Kinematics and binaries in young stellar aggregates. II. NGC 6913 = M29

arXiv:astro-ph/0310090v1 3 Oct 2003Astronomy&Astrophysicsmanuscriptno.BoecheFebruary2,2008

(DOI:willbeinsertedbyhandlater)

Kinematicsandbinariesinyoungstellaraggregates.

II.NGC6913≡M29

C.Boeche1,U.Munari1,L.Tomasella1,andR.Barbon2

1OsservatorioAstronomicodiPadova,SedediAsiago,I-36012Asiago(VI),Italy2OsservatorioAstrofisicodelDipartimentodiAstronomia,Universit´adiPadova,I-36012Asiago(VI),Italy

Receiveddate..............;accepteddate................

Abstract.Between1996and2003wehaveobtained226highresolutionspectraof16starsinthefieldoftheyoungopenclusterNGC6913,totheaimofconstrainingitsmainpropertiesandstudyitsinternalkinematics.Twelveoftheprogramstarsturnedouttobemembers,oneofthemprobablyunbound.Ninearebinaries(oneeclipsingandanotherdoublelined)andforsevenofthemtheobservationsallowedtoderivetheorbitalelements.Allbuttwooftheninediscoveredbinariesareclustermembers.Inspiteoftheyoungage(afewMyr),theclusteralreadyshowssignsthatcouldbeinterpretedasevidenceofdynamicalrelaxationandmasssegregation.However,theymaybealsotheresultofanunconventionalformationscenario.Thedynamical(virial)massasestimatedfromtheradialvelocitydispersionislargerthantheclusterluminousmass,whichmaybeexplainedbyacombinationoftheopticallythickinterstellarcloudthatoccultspartofthecluster,theunboundstateorundetectedverywidebinaryorbitofsomeofthemembersthatinflatethevelocitydispersionandahighinclinationfortheaxisofapossibleclusterangularmomentum.Alldiscoveredbinariesarehardenoughtosurviveaveragecloseencounterswithintheclusteranddonotyetshowsignofrelaxationoftheorbitalelementstovaluestypicaloffieldbinaries.

Keywords.Binaries:spectroscopic–Stars:earlytype–ISM:bubbles–Openclustersandassociations:general–Openclustersandassociations:individual(NGC6913)

1.Introduction

Thisisthesecondpaperofaseriesdevotedtotheresults

ofalongterm,highresolutionspectroscopicstudyofearly

typemembersofyoungopenclusters,trapeziumsystems

andOBassociations.Theaimsofthisseriesarediscussed

inPaperI(MunariandTomasella1999).

NGC6913,thetopicofthispaper,isayoungopen

clusterharboringO-typemembersandlyingclosetothe

planeoftheGalaxy(α=20h23m·9,δ=+38◦32′(J2000);

l=76◦·92,b=+0◦·61).DespiteappearingintheMessier

catalogasM29,fewpapersinliteraturedealwithit,

furthermoreshowingsomedisagreementintheresults.

Clusterdistanceisreportedtobe2.2kpcbyMorgan

andHarris(1956)andMasseyetal.(1995),1.5kpcby

Joshietal.(1983),and1.1kpcbyHoagetal.(1961),

whileTifft(1958)suggestedthatNGC6913isindeedthe

resultsoftwoseparategroupsofstars,oneat1.6kpc

andtheothersomewherebetween1.9and2.4kpc.The

meananddifferentialreddeningspanarangeofvaluestoo:

=0.78,∆EB−V=0.64accordingtoJoshietal.

(1983),=0.71and∆EB−V=1.82forWangand

Hu(2000),and=1.03followingMasseyetal.2C.Boecheetal.:Kinematicsandbinariesinyoungstellaraggregates.II.NGC6913≡M29

Fig.1.FindingchartforNGC6913programstars.

galacticmotion,theindividualrotationalvelocities,and

theinternalkinematicalandevolutionarystatusofthe

cluster.Spectroscopicorbitsarecalculatedforthediscov-

eredbinarystars.

2.Spectroscopicobservations

Table1summarizesthemainpropertiesofthe16se-

lectedprogramstars,andFigure1providesafinding

chartforthem.Theprogramstarshavebeenspectro-

scopicallyobservedovertheperiod1996-2003withthe

1.82cmtelescopeandEchelle+CCDspectrographofthe

AstronomicalObservatoryofPadovaatAsiago(Italy).

Table2providesthejournalofobservations.Theinstru-

mentalset-up,spectraextractionandcalibration,accura-

cies,etc.areidenticaltoPaperIandthereaderisreferred

toitfordetails.

2.1.Spectralclassificationandradialvelocities

Scantyinformationexistsonthespectral

classificationof

theprogramstars.WangandHu(2000)derivedspectral

typesfromlowresolutionspectra(5.3˚A/pix)covering

therange4200-6900˚A.KazlauskasandJasevicius(1986)

obtainedphotoelectricphotometryintheVilniussystem,

thatwehaveconvertedintospectraltypesusingthered-

deningfreecolorparametersQdefinedbyStrayˇzis(1977)

appropriatefortheRV=AV/EB−V=3.6reddeninglaw

thatappliestoNGC6913accordingtoJohnson(1962).

Wehavealsoderivedspectralclassificationofthepro-

gramstarsusingourEchellespectra,classifiedagainst

theYamashitaetal.(1977)spectralatlas.Evenifspec-

tralclassificationofEchellespectrahastobecarriedout

withcare(linestobecomparednormallyfallondifferentFig.2.TheprogramstarsonthereddeningcorrectedV◦,

(B−V)◦diagramusingEB−VfromTable3andV,B−V

photometryfromTable1forRV=AV/EB−V=3.6ap-

propriateforNGC6913accordingtoJohnson(1962).The

isochroneforsolarmetallicityand5MyrisfromBertelli

etal.(1994)anditisscaledtom−M=10.5(dottedline),

m−M=11.0(solidline)andm−M=11.7(dashedline).

Itisevidenthowtheclusterdistancecannotbewellcon-

strained.Inthispaperweadopta1.6kpcdistance.

Echelleorders),neverthelesstheresultingspectraltypes

lookquitereasonable,and,giventhefarsuperiorspec-

tralresolutionandhighS/N,alsopossiblymoreaccurate

thanthoseofWangandHu(2000).Thethreeestimates

ofthespectraltypearecomparedinTable3.Thelasttwo

columnsofthetablegivethereddeninganddistancewhen

Fitzgerald(1970)intrinsiccolorsandourspectralclassifi-

cationarecomparedtoV,B−VphotometryinTable1.

Thepositionsoftheprogramstarsonthereddeningcor-

rectedHRdiagramareshowninFigure2.

Radialvelocitiesfromindividualobservations(here-

afterreferredtoasepochradialvelocities)oftheprogram

starsaregiveninTable4.ForOandBtypeprogram

starstheyrestonindividualmeasurementofHeIandHeII

lines.Fortheother,coolerprogramstarstheradialveloc-

itiescomefrommeasurementofthemetallicabsorptions

lines(mainlyFeI,MgI,TiII).Theradialvelocitiesofthe

Beprogramstar#5pertaintotheemissionlines,which

completelyfillheliumandhydrogenabsorptionlines.

2.2.Binariesandorbitalsolutions

Abouthalfoftheprogramstarshaveturnedouttobe

spectroscopicbinaries.Table5summarizesthebarycen-

tricvelocity,themembershipandthebinarystatusbased

onepochradialvelocitiesinTable4.Table6givesthe

spectroscopicorbitscomputedforallthebinarystarsbut