Search for Associations Containing Young stars (SACY): Chemical tagging IC 2391 & the Argus association

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by  G. M. De Silva

Mon.Not.R.Astron.Soc.000,000–000(0000) Printed28January2013 (MNLATEXstylefilev2.2)
Search for Associations Containing Young stars (SACY): Chemical
tagging IC 2391 & the Argus association⋆
G. M. De Silva1†, V. D’Orazi2,6, C. Melo3, C. A. O. Torres4, M. Gieles5,7, G. R. Quast4,
3
1 M. Sterzik3
0
2
1AustralianAstronomicalObservatory,105DelhiRd,NSW2113,Australia

n 2MacquarieUniversityResearchCentreinAstronomy,Astrophysics&AstrophotonicsNSW2109,Australia;
a DepartmentofPhysics&Astronomy,MacquarieUniversity,NSW2109,Australia
J 3EuropeanSouthernObservatory,Casilla19001,Santiago19,Chile
5 4Laborato´rioNacionaldeAstrof´ısica/MCT,37504-364,Itajuba´,Brazil
2 5InstituteofAstronomy,UniversityofCambridge,MadingleyRoad,CambridgeCB30HA,UK
6MonashCentreforAstrophysics,SchoolofMathematicalSciences,Building28,MonashUniversity,VIC3800,Australia
] 7DepartmentofPhysics,UniversityofSurrey,Guildford,GU27XH,UK
R
S
. 28January2013
h
p
-
o ABSTRACT
r We explorethe possible connectionbetweenthe opencluster IC2391andthe unboundAr-
t
s gusassociationidentifiedbythe SACY survey.In additionto commonkinematicsand ages
a betweenthesetwosystems,hereweexploretheirchemicalabundancepatternstoconfirmif
[
the two substructuresshared a commonorigin.We carry outa homogenoushigh-resolution

1 elementalabundancestudyof eightconfirmedmembersofIC 2391as wellas six members
v oftheArgusassociationusingUVESspectra.Wederivespectroscopicstellarparametersand
7 abundancesforFe,Na,Mg,Al,Si,Ca,Ti,Cr,NiandBa.
6 AllstarsintheopenclusterandArgusassociationwerefoundtosharesimilarabundances
9 with thescatterwellwithin theuncertainties,where[Fe/H]= −0.04± 0.03forclusterstars
5
and [Fe/H] = −0.06 ± 0.05 for Argus stars. Effects of over-ionisation/excitationwere seen
.
1 forstarscoolerthanroughly5200Kaspreviouslynotedintheliterature.Also,enhancedBa
0 abundancesofaround0.6dexwereobservedinbothsystems.Thecommonages,kinematics
3 and chemical abundances strongly support that the Argus association stars originated from
1 theopenclusterIC2391.Simplemodelingofthissystemfindthisdissolutiontobeconsistent
:
v withtwo-bodyinteractions.
i
X Keywords: (Galaxy):openclustersandassociations:general–(Galaxy):openclustersand
associations:individual:IC2391andArgusassociation–stars:abundances
r
a
1 INTRODUCTION population.
Thevastmajorityofstarsthatweobserveinanygivengalaxyare
Based on near-infrared studies of embedded clusters,
field stars, a complex mixture of stellar populations of different
Lada&Lada(2003)suggestthatlessthanabout10%oftheclus-
ages and metallicities. The present view is that star formation
tersformedinmolecularcloudssurvivelongerthan10Myr.They
taking place in molecular clouds gives rise to groups of stars
propose that this“infant mortality”of clustersisdue tofeedback
- stellar associations and open clusters. These groups of stars
of massive stars that expels the gas from the star forming region
must dissolve with time to connect star formation with the field
leaving the stars behind with super-virial velocities. For compact
systemsofafewhundredstarsthedynamicaltime-scaleisonlya
fewMyrs.Thismeansthatclusterexpansionandstarlossbytwo-
body relaxation is also a relevant process at these ages for such
⋆ BasedonobservationsobtainedattheEuropeanSouthernObservatory,
systems(Moeckeletal.2012;Gielesetal.2012).Onlongertime-
Paranal,Chile(ESOprogram082.C-0218andonobservationsmadeunder
scales(few100Myrs)variousprocessessuchastheGalactictidal
theON-ESOagreementforthejointoperationofthe1.52mESOtelescope
field and interactions with Giant Molecular Clouds (GMCs) be-
and at the Observato´rio do Pico dos Dias operated by MCT/Laborato´rio
NacionaldeAstrof´ısica(LNA/MCT),Brazil. come important. The paucity of open clusters in the solar neigh-
† E-mail:[email protected] bourhoodwithagesbetween0.5andafewGyrswasalreadynoted
(cid:13)c 0000RAS
2
inthelate50s(Oort1958).Spitzer(1958)analyticallyderivedthe 2 IDENTIFYINGTHEARGUSASSOCIATIONINTHE
cluster disruption times due to encounters with GMCs, showing SACYSURVEY
that such phenomena would account for the scarcity of Galactic
2.1 Theconvergencemethod
clustersolderthan109years.Amoredetailedtreatmentoftheprob-
lemtakingintoaccountotherdisruptiveagentshasbeencarriedout Usually we think about an association as being a group of stars
intheliterature(e.g.Lamers&Gieles2006). appearing concentrated in a small volume in space sharing some
Torresetal. (2006) reported the results of a high-resolution common properties such as age, chemical composition, distance
optical spectroscopic survey tosearchforassociations containing and kinematics. However, if such a group is close enough to the
young stars (SACY) among optical counterparts of ROSAT All- Sun,itsmemberswillappeartocoveralargeextentinthesky(as
SkyX-raysourcesinthesouthern hemisphere. Usingthemethod an example, Orion at 50 pc would cover almost the whole sky).
describedinTorresetal.(2006),Torresetal.(2008)identifiednine Thus, to find a group, projected spatial concentrations (i.e., in
newyoungassociations,namely,Chamaleontis(ChA),TWHydrae termsofrightascensionanddeclinationonly)andpropermotions
(TWA), β Pictoris, Octants (OctA), Tucana-Horologium (THA), maynotbeenough.Abettercriterionistolookforobjectssharing
Columba(ColA),Carina(CarA),Argus(ArgA),andABDoradus similar heliocentric space motions (UVW) all around the sky
(ABDA).Theagespanderivedbasedontheisochronefittingand (whereweuseUpositivetowardstheGalacticcenter,Vpositivein
the Li abundance analysis ranges from 6-70 Myr (daSilvaetal. thedirectionof Galacticrotation, and Wpositiveinthedirection
2009). oftheNorthGalacticPole).
Of relevance to the context of cluster disruption discussed Torresetal. (2006) describe in some detail the convergence
above, aretwoyoung associations identifiedintheSACYsurvey method developed to search for members of an association.
which appear to be associated with known open clusters. These This method examines the stars in the hexa-dimensional space,
are the Argus association and the open cluster IC 2391 and the UVWXYZ,asdefinedbythespacemotionsrelativetotheSunand
ǫ Cha Association and the η Cha cluster (Mamajeketal. 2000; the physical space coordinates centered on the Sun (XYZ, in the
Torresetal. 2008). In this paper we investigate in detail the same directions as UVW). We represent with m , M and M
v v v,iso
possibleconnectionbetweentheArgusAssociationandtheyoung the apparent visual magnitude, the resultant absolute magnitude
openclusterIC2391. withthedistanceobtainedfromtheconvergence method, and the
absolutemagnitudegivenbytheadoptedisochroneforthe(V−I)
C
Inparticular,wecantestforacommonoriginoftheassoci- stellarcolor;andµ ,µ andV arethepropermotionsandthera-
α δ r
ation and cluster by examining the stellar elemental abundances. dialvelocity.Inbrief,ifthereisnoreliabletrigonometricdistance
If the abundance analysis reveals high chemical homogeneity available1,theconvergencemethodfindsthedistance(d)foreach
among the association stars comparable to the open cluster starinthe sample that minimizes theF value of Equation 1. The
abundance patterns, it supports a common origin scenario where first term is a photometric distance modulus and the second is a
the Argus association stars were once bound members of open kinematicalone.Themethodneeds,asinput,anassumedageand
cluster IC 2391. This assumes that the proto-cluster gas cloud initial velocity values (U ,V ,W ) for the proposed association,
0 0 0
was sufficiently mixed hence all stars born from the same site and a cutoff value for F above which stars should be considered
shouldsharethesameelementalabundance pattern.Thisconcept spurious.Thiscutoffvaluevariesforeachassociationbutusually
ofelementalabundance matchingisoftenreferredtoas chemical we begin with 3.5 (this approximately means 0.7 magnitudes for
tagging (Freeman&Bland-Hawthorn 2002). Conversely if the thedistancemodulusand3kms−1forthevelocitymodulus).
abundances of the Argus association stars are found to be either
heterogeneous, matching the general disk stellar population (e.g. The method is iterative, and for each iteration a list of stars
theHerculesstream,seeBensbyetal.(2007))ortheydonotmatch withnew(U ,V ,W )isobtained.Theprocessendswhenthelist
0 0 0
theabundancepatternsofIC2391,thenitismorelikelythatthese ofstarsandthevelocities(U ,V ,W )donotchangesignificantly.
0 0 0
aretwoseparatesystemseveniftheysharethesamekinematics.
Given that disk dynamics make the kinematical information F(mv,µα,µδ,Vr;d)=
somewhat unreliable to explore the history of these stellar struc- [p×(M −M )2+(U−U )2+(V−V )2+(W−W )2]1/2(1)
v v,iso 0 0 0
tures, for the first time we carry out a high resolution elemental
wherepisaconstantweightingtheimportanceoftheevolutionary
abundance analysis of the Argus association in a homogeneous
distancewithrespecttothekinematicdistance.Usually,ifthereare
manner withstars inIC 2391 in order to carry out chemical tag-
several starswithtrigonometric parallaxes among thecandidates,
ging.Thiswillalsobethefirstattemptatchemicaltaggingayoung
weuse p=0.Butiftherearenostarswithreliableparallaxes,asis
stellarsystem.Allpublishedexamplesofchemicaltagging inthe
thecasefortheIC2391,themethodmaynotconverge,soweused
diskareforstellarsystems olderthantheHyades (Bubar&King
p=20forthiscase.
2010;DeSilvaetal.2007b).
We also define a membership probability to verify the com-
InSection2,webrieflydescribetheSACYsurveyandpresent
pactnessoftheassociationin7dimensions,namelyUVWXYZand
space velocities and spatial locations of the stars in the two sys-
(M −M ).Normalizedvaluesofthesevariablesarecomputedby
tems.InSection3wepresentadetailedelementalabundancestudy v v,iso
subtractingtheirmeanvaluesanddividingbytheirdispersions.A
of stars in both the Argus association and open cluster IC 2391.
In Section 4 we discuss our results in terms of chemical tagging
thetwostellarsystems.Weconcludewithadiscussionofpossible
disruptionmechanismsgiventheevidencegatheredintheprevious 1 Weconsiderthetrigonometricparallaxesasunreliableiftheyhaveerrors
sections. largerthan2mas.
(cid:13)c 0000RAS,MNRAS000,000–000
3
Figure1.CombinationsoftheUVWXYZ–spacederivedbasedontheconvergencemethodfortheArgusAssociationstars(redsquares),theopencluster
IC2391(bluetriangles)andwhenthetwosystemsareconsideredasasingleassociation(graycircles).Welldefinedclusteringcanbeseeninbothkinematical
andspatialcoordinates,withnosignificantdifferencewhenconsideringthetwostellarsystemindependentlyortogether.
quantityKisobtainedasthequadraticsumofthesedimensionless
variablesandtheprobabilityiscomputedas
P=1−(erf(k))7 (2)
whereerf istheerrorfunction.
Finally,wealsotakeintoaccounttheLicontentofthecandi-
datestarstoseeifitiscompatiblewiththeLidepletionforitsage
(Neuha¨user1997).
(cid:13)c 0000RAS,MNRAS000,000–000
4
Table1.RevisedlistofhighprobabilitymembersfortheArgusassociation1
Name α δ µα µδ Vr Vsini Pphot VJ V-IC SpT ELi πkin (U,V,W) (X,Y,Z) Prob notes
2000 2000 masyr−1 kms−1 days mÅ mas kms−1 pc
(*)CD-292360 053459.2 -295404 17.1 33.1 26.0 3.519 10.53 1.11 K3Ve 180 14.8 -22.7-16.6-5.2 -34.9 -47.9-32.4 0.72 K
(*)APCol 060452.2 -343336 27.3 340.9 22.4 11 12.96 3.36 M4Ve 280 119.2 -22.0-13.6-4.4 -3.7 -6.7 -3.4 0.90 1
CD-561438 061153.0 -561905 -2.5 38.3 14 130 0.418848 11.32 0.93 K0V 230 8.7 -21.9-11.2-5.1 -9.1-101.3-53.3 0.68 K
CD-283434 064945.4 -285917 -11.3 20.8 26.8 6.4 3.823 10.62 0.85 G7V 230 9.8 -23.1-16.3-7.0 -51.2 -85.3-23.1 0.45 K
CD-422906 070153.4 -422756 -13.8 35.5 23.6 10.8 3.96 10.64 1.02 K1V 275 10.7 -22.8-16.8-6.3 -26.5 -85.8-26.0 0.75 K
CD-482972 072822.0 -490838 -26.7 44.4 21.1 52.0 1.0373 9.85 0.86 G8V 250 12.6 -22.3-16.6-7.3 -11.9 -76.0-20.0 0.88 K
(*)HD61005 073547.5 -321214 -55.7 74.6 22.5 8.2 5.04 8.22 0.80 G8V 171 28.3 -23.0-14.1-4.4 -14.1 -32.2 -3.5 0.73 H,2
CD-483199 074726.0 -490251 -22.4 36.1 18.5 24.7 2.192 10.56 0.86 G7V 230 10.0 -22.2-15.1-5.0 -12.9 -96.7-20.2 1.00 K
CD-433604 074849.8 -432706 -26.8 39.6 21.4 40.4 0.4706 11.13 1.40 K4Ve 320 12.2 -22.6-16.4-4.8 -17.5 -79.1-12.6 0.82 K
TYC8561-0970-1 075355.5 -571007 -18.3 26.9 15.8 5.0 11.50 K0V 210 7.0 -21.9-14.7-6.5 0.6-139.0-36.4 0.98 T
HD67945 080938.6 -201350 -37.8 20.4 22 120 8.08 F0V 0 14.3 -22.1-13.4-4.2 -34.7 -60.0 8.5 0.84 T,SB2?
CD-582194 083911.6 -583428 -32.7 35.4 15.6 85.0 1.240 10.18 0.83 G5V 270 9.8 -21.9-16.7-5.4 8.3-100.3-18.3 0.98 K
CD-572315 085008.1 -574559 -34.1 31.0 11.7 52.0 10.21 K2Ve 308 9.3 -22.1-13.0-5.5 9.2-105.8-16.2 0.90 T
TYC8594-0058-1 090203.9 -580850 -24.2 26.2 12.1 34.1 11.30 0.83 G8V 300 7.2 -22.2-14.4-2.4 15.2-137.7-18.7 0.95
CD-621197 091330.3 -625909 -34.1 27.4 12.7 84.0 0.54841 10.46 0.93 K0V(e) 280 8.4 -21.7-16.3-5.8 22.1-114.9-20.4 0.91 K
TYC7695-0335-1 092854.1 -410119 -28.7 13.6 14 120 0.3917 11.65 1.10 K3V 300 6.7 -22.0-13.6-5.3 -7.9-147.3 18.8 0.87 K
(*)BD-202977 093951.4 -213417 -50.0 7.8 18.3 10.1 10.22 G9V 260 11.3 -22.3-16.5-4.7 -21.9 -79.0 34.4 0.58 T
TYC9217-0641-1 094247.4 -723950 -30.1 18.9 6.7 23.4 2.3026 12.30 1.11 K1V 240 6.5 -22.1-13.6-6.2 50.4-139.6-39.1 0.88 K
CD-395833 094719.9 -400310 -40.6 16.3 15.0 10.5 10.89 K0V 260 9.0 -22.2-15.7-4.6 -2.1-109.5 20.0 0.92 T
HD85151A 094843.2 -445408 -68.4 33.5 14.3 9.61 G7V 220 15.3 -22.3-15.8-3.9 2.5 -64.8 7.7 0.92 T
HD85151B 094843.4 -445409 -68.4 33.5 13.4 10.21 G9V 250 15.3 -22.3-14.9-4.0 2.5 -64.8 7.7 0.84 T
CD-65817 094909.0 -654021 -32.3 19.9 7.3 19.2 2.7388 10.17 0.75 G5V 200 7.1 -22.4-13.1-4.8 37.5-134.5-22.5 0.97 T,K,D2.0”
HD309851 095558.3 -672122 -42.4 25.7 6.6 19.8 1.8164 9.90 0.68 G1V 170 9.2 -22.5-13.2-4.5 31.8-102.6-19.0 0.98 K
HD310316 104956.1 -695122 -43.5 13.5 4.7 16.3 3.625 10.07 0.78 G8V 224 8.2 -22.4-13.6-5.2 46.5-110.7-19.9 0.97 K,D0.6”
CP-691432 105351.5 -700216 -32.6 12.2 3.6 55.0 1.0305 10.66 0.71 G2V 195 6.3 -22.8-13.1-3.3 61.8-144.6-26.1 0.64 K
(*)CD-427422 120632.9 -424751 -50.3 -2.5 3.3 28.3 1.984 10.66 0.92 K0V 260 8.9 -21.6-15.4-4.8 43.4 -96.5 37.1 0.50 K
CD-74673 122034.4 -753929 -109.4 4.2 2.7 7.3 3.477 10.72 1.17 K3Ve 230 19.5 -21.7-15.5-2.8 25.8 -42.9-11.5 0.75 K,3,4
CD-75652 134912.9 -754948 -62.2 -31.5 -0.9 20.1 2.2743 9.67 0.76 G1V 200 12.3 -22.3-13.6-6.0 47.0 -63.4-18.8 0.93 K
HD129496 144621.4 -674616 -47.6 -39.7 -4.8 88.0 8.78 0.60 F7V 150 11.1 -22.5-13.6-5.9 61.6 -64.9-11.5 0.79
NYAps 151223.4 -751516 -73.9 -73.1 3.4 10.8 4.084 9.42 0.86 G9V 182 19.9 -21.2-12.6-4.3 32.3 -36.3-12.9 0.62 K,H
(*)HD145689 161705.4 -675629 -50.0 -84.0 -9 106.4 5.95 0.18 A6V 19.2 -22.7-11.7-3.7 39.2 -32.4-11.2 0.68 H,5,6,7
CD-529381 200723.8 -514727 85.9 -143.9 -13.3 42.0 0.8368 10.59 1.52 K6Ve 60 33.6 -22.4-14.8-4.2 24.4 -5.7-16.0 0.80 K
c(cid:13) 1Datainitalics arefromtheliterature asfollows:H =datafromHipparcos; T =datafromTYCHO;K=datafromKiraga(2012);(1)Riedeletal.(2011);(2)Desideraetal.(2011a);(3)SB1,Porb=613.9d
00 Guentheretal.(2007);(4)Covinoetal.(1997);(5)Browndwarfat6.7”Hue´lamoetal.(2010);(6)Zuckermanetal.(2011);(7)D´ıazetal.(2011).
0
0 Entriesmarked(*)arenewinclusionspreviouslynotlistedbyTorresetal.(2008).
R
A
S
,
M
N
R
A
S
0
0
0
,
0
0
0
–
0
0
0
c(cid:13)
0 Table2.RevisedlistofhighprobabilitymembersproposedfortheopenclusterIC23911
0
0
0
RA Name α δ µα µδ Vr Vsini Pphot VJ B-Vor SpT ELi πkin (U,V,W) (X,Y,Z) Prob notes
S, 2000 2000 masyr−1 kms−1 days V−IC mÅ mas kms−1 pc
M
N
R (*)PMM5314 082834.6 -523704 -24.1 22.7 13.4 63.0 10.40 0.45 7.1 -21.7-12.2-6.3 -2.5-138.5-19.6 0.92
A PMM7422 082845.6 -520527 -22.6 24.6 14.8 33.0 1.515 10.49 0.69 G6 233 7.2 -22.1-13.7-4.8 -3.5-137.1-18.6 1.00 K
S
0 PMM7956 082951.9 -514040 -23.0 18.8 15.3 13.8 10.62 0.98 e 289 6.5 -21.3-13.8-7.7 -4.6-152.7-19.7 0.71
0
0, PMM6974 083418.1 -521558 -22.6 23.0 15.2 5.2 7.80 12.26 1.04 74 6.9 -22.1-14.4-5.1 -2.1-143.5-18.0 1.00 1
0
0 PMM4280 083420.5 -525005 -21.0 22.5 12.3 16.0 10.34 0.67 G5 151 6.6 -22.1-11.7-4.3 -0.9-151.0-19.8 0.90
0
–0 PMM6978 083501.2 -521401 -23.3 22.3 15.2 7.1 5.1 12.07 1.02 179 6.9 -22.0-14.3-5.7 -2.0-143.6-17.7 1.00 1
0
0 PMM2456 083543.7 -532120 -23.6 24.3 14.8 48.0 12.20 0.92 K3e 301 7.2 -22.1-14.3-5.0 0.5-138.0-18.4 1.00
PMM351 083624.2 -540106 -24.4 20.7 15.4 90.0 1.923 10.18 0.57 G0 90 6.8 -21.6-14.8-7.3 2.0-146.2-20.3 0.92 1
PMM3359 083655.0 -530834 -23.1 23.6 14.7 8.2 3.840 11.51 0.76 226 7.0 -22.1-14.2-4.9 0.3-141.6-18.2 1.00 1
PMM5376 083702.3 -524659 -20.8 20.9 15.6 9.8 14.30 1.37 e 0 6.3 -22.1-15.0-5.1 -0.4-158.2-19.7 1.00
PMM4324 083747.0 -525212 -22.4 22.0 14.3 41.0 9.66 0.53 F5V 97 6.7 -22.1-13.8-5.2 0.0-148.9-18.5 0.97
PMM665 083751.6 -534546 -21.1 22.7 14.5 7.9 4.510 11.35 0.75 G8e 189 6.5 -22.2-14.3-4.4 1.9-151.7-20.2 1.00 1
PMM4336 083755.6 -525711 -23.0 24.9 15.6 8.0 0.371 11.55 0.98 G9 7.2 -22.2-15.2-4.3 0.1-137.9-17.2 1.00 1,2
PMM4362 083822.9 -525648 -23.5 22.4 15.4 9.0 3.93 10.95 0.74 191 6.9 -22.0-14.7-5.6 0.2-144.1-17.8 1.00 1,2
PMM4413 083855.7 -525752 -22.3 21.6 14.4 8.6 5.05 10.31 0.74 62 163 6.6 -22.1-13.9-5.2 0.4-150.9-18.5 1.00 1,2,3
PMM686 083922.6 -535506 -21.3 21.7 15.2 12.7 4.41 12.63 1.04 e 215 6.4 -22.1-15.1-4.9 2.6-155.1-20.4 1.00 1
PMM4467 083953.0 -525757 -21.1 19.3 15.2 12.6 3.85 11.86 0.91 K0(e) 190 6.1 -21.9-14.7-5.9 0.7-164.0-19.8 1.00 1,2,4
PMM1083 084006.2 -533807 -20.0 23.4 12.2 67.9 1.333 10.45 0.69 G0 162 6.5 -22.2-12.2-2.9 2.1-152.6-19.4 0.92 1,2,4
PMM8415 084016.3 -535629 -25.7 24.9 15.7 20.7 11.84 0.94 G9(e) 302 7.6 -22.1-15.3-5.2 0.6-131.2-15.7 1.00 2,4
PMM1759 084018.3 -533029 -26.2 21.2 13.5 3.8 13.54 1.53 K4e 55 7.1 -21.6-13.0-7.0 1.8-139.1-17.4 0.94 2,4
PMM1142 084049.1 -533745 -18.4 23.6 14.0 7.3 11.08 0.80 G1 6.3 -22.1-14.2-2.0 2.4-156.4-19.6 0.91 2
PMM1174 084122.7 -533809 -23.6 22.7 13.5 60.0 9.54 0.43 F3V 79 6.9 -22.1-13.3-5.1 2.3-144.2-18.0 1.00
PMM4636 084157.8 -525214 -24.0 23.9 13.6 4.9 5.80 13.57 1.55 K7e 100 7.2 -22.2-13.3-4.5 0.9-138.8-16.0 1.00 1,2,4
PMM756 084300.4 -535408 -23.5 20.4 15.9 16.5 3.14 11.16 0.68 G9 217 6.5 -21.8-15.7-6.4 3.4-152.1-18.9 0.98 1
PMM5811 084317.9 -523611 -21.3 21.6 14.3 59.0 9.16 0.37 F2V 107 6.4 -22.2-14.0-4.2 0.7-155.0-17.0 0.97
PMM2888 084352.3 -531400 -25.3 20.2 15.0 66.5 9.76 0.56 F5 6.9 -21.6-14.6-7.0 2.1-144.7-16.6 0.96 2
PPM2012 084359.0 -533344 -24.7 25.1 13.9 17.4 2.210 11.67 0.90 K0(e) 251 7.4 -22.2-13.9-4.3 2.6-134.5-15.9 1.00 1,2,4
PMM4809 084405.2 -525317 -21.3 19.7 13.8 17.5 2.600 10.85 0.75 G3(e) 175 6.1 -22.1-13.5-5.2 1.6-162.6-18.0 1.00 1,2
PMM1373 084410.2 -534334 -23.3 24.4 14.6 7.0 5.38 12.25 0.96 156 7.1 -22.2-14.7-4.0 3.1-140.5-16.8 1.00 1
PMM5884 084426.2 -524232 -21.4 17.5 14.5 13.9 3.03 11.46 0.84 G9(e) 225 5.8 -21.8-14.1-6.5 1.4-170.2-18.4 0.98 1,2,4
PMM4902 084526.9 -525202 -25.2 23.4 14.7 7.7 4.820 12.76 1.24 K3e 204 7.2 -22.1-14.5-5.1 1.6-137.5-14.8 1.00 1,2,4
PMM6811 084539.1 -522600 -24.4 24.8 17.4 90.0 0.653 9.91 0.60 F8) 137 7.3 -22.2-17.2-4.2 0.9-135.4-13.9 0.96 2
PMM2182 084548.0 -532551 -22.4 22.2 14.7 78.0 1.437 10.22 0.57 187 6.6 -22.2-14.7-4.4 3.1-150.4-17.0 1.00 1
1DatamainlyfromPlataisetal.(2007);thoseinitalicsarefromotherliteraturesourcesasindicated(forpropermotions,datainitalicsarefromPlataisetal.(2007),otherwisefromUCAC4;forcolorindices,data
initalicsrepresentV-IC.):K=datafromKiraga(2012);(1)Desideraetal.(2011b);(2)Patten&Simon(1996);(3)SB2,Porb=90.617dPlataisetal.(2007)(4)Randichetal.(2001).
Theentrymarked(*)isanewinclusionpreviouslynotlistedbyTorresetal.(2008).
5
6
2.2 KinematicsandagesofIC2391andtheArgus
Association
TheArgusassociationwaseasilydiscoveredintheSACYsurvey
due to its special U velocity of U = −22.0km s−1 as illustrated
in Figure 2 of Torresetal. (2008). The evidences presented in
Torresetal. (2008) strongly suggested that the Argus association
andIC 2391 haveacommon origin. Wehaveupdated theSACY
catalog in order to be able to test if members proposed by
Desideraetal.(2011a),Zuckermanetal.(2011), andRiedeletal.
(2011)couldbeconvergedintotheIC2391andArgusassociation
memberlist.Newlyobtaineddatapresentedinthispaperwerealso
used.
The convergence was done using the new reduction of
Hipparcos (vanLeeuwen 2007), the new proper motion catalog
UCAC4(Zachariasetal.2012)andtheASASphotometricsurvey
(Kiraga2012),forVandV-Icolors,forstarsnotobservedby us.
Figure2.TheHRdiagramofthemembersproposedfortheArgusAsso-
We did not change the published proper motions for stars in the
ciation. Bluetriangles arethe ArgAfield members whereas the IC2391
Hipparcos catalogue, and for double stars in UCAC4 with sepa-
members are represented by red triangles. The overplotted isochrones
rations between about 1′′ to 10′′ because the image de-blending
(Siessetal.2000)aretheonesfor20(dot-dashed),30(solid)and40Myr
process in UCAC4 seems unreliable. As we did in (Torresetal.
(dashed).
2008), we correct the photometric values for duplicity where
possible.NoteforthepairHD85151A/B, therearenoindividual
propermotionsinTycho-2.Thereforeweusedtheprimaryproper
the IC 2391 members only (blue triangles), with those obtained
motions for both stars. Our spectroscopic observations and the
whenthetwogroupsareconsideredasonesingleassociation(gray
visual binary measurements from the Washington Double Star
circles).Nosignificantdifferenceisfound.
cataloguesuggesttheymustbeaphysicalpair.
Also as shown in Table 3 no significant difference is found
ThedataforIC2391membersaremainlyfromPlataisetal.
intheagesofthetwogroups.Figure2showsthattheposition of
(2007) butusingUCAC4propermotions, except fordouble stars
theArgusassociationintheHRdiagramisconsistentwithaunique
where we use Plataisetal. (2007) and Torresetal. (2008) proper
isochroneof30Myr±10Myr,whichiscomparabletoourobtained
motionsduetoimagede-blendingissueswithUCAC4asexplained
age of about 26 Myr. Ourages are fromthird degree polynomial
above.Thereare41clusterstarswithallkinematicaldata. Except
isochornecurves(Torresetal.2008)basedonsomewellobserved
for PMM 4413 whose systemic velocity is known, all other 20
stargroupsandcomparedpartiallywithSiessetal.(2000).Asthey
knownspectroscopicbinarieswereexcludedfromthestudy.
arenot strictlycalibrated, we must take these heuristicages with
caution, but of course the relative ages are more reliable. These
InTable1andTable2welistthehighprobability members results imply that both samples have the same age. Additionally,
proposed for the Argus association stars and the open cluster daSilvaetal.(2009)computedLiabundancesforallSACYasso-
IC 2391 respectively, obtained by applying the convergence ciationsandshowedthattheLithiumDepletionPatternfortheAr-
methodforbothassociationandclusterstarstogether.Wepresenta gusassociation(definedbyfittingtheLiabundance asafunction
totalof65highprobabilitymemberswhichcompriseof32Argus ofeffectivetemperature),fitswellinatimesequenceoftheSACY
associationstarsand33IC2391members.Weacceptprobabilities associationswithnodifferencebetweentheArgusassociationstars
down to about 50 percent as the stellar system is formed by the andtheIC2391members.
concentratedIC2391clusterandthewidelyscatteredassociation
stars around it, thus forming a spatial distribution that is far
from gaussian. When we apply the convergence criteria on the
2.3 Commentsonexcludedstars
associationstarsalone(withoutincludingIC2391members),then
theprobabilitiesoftheassociationmembersincreasesignificantly Table 1 and Table 2 are not the same as those presented in
withthelowestmembershipprobabilitybeingabout70percent.) Torresetal. (2008), as we used updated data and applied a new
statisticalmodel.Belowwecommentontheexcludedstars.
In order to test the true kinematical connection between
IC 2391 and the Argus association stars, we also applied our FortheArgusassociationsample,3starswereexcludedfrom
convergence test to the two groups independently. The results the list of Torresetal. (2008). Star BW Phe was eliminated on
are presented in Table 3 and in Figure 1. 1 The convergence kinematical grounds, starCD-49 1902 was eliminated forhaving
method resulted in a near perfect match between the kinematics too large value in Z and star HD 84075 has a U velocity well
of IC 2391 stars and those of the Argus association stars from outsideoftheArgusassociationrange.
Torresetal. (2003). Thevelocitiesof thetwosamples agree well
withintheerrors.Togiveabetterideaofthevariationsinthefinal FortheIC2391sample,3starswereexcludedfromthelistof
UVWXYZvalueswhentheArgusassociationstarsareconsidered Torresetal. (2008). PMM 1560 and PMM 1820 were eliminated
together with the IC 2391 members, we have over-plotted the due to low membership probabilities based on kinematics. For
results obtained for the Argus field stars only (red squares), and PMM 3695 the proper motions are significantly distinct between
(cid:13)c 0000RAS,MNRAS000,000–000
7
Table3.ConvergencemethodresultsfortheArgusfieldstarsandthemembersofopenclusterIC2391.
Sample Distance[mas] U[km/s] V[km/s] W[km/s] N.members Age[Myr]
Argusonly(field) 15.5±19.9 -22.6±0.4 -14.6±1.5 -5.0±1.2 32 26
IC2391 6.9±0.4 -21.7±0.6 -14.2±1.0 -5.1±1.2 33 26
Argus&IC2391 11.2±14.5 -22.1±0.4 -14.4±1.3 -5.0±1.2 65 26
Plataisetal.(2007)andUCAC4,forunclearreasons. lineslist,atomicdataadoptedandmeasuredequivalentwidthsofa
samplestar(PMM3359)canbefoundinAppendixA.
3 ELEMENTALABUNDANCES
We derive the stellar parameters based on spectroscopy. Abun-
3.1 Highresolutionobservations dancesforallFeiandiilineswerecomputed fromthemeasured
EWs. Effective temperature (T ) was derived by requiring exci-
Inordertofurtherexploretheconnectionbetweentheopencluster eff
tation equilibrium of the Fe i lines. Microturbulence was derived
IC 2391 and the Argus association, we examine their elemental
from the condition that abundances from Fe i lines show no
abundances. High resolution and high signal-to-noise spectra of
trendwithEW.Surface gravity(logg) was derivedviaionisation
members of the Argus association and IC 2391 were observed
equilibrium,i.e.requiringtheabundancesfromFe ilinestoequal
usingVLT-UVESintheframeworkofprogramID082.C-0218(PI
those from Fe ii lines. For stars cooler than 5500 K, we adopted
Melo). Observations were undertaken in service mode using the
alogg value of 4.5, which isconsistent withthe cluster age. We
UVESDIC#1(390+580nm) standardsettingwitha0.8arcsecslit
did not attempt to optimize logg via ionisation equilibrium for
to achieve a spectral resolving power of R=60,000. The typical
the cool stars because of the known over-ionisation effects (e.g.,
S/N ratio was ∼100 per pixel at 600 nm. The data were reduced
Schuleretal. (2010), see Section 3.4 for further discussion). The
withthelatestUVESESO-MIDASpipeline.Theresultingspectra
finaladoptedstellarparametersandtheelementalabundancesare
werenormalisedusingthecontinuumtaskintheIRAF2package.
giveninTable4.
Wecarry out a detailedelemental abundance analysis of six
Argus association stars and eight IC 2391 stars with vsini ≤ 10
kms−1. The other stars showed broadened spectral lines which
3.3 Errorbudget
meanttherewasgreaterblendingbetweenthelines,makingindi-
viduallineanalysismoreuncertain.Table4liststhesampleofstars Chemical abundances are mainly affected by two kinds of un-
analysedinthisstudytogetherwiththeirmeasuredproperties. certainties, namely (i) errors due to the EW measurements or to
the best-fit determination (in the case of spectral syntheses), and
(ii) errors due to the stellar parameters (T , logg, and ξ). The
eff
3.2 Abundanceanalysis impact of theuncertainties on the atomicparameters, i.e. loggf,
should be instead almost negligible since our analysis is strictly
Theelementalabundanceswerederivedbasedonequivalentwidth
differential withrespect tothe Sun(seeSection 3.2). Concerning
(EW) measurements and spectral synthesis, making use of the
the EW analysis, random errors in the [Fe/H] ratios due to EW
latest versionof the MOOG code (Sneden1973).TheEWswere
measurementsarewellrepresentedbythestandarddeviation(rms)
measured using the automated ARES code (Sousaetal. 2007)
from the mean abundance based on the whole set of lines, while
withfrequent manual checking of theEWs.Ba abundances were
the ones for the [X/Fe] ratios were computed by quadratically
derived from the 5853 Å line (which is known to be strong,
addingthermsfor[Fe/H]andrmsfor[X/H].Errorsduetostellar
isolated, un-blended and not affected by NLTE effects, e.g.,
parameterswereestimatedbyvaryingoneparameteratatime,and
Mashonkinaetal. (2007)) using the driver synth in MOOG. We
checking the corresponding variation in the resulting abundance.
retrieved hyperfine structure data from McWilliametal. (1995),
We adopted variations of ±50 K in T and ±0.15 kms−1 in ξ,
adopting anisotopicsolarmixtureof81%(134Ba+136Ba+138Ba) eff
and19%(135Ba+137Ba).AnexampleofthesynthesisoftheBaii becauselargerchanges inthosequantitieswouldhaveintroduced
asignificanttrendoflog n(Fe)vstheexcitationpotentialsandthe
lineat5853ÅisprovidedinFigure3foroneofoursamplestars
linestrength, respectively. Forstarsfor which wecould optimise
(PMM3359).
thesurfacegravities,theuncertaintiesinlog gwereestimatedby
varying this quantity until the difference between log n(Fei) and
Interpolated Kurucz model atmospheres based on the AT-
log n(Feii) is larger than 0.1 dex, i.e. the ionisation equilibrium
LAS9code(Castellietal.1997)withnoconvective overshooting
conditionisnolongersatisfied.Thetypicalerrorinloggwasfrom
were used throughout this study. We carried out the abundance
0.1 to 0.15. The final errors are then obtained by quadratically
analysis relative to the solar spectrum, taken with the same
summingerrorsduetotheEWmeasurementsandtheonesdueto
instrument and the same resolution of our sample stars. The full
stellarparameters(seeTable4).
2 IRAF is distributed by the National Optical Astronomy Observatory, Focusingonabundancesderivedfromspectralsyntheses,we
whichisoperatedbytheAssociationofUniversitiesforResearchinAstron- have errors due to the best-fit determination of typically 0.1 dex,
omy,Inc.,undercooperativeagreementwiththeNationalScienceFounda- reflectinguncertaintiesduetothecontinuumplacementandtothe
tion. strongnatureoftheBafeatures;similarly,sincewearedealingwith
(cid:13)c 0000RAS,MNRAS000,000–000
8
Figure3.ExampleoftheBasynthesisforstarPMM3359,withsolidredlineshowsthebestfit.
averystrongtransitionline,theerrorsduetostellarparametersare this phenomenon, being about 0.2−0.3 dex lower in the cooler
dominatedbythemicroturbulencevalues(∆[Ba/Fe]=0.05-0.07dex stars; similarly, Al and Cr (from the Cr i lines) seem to show
forξchangesof0.15kms−1),whiletheeffectivetemperaturesand the same effect, although to a lesser extent. Because we could
gravitieshaveaminorimpact,i.e.within0.03dex. alsomeasure abundances forCrandTi fromtheirfirst-ionisation
stage transitions, we can confirm that we are observing an over-
ionisation phenomenon: as can be seen in Table 4, abundances
3.4 Abundanceresults from Ti ii and Cr ii lines result in larger values for the cooler
stars,wherethe[X/Fe]ratiosareenhanced upto0.4dex(seethe
FinalabundancesarereportedinTable4,wherestellarparameters
[Crii/Fe] inthemost extremecases,e.g.,PMM 4902 inIC2391
(T , logg, ξ) and [X/Fe] ratios along with the corresponding
eff or TYC 8561-0907-1 in Argus association). This fact reflects
errorsareprovided foralloursample stars.InFigure4weshow
a pumping of the electrons to the ionised stage because of the
the [X/Fe] ratios as a function of the effective temperatures for
strongUVfluxcomingfromthehotchromospheres(wereferthe
bothIC2391(filledcircles)andtheArgusassociation(triangles).
reader to Schuleretal. (2003, 2010); D’Orazi&Randich (2009)
Thefirstevidencecomingoutfromtheseplotsisthatweconfirm
for a more detailed discussion on this topic). Si deserves special
previousfindingsbySchuleretal.(2003,2004);Shenetal.(2005);
attentionsincewefoundanover-excitationeffect(alltheemployed
D’Orazi&Randich (2009); Schuleretal. (2010); Biazzoetal.
lines have high excitation potentials, i.e. 4.92 < χ <5.98 eV)
(2011): over-ionisation/excitation effects are at work for stars
acting on the cooler stars, that was also previously detected by
cooler than T .5200 K. The photospheric abundances of Ca,
Na, andTi (froeffmthe Ti i lines)show the most dramaticeffect of Schuleretal. (2003) in M34, but not by D’Orazi&Randich
(cid:13)c 0000RAS,MNRAS000,000–000
9
(2009).Over-ionisation/excitation effectsarealsoobserved inthe
cool field stars (see e.g., AllendePrietoetal. (2008)). Note that
no temperature effects were observed for the Ni abundances in
eithertheIC2391ortheArgusassociationstars.Therefore theNi
abundanceisnotplottedinFigure4.
Due totheover-ionisation effects, themean abundances and
the associated rms error were computed considering all the stars
for Fe i, Mg, Ni, and Ba, while we considered only the warmer
stars (T > 5200K) when averaging [X/Fe] ratios for Na, Al, Si,
eff
Ca, Ti and Cr (see Table 4). We found average metallicity of
[Fe/H]=−0.04±0.03 and [Fe/H]=−0.05±0.04 for IC 2391 and
Argus association, respectively: the cluster and the association
exhibitasolarmetallicity,withnoevidenceofinternalscatter(the
rms is significantly smaller than the observational uncertainties).
Moreover,alltheotherα(Mg,Si,Ca,andTi),odd-Z(Na,Al),and
iron-peakelementsshowsolarratios,pointingoutthatthe cluster
andtheassociationareindistinguishableconcerningtheirchemical
composition (see Section 4 for a discussion on the scientific
implicationofourresults).
Ba is notably over-abundant, with a mean of
[Ba/Fe]=0.62±0.07 (IC 2391) and [Ba/Fe]=0.53±0.08 (Ar-
gus association). The slightly higher variation across the two
stellar aggregates mainly reflect the larger uncertainties affecting
Baabundances.TheBalinefeatureisclosetothesaturationregime
inthecurveofgrowth,andasaconsequenceitishighlysensitive
to the adopted microturbulence values. The extremely high Ba
contentthatweobtainedforoursamplestarsisnotsurprisingand
confirms previous measurements (see Section 4). We stress, in
passing,thatover-ionisationeffectscanberuledoutasapossible
explanationgiventhat,attheeffectivetemperaturesofoursample
stars,Baisalmosttotallyionised.FurtherthepossibilityofNLTE
effectsisminimalasthe5853lineusedinthisstudyisnotknown
tobeaffectedbyNLTEeffects(Mashonkinaetal.(2007)).
(cid:13)c 0000RAS,MNRAS000,000–000
1
Table4.Stellarparametersandabundances(seesection3.4fordetailsonthecomputationoftheaveragevalues) 0
star RV vsini Teff logg ξ [Fei/H] [Feii/H] [Na/Fe] [Mg/Fe] [Al/Fe] [Si/Fe] [Ca/Fe] [Tii/Fe] [Tiii/Fe] [Cri/Fe] [Crii/Fe] [Ni/Fe] [Baii/Fe]
kms−1 kms−1 (K) kms−1
IC2391
PMM4902 14.7 7.10 4440 4.5 1.3 −0.03±0.12 ....... −0.25±0.15 −0.06±0.09 −0.08±0.06 0.09±0.08 −0.25±0.11 −0.32±0.17 0.12±0.14 −0.07±0.15 0.41±0.23 0.03±0.09 0.65±0.15
PMM6974 15.1 4.99 4600 4.5 1.2 −0.06±0.08 ....... −0.21±0.14 −0.06±0.09 −0.09±0.04 0.17±0.09 −0.29±0.10 −0.33±0.09 0.14±0.15 −0.14±0.13 0.32±0.14 0.03±0.10 0.60±0.15
PMM6978 15.4 6.77 4600 4.5 1.1 −0.01±0.10 ....... −0.26±0.13 −0.07±0.05 −0.12±0.09 0.12±0.10 −0.29±0.11 −0.31±0.13 0.11±0.13 −0.10±0.09 0.22±0.18 0.03±0.09 0.65±0.15
PMM1373 13.9 6.68 4750 4.5 1.2 −0.05±0.10 ....... −0.16±0.12 −0.11±0.09 ....... 0.10±0.09 −0.19±0.09 −0.27±0.15 0.15±0.15 −0.03±0.07 0.44±0.22 0.02±0.10 0.65±0.15
PMM3359 14.7 7.88 5380 4.5 1.2 0.00±0.10 ....... 0.02±0.08 0.01±0.07 −0.01±0.07 −0.05±0.08 0.08±0.10 −0.03±0.12 0.04±0.11 0.00±0.09 0.09±0.12 −0.03±0.11 0.65±0.15
PMM665 14.6 7.47 5550 4.5 1.6 −0.07±0.08 −0.11±0.12 0.05±0.08 0.05±0.09 0.09±0.04 0.07±0.07 0.08±0.09 0.07±0.09 0.04±0.14 0.03±0.09 0.08±0.09 0.03±0.09 0.46±0.15
PMM4362 15.1 8.61 5650 4.3 1.4 −0.05±0.10 −0.08±0.09 0.09±0.07 0.03±0.08 0.08±0.04 0.05±0.07 0.11±0.09 0.01±0.11 0.03±0.11 0.11±0.11 0.10±0.07 0.02±0.11 0.60±0.20
PMM1142 14.0 6.71 5700 4.3 1.5 −0.07±0.09 −0.05±0.11 0.10±0.09 0.03±0.09 0.06±0.04 0.02±0.06 0.08±0.09 0.09±0.11 −0.02±0.11 0.06±0.06 0.02±0.06 −0.01±0.10 0.70±0.15
AVERAGE±rms −0.04±0.03 −0.08±0.03 0.06±0.04 0.03±0.02 0.06±0.05 0.02±0.05 0.09±0.02 0.04±0.06 0.02±0.03 0.05±0.05 0.07±0.04 0.02±0.03 0.62±0.07
ARGUS
CD−74673 4.8 7.28 4600 4.5 1.3 0.04±0.10 ....... −0.22±0.13 −0.11±0.06 −0.01±0.11 0.10±0.10 −0.22±0.12 −0.19±0.15 0.03±0.13 −0.11±0.13 0.19±0.13 0.01±0.11 0.55±0.15
TYC8561-0907-1 15.6 4.41 4900 4.5 1.2 −0.10±0.09 ....... −0.20±0.13 −0.11±0.06 −0.09±0.08 0.17±0.09 −0.14±0.11 −0.37±0.13 0.21±0.11 −0.19±0.11 0.44±0.15 0.01±0.09 0.65±0.15
CD−422906 23.5 10.35 5200 4.5 1.6 −0.07±0.09 ....... 0.01±0.08 0.04±0.08 0.02±0.05 0.08±0.09 0.04±0.11 −0.02±0.09 0.01±0.14 0.03±0.09 0.12±0.09 0.02±0.06 0.45±0.20
NYAps -3.2 10.95 5340 4.5 1.5 −0.03±0.10 ....... 0.09±0.11 0.08±0.09 0.04±0.06 0.05±0.09 0.14±0.11 0.07±0.10 0.02±0.11 0.07±0.10 0.17±0.19 −0.03±0.07 0.45±0.15
CD−395833 14.9 10.17 5500 4.6 1.6 −0.02±0.10 −0.04±0.09 0.01±0.06 −0.02±0.08 0.00±0.05 −0.05±0.09 0.09±0.09 0.06±0.10 0.05±0.13 0.11±0.06 0.15±0.11 0.00±0.07 0.60±0.20
CD−283434 26.8 6.36 5600 4.3 1.5 −0.09±0.09 −0.10±0.08 0.07±0.07 0.02±0.08 0.05±0.07 0.01±0.09 0.13±0.09 0.00±0.11 −0.07±0.15 0.11±0.09 0.13±0.15 0.00±0.06 0.50±0.15
AVERAGE±rms −0.05±0.05 −0.07±0.04 0.05±0.04 0.03±0.04 0.03±0.02 0.02±0.06 0.09±0.05 0.03±0.04 0.00±0.05 0.08±0.04 0.14±0.02 0.00±0.02 0.53±0.08
c(cid:13)
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