 −1.31 4.9 −1.33 17.6
CeII 4628.16 0.52 0.14 −0.36 33.5 ··· ··· ··· ··· ··· ··· −1.30 14.9
NdII 4012.70 0.00 −0.60 ··· ··· ··· ··· ··· ··· −1.20 5.6 ··· ···
NdII 4021.33 0.32 −0.10 −0.55 14.1 −0.77 25.0 −1.43 14.1 −1.43 4.4 ··· ···
NdII 4059.95 0.20 −0.52 ··· ··· −0.76 15.9 ··· ··· ··· ··· ··· ···
NdII 4061.08 0.47 0.55 −0.35 33.1 −0.68 53.8 −1.35 33.1 −1.31 14.8 ··· ···
NdII 4368.63 0.06 −0.81 ··· ··· −0.64 18.0 −1.24 10.3 ··· ··· −1.33 11.4
NdII 4446.38 0.20 −0.35 −0.42 15.5 −0.63 29.1 −1.28 15.9 ··· ··· −1.32 19.3
NdII 4501.81 0.20 −0.69 ··· ··· ··· ··· ··· ··· −1.11 3.6 ··· ···
NdII 4513.33 0.06 −1.33 −0.29 5.3 −0.54 7.5 −1.03 5.3 ··· ··· −1.24 4.8
NdII 4563.22 0.18 −0.88 −0.27 9.6 ··· ··· ··· ··· −1.11 2.6 ··· ···
EuII 3819.64 0.00 0.51 ··· ··· −1.62 113.8 −2.23 78.3 ··· ··· −2.10 104.7
EuII 4129.70 0.00 0.22 −1.38 111.4 −1.42 110.7 −2.00 76.1 −2.01 31.2 -1.96 94.4
EuII 4205.04 0.00 0.21 ··· ··· −1.46 125.5 −2.02 81.2 ··· ··· ··· ···
PbI 3683.46 0.97 −0.54 −0.05 7.6 −0.43 2.7 <−0.65 ··· ··· ··· ··· ···
Neutron-captureElementsinVery-Metal-poorStars 5
TABLE3
STELLARPARAMETER
ObjectName Teff [Fe/H] logg1 loggr2 ξ ξBa3 References
(K) (dex) (dex) (dex) (kms−1) (kms−1)
BD+6◦648 4400 −2.11 0.90 1.30 1.85 1.80 1
HD23798 4450 −2.26 1.06 1.26 2.17 1.92 2
HD85773 4268 −2.62 0.87 0.87 1.98 2.00 3
HD107752 4760 −2.85 ··· 1.40 1.90 1.85 ···
HD110184 4240 −2.52 0.30 0.50 2.21 2.32 4
1loggfromprecedentresearch
2loggregulatedfromabundancesderivedfromFeIandFeII
3ξdeterminedfromJVOdatausedforBaabundancemeasurements
REFERENCES.—(1)Aoki&Honda (2008),(2)Burrisetal.(2000),(3)Simmereretal.
(2004),(4)Hondaetal.2004
1.00 1.00
0.80 0.80
FeI

0.60 MoI 0.60 RuI
ErI ZrI
FeII FeI
0.40 0.40
0.20 0.20
3863.5 3864 3864.5 3498.5 3499 3499.5
Wavelength [Å] Wavelength [Å]
1.00 1.00
0.80 0.80
0.60 0.60
RuI PdI CoI
0.40 F¯eI 0.40 ¯
VII ZrII
0.20 0.20
FeI
3727.5 3728 3728.5 3404 3404.5 3405
Wavelength [Å] Wavelength [Å]
FIG.2.—Observedlightneutron-captureelementsofHD110184:theMoIline(3864.10A˚),twoRuIlines(3498.94and3728.03A˚)andthePdIline(3404.58
A˚).Dottedlinesaretheobservations;solidgreen(middle)linesarethespectracalculatedfortheadoptedabundances.Theblue(upper)andred(lower)linesare
thespectracalculatedchangingthevaluesby±0.2dex.
al.
et
ki
o
A
TABLE4
ELEMENTALABUNDANCES
BD+6◦648 HD23798 HD85773 HD107752 HD110184
Species logε σran σtot [X/Fe] N logε σran σtot [X/Fe] N logε σran σtot [X/Fe] N logε σran σtot [X/Fe] N logε σran σtot [X/Fe] N
SrII 0.95 0.15 0.19 0.19 2 0.86 0.15 0.19 0.25 2 0.00 0.16 0.20 −0.25 2 −0.26 0.15 0.28 −0.28 2 0.46 0.15 0.28 0.11 1
YII 0.02 0.15 0.26 −0.09 3 −0.04 0.15 0.26 0.01 4 −0.96 0.18 0.28 −0.55 3 −0.87 0.15 0.16 −0.23 4 −0.82 0.17 0.18 −0.51 4
ZrII 0.76 0.15 0.18 0.29 2 0.71 0.18 0.20 0.39 9 −0.23 0.16 0.19 −0.19 4 −0.22 0.15 0.16 0.05 7 −0.07 0.21 0.21 −0.13 6
MoI 0.03 0.15 0.25 0.25 1 −0.11 0.15 0.25 0.27 1 −0.97 0.16 0.26 −0.23 1 −0.90 0.15 0.17 0.07 1 −0.70 0.15 0.17 −0.06 1
RuI −0.31 0.15 0.28 0.05 2 −0.17 0.15 0.28 0.34 2 −1.01 0.16 0.28 −0.14 2 −0.96 0.15 0.21 0.14 2 −0.85 0.15 0.21 −0.08 2
PdI −0.78 0.15 0.21 −0.25 1 −0.74 0.15 0.21 −0.05 1 −1.30 0.16 0.22 −0.25 1 −1.35 0.15 0.18 −0.07 1 −1.22 0.15 0.18 −0.27 1
BaII 0.22 0.15 0.18 0.15 4 −0.10 0.15 0.18 −0.02 5 −0.77 0.16 0.18 −0.33 4 −1.09 0.15 0.16 −0.52 4 −1.06 0.17 0.18 −0.72 4
LaII −0.72 0.15 0.17 0.28 3 −0.99 0.15 0.17 0.17 3 −1.75 0.16 0.18 −0.23 3 −1.83 0.15 0.16 −0.08 2 −1.73 0.15 0.16 −0.31 4
CeII −0.40 0.15 0.18 0.13 4 −0.66 0.15 0.18 0.02 5 −1.40 0.16 0.19 −0.36 4 −1.26 0.15 0.16 0.01 7 −1.26 0.15 0.16 −0.32 5
NdII −0.38 0.15 0.18 0.30 5 −0.67 0.15 0.18 0.17 6 −1.27 0.16 0.19 −0.07 5 −1.23 0.15 0.16 0.20 5 −1.30 0.15 0.16 −0.20 3
EuII −1.38 0.15 0.19 0.20 1 −1.50 0.15 0.19 0.24 3 −2.08 0.16 0.20 0.02 3 −2.02 0.15 0.16 0.31 1 −2.03 0.15 0.16 −0.03 2
6
Neutron-captureElementsinVery-Metal-poorStars 7
andHD23798(Fig.3). AnupperlimitofPbisestimatedfor
TABLE5
HD85773(Fig.3). ThePbI4058A˚ lineisnotusedbecause ABUNDANCEOFHD122563
ofthesevereblendwithCHmolecularlines. Theabundance
ofPbofBD+6◦648wasmeasuredusingthe4058A˚ lineina logε
previousstudybyAoki&Honda (2008).Althoughtheabun-
Species Z OurMeasurement Hondaetal.(2006)
danceismeasuredusingdifferentlines, ourresultisingood
SrII 38 −0.17 −0.11
agreement with their Pb abundance. No meaningful upper
YII 39 −0.99 −0.93
limitisobtainedfortheothertwoobjects.
ZrII 40 −0.35 −0.28
3.3. Re-analysisofHD122563 MoI 42 −0.91 −0.87
RuI 44 −0.87 −0.86
Toverifytheconsistencyofouranalysiswithpreviousstud- PdI 46 −1.38 −1.36
ies,weapplyouranalysisproceduretothedataofHD122563 BaII 56 −1.66 −1.62
reported by Hondaetal. (2006). We adopt the equivalent LaII 57 −2.73 −2.66
widthsofHondaetal. (2006)aswellasatomicdata,except CeII 58 −2.02 −1.83
forCe.Forthiselement,atomicdataupdatedbyLawleretal. NdII 60 −2.05 −2.01
(2009)areadopted. TheresultsaregiveninTable5forcom- EuII 63 −2.84 −2.77
parisonwiththoseobtainedbyHondaetal. (2006).
The chemical abundances of HD 122563 obtained by our 107752([Fe/H]=−3.01)andBD+6◦648([Fe/H]=−2.10),re-
analysis agree well with those obtained by Hondaetal. spectively, which hold the lowest and highest metallicity in
(2006). The abundances of most elements are systemati- our five samples. The rest of the stars, which hold similar
cally lower by around0.05–0.10dex than those obtainedby properties,areexpectedtoinvolvesimilarerrors.Tablesshow
Hondaetal. (2006). This difference can be attributed to differencesin abundance measurements(originalabundance
the difference in model atmospheres. Hondaetal. (2006) −abundancemeasuredusingchangedparameter)bychang-
adopts models provided by Kurucz (1993) in which con- ing±100KforT ,±0.3dexforlogg,±0.3dexfor[Fe/H],
eff
vective overshooting is assumed, while we adopt models of and±0.3kms−1 forξ.
Castelli&Kurucz (2003), which are calculated assuming Inparticular,effectsofchanging[Fe/H]andloggaresys-
pure mixing-length convection with the new opacity distri- tematically different between abundances derived from neu-
bution functions. The temperature structure of the models tralandionizedspecies. AbundancesofMo, Ru, andPdare
ofCastelli&Kurucz (2003)isseveraltenthsofkelvinlower determined by lines of neutral species, whereas other heavy
thanthoseofKurucz (1993). elements are measured from ionized species. This indicates
Aside from the effect of the difference in the model at- that the abundance ratios derived from neutral and ionized
mosphere, Ce abundance shows a relatively large difference species (e.g., Mo/Zr) could be significantly affected by pa-
(0.19dex). TheupdatedatomicdatabyLawleretal. (2009) rameters (e.g., T ) adopted in the analysis. We investigate
eff
partially explains the discrepancy between the two studies, the effect on the abundance pattern of HD 107752, by con-
thoughtheeffectislessthan0.1dexasmentionedabove. ductingabundanceanalysesforT =4400and4760K.The
eff
log g, [Fe/H], and ξ are derived for each T , resulting in
3.4. ErrorEstimates eff
0.7dex higher log g for the higher T . The abundance ra-
eff
Thevaluesadoptedforerrorbars(totalerror)forcompar- tiosofheavyelementsare∼0.5dexsystematicallygreaterfor
isons of abundance patterns are estimated from the random 4760K.Interestingly,however,theabundancepatternsarenot
errors and the systematic errors affected by uncertainties of significantlydependentonthechoiceofT . Theeffectofthe
eff
modelatmosphereparameters. We estimatedthe randomer- changesof T adoptedin the analysisis largerforelements
eff
rorsfromthestandarddeviationofabundancesderivedfrom measuredfromneutralspecies. Ontheotherhand,changesof
individuallines. Asfortheabundancesofelementsthathave log g have little effect on the abundancesof these elements,
onlyone or two available lines, the standarddeviationof Fe while elements measured from ionized species are affected
abundance ratios from individual FeI lines is adopted. The by changes of log g as well by the changes of T . Hence,
eff
standarddeviationofFeisalsoadoptedforelementsthathave eventhoughbothneutralandionizedspeciesarestudied,the
severalavailablelinesbutthestandarddeviationoftheabun- overallabundancepatternobtainedbythepresentworkisro-
dances derived from these lines is smaller than that of Fe. bust. We addedtherandomandsystematicerrorsinquadra-
Wealsoindependentlyestimatederrorsduetotheuncertain- turetoderivethevaluesadoptedforerrorbarsforcomparison
ties of equivalentwidth measurementsbased on the relation ofabundancepatterns,whicharelistedas“σ ”inTable4.
tot
σ ≃(λn1/2)/(R[S/N]) (Norrisetal. 2001). For example,
w pix 3.5. ComparisonwithPreviousStudies
theerrorfromtheuncertaintiesofequivalentwidthoftheMo
line at 3864 A˚ of BD+6◦648, which has the lowest S/N at To verifyourresults, we compareto the abundancesmea-
4100A˚ amongthe sample as shown in Table 1, is 0.10 dex. sured by previousstudies for the same objects. We exclude
The errors due to the uncertainty of continuum placement, Sr fromthecomparison,since their abundancesare lesscer-
which is at most 2%, is 0.07 dex. These errors are smaller tain (§3.1). Differences of adopted atmospheric parameters
than those estimated from the standard deviation described and abundanceresults (our result minus the values of previ-
above. Therefore,we adoptedthe standarddeviationsas the ousstudies)aresummarizedinTable8.
randomerrors,listedas“σ ”inTable4.
ran 3.5.1. BD+6◦648
Theuncertaintiesofmodelatmosphereparametershaveef-
fects mostly on the systematic errors of the abundances of Three neutron-capture elements (La, Eu, and Pb) of
neutron-capture elements. The effects of stellar parameters BD+6◦648 have been studied by Aoki&Honda (2008). A
on abundances are given in Table 6 and Table 7 for HD relativelylargedifferenceofatmosphericparametersisfound
8 Aokietal.
BD+6 648 HD 23798
1.00 1.00
nt nt
u 0.90 u 0.90
o o
C C Pb

FeI

0.80 0.80
Pb
FeI
3683.2 3683.4 3683.6 3683.2 3683.4 3683.6
Wavelength [Å] Wavelength [Å]
1.05
HD 85773
1.00
0.95
nt
u 0.90
o
C FeI
0.85 Pb
0.80
0.75
3683.2 3683.4 3683.6
Wavelength [Å]
FIG.3.—ObservedPbIlinesofBD+6◦648,HD23798andHD85773. Dottedline:observations;solidgreenlinesarethespectracalculatedfortheadopted
abundances. ThegreenlineisadoptedastheupperlimitofthePbabundance forHD85773. Blue(upper)andred(lower)lines arethespectracalculated
changingthevaluesby±0.3.Thegraysolidline(top)showsthecalculatedspectrawithoutPbcontamination.
TABLE6
ERRORESTIMATESFORHD107752
∆Teff ∆logg ∆[Fe/H] ∆ξ
Species −100K +100K −0.3 +0.3 −0.3 +0.3 −0.3 +0.3
FeI −0.12 0.12 0.04 −0.04 0.00 −0.01 0.14 −0.12
FeII −0.01 0.02 −0.09 0.10 0.00 0.01 0.02 −0.02
SrII −0.08 0.10 −0.03 0.05 0.09 −0.07 0.26 −0.22
YII −0.08 0.08 −0.07 0.08 −0.01 0.01 0.07 −0.05
ZrII −0.08 0.08 −0.08 0.09 −0.01 0.01 0.05 −0.04
MoI −0.15 0.15 0.01 −0.01 0.00 −0.02 0.00 0.00
RuI −0.24 0.18 0.00 −0.01 0.02 −0.04 0.01 −0.02
PdI −0.17 0.15 0.02 −0.03 0.02 0.00 0.02 −0.01
BaII −0.07 0.11 −0.10 0.09 −0.03 0.04 0.04 −0.02
LaII −0.08 0.08 −0.09 0.10 −0.01 0.01 0.00 −0.01
CeII −0.08 0.08 −0.09 0.09 −0.01 0.01 0.00 0.00
NdII −0.08 0.08 −0.09 0.09 −0.01 0.01 0.01 0.00
EuII −0.08 0.07 −0.09 0.09 −0.01 0.00 0.00 0.00
in the value of log g (0.4 dex). According to our error esti- betweenourresultsofLaandEu,andthoseofAoki&Honda
mates(Table7),suchadifferenceinthevalueofloggresults (2008). This suggests that the differences in abundance are
inachangeoftheabundanceofheavyelementsaround+0.09 mostly due to the difference of the adopted atmospheric pa-
dex. Thedifferenceinthevalueofξis−0.35kms−1,which rameters.
affects the abundance of La around +0.09 dex according to ThisobjectwasalsostudiedbyBurrisetal. (2000). They
Table7. Theestimationsare comparablewiththe difference have measured the abundances of Y, Zr, Ba, La, Nd, and
Neutron-captureElementsinVery-Metal-poorStars 9
TABLE7
ERRORESTIMATESFORBD+6◦648
∆Teff ∆logg ∆[Fe/H] ∆ξ
Species −100K +100K −0.3 +0.3 −0.3 +0.3 −0.3 +0.3
FeI −0.19 0.18 0.08 −0.07 0.02 −0.03 0.17 −0.18
FeII −0.03 0.03 −0.11 0.10 −0.04 0.05 0.08 −0.06
SrII −0.11 0.13 −0.01 0.01 0.02 −0.03 0.18 −0.17
YII −0.07 0.06 −0.08 0.07 −0.05 0.06 0.28 −0.24
ZrII −0.04 0.04 −0.10 0.10 −0.06 0.07 0.09 −0.06
MoI −0.27 0.21 0.02 −0.02 0.08 −0.12 0.03 −0.04
RuI −0.29 0.25 0.05 −0.04 0.07 −0.09 0.02 −0.01
PdI −0.21 0.21 0.04 −0.01 0.02 −0.02 0.03 −0.04
BaII −0.04 0.07 −0.09 0.09 −0.05 0.06 0.05 −0.07
LaII −0.05 0.04 −0.10 0.09 −0.04 0.06 0.09 −0.07
CeII −0.05 0.05 −0.10 0.11 −0.06 0.07 0.05 −0.03
NdII −0.05 0.05 −0.11 0.10 −0.06 0.07 0.06 −0.04
EuII −0.05 0.04 −0.10 0.09 −0.05 0.06 0.00 0.00
PbI −0.24 0.20 0.07 −0.03 0.01 −0.02 0.00 0.00
Eu. Overall,theirresultsareingoodagreementwithoursas 3.5.5. HD110184
shownin Table 8. Largestdisagreementis the measurement
Neutron-captureelementsofHD110184havebeenstudied
ofNd,forwhichtheirresultis0.12dexhigherthanours.The
byHondaetal. (2004). TheymeasuredabundancesofY,Zr,
T adoptedbyBurrisetal. (2000)islowerthanoursby100
eff Ba, La, Ce, Nd, and Eu. The abundancesof heavy neutron-
K.SuchadifferenceinthevalueofT resultsinlowerabun-
eff captureelementsfromLatoEuobtainedbyouranalysisare
dance,whilethehigherlogg(0.20dex)ofourworkpartially
ingoodagreementwiththeirresults.TheBaabundancedeter-
cancelstheeffect.
minedbyourstudyisabout0.24dexlowerthantheirresult.
Thisisduetothedifferenceintheadoptedlines.Hondaetal.
3.5.2. HD23798
(2004)usethetwolinesat4554and4934A˚,whichgiveout
Neutron-capture elements including La and Eu of HD higherabundancecomparedtolinesintheredrangesuchas
23798 have been studied by Simmereretal. (2004). We 5854and6497A˚ weadopted(see§2).
adoptT fromtheirstudy. Thedifferenceofξbetweentheir
eff OurresultsofYandZrhavedifferencesofaround0.4dex
study and ours is 0.32 km s−1, which does not significantly fromtheirs. The discrepancyis also due to the differencein
affecttheanalysisofweaklines. Theabundancesofthetwo theadoptedlines. Hondaetal. (2004)usefivelinestodeter-
elementsareingoodagreementwithours. minetheabundanceofY.Ofthesefivelines, only3950A˚ is
includedinthelinesweadopt. Theequivalentwidthof3950
3.5.3. HD85773 A˚ measuredbyourstudyis89.7mA˚ andabundance(logε(Y))
We adoptT from Simmereretal. (2004) for this target. derived is −0.53 dex. Hondaetal. (2004) measured 88.7
eff
The atmospheric parameters are overall in good agreement. mA˚ for this line, which is consistent with our measurement.
Theabundancesofthe twoneutron-captureelementsLaand Other lines used by Hondaetal. (2004) are not included in
Eu measured by them are both higher than our results by our study, because some are within the gap of our spectrum
around 0.2 dex. The ratio between La and Eu (La/Eu) is andsomearesuspectedtobeaffectedbycontamination. The
comparable. The reason for the discrepancy in abundances abundances obtained from other lines we adopted are lower
is unclear, due to the few available details of analysis in thanthatderivedfromthe3950A˚ line,whichrendersourre-
Simmereretal. (2004). sult,whichislowerthanthatinHondaetal. (2004).
Thetargethasalso beenstudiedby Ishigakietal. (2013). TheabundanceofZrderivedbyHondaetal. (2004)is0.32
TheymeasuredabundancesofY,Zr,Ba,Nd,andEu.TheTeff dex higher than our result. We adopt six lines to determine
weadoptis102KlowerthantheirTeff. [Fe/H]derivedbyour the abundance of Zr, whereas Hondaetal. (2004) adopted
analysisisalsolowerthanthatofIshigakietal. (2013). Our threelines(4161,4209,and4317A˚).Amongtheselines,we
overallabundanceratiosarelowerby0.1to0.2dexthantheir donotadopt4161A˚ due to contaminationsuspectedforthe
results, and the discrepancyis atleast partially explainedby
line. The average of the abundance obtained from the other
thedifferencesinadoptedeffectivetemperature.
two lines (4209 and 4317 A˚) is 0.20 dex, which is in good
agreementwithHondaetal. (2004),whereastheabundance
3.5.4. HD107752
derivedfromtherestofthelinesadoptedinourstudyishigher
Neutron-captureelementsofHD107752havebeenstudied thantheirs.
by Ishigakietal. (2013). They measured the abundance of TheabundancesofY,Zr,Pd,Ba,andEuarealsostudiedby
Y,Zr,Ba,Nd, andEu. TheT andlogginouranalysisare Hansenetal. (2012). Inaddition,Hansenetal. (2014)mea-
eff
66 K and 0.2 dex lower than those in Ishigakietal. (2013). suredMoandRuforthisobject. Theabundances(logεval-
According to our investigation on error estimates (Table 6), ues)oftheseelementsderivedbyHansenetal. (2012,2014)
suchdifferencesinloggandT shouldaffecttheabundance are0.5–0.8dexhigherthanourresults. Thisispartiallyex-
eff
about−0.1dex. Thediscrepancyoftheabundancesbetween plainedbythedifferenceofadoptedatmosphericparameters:
the two studies is attributed to the difference in the adopted the T we adopt is 210 K lower than theirs. Differencesof
eff
atmosphericparameters. abundancesbetween the two studies, however, remain to be
10 Aokietal.
TABLE8
COMPARISONWITHPREVIOUSSTUDIES
Star ∆Teff ∆logg ∆[Fe/H] ∆ξ ∆logε (dex)
(K) (dex) (dex) (kms−1) Y Zr Ba La Ce Nd Eu References
BD+6◦648 0 +0.40 −0.01 −0.35 ··· ··· ··· +0.23 ··· ··· +0.12 1
−100 +0.20 −0.01 −0.15 −0.09 −0.07 +0.04 +0.03 ··· −0.12 −0.08 2
HD23798 0 +0.20 0.00 −0.32 ··· ··· ··· +0.02 ··· ··· −0.14 3
HD85773 0 0.00 0.00 −0.02 ··· ··· ··· −0.19 ··· ··· −0.24 3
−102 +0.14 −0.21 0.00 −0.22 −0.20 −0.02 ··· ··· −0.12 −0.15 4
HD107752 −66 −0.21 −0.02 +0.05 −0.02 −0.19 −0.19 ··· ··· −0.07 −0.06 4
HD110184 0 +0.20 0.00 +0.30 −0.42 −0.32 −0.24 +0.03 +0.19 +0.03 −0.12 5
REFERENCES.—(1)Aoki&Honda (2008),(2)Burrisetal.(2000),(3)Simmereretal.(2004),(4)Ishigakietal.(2013),(5)Hondaet
al.(2004)
NOTE.—Thedifferenceistakeninasenseofourresultsminusotherworks
of the 0.2dexlevel, evenafter the differencesof parameters 4.2. ContributionsofMainr-process,Weakr-processand
aretakenintoconsideration.Thereasonforthediscrepancyis s-process
unclear,duetothefewavailabledetailsofabundanceanalysis
Weinterprettheexcessoflightneutron-captureelementsin
for this object in their studies. It should be noted, however,
ourobjectsascontaminationoftheweakr-process. Ifthisis
that these differences are systematic, and the overall abun-
thecase,theabundancesofourtargetstarswillbeexplained
dancepatternsobtainedbythetwostudiesaresimilartoeach
by the average of the abundances of the main and weak r-
other.
processeswithcertainweights,assumingthatbothprocesses
haveuniversalpatterns. Weattempttoexplaintheabundance
distributionsofourtargetstarsbyadoptingtheabundancesof
4. RESULTSANDDISCUSSION
CS22892-052andHD 122563asrepresentativeofthemain
4.1. ComparisonwiththeSolarr-processPattern r-processandtheweakr-process,respectively. Thecontribu-
Figures 4 (a) - (e) show abundances of the five tar- tionsofthetwocomponentsaredeterminedtoreproducethe
get stars compared to the Eu scaled solar r-process pat- abundanceratios between Zr and Eu of each targetstar. We
tern (Simmereretal. 2004). The abundance pattern calculate the weights of the main r-process (f) and the weak
of HD 122563 (Hondaetal. 2006) and CS 22892-052 r-process(1−f)as
(Snedenetal. 2003)arealsocomparedtothesolarr-process
patterninFigure4(f).TheabundancepatternsofHD122563 (cid:0)fεm,Zr+(1−f)εw,Zr(cid:1):(cid:0)fεm,Eu+(1−f)εw,Eu(cid:1)=ε⋆,Zr:ε⋆,Eu,
(1)
andthesolarr-processarescaledtotheEuabundanceofCS
where ε and ε are the abundances of element i for CS
22892-052. m,i w,i
22892-052 and HD 122563, respectively. ε is the abun-
As shown in Figure 4 (a) and (b), the abundance pat- ⋆,i
dance of element i for each of target star. The parameter f
ternsofheavyneutron-captureelementsofthetwostars,HD
isthecontributionweightofthemainr-processtoZrforeach
107752andHD110184,agreewiththatofthescaledsolarr-
star, as presented in Table 9. We apply the weight of main
process. AnexceptionisCe,whichisoverabundantatabout
r-processcontributiontootherelementsbycalculating
0.4dexcomparedtothescaledsolarr-processpatterninboth
stars. Similardisagreementisalsofoundintheabundanceof ε′= fε +(1−f)ε (2)
i m,i w,i
HD 122563; its measurementof Ce results in the overabun-
whereε′isthelinearabundanceofeachelement.Table9also
danceofabout0.5dex(Figure4(f)). i
givestheweakr-processcontributionratiocalculatedforZr
On the contrary, the abundances of light neutron-capture
elements of all the target stars tend to be overabundant (1−f)ε
compared to the solar r-process pattern normalized at Eu. Rw(Zr)= w,Zr (3)
fε +(1−f)ε
The levels of overabundance are not as evident as found in m,Zr w,Zr
HD122563(Fig. 4(f)). Thisisexpectedfromourtargetse- Figures5(a)-(e)showourtargetstarscomparedwiththemix-
lection (Fig. 1), since our targets are known to have higher turesof themain andweakr-processpatternswith contribu-
[Zr/Ba] ratios than the solar values, but not as extreme as tionweightsofthemainr-processpresentedinTable9. The
HD122563. Suchanoverabundanceoflightneutron-capture lines fit well to the abundance distributions of the stars HD
elementssuggestscontributionoftheweakr-process. Inpar- 107752andHD110184(Fig.5(a)and(b)).
ticular, the abundances of Sr and Zr are significantly over- Figures 5 (c)–(e) show results of BD+6◦648, HD 23798,
abundant. An exception is Y, which shows no excess com- and HD 85773, respectively. Contrary to the cases for HD
paredtothescaledsolarr-processpattern. SimilartrendofY 107722andHD110184,somelightelementssuchasRucan-
isalsofoundinCS22892-052andHD122563(Fig.4(f)).Mo not be reproduced by the mixed abundance patterns. Fur-
isoverabundantinalltheobjects.However,notallofthestars thermore, already pointed out in §4.1, some heavy neutron-
showanoverabundanceinRu,whilePdisunder-abundantin capture elements such as Ba and La are overabundantcom-
alloftheobjects. paredtoEuinthesethreestars. InFigure5(c)–(e),weadd
The three target stars BD+6◦648, HD 23798, and HD cyan lines, which represent contribution of the s-process to
85773 (Fig. 4 (c)–(d)) also show overabundances of heavy heavy neutron-captureelements. For the s-process, the con-
neutron-captureelements,especiallyofBa. Wediscusspossi- tribution weights are calculated in the same manner applied
blecontributionsofthes-processtotheseelementsinsection to the contribution of the weak r-process, so that the abun-
§4.2. danceratiosofLatoEumatchtheobservationalresults. We