Stellar substructures in the solar neighbourhood IV. Kinematic Group 1 in the Geneva-Copenhagen survey

3 MB·English

by  R. Ženovienė

Astronomy&Astrophysicsmanuscriptno.25088˙ZenovieneIV (cid:13)c ESO2015
January27,2015
Stellar substructures in the solar neighbourhood
IV. Kinematic Group 1 in the Geneva-Copenhagen survey
R.Zˇenoviene˙1,G.Tautvaisˇiene˙1,B.Nordstro¨m2,3,E.Stonkute˙1,andG.Barisevicˇius1
1 InstituteofTheoreticalPhysicsandAstronomy,VilniusUniversity,A.Gostauto12,01108Vilnius,Lithuania
e-mail:[renata.zenoviene;grazina.tautvaisiene;edita.stonkute;gintaras.barisevicius]@tfai.vu.lt
2 DarkCosmologyCentre,NielsBohrInstitute,CopenhagenUniversity,JulianeMariesVej30,DK-2100,Copenhagen,Denmark
3 StellarAstrophysicsCentre,DepartmentofPhysicsandAstronomy,AarhusUniversity,NyMunkegade120,DK-8000AarhusC,
5
Denmark
1
e-mail:[email protected]
0
2 ReceivedSeptemberXX,2014;acceptedOctoberXX,2014

n
a ABSTRACT
J
Context.AcombinedstudyofkinematicsandchemicalcompositionofstarsisoneofthemostpromisingtoolsofresearchinGalaxy
6
formation.ThemaingoalinthisfieldofresearchistoreconstructtheformationhistoryofourGalaxy,torevealtheoriginofthethick
2
disc,andtofindremnantsofancientmergers.

Aims.Wedeterminedetailedelemental abundances instarsbelonging totheso-calledGroup1oftheGeneva-Copenhagen survey
]
R (GCS)andcomparethechemicalcompositionwiththeGalacticthin-andthick-discstars,withtheGCSGroup2andGroup3stars,as
wellaswithseveralkinematicstreamsofsimilarmetallicities.Theaimistosearchforchemicalsignaturesthatmightgiveinformation
S
abouttheformationhistoryofthiskinematicgroupofstars.
.
h Methods.High-resolutionspectrawereobtainedwiththeFibre-fedEchelleSpectrograph(FIES)spectrographattheNordicOptical
p Telescope,LaPalma,andwereanalysedwithadifferentialmodelatmospheremethod.Comparisonstarswereobservedandanalysed
- withthesamemethod.
o Results.Theaveragevalueof[Fe/H]forthe37starsofGroup1is−0.20±0.14dex.InvestigatedGroup1starscanbeseparatedinto
r
threeagesubgroups.Alongwiththemain8-and12-Gyr-oldpopulations,asubgroupofstarsyoungerthan5Gyrcanbeseparated
t
s as well. Abundances of oxygen, α-elements, and r-process dominated elements are higher than in Galactic thin-disc dwarfs. This
a elementalabundancepatternhassimilarcharacteristicstothatoftheGalacticthickdiscanddiffersslightlyfromthoseinHercules,
[
Arcturus,andAF06stellarstreams.
1 Conclusions.ThesimilarchemicalcompositionofstarsinGroup1,aswellasinGroup2and3,withthatinstarsofthethickdisc
v mightsuggestthattheirformationhistoriesarelinked.Thechemicalcompositionpatterntogetherwiththekinematicpropertiesand
1 agesofstarsintheinvestigatedGCSgroupsprovideevidence oftheircommonoriginandpossiblerelationtoanancient merging
0 event.Agas-richsatellitemergerscenarioisproposedasthemostlikelyorigin.
4
Keywords.stars:abundances–Galaxy:disc–Galaxy:formation–Galaxy:evolution
6
0
.
1 1. Introduction essentially older and more highly enriched in α-elements than
0
thin-discstars(e.g.Soubiran&Girard2005).Theenhancement
5
In this series of papers (Stonkute˙etal. 2012, 2013, hereafter inα-elementssuggeststhatthethick-discstarswereformedon
1
Paper I and Paper II; and Zˇenoviene˙etal. 2014, hereafter relatively short timescales (∼ 1 Gyr), thus offering us a hint
:
v Paper III), we investigate the detailed chemical composition totheformationhistoryofourGalaxy(Kordopatisetal.2013a).
i of stellar kinematic groups that were identified in the Geneva- Ancientminormergersofsatellitegalaxiesareconsideredasone
X
Copenhagensurvey(GCS,Nordstro¨metal.2004)andweresug- possiblescenarioofthethick-discformation.
r
a gested to belong to remnants of ancient merger events in our Numerical simulations of merger events have shown that
Galaxy(Helmietal.2006). suchdebrisstreamssurviveascoherentstructuresovergigayears
TheformationandevolutionoftheMilkyWaygalaxyisone (Helmi 2004; Lawetal. 2005; Pen˜arrubiaetal. 2005). Stellar
ofthegreatestoutstandingquestionsofastrophysics.According streams may be discovered as overdensitiesin the phase space
Lambda cold dark matter (ΛCDM) cosmological model, large distributionofstarsinthesolarvicinity.ExaminingtheGCScat-
galaxieslikeourGalaxyemergeasanend-pointofhierarchical alogue,Helmietal.(2006)lookedforstellarstreamsinaspace
clustering,merging,andaccretion,butwestilllackthedetailed of orbitalapocentre,pericentre,and z-angularmomentum(Lz),
physical picture of how individual stellar populations can be the so-called APL-space. In this kind of space stellar streams
associated with elements of the proto-cloud,and how different cluster around lines of constant eccentricity. They found three
Galactic componentsformed and evolved. Even the thick disc, new coherent stellar groups (Group 1, 2, and 3) with distinc-
asauniqueGalacticcomponentdiscoveredmorethan30years tiveages,metallicities,andkinematics,andsuggestedthatthose
ago(Gilmore&Reid1983),stillhasnoapprovedformationsce- groupsmightcorrespondtoremainsofdisruptedsatellites.
nario.SeparationoftheGalacticdiscinto”thick”and”thin”disc Arifyanto&Fuchs (2006), Dettbarnetal. (2007), and
populationsreferstodifferenttypesofstars.Thick-discstarsare Klementetal. (2008) proposed an alternative projection of the
1
R.Zˇenoviene˙etal.:Stellarsubstructuresinthesolarneighbourhood.IV.
eccentricity and L space, which accordingto Dekker’s (1976) to have a significantly smaller z-velocity dispersion. We inves-
z
theoryofGalacticorbitscanbeapproximatedby(U2+2V2)1/2 tigated chemical compositionfor 37 Group 1 stars, eight com-
andVvelocities,respectively(UispointingtowardstheGalactic parison Galactic thin-disc stars, and five comparison Galactic
center, V into the direction of Galactic rotation). The assump- thick-discstars.Wedeterminedabundancesofupto22chemical
tion is that stars in the same stellar stream move on orbits elementsand comparedthem withthe Galactic disc abundance
that stay close together, which is justified by numerical sim- pattern.
ulations of satellite disruptions (Helmietal. 2006). With this In Fig. 1, we show the velocity distribution of the Galactic
method, Arifyanto&Fuchs (2006) identified several known discstarsfromHolmbergetal.(2009).StarsbelongingtoGCS
streams(Hyades-Pleiades,Hercules,andArcturus)andonenew, Group 1 are marked with open and filled circles (the latter are
the so-called AF06 stream (later confirmed by Klementetal. usedto markstarswe investigated).An averageerrorofstellar
2009andbyKlementetal.2011). spacemotionsineachcomponent(U,V,andW)is1.5 kms−1
Dettbarnetal. (2007) analysed the phase space distribution (Nordstro¨metal.2004).ItisclearfromFig.1thatthedistribu-
inasampleofnon-kinematicallyselectedlowmetallicitystarsin tionofGroup1starsinthevelocityspacediffersincomparison
thesolar vicinityanddeterminedthe orbitalparametersofsev- tootherstarsoftheGalacticdisc.Forexampleinthe(U,V)plane
eralhalostreams.Oneofthosestreamsseemedtohaveprecisely Group1starsaredistributedinabananashape,whereasthethin-
thesamekinematicsastheSagittariusstream. disc stars define a centrally concentratedclump.These charac-
Klementetal. (2008) searched for stellar streams or mov- teristicpatternsarethesameasthoseofinfallingdwarfsatellites
ing groups in the solar neighbourhood,using data providedby afterseveralgigayears(see Helmietal. 2006fora comprehen-
the first RAVE public data release. They estimated overden- sivediscussion).InFig.2,thestarsareshownintheAPL-space.
sities related to the Sirius, Hercules, Arcturus, and Hyades- Kinematicgroupstarsspreadoutintheedgeofthedistribution
Pleiadesmovinggroups.Besides,theyfoundanewstreamcan- of data points. The location of a star in the APL space is very
didate (KFR08), suggesting that its origin is external to the accurate.AlimitedknowledgeoftheformoftheGalacticpoten-
Milky Way’s disc. The KFR08 stream later was confirmed by tial,usedtodeterminevaluesofapocentreandpericentre,does
Klementetal. (2009) and Bobylevetal. (2010). Antojaetal. notaffectthe distributionofpointsin the APL space muchbe-
(2012)alsousedtheRAVEdataanddiscoveredanewgroupat causethevolumeprobedbytheGCSsampleissosmallthatthe
(U,V) = (92,−22)kms−1 inthesolarneighbourhood.Thenew Galactic potential is close to a constant inside this region (c.f.
group was detected as a significant overdensity in the velocity Helmietal. 2006). This implies that the energy,and hence the
distributionsusingatechniquebasedonthewavelettransform. orbital parameters or the location of a star in the APL space,
A mechanism, known as ’ringing’ was introduced by are determinedmostly by its kinematicsrather than by its spa-
Minchevetal. (2009). Minchevetal. (2009) showed that the tiallocation(ortheGalacticpotential).ChangesintheGalactic
sudden energy kick imparted by the gravitational potential of potentialproduceonlysmallvariationsintheorbitalparameters.
the satellite as itcrossesthe planeof thedisc can stronglyper- AccordingtoHelmietal.(2006),inthecaseofthepotentialpro-
turb the velocity field of disc stars located in local volumes posedbyFlynnetal.(1996),whichwasusedfortheorbitinte-
such as the solar neighbourhood. These perturbations can be grationsin Nordstro¨metal. (2004) and the following papers, a
observed in the (U,V) plane as arc-like features travelling in typicalchangeintheapocentreandpericentreisofabout1–2%.
the direction of positive V (see Go´mezetal. 2012). Using this Despite the fact that the kinematic group members, over
technique Minchevetal. (2009) confirmed presence of several time, were dispersed through the Galactic disc, their chemi-
known moving groupsand predicted four new features at V ∼ calcompositionshouldremainunchanged.Thehigh-resolution
−140,−120,40, 60 kms−1. By matchingthe numberand posi- spectroscopic analysis is an importantsupplementalmethod in
tionsoftheobservedstreams,theyestimatedthattheMilkyWay revealingandconfirmingtheoriginandhistoryoftheGCSstel-
discwasstronglyperturbedabout1.9Gyrago. largroups.
Variousstudieshave beensearchingforevidenceofminor-
merger events in the Milky Way (see review by Klement
2. Observationsandanalysis
2010). Large and deep chemodynamical surveys such as Gaia
(Perrymanetal.2001)willenableustocharacterisetheGalaxy We obtained high-resolution spectra of the programme and
morepreciselyandtoexamineitsformationscenariosaswellas comparison stars with the Fibre-fed Echelle Spectrograph on
thekinematicstellargroupidentificationmethods. the Nordic Optical 2.5 m telescope (NOT) during 2008,
Anindispensablemethodtochecktheoriginofvariouskine- 2011, and 2012. This spectrograph gives spectra of resolv-
maticstreamsistheinvestigationoftheirchemicalcomposition. ing power (R ≈ 68000) in the wavelength range of 3680–
In this paper, we present the detailed chemical composition of 7270 Å. We exposed the spectra to reach a signal-to-noise ra-
stars belonging to Group 1 of the Geneva-Copenhagensurvey, tio higher than 100. We elected the programme stars so that
whichwasidentifiedbyHelmietal.(2006). they were observable from the NOT and were brighter than
High-resolution spectra of 21 stars in the most metal- V = 10 mag. Reductions of CCD images were made with the
deficientGCSGroup3wereinvestigatedinPaperIandPaperII. FIESpipelineFIEStool,whichperformsacompletereduction:
PaperIIIpresentedresultsoftheanalysisof32starsintheGCS calculation of reference frame, bias and scattering subtraction,
Group2.Galacticthin-discstarswereanalysedforacomparison flat-field division, wavelength calibration and other procedures
aswell.Wepresentthedetailedchemicalcompositionresultsfor (http://www.not.iac.es/instruments/fies/fiestool).Alistoftheob-
Group 1, the last group of GCS. This kinematic group, which servedstarsandsomeoftheirparameters(takenfromthecata-
is the most numerous and metal-rich among the GCS groups, logue of Holmbergetal. 2009 and SIMBAD) are presented in
consists of 120 stars. Accordingto the (Nordstro¨metal. 2004) Table1.
catalogue, its mean photometric metallicity, [Fe/H], is about We analysed the spectra using the differential model at-
−0.45dex.TheGroup1starsaredistributedintotwoagepopu- mosphere technique described in Papers I, II, and III. Here
lationsof8Gyrand12Gyr.Group1alsodiffersfromtheother we will revisit only a few details. The Eqwidth and BSYN
two groups by different metallicity and kinematics, and tends program packages, developed at the Uppsala Astronomical
2
R.Zˇenoviene˙etal.:Stellarsubstructuresinthesolarneighbourhood.IV.

) ) )
sm m s m s
(V k (W k (V k
U (km s ) U (km s ) W (km s )
Fig.1.VelocitydistributionforallstarsinthesampleofHolmbergetal.(2009)(plussigns),starsofGroup1(circles),andthestars
investigatedhere(filledcircles).

13
12
)
c
p
k 11
(
o
p
a
10
9
8
800 1000 1200 1400 1600 1800 2000 3 4 5 6 7 8
peri (kpc)
L (kpc km s )
z
Fig.2.DistributionforthestarsintheAPLspace.PlussignsdenotetheGCSsample(Holmbergetal.2009),circlesdenoteGroup1,
thefilled circlesarethe starswe investigated.Note thatthese starsaswellas allGroup1stars aredistributedinAPL space with
constanteccentricity.
Observatory, were used to carry out the calculation of abun- respondingtotheminimalline-to-lineFeiabundancescattering
dancesfrommeasuredequivalentwidthsandsyntheticspectra, were chosen as correct values. We performedthe spectral syn-
respectively. A set of plane-parallel, line-blanketed, constant- thesis method for the determination of oxygen, yttrium, zirco-
fluxLTEmodelatmospheres(Gustafssonetal.2008)weretaken nium,barium,lanthanum,cerium,praseodymium,neodymium,
from the MARCS stellar model atmosphere and flux library samarium, and europium abundances. Several fits of the syn-
(http://marcs.astro.uu.se/). theticlineprofilestotheobservedspectraareshowninFigs.5–
We used the Vienna Atomic Line Data Base (VALD, 8. We complied atomic parameters of lines in the intervals of
Piskunovetal. 1995) to prepareinputdataforthe calculations. spectral syntheses from the VALD database. We calibrated all
We took the atomic oscillator strengths for the main spectral loggf valuestofittothesolarspectrumbyKurucz(2005)with
lines we analysed from an inversesolar spectrum analysis per- solarabundancesfromGrevesse&Sauval(2000). We tookhy-
formedinKiev(Gurtovenko&Kostyk1989). perfinestructuresandisotopeshiftsintoaccountasappropriate.
Initialvaluesoftheeffectivetemperaturesfortheprogramme Abundances of other chemical elements were determined us-
starsweretakenfromHolmbergetal.(2009)andthencarefully ingequivalentwidthsoftheirlines.Wedeterminedabundances
checkedandcorrected,if needed,byforcingFe i linesto yield of Na and Mg taking non-local thermodynamical equilibrium
nodependencyofironabundanceonexcitationpotentialthrough (NLTE) into account. The equivalent widths of the lines were
changes to the model effective temperature. We used the ioni- measuredbyfittingofaGaussianprofileusingthe4Asoftware
sation equilibrium method to find surface gravities of the pro- package(Ilyin2000).
grammestars by forcingneutraland ionised iron lines to yield Theuncertaintiesin abundancesare due to severalsources:
thesameironabundances.Microturbulencevelocityvaluescor- uncertainties caused by analysis of individual lines, including
3
R.Zˇenoviene˙etal.:Stellarsubstructuresinthesolarneighbourhood.IV.
Table1.Parametersofprogrammeandcomparisonstars
Star Sp.type Age Age M d U V W e z R R
V max peri apo
H09∗ C11∗ mag pc kms−1 kms−1 kms−1 kpc kpc kpc
Group1stars
HD3795 K0V 10.6 9.9 3.84 29 -48 -90 42 0.37 1.07 3.80 8.19
HD4607 F5 4.2 3.5 3.08 85 -108 24 27 0.31 0.86 6.73 12.79
HD15777 G0 11 11.1 4.28 50 -77 -69 20 0.32 0.50 4.45 8.74
HD22872 F9V 6.2 5.9 3.59 74 -99 -49 -11 0.32 0.06 4.96 9.57
HD25123 G0 11 ... 4.01 67 93 -30 -9 0.32 0.03 5.38 10.43
HD40040 G0 12.4 11 4.12 65 44 -70 -12 0.31 0.07 4.45 8.53
HD49409 G0V 11.2 8.2 4.30 53 110 -35 -62 0.37 1.53 5.10 11.07
HD52711 G4V 7.9 5.1 4.53 19 -18 -78 -9 0.30 0.03 4.30 8.03
HD60779 G0 6.8 5.3 4.38 36 -126 -57 -14 0.40 0.11 4.40 10.23
HD67088 G5 13.7 ... 4.63 67 95 -20 -7 0.31 0.05 5.67 10.86
HD67587 F8 4.7 3.9 3.29 47 74 -56 0 0.32 0.12 4.77 9.28
HD76095 G5V 5.5 5.3 3.28 49 2 -108 8 0.44 0.25 3.12 8.05
HD77408 F6IV 3.7 4.3 3.52 50 -120 -10 -27 0.32 0.40 5.86 11.42
HD78558 G0 11.6 10.9 4.44 37 -69 -74 -65 0.32 1.24 4.43 8.59
HD88371 G2V 13.2 9.4 4.55 59 -135 -22 9 0.37 0.31 5.34 11.56
HD88446 F8 7.3 7.8 3.77 67 -42 -98 4 0.41 0.17 3.45 8.16
HD90508 G1V 10.4 7.4 4.63 23 21 -92 22 0.37 0.51 3.72 8.14
HD109498 G3V 10.4 ... 4.53 69 59 -46 -46 0.26 0.79 5.32 9.03
HD111367 G1V 9.5 8.5 4.11 86 -69 -73 -45 0.33 0.76 4.34 8.53
HD135694 K0 12.8 6.7 4.82 71 -36 -98 -39 0.40 0.57 3.50 8.10
HD138750 F8 3.3 3.2 2.60 116 -72 -105 -16 0.46 0.18 3.07 8.37
HD140209 G0 10.4 9.6 4.03 71 -73 -63 -29 0.30 0.37 4.62 8.62
HD149105 G0V 6 5.8 3.37 53 49 -71 -10 0.32 0.06 4.35 8.53
HD149890 F8V 6.8 6.1 4.15 39 60 -61 -29 0.31 0.38 4.69 8.82
HD156617 G5 9.3 9.1 3.92 66 -102 -39 0 0.30 0.11 5.18 9.68
HD156893 G5 8.9 8.5 3.74 77 -97 -65 56 0.35 1.60 4.47 9.19
HD157214 G0V 13.9 8.1 4.60 14 25 -81 -64 0.32 1.33 4.21 8.17
BD+403374 K1 ... 6.4 6.43 49 -109 -49 -58 0.34 1.15 4.88 9.88
HD171009 G5 11.7 6.7 4.24 66 -68 -66 29 0.30 0.68 4.59 8.53
HD171242 G0 9.2 8.5 4.02 62 103 -37 -2 0.35 0.10 5.03 10.55
HD178478 G5 15.8 ... 4.90 47 -86 -70 -12 0.35 0.08 4.26 8.86
HD188326 G8IV ... 8.7 3.83 56 -91 -56 48 0.31 1.24 4.87 9.24
HD206373 G0V 5.5 5.6 3.19 104 -37 -95 0 0.39 0.11 3.56 8.08
HD210483 G1V 9.3 6.6 4.03 51 -77 -80 -17 0.37 0.15 3.96 8.63
HD211476 G2V 7.1 6.7 4.59 31 -115 -35 -46 0.33 0.87 5.23 10.39
HD217511 F5 2 2.1 2.23 122 -84 -55 -4 0.30 0.08 4.84 9.01
HD219175 F9V 3.5 2.9 4.61 39 -92 -55 -14 0.31 0.10 4.79 9.19
Thin-discstars
HD115383 G0V 4.3 5.1 3.97 18 -38 2 -18 0.09 0.18 7.54 8.99
HD127334 G5V 10.5 10.7 4.51 24 30 -4 -2 0.12 0.08 7.14 9.10
HD136064 F9IV 3 3.7 3.12 25 62 -29 -24 0.23 0.29 5.83 9.37
HD163989 F6IV 2.2 2.5 2.51 32 -29 -27 -22 0.10 0.22 6.64 8.16
HD187013 F7V 2.8 ... 3.34 22 38 -8 -25 0.14 0.28 6.88 9.20
HD187691 F8V 3.3 3.1 3.71 19 -3 -3 -25 0.03 0.27 7.88 8.28
HD200790 F8V 2.2 2.6 2.51 49 19 -37 10 0.15 0.27 6.03 8.21
HD220117 F5V 1.8 1.6 2.66 42 -15 -21 -16 0.07 0.12 7.03 8.02
Thick-discstars
HD150433 G0 13.8 7.1 4.86 30 -8 -58 -46 0.21 0.73 5.21 7.97
HD181047 G8V 13.9 10.2 4.96 47 -99 -43 -17 0.30 0.15 5.12 9.57
HD186411 G0 6 ... 3.36 88 -64 -56 -6 0.26 0.02 4.96 8.52
HD195019 G3IV-V 9.7 8.6 3.96 39 -73 -76 -39 0.34 0.61 4.18 8.57
HD198300 G0 10.3 7.1 4.89 53 81 -20 -24 0.28 0.28 5.84 10.26
Notes.∗H09-agestakenfromHolmbergetal.(2009),C11-fromCasagrandeetal.(2011).
randomerrorsofatomicdataandcontinuumplacementandun- abundancesseriously;the element-to-ironratios, which we use
certaintiesinthestellarparameters.Thesensitivityoftheabun- inourdiscussion,areevenlesssensitive.
danceestimatestochangesintheatmosphericparametersbythe
assumed errors ∆[El/H]1 are illustrated for the star HD52711
Thescatter of the deducedabundancesfromdifferentspec-
(Table 2). Clearly, possible parameter errors do not affect the
trallines σ givesan estimate of the uncertaintydueto the ran-
dom errors. The mean value of σ is 0.04 dex, thus the uncer-
1 We use the customary spectroscopic notation [X/Y]≡ taintiesin thederivedabundancesthatarethe resultof random
log (N /N ) −log (N /N ) . errorsamounttoapproximatelythisvalue.
10 X Y star 10 X Y ⊙
4
R.Zˇenoviene˙etal.:Stellarsubstructuresinthesolarneighbourhood.IV.

1.0
1.0
0.9 TiI 0.9
Relative intensity 00..78 CrI Relative intensity 00..78 CeII CrII+FeCIrI+FeI CrI
0.6 0.6
YII
0.5
0.5 FeI+CrI
FeI FeI
0.4 HD 149890 HD 149890
HD 88446 FeI 0.4 HD 88446 FeII+CrI+CoI
5198.5 5199.0 5199.5 5200.0 5200.5 5201.0 5201.5 5202.0 5202.5 5273.0 5273.5 5274.0 5274.5 5275.0 5275.5 5276.0
, ¯ ¯
Fig.6.NOT-FIESspectraofGroup1 starsHD149890andHD88446.Thesetwo spectraareover-plottedtosee the differencein
spectrallinesofelementsproducedins-process,whilelinesofotherchemicalelementsaresimilar.

1.00 1.0 1.00
0.99 0.98
0.9
Relative intensity0000....99995678 00.. 78 Relative intensity 00000.....8999980246 Pr II
0.94 0.6 0.86
[O I]
0.93 Ba II 0.84 Sm II
6300.1 6300.2 6300.3 6300.4 5853.4 5853.6 5853.8 5322.6 5322.7 5322.8 5322.9 4467.2 4467.3 4467.4
¯ , ¯
Fig.3.Syntheticspectrumfittotheforbidden[Oi]lineat6300.3 Fig.4. Synthetic spectrum fit to the praseodymium line at
ÅintheobservedspectrumofHD157214(leftpanel)andtothe 5323 Å in the observed spectrum of HD3795 (left panel) and
barium line at 5853 Å in the observed spectrum of HD52711. to the samarium line at 4467 Å in the observed spectrum
The observed spectra are shown by solid lines with dots. The of HD22872. The observed spectra are shown by solid lines
darkgreysolidlinesaresyntheticspectrawith[O/Fe]=0.42± with dots. The dark grey solid lines are synthetic spectra with
0.1andwith[Ba/Fe]=−0.08±0.1,respectively. [Pr/Fe]=0.54±0.1and[Sm/Fe]=0.01±0.1,respectively.
Somestarsfromoursamplewerepreviouslyinvestigatedby
Effective temperatures for all stars investigated here are otherauthors.InTable 3, we presenta comparisonwith results
by Bensbyetal. (2014) and Reddyetal. (2006), who investi-
also available in Holmbergetal. (2009) and Casagrandeetal.
gated several stars in common with our work. Eight thin-disc
(2011).Casagrandeetal.(2011)providedastrophysicalparam-
stars thatwe investigatedfora comparisonhavebeen analysed
eters for the Geneva-Copenhagen survey by applying the in-
frared flux method to determine the effective temperature. In previouslybyEdvardssonetal.(1993).Slightdifferencesinthe
loggvaluesliewithintheerrorsofuncertaintiesandarecaused
comparisonto Holmbergetal. (2009), stars in the catalogueof
mainlybydifferencesintheapplieddeterminationmethods.We
Casagrandeetal. (2011) are on average 100 K hotter. For the
see that titanium and zirconium abundances determined using
stars investigated here, our spectroscopic temperatures are on
average only 40 ± 70 K hotter than in Holmbergetal. (2009) both neutraland ionised lines agree well and confirm the logg
and 40 ± 90 K cooler than in Casagrandeetal. (2011). The valuesdeterminedusing iron lines. Overall, our [El/Fe] for the
[Fe/H] values for all of the stars we investigated are avail- starsincommonagreeverywellwiththoseinotherstudies.
able in Holmbergetal. (2009) as well as in Casagrandeetal.
(2011). A comparison between Holmbergetal. (2009) and
3. Resultsanddiscussion
Casagrandeetal. (2011) shows that the latter gives[Fe/H] val-
ues that are more metal-rich on average by 0.1 dex. For our Theatmosphericparameters,T ,logg,v,[Fe/H]andtheabun-
eff t
programme stars, we obtain a difference of 0.1 ± 0.1 dex in dances of 21 chemical elements relative to iron [El/Fe] of the
comparisonwith Holmbergetal. (2009) and we obtainno sys- programmeandcomparisonstarsarepresentedinTables4 and
tematic difference,buta scatter of 0.1dex,in comparisonwith 5. The numberof lines and the line-to-line scatter (σ) are pre-
Casagrandeetal.(2011).Thesameresultwasfoundincompar- sentedaswell.
ingtheatmosphericparametersdeterminedforGroup3starsin The results are graphically displayed in Figs. 7 and 8. We
ourPaperIandforGroup2inPaperIII. display elemental abundance ratios of Group 1 stars together
5
R.Zˇenoviene˙etal.:Stellarsubstructuresinthesolarneighbourhood.IV.

O Na
e]
El/F
[

Mg Al
e]
El/F
[

Si Ca
e]
El/F
[

Sc Ti
e]
El/F
[

V Cr
e]
El/F
[

Co Ni
e]
El/F
[
[Fe/H] [Fe/H]
Fig.7. [El/Fe] ratio as a function of [Fe/H] for Group 1 stars (filled circles) investigated here and for comparisonthin-disc stars
analysedinthiswork,PaperI,andPaperIII(opencircles).ThedatafortheMilkyWaythin-discdwarfstakenfromotherstudies
areshownasagreydots.SolidlinesareGalacticthin-discchemicalevolutionmodelspresentedbyPagel&Tautvaisˇiene˙ (1995).
AverageuncertaintiesareshownintheboxforNa.
6
R.Zˇenoviene˙etal.:Stellarsubstructuresinthesolarneighbourhood.IV.

Y Zr
e]
El/F
[

Ba La
e]
F
El/
[

Ce Pr
e]
El/F
[

Nd Sm
e]
F
El/
[

Eu
[Fe/H]
e]
F
El/
[
[Fe/H]
Fig.8. [El/Fe] ratio as a function of [Fe/H] for Group 1 stars (filled circles) investigated here and for comparisonthin-disc stars
analysedinthiswork,PaperII,andPaperIII(opencircles).Thes-processenhancedstar(HD88446)ismarkedasanasterisk.Grey
dotscorrespondtothedatafortheMilkyWaythin-discdwarfstakenfromotherstudies.TheGalacticthin-discchemicalevolution
modelisshownasasolidline(Pagel&Tautvaisˇiene˙ 1997).AverageuncertaintiesareshownintheboxforZr.
with data of thin-disc stars investigated here and in Papers I, The chemical evolution models of the thin-disc were taken
II, and III, as well as with results taken from other thin- from Pagel&Tautvaisˇiene˙ (1995, 1997). The thin-disc stars
disc studies (Edvardssonetal. 1993; Gratton&Sneden from Edvardssonetal. (1993) and Zhang&Zhao (2006) were
1994; Koch&Edvardsson 2002; Bensbyetal. 2005; selected using the membership probability evaluation method
Reddyetal. 2006; Zhang&Zhao 2006; Brewer&Carney described by Trevisanetal. (2011), since their lists contained
2006; Mashonkinaetal. 2007, and Misheninaetal. 2013). stars of other Galactic components as well. The same kine-
7
R.Zˇenoviene˙etal.:Stellarsubstructuresinthesolarneighbourhood.IV.

Table3.Comparisonwithpreviousstudies.
1.00
0.9 0.99 Quantity Ours–B14 Ours–R06 Ours–E93
Relative intensity 00..78 000...999 678 T[[l[oFMNegffeag//g/HFFe]e]] −−0000....0010−45813±±±±1±0000....0100450388 −−−0000....010096577±±±±4±0000....0100565551 −−0000....1020−56851±±±±2±0000....0120661475
0.6 0.95 [Al/Fe] 0.00±0.06 −0.02±0.06 −0.11±0.05
[Si/Fe] 0.01±0.03 −0.03±0.03 −0.02±0.03
0.94 [Ca/Fe] 0.02±0.03 0.04±0.02 0.03±0.05
0.5 EuII EuII
[Sc/Fe] ... 0.01±0.07 ...
0.93
4129.6 4129.7 4129.8 6645.0 6645.2 [Ti/Fe] 0.02±0.04 0.06±0.04 −0.04±0.06
¯ [V/Fe] ... 0.03±0.05 ...
Fig.5. Synthetic spectrum fit to the europium lines at 4129 Å [Cr/Fe] 0.01±0.02 0.02±0.03 ...
and 6645 Å. The observed spectrum for the programme star [Co/Fe] ... −0.01±0.04 ...
HD149105 is shown as a solid line with dots. The dark grey [Ni/Fe] 0.00±0.03 −0.02±0.02 −0.12±0.05
solidlinesaresyntheticspectrawith [Eu/Fe] = 0.04±0.1and [Y/Fe] −0.05±0.06 −0.10±0.09 −0.12±0.09
[Ba/Fe] −0.02±0.14 0.06±0.13 −0.06±0.18
[Eu/Fe]=0.11±0.1forthesetwolines,respectively.
[Ce/Fe] ... 0.01±0.15 ...
[Nd/Fe] ... −0.06±0.10 ...
Table 2. Effects on derived abundances resulting from [Eu/Fe] ... −0.02±0.11 ...
modelchangesforthestarHD52711.
Notes. Mean differences and standard deviations of the main param-
eters and abundance ratios [El/Fe] for 13 stars of Group 1 in com-
mon with Bensbyetal. (2014, B14), 7 stars of Group 1 in common
Ion ∆Teff ∆logg ∆vt Total with Reddyetal. (2006, R06), and 8 thin-disc stars in common with
+100K +0.3 +0.3kms−1
Edvardssonetal.(1993,E93).
[Oi] 0.03 0.13 0.01 0.13
Nai 0.05 −0.02 −0.01 0.05
Mgi 0.05 −0.02 −0.03 0.06
Ali 0.04 0.00 0.00 0.04 and [Ba/Eu] = 0.70,falls in the categoryof the s-process-rich
Sii 0.03 0.01 −0.01 0.03 stars.
Cai 0.07 −0.01 −0.02 0.07 The metallicity of Group 1 stars we investigated lie in a
Scii −0.00 0.11 −0.04 0.12 broadintervalof0.04≥[Fe/H]≥−0.57withtheaverage[Fe/H]
Tii 0.09 0.00 −0.01 0.09 equal to −0.20± 0.14 dex. Abundances of chemical elements
Tiii 0.01 0.12 −0.05 0.13 areratherhomogeneousandshowsimilaroverabundancesofα-
Vi 0.11 −0.00 −0.01 0.11
elements and r-process-dominated chemical elements with re-
Cri 0.08 −0.02 −0.06 0.10
spect to thin-disc stars, as we also found for the stars of GCS
Crii −0.03 0.10 −0.09 0.14
Fei 0.07 −0.02 −0.07 0.10 Group 2 and 3. This elemental abundance pattern has similar
Feii −0.02 0.11 −0.07 0.13 characteristicswiththatintheGalacticthick-disc.
Coi 0.08 0.01 −0.01 0.08
Nii 0.06 −0.00 −0.04 0.07
Yii 0.02 0.10 −0.12 0.16 3.1.Comparisonwiththethick-disc
Zri 0.11 0.01 0.01 0.11 In Table 6 we present a comparison of mean [El/Fe] ratios
Zrii 0.02 0.13 0.01 0.13
calculated for stars of Group 1 and thick-disc stars at the
Baii 0.06 0.08 −0.18 0.21
same metallicity interval −0.57 < [Fe/H] < 0.04. Twenty-six
Laii 0.03 0.12 0.01 0.12
Ceii 0.03 0.11 0.01 0.11 thick-disc stars in this metallicity intervalwere investigatedby
Prii 0.02 0.12 0.01 0.12 Bensbyetal. (2005), 37 stars by Reddyetal. (2006), 10 stars
Ndii 0.03 0.12 −0.01 0.12 by Mashonkinaetal. (2007), 51 stars by Stanford&Lambert
Smii 0.04 0.11 −0.01 0.12 (2012), and 7 stars by Misheninaetal. (2013). When compar-
Euii 0.03 0.12 0.01 0.12 ing oxygen abundances, we did not use the results reported
byReddyetal.(2006)andStanford&Lambert(2012)because
Notes.Thetableentriesshowtheeffectsonthelogarithmicabundances theyinvestigatedtheOiline,whilewestudied[Oi].Thestudies
relativetohydrogen,∆[El/H].
by Mashonkinaetal. (2007) and Misheninaetal. (2013) were
includedinthecomparisontoenlargetheinformationonneutron
captureelements.Theaveragevaluesofα-elementabundances
matic approach in assigning thin-disc membership was used includedMg,Si,andCa.Titaniumwasexcludedbecausethisel-
in Bensbyetal. (2005) and Reddyetal. (2006), which means ementwasnotdeterminedinoneofthestudies(Misheninaetal.
thatthe thin-disc stars used for comparisonare uniformin that 2013).Thecomparisonshowsthatthedeviationsdonotexceed
respect. theuncertainties.
One star in Group1 is richin elementspredominantlypro- Fig. 9 displays the comparison of [El/Fe] ratios for some
duced in s-process. As shown in Table 5 and Figs. 6 and 8, chemicalelementsbetweenindividualstarsinGroups1,2,and
HD 88446 has much stronger lines of elements predomi- 3 and the thick-disc stars of the above-mentioned studies. For
nantly produced in s-process and consequently much higher comparison,weselectedoxygen,theaveragedvaluesfortheα-
abundances of these elements. According to the definition of elements Mg, Si, and Ca, and the s- and r-process-dominated
Beers&Christlieb (2005), HD 88446,with its [Ba/Fe] = 1.04 elements barium and europium. Stars of the kinematic groups
8
R.Zˇenoviene˙etal.:Stellarsubstructuresinthesolarneighbourhood.IV.

Table6.Comparisonwiththick-discstudies.
2
[El/Fe] OBu1rs4− ORu0rs6− OMuar0s−7 OSu1r2s− OMuir1s3− 3
[O/Fe] −0.02 ... ... ... 0.08
[Na/Fe] −0.02 −0.06 ... −0.02 ...
[Mg/Fe] −0.03 −0.06 ... −0.01 −0.03 4
[Al/Fe] −0.07 −0.12 ... −0.07 ... MV

[Si/Fe] −0.04 −0.10 ... −0.10 −0.06
[Ca/Fe] 0.01 −0.02 ... −0.02 −0.04 5
[Sc/Fe] ... −0.06 ... −0.11 ...
[Ti/Fe] −0.05 −0.04 ... −0.01 ...
[V/Fe] ... −0.06 ... −0.04 ... 6
[Cr/Fe] 0.01 0.03 ... −0.05 ... 8 Gyr 12 Gyr
[Co/Fe] ... −0.06 ... −0.05 ... [Fe/H]= 0.0 [Fe/H]= 0.2
[Fe/H]= 0.2 [Fe/H]= 0.6
[Ni/Fe] −0.03 −0.05 ... −0.02 −0.05
[Y/Fe] −0.09 −0.08 −0.13 −0.11 −0.11 3.80 3.75 3.70 3.65 3.80 3.75 3.70 3.65
[Zr/Fe] ... ... −0.07 ... 0.02 log Teff
[Ba/Fe] 0.05 0.12 0.11 0.07 0.06 Fig.10.HRdiagramsoftheGroup1stars.Isochronesaretaken
[La/Fe] ... ... ... ... 0.10 from Bressanetal. (2012). The isochrones for metal-deficient
[Ce/Fe] ... −0.07 −0.06 −0.08 0.02 stars are with [α/Fe ]= 0.2. The open circles represent the
[Nd/Fe] ... −0.09 ... −0.06 −0.05 youngerstarswithagesof2–5Gyr.
[Sm/Fe] ... ... ... ... 0.02
[Eu/Fe] ... −0.14 ... −0.17 −0.06
Notes. Differences of mean [El/Fe] values for stars of Group 1 and youngmain-sequencestarscanbeseparatedamong86Group2
thick-disc stars at the same metallicity interval −0.57 < [Fe/H] stars as well (Paper III). The chemical composition pattern of
< 0.04. Sixty-three stars from Bensbyetal. (2014, B14), 37 stars theyoungstarsweinvestigatedissimilartotherestoftheGCS
fromReddyetal.(2006,R06),10starsfromMashonkinaetal.(2007, kinematicgroupstarsofthesamemetallicity.
Ma07), 51 stars from Stanford&Lambert (2012, S12), and 10 stars
fromMisheninaetal.(2013,Mi13).
3.3.Comparisonwithkinematicstreams
and of the thick disc have very similar chemical compositions. In this subsection, we discuss the GCS kinematic groups in
Wehaveobservedandanalysedseveralthick-discstarsaswell. the context of three other Galactic kinematic substructures
Their elementalabundanceratiosare also plotted in Fig. 9 and of similar metallicities. Our attention was attracted by the
agreewellwithresultsoftheprogrammestars.Thus,thechem- Hercules stream. Stars of this kinematic stream have a simi-
ical composition of all three GCS kinematic groups is similar lar range of metallicities and ages to those in the GCS groups
tothethick-discstars,whichmightsuggestthattheirformation (Bobylev&Bajkova 2007; Antojaetal. 2008; Bensbyetal.
historiesarelinked. 2007,2014).TheHerculesstreamwasfirstidentifiedbyEggen
(1958)asagroupof22starswithvelocitiessimilartothehigh
velocity star ξ Herculis (HD 150680). It is believed that the
3.2.Age
Herculesstreamisaresultofresonantinteractionsbetweenstars
According to Helmietal. (2006), the stars in Group 1 fall in the outer disc and the Galactic bar. This stellar stream is a
into two age populations: 33% of the stars are 8 Gyr old, heterogeneous group of objects from the thin and thick discs
and 67% are 12 Gyr old. The ages were later redetermined (Dehnen 2000; Fux 2001; Quillen 2003; Famaeyetal. 2005;
by Holmbergetal. (2009) and Casagrandeetal. (2011) and Soubiran&Girard 2005; Pakhomovetal. 2011; Antojaetal.
agree with each other within uncertainties(we present them in 2014;Bensbyetal.2014).
Table 1). Holmbergetal. (2009) present upper and lower age The origin of the Arcturus stream has been debated for
limitsforeverystar.Theaveragelowerandupperlimitis10and years (Eggen 1971, 1996, 1998; Arifyanto&Fuchs 2006;
14 Gyr for stars of the 12 Gyr age population, 7 and 10 Gyr Gilmoreetal. 2002; Wyseetal. 2006; Bensbyetal. 2014, and
for the 8 Gyr population, and 4 and 5 Gyr for the younger references therein). Stars of the Arcturus stream were identi-
stars,respectively.Fig.10showstheGroup1starsinvestigated fiedbyGilmoreetal.(2002),andlaterWyseetal.(2006),asa
here with our spectroscopic effective temperatures and abso- group of stars lagging behind the local standard of rest (LSR)
lute magnitudes M , taken from Holmbergetal. (2009), in a by about 100 kms−1. This stream was associated with a dis-
v
Hertzsprung-Russell(HR) diagram.The isochronesweretaken rupted satellite that merged with the Milky Way 10–12 Gyr
from Bressanetal. (2012). The overall features of stars in the ago.Navarroetal.(2004)suggestedthatthesestarsarethesame
diagramsarewellreproducedbyisochronesofthetwoindicated groupofstarsthatEggen(1971)associatedwiththebrightstar
ages.Themoremetal-abundantstarsfitthe8Gyrisochronequite Arcturus, whose Galactic orbital velocity also lags at the same
well, while more metal-deficientstars fit the 12 Gyr isochrone value. Navarroetal. (2004) analysed the group of stars associ-
(formetal-deficientstars,theisochronesarewith[α/Fe]=0.2). atedkinematicallywithArcturusandconfirmedthattheyconsti-
A subgroup of ten stars in our sample are younger (2 Gyr ≤ tuteapeculiargroupingofmetal-poorstarswithasimilarapoc-
age ≤ 5Gyr)andintheHRdiagramliehigherthantheturnoff entricradius,acommonangularmomentum,anddistinctmetal
luminosityofthe8Gyrisochrone.Thenumberofstarsyounger abundance patterns. These properties are consistent with those
than5Gyramong120starsinGroup1is18(accordingtoages expectedforagroupofstarsoriginatingfromthedebrisofadis-
determined by Holmbergetal. 2009). A subgroup of about 15 ruptedsatellite.Navarroetal.(2004)alsonoticedthattheangu-
9
R.Zˇenoviene˙etal.:Stellarsubstructuresinthesolarneighbourhood.IV.

O Alpha
e]
El/F
[

Ba Eu
e]
El/F
[
[Fe/H] [Fe/H]
Fig.9.[El/Fe]ratioasafunctionof[Fe/H]intheGroup1stars(greendots)investigatedhere,Group2(bluetriangles,PaperIII),
Group3 (redsquares,PapersI andII),andcomparisonthick-discstars(purplediamonds).The literaturedata fortheMilkyWay
thick-discstarsareshownasagreydots.Theaveragedvaluesforα-elementsconsistofMg,Si,andCaabundances.
larmomentumofsuchagroupistoolowtoarisefromdynam- dancesasseeninthethinandthickdiscs(c.f.Soubiran&Girard
ical perturbations induced by the Galactic bar. More recently, 2005;Bensbyetal.2007,2014;Pakhomovetal.2011).
Gardner&Flynn (2010) and Monarietal. (2013) showed that Thus, the presented comparison of kinematic and chemi-
theGalacticlongbarmayproduceakinematicfeatureinveloc- cal compositionpatternsin the Galactic substructures,leads us
ityspacewiththesameparametersasoccupiedbytheArcturus to the conclusion that the origin of the GCS kinematic stellar
movinggroup. groupswas differentfromGalactic streams thatoriginatedin a
Another so-called AF06 stellar stream was discovered by courseoftheresonantinteractionsbetweenstarsintheouterdisc
Arifyanto&Fuchs (2006) while analysing the fine structure of andtheGalacticbar.
thephasespacedistributionfunctionofnearbysubdwarfsusing
dataextractedfromvariouscatalogues.Accordingtothediscov-
3.4.Origin
erers,AF06possiblyresemblestheArcturusstream.
Fig.11presentsaToomrediagramandvelocitydistributions Asampleof274starswasidentifiedasanoverdensityintheec-
of stars in the GCS kinematic groups and Hercules, Arcturus, centricityrange0.3<ǫ <0.5intheGeneva-Copenhagensurvey
and AF06 streams. This diagram shows that the kinematics of byHelmietal. (2006).Theauthorsprovidestatisticalevidence
stars in the GCS groups is quite different from the displayed thattheseoverdensitiesarereal,andthattheydonotresultfrom
streams,andonlyAF06partiallyoverlapsthepattern. a poor choice of the comparison model of the Galaxy or un-
A comparison of [El/Fe] ratios for α-elements between in- certaintiesofeccentricitydeterminations.Itwasfoundthatstars
dividual stars in the GCS groups and stars of the Arcturus, with these dynamicalcharacteristics do not constitute a homo-
AF06, and Hercules streams is presented in Fig.12. The av- geneous population. The metallicity distribution of the stars in
eraged values for α-elements consist of Mg, Si, Ca, and Ti thisoverdenseregionoftheAPL-spacevariedwitheccentricity
abundances. We took the elemental abundancesfor 33 stars of inadiscontinuousfashion.Thisallowedtheauthorstoseparate
theHerculesstreamfromSoubiran&Girard(2005) andfor35 these stars into three kinematic groups. These three groups of
stars from Bensbyetal. (2014). We took the elemental abun- starsaredissimilarnotonlyintheirmetallicitydistribution,but
dances for 18 stars of the Arcturus stream and for 26 stars of they also have differentkinematicsin the vertical(z) direction.
the AF06 stream from Ramyaetal. (2012). In Fig.12 we also TheGroup1velocitydispersionhasσ about28kms−1,thatof
z
showthechemicalevolutionmodeloftheGalacticthin-discby Group2about39kms−1,andthatofGroup3about52kms−1.
Pagel&Tautvaisˇiene˙ (1995)andasimplesecond-orderpolyno- Helmietal.(2014)determinedthedetailedchemicalcompo-
mialfittotheGCSkinematicgroupstars. sitionfor36starsoftheGCSGroup1,for22starsofGroup2,
The element-to-iron ratios in the GCS kinematic stellar and 14 stars of Group 3 located in the southern hemisphere.
groups lie higher than in the majority of stars belonging to In this study, they noticed a relatively sharp transition in dy-
Arcturus,AF06,andHerculesstreams.TheAF06grouphasthe namical and chemical propertiesthat occurs at a metallicity of
chemicalcompositionmostsimilartotheGCSstars.InPaperIII, [Fe/H] ∼ −0.4.Intheirsample,starswith[Fe/H] > −0.4have
we pointedoutthatthe Arcturusgrouphasthick-disckinemat- mostlylowereccentricities,smallerverticalvelocitydispersions,
ics,butithasseeminglythin-discabundances.Finally,asnoted are α-enhanced,and define a rather narrow sequence in [α/Fe]
previously,thestarsassociatedwiththeHerculesstreamdonot versus [Fe/H], clearly distinct from that of the thin disc. Stars
haveadistinctchemicalsignature,butshowamixtureofabun- with [Fe/H] < −0.4 have a range of eccentricities, are hotter
10