Formation of new stellar populations from gas accreted by massive young star clusters

13.8 MB·

by  Chengyuan Li

LETTER
Formation of new stellar populations from gas
accreted by massive young star clusters
ChengyuanLi1,2,3,RicharddeGrijs1,4,LicaiDeng2,AaronM.Geller5,6,YuXin2,YiHu2andClaude-Andre´ Faucher-Gigue´re5
6
1 1KavliInstituteforAstronomy&Astrophysics,PekingUniversity,YiHeYuan bulkstellarpopulation(inparticularintermsoftheheliumandheavier-
0 Lu5,HaiDianDistrict,Beijing100871,China elementabundances),giventheabsenceofanycleardifferencesinthe
2 2KeyLaboratoryforOpticalAstronomy,NationalAstronomicalObservatories, observedridge-linecolours. SequenceAmayalsoincludeasubpopu-
ChineseAcademyofSciences,20ADatunRoad,ChaoyangDistrict,Beijing lationofstarsassociatedwiththeclustersredclump,the0.7to2solar-
n
a 100012,China mass analogues of the helium-burning horizontal-branch stars (indi-
J 3PurpleMountainObservatory,ChinesAcademyofSciences,2WestBeijing catedbytheorangearea). Figure1bshowsthecolour-magnitudedia-
7 Road,Nanjing,210008,China grampertainingtoNGC1696. Italsoexhibitsabright,youngsimple-
2 4InternationalSpaceScienceInstitute-Beijing,1Nanertiao,HaiDianDistrict, stellar-population sequence characterized by log(t yr−1) = 8.70 (500
Beijing100190,China millionyears),whereasthebulkoftheNGC1696starsarebestrepre-
] 5Center for Interdisciplinary Exploration and Research in Astrophysics sentedbyanolderageoflog(tyr−1)=9.18(1.5billionyears). Com-
A
(CIERA) and Department of Physics and Astronomy, Northwestern Univer- paredwiththeclustersdominantmainsequence,theyoungersequence
G sity,2145SheridanRoad,Evanston,IL60208,USA exhibits a colour offset of approximately -0.06 mag (a shift to bluer
. 6DepartmentofAstronomyandAstrophysics,UniversityofChicago,5640S. colours),whichisconsistentwithastellarpopulationcharacterizedby
h EllisAvenue,Chicago,IL60637,USA anenhancedheliumabundanceofY =0.330(33.0%ofheliumatoms
p
bymass),whileY =0.256forthebulkoftheclusterstars. Asimilar
-
o Stars in star clusters are thought to form in a single burst from resultforNGC411isshowninFigure1c,wherewealsofindanaddi-
r acommonprogenitorcloudofmoleculargas. However, massive, tional,brighterstellarsequencethatiswell-representedbyayounger
st oldglobularclusters–withagesgreaterthan10billionyearsand isochroneoflog(tyr-1)=8.50(320millionyears)andY =0.400,com-
a massesofseveralhundredthousandsolarmasses–oftenharbour paredwithlog(tyr−1)=9.14(1.4billionyears)andY =0.252forthe
[ multiplestellarpopulations1–4,indicatingthatmorethanonestar- bulkoftheclustersstellarpopulation. Thesewell-populated,younger

forming event occurred during their lifetimes. Colliding stellar andhelium-enhancedstellarsequencesrepresentthestrongestevidence
1
v winds from late-stage, asymptotic-giant-branch stars5–8 are often yetthatadditional,post-formationstarbursteventsmayhaveoccurred
6 invoked as second-generation star-formation trigger. The initial inoursampleclusters.Theenhancedheliumabundancesmayalsolead
9 clustermassesshouldbeatleast10timesmoremassivethanthey tosmallchangesinthebest-fittingclusterages,buttherearecurrently
2 aretodayforthistowork8. However,largepopulationsofclusters noappropriatemodelisochronesavailabletoaccuratelyexplorethisef-
7 withmassesgreaterthanafewmillionsolarmassesarenotfound fectforstellarpopulationsyoungerthan1billionyears. Nevertheless,
0 inthelocalUniverse. Herewereportonthree1-2billion-year-old, thegeneralsenseofhelium-enhancedyoungsequencesshownhereis
1. massivestarclustersintheMagellanicClouds, whichshowclear robust. Todate,noothermassiveclustersofequivalentageareknown
0 evidenceofburst-likestarformationthatoccurredafewhundred tohostsimilarlysignificantpopulationsofyoungerstars10.
6 millionyearsaftertheirinitialformationera. Weshowthatsuch
1 clusterscouldaccretesufficientgasreservoirstoformnewstarsif Weexploredwhether‘bluestragglerstars’–starsthathavebeen
: the clusters orbited in their host galaxies gaseous discs through- rejuvenatedthrougheitherstellarcollisionsormasstransferinbinary
v
outtheperiodbetweentheirinitialformationandthemorerecent stellarsystems11,12 –couldbeentirelyresponsibleforthepresenceof
i
X burstsofstarformation.Thismayeventuallygiverisetotheubiq- theseyoungersequences. Iftheyareformedthroughmasstransferin
r uitousmultiplestellarpopulationsinglobularclusters. unresolved, compact binary systems, they would appear brighter and
a slightlyredderthanthecorrespondingisochrone13describingzero-age
The colour-magnitude diagrams of NGC 1783, NGC 1696 and single stars (stars whose output luminosities are no longer powered
NGC411areshowninFigure1.Thesestellardistributions–theobser- bytheexcessenergygainedfromgravitationalcontractionbutwhich
vationalcounterpartsofthetheoreticalHertzsprung-Russelldiagrams, are instead driven by nuclear fusion of hydrogen atoms). However,
which relate the stellar surface temperatures to their luminosities – the younger sequences in both clusters are too blue to account for a
havebeenfield-stardecontaminatedbycarefulapplicationofstatisti- binary origin and can instead be very well described by single-star
calbackground-subtractiontechniques(seeMethods).Figure1ashows isochrones.Giventheiryoungagesandthetimescalesinvolvedinevo-
thatthemajorityofstarsassociatedwithNGC1783arewell-described lutionthroughbinarymasstransfer, ifanyoftheseyoungerstarsare
by an isochrone9 – a theoretical ridge-line that describes stars with indeedbluestragglers,theywillmostlikelyhaveformedthroughstel-
identicalagesbutcoveringarangeofinitialmasses–characterizedby larcollisions.Collisionallyformedbluestragglers,likesecondarystel-
anageoflog(tyr−1)=9.15(1.4billionyears;seeMethods).However, largenerationsoriginatingfromcollidingstellarwinds,areexpectedto
one can also clearly discern two additional, bright stellar sequences, bemorecentrallyconcentratedthantheclustersdominant(bynumber)
denoted A and B, with younger ages of, respectively, log(t yr−1) = stellar populations6. Figure 2 compares the normalized radial distri-
8.65 (450 million years) and log(t yr−1) = 8.95 (890 million years). butions of the young sequences with those of normal cluster stars of
Thelatterappeartohavesimilarchemicalcompositionsastheclusters similarluminosity.Thestarsintheyoungsequencesaremarkedlyless
1
A
18
)
g B
a
m
(20
B
a
22
−0.1 0 0.1 0 0.5 1 1.5 2 2.5
∆ (B– I ) (mag) B–I (mag)
18 18
) )
g g
a a
m m
(20 (20
B B
b c
22 22
0 0.5 1 1.5 2 0 0.5 1 1.5
B–I (mag) B–I (mag)
Figure1|Colour–magnitudediagrams,includingthebest-fittingisochrones,andtrue-colourimagesforallthreeclusters.a:NGC1783.Purpleanddarkgreen
squares:sequence-Aand-Bstars,respectively;redsolid,bluesolidandbluedashedlines:isochronesforlog(tyr−1)=9.15,8.95and8.65,respectively.Theorange
boxindicatestheregionwherered-clumpstarsassociatedwithsequenceAmaybefound. Representative1σmeasurementuncertaintiesareshownintheleft-hand
subpanel;notethatitsx-axisscaleisdifferentfromthatofthemainpanel.b:NGC1696.Purplesquares:youngersequence;redsolidandbluedashedlines:isochrones
forlog(tyr−1)=9.18and8.70,respectively. c:NGC411. Purplesquares:youngersequence;redsolidandbluedashedlines:isochronesforlog(tyr−1)=9.14
and8.50,respectively. Themeasurementuncertaintiesinpanelsbandcareequivalenttothoseshownforpanela. FortheyoungsequencesinNGC1696andNGC
411,enhancedheliumabundanceshavebeenadopted(seetext).
centrally concentrated than the dominant older population of cluster ofmillionsofyears20,21owingtotheoccurrenceofTypeIIsupernovae
members. resultingfromthedeathsofthemostmassivefirst-generationstars22,23.
Themoreextendednatureoftheyoungpopulationssuggeststhat Thisleavesaclusterembeddedinagas-poor‘cavity’. Astheyoung
theymayhaveanexternalorigin. Indeed,sincethetypicalmassesof starclustermovesthroughtheinterstellarmedium,itcouldpotentially
NGC1783,NGC1696andNGC411areonly1.8×105,5.0×104and accrete sufficient gas to fuel renewed star formation. In theory, sec-
3.2×104solarmasses14,15,respectively,theyareinsufficientlymassive ondary star formation can be triggered23–25 and proceed rapidly once
to efficiently capture the stellar winds from asymptotic-giant-branch thegasdensityreachestherelevantthreshold,forsufficientlylowtem-
stars16. Note that our NGC 1696 mass is based on extrapolation of peratures. This would result in the appearance of a younger ‘simple
theobservedstellarluminosityfunctiondowntoastellarmassof0.08 stellarpopulation’. However,todatetherealityofthisproposedidea
solarmasses(theminimummassforhydrogenfusion, somewhatde- hasnotbeenconfirmed.
pending on the stars chemical composition), adopting a Kroupa-like Althoughtheyareallcontainedwithintwicetheirhostclusterscore
initialmassdistribution17.Inaddition,theobservedupperlimitsinthe radii, theobservedspatialdistributionsofallyoungstellarsequences
mass-agediagramspopulatedbystarclustersintheMagellanicClouds inoursampleclustersaremoreextendedthanthoseoftheirbulkstellar
arewell-understoodintermsof‘size-of-sample’effects. Theycannot populations. Thiscouldindicatethattheseclustersmayhaveaccreted
bereconciledwithinitialclusterpopulationscontaininglargenumbers ambientgas,allowingstarformationtoproceed. Wethusexploredthe
ofyoungclusterswithmassesfargreaterthanamillionsolarmasses. expectedgas-accretionrateasafunctionofthelocalgasdensity. In-
AdoptingaKroupa-likeinitialstellarmassdistribution17,weesti- deed,itappearspossibleforNGC1783-likeclusterstoaccreteenough
mate that the total stellar masses in the younger sequences, down to gastoformnewstars23(seeMethods).
thehydrogen-burninglimit,are372and250solarmasses(NGC1783 AlmostallGalacticglobularclustershostmultiplestellarpopula-
sequences A and B, respectively), 527 and 560 solar masses (NGC tions.However,itisstillunclearwhethertheselatterpopulationsorig-
1696 and NGC 411, respectively). Compared with the total cluster inatefromyoungclustersthatformedassingle-agestellarpopulations.
masses,theyoungsequencesrepresentmere0.2-2.0%massfractions. Ourobservationsofsecondarystellarpopulationsinintermediate-age
TheagedifferencesbetweentheNGC1783starsinsequencesAandB, MagellanicCloudclusterssuggestthatthesameprocessgivingriseto
andthoseonthemainsequenceare440millionyearsand520million themmayalsoexplainthemultiplestellarpopulationsseeninatleast
years,respectively. ForNGC1696andNGC411,theagedifferences someGalacticglobularclusters. Weaimataddressingthisissueina
betweentheclustersyoungandmainsequencesare1.02billionyears follow-upstudy.
and1.06billionyears,respectively. ManystarclustersintheMagellanicCloudscontainlargenumbers
The Large and Small Magellanic Clouds, the host galaxies of ofstarsoccupyingtheparameterspaceatbluercoloursandbrighterlu-
our sample clusters, contain numerous, densely distributed giant minositiesthantheirmain-sequenceturn-offregions26,27. Theseclus-
molecular-gas clouds18,19. Massive clusters like those targeted here tersincludeNGC121,NGC1652,NGC1751,NGC1795,NGC1806,
quenchtheirinitialstar-formationactivityontimescalesofafewtens NGC 1846, NGC 1852, NGC 1917, NGC 1978, NGC 2121, NGC
2
2154, NGC 339, NGC 416, NGC 419, Hodge 7, Kron 3, Lindsay 1,
andLindsay38.Thesebrighterandbluerstarsareusuallydismissedas
residualfield-starcontamination. However,ifwell-populatedyounger
sequencesare,infact,embeddedinthisparameterspace,thepresence
of such simple stellar populations indicates that many clusters may
haveexperiencedstarbursteventssometimeaftertheirinitialforma-
tion epoch. Among the clusters highlighted here, some – including
NGC1806,NGC1846,Lindsay38andNGC419–appeartoexhibit
similarfeaturesasNGC1783,NGC1696andNGC411,althoughata
lowerlevelofsignificance. ParticularlyforLindsay38andNGC419,
theradialdistributionsofthebrightstarsbeyondtheirmain-sequence
turn-offregionsareknowntobelessconcentratedthanthebulkcluster
starswithsimilarluminosities28,andhenceapopulationofbluestrag-
glerslikelycannotexplainthepropertiesofallthosestars. Somemay
haveoriginatedfromgasaccretion. Theseclusterswillbetargetedin

0.04 our follow-up studies. Our discovery of clear, well-populated young
Sequence A
sequencesinNGC1783, NGC1696andNGC411hasrevealedthat
0.03 Sequence B
starclustersmayindeedhavethecapacitytoaccretegasfromtheiren-
0.02 vironment. This could, in fact, be the most important route to form
secondarystellarpopulationsinyoungmassiveclusters.
0.01 a
0
10 20 30 40 50 60 70 80 90 100 110
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were observed for 90 s in the B band and 8 s in the I band, while
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27. Rich, R. M., Shara, M. M., & Zurek, D. New photometry for the long-exposureimageswereobservedfor680sinbothbands.
intermediate-ageLargeMagellanicCloudglobularclusterNGC2121and
BecauseNGC1783hasanextendedcore(seebelow),thecluster
thenatureoftheLMCagegap.Astron.J.122,842–848(2001)
28. Glatt,K.,etal. Agedeterminationofsixintermediate-ageSmallMagel- region occupies almost the entire image. Therefore, we obtained an
lanicCloudstarclusterswithHST/ACS.Astron.J.136,1703–1727(2008) additional set of observations towards the southeast of NGC 1783 as
representativefieldregion(HSTprogrammeGO-12257; principalin-
Acknowledgements WethankF.Ferraro,P.KroupaandX.K.Liufordis-
vestigator: L.Girardi). Itscentreislocatedatadistanceofmorethan
cussions. PartialfinancialsupportforthisworkwasprovidedbytheNational
300 arcseconds from the cluster centre, so that it is unlikely signifi-
Natural Science Foundation of China through grants 11073001, 11373010
and11473037.C.L.wasalsopartiallysupportedbytheStrategicPriorityRe- cantlyaffectedbytidallystrippedclusterstars. Thedatasetspertain-
searchProgramme‘TheEmergenceofCosmologicalStructures’oftheChi- ingtothefieldwerealsoobtainedwiththeACS/WFCintheF435W
nese Academy of Sciences (grant XDB09000000). A.M.G. is funded by a andF814Wfilters. Theirtotalexposuretimesare700sand720sfor
NationalScienceFoundationAstronomyandAstrophysicsPostdoctoralFel-
theBandIbands,respectively.Theseexposuretimesaresufficientto
lowshipunderAwardNo. AST-1302765. C.-A.F.-GissupportedbyNational
resolvetheyoungsequences.
ScienceFoundationgrantAST-1412836.
NGC 1696: The NGC 1696 data sets were also obtained as part
AuthorContributions C.L., R.d.G. and L.D. jointly designed and coordi-
ofHSTprogrammeGO-10595,usingtheACS/WFC.Theclusterwas
natedthisstudy. C.L.performedthedatareduction. C.L.,R.d.G.,Y.X.and
Y.H.collaboratedonthedetailedanalysis. L.D.providedideasthatimproved observedthroughtheF435WandF814Wfilters. TheimagesofNGC
thestudysrobustness.A.M.G.andC.-A.F.-Gledthetheoreticalanalysisofthe 1696arealsocomposedofshort-andlong-exposureframes. Thelong
gas-accretionphysics. Allauthorsread,commentedonandjointlyapproved (short)exposuretimesintheB andI bandswere680(90)sand680
submissionofthisarticle.
(8)s,respectively. SinceNGC1696isnotasextendedasNGC1783
(seebelow),arepresentativefieldregionwasselectedclosetotheedge
AuthorInformation Reprints and permissions information is available at
www.nature.com/reprints.Correspondenceandrequestsformaterialsshould oftheNGC1696images.FewstudiestodatehavetargetedNGC1696.
beaddressedtoC.L.([email protected]@pmo.ac.cn). Therefore,wederivedthephysicalparametersofNGC1696ourselves.
NGC 411: The data sets of NGC 411 were obtained as part of
CompetingInterests Theauthorsdeclarethattheyhavenocompetingfi-
nancialinterests. HSTprogrammeGO-12257,usingtheWideFieldCamera-3(WFC3).
TheclusterwasobservedthroughtheF475WandF814Wfilters. The
F475Wfilteriscenteredatawavelengthof475nm; itstransmission
curvealsocorrespondsapproximatelytothatoftheJohnsonBband.
TheexposuretimesinboththeB andI bandswere700s. NGC411
has a relatively small core (see below), which allowed us to select a
representativefieldregionclosetoedgeofourscienceimages.
PhotometryandDataReduction
We used two independent software packages to perform point-
spread-function (PSF) photometry, including DAOPHOT29 within the
IRAF environment and DOLPHOT30–32. Our stellar catalogues are
based on the DAOPHOT results. We performed the DOLPHOT analy-
sisforcomparison,toensurethatourfinalphotometryisnotbiased.
The DAOPHOT-generated raw stellar catalogue contains a sharp-
nessparameter, whichdescribesthegoodnessofthePSFfit29. Fora
‘good’star,thesharpnessshouldbecloseto0.Wethusconstrainedour
sampletostarswithasharpnessbetween-0.5and0.5,whichremoved
approximately4%ofobjectsfromourcatalogue.Wecarefullychecked
the resulting colour-magnitude diagram and found that the main fea-
tures of interest were not affected by this selection. For NGC 1783
andNGC1696,wemergedthestellarsamplesresultingfromtheshort
and long exposure times, carefully cross-referencing both catalogues
toavoidduplicationofobjectsinthecombinedoutputcatalogue. For
NGC1783,theshort-exposure-timecataloguecontributesverylittleto
sequencesAandB.ForNGC1696,theshort-exposure-timestarscon-
tributeonlymarginallytothefeatureofinterest.
4
regiontowardsthesoutheast(‘field2’). Becausethefeaturesofinter-
0.5 a estareverybright,weonlyconsiderstarswithB≤23mag.Forthese
stars, NGC1783iscompact(itscoreradiusis25arcsec)andfield1
0
indeedadequatelyrepresentsthebackground. Wefoundthattheeast-
−0.5 ern part of field 2 exhibits a clear, slightly brighter stellar sequence
parallel to the main sequence, indicating a significant population of
−1
0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 unresolvedbinarysystems. Sincethefieldsstellarnumberdensityis
low, blending of unrelated stars along the line of sight is negligible;
2] 0.5 b
c instead,thisbinarypopulationmayreflectcontaminationbyanearby
e
s
arc 0 star-forming region. We hence selected the eastern part of field 2 as
ρ[−0.5 representativefieldregion. Weselectedthreerectangles, eachcover-
og ing an area of 1000×1000 pixels2 (∼50×50 arcsec2), near the edge
l −1
0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 of field 1, as well as four rectangles from the western part of field 2
(coveringthesamearea), toconstructacomplete, combinedfieldre-
c
0.5 gion. Toassignequalweightstothestellarcataloguesfrombothre-
0 gions,werandomlyselected ofthefullsampleofstarsdetectedinthe
−0.5 fourrectanglesoffield2sstellarcatalogue. Thecombinedstellarcat-
aloguebasedonthesesevenrectanglesrepresentsthesynthesizedfield
−1
0.8 1 1.2 1.4 1.6 1.8 2 2.2 regionscolour-magnitudediagram. Theclusterregionisroughly1.6
R(arcsec)
timeslargerthanthefieldregion(seetheleft-handpanelofExtended
DataFigure2).
For NGC 1696, we selected an area of 600×4000 pixels2
Figure1|Normalizedradialdistributionsoftheyoungsequenceswithre- (∼30×200arcsec2)neartheedgeoftheimageasrepresentativefield
specttonormalclusterstarsofsimilarluminosity. RGB:red-giantbranch;
region. TheNGC1696clusterregionisroughly1.7timeslargerthan
RC:redclump.a:NGC1783;b:NGC1696;c:NGC411.Theerrorbarsreflect
Poissonian1σuncertainties. the field region. The selected cluster and field regions pertaining to
NGC1696areshowninthemiddlepanelofExtendedDataFigure2.
For NGC 411, we selected a field region covering an area of
800×3500pixels2(∼32×140arcsec2)fromtheclustersperiphery.We
DeterminationoftheClusterandFieldRegions avoidedregionsthatwerelocatedclosetotheedgeoftheimage,where
thephotometricqualityismarkedlyinferior,likelyowingtotherela-
We divided the stellar spatial distribution into 15-20 bins along
tively large offsets between the individual science images, combined
both the right ascension (α ) and declination (δ ) axes. We
J2000 J2000
withinstrumentalpropagationeffects. Althoughourselectionmayin-
variedthebinnumberstoensurethatstatisticalscatterwouldnotsig-
cludesometidallystrippedclusterstars,thisdoesnotaffectthemag-
nificantlyaffecttheshapeofthenumber-densitydistributions.Weused
nituderangeofinterest,whichisbright. TheNGC411clusterregion
aGaussianfunctiontofitthelatteralongbothaxesanddefinedtheclos-
is roughly 1.8 times larger than the field region. The selected clus-
estpositionalcoincidenceofbothGaussianpeaksastheclustercentre.
TheNGC1783clustercentreislocatedatα = 04h59m08.47s, terandfieldregionsofNGC411areshownintheright-handpanelof
J2000
δ = −65◦59(cid:48)17.81(cid:48)(cid:48). ForNGC1696,thecentrecoordinatesare ExtendedDataFigure2.
J2000
α = 05h02m11.16s,δ = −67◦59(cid:48)07.66(cid:48)(cid:48),whileforNGC
J2000 J2000
411,α = 01h07m56.22s,δ = −71◦46(cid:48)04.40(cid:48)(cid:48). OurNGC 1 Reducing Background Contamination and
J2000 J2000
1783andNGC411clustercentresareveryclosetotheclustercentres IsochroneFitting
determinedpreviously33,34.WeusedaMonteCarlomethodtoexamine Once we had obtained the cluster and background colour-
thespatialdistributionoftheNGC1696starsandestimatetheareasof magnitudediagrams,wegeneratedacommonmagnitude×colourgrid
annuliatdifferentradii. Thestellarnumberdensityineachannulusis withcellsizesof0.5×0.25mag2, spanningtherangesfrom(B−I)
N(R)/A(R),whereN(R)isthenumberofstarsobservedinanan- =-2.5magto3.5magandfromB =16magto27mag. Thisrange
nuluswithradiusR,andA(R)istheareaoftheannulus. Wedefined issufficientlylargetocoverthefullcolour-magnitudediagramsofour
theclusters’(2D-projected)coreradiiasthoseradiiwherethedensity targetclusters. Thecellsizeisrelativelylargeforthemainsequences,
profiles drop to half the respective central densities. We selected the becauseitwasspecificallydesignedtobepracticallyusefulinthere-
areascontainedwithin2coreradiiastheclusterregions. NGC1783 gionsoccupiedbytheyoung, bluesequences, wherethestellarnum-
hasalargecoreradius(45-50arcsec,whichisidenticaltothatadopted ber density is lower. We counted the number of background stars in
byref. 35,althoughourcoreradiusisslightlylargerthantheirvalue each cell and calculated the number of possible contaminating stars
of36.7arcsec. Wecomparedourcataloguewiththeirsandfoundthat in the same cell (corrected for differences in areas covered), which
ourphotometryisdeeperand, hence, containsmorefaintstars). The wethenrandomlyremoved. Weconfirmedthatvaryingthecellsizes
coreradiusofNGC1696issmaller(∼30arcsec), whilethecorera- from0.3×0.15mag2 to0.5×0.25mag2 forNGC1783wouldnotaf-
diusofNGC411isapproximatelyhalfthatofNGC1783(20-25arc- fectthesignificanceofanyofthefeaturesofinterest. ForNGC1696,
sec). Hence, theclusterradiiweadoptedforNGC1783, NGC1696 the typical practically useful cell sizes range from 0.4×0.4 mag2 to
andNGC411were100arcsec,60arcsecand50arcsec,respectively. 0.5×0.5mag2,whileforNGC411,viablecellsizesrangefromroughly
We selected these radii as cluster radii, because of the need to avoid 0.3×0.15mag2 to0.5×0.25mag2. Adoptingmuchlargerorsmaller
backgroundcontaminationasmuchaspossible,whilesimultaneously cellsizeswoulderasetheobservedsequences. Wecarefullyexamined
ensuring statistically robust results. In Extended Data Figure 1, we theperformanceofourdecontaminationmethodandfoundthattheob-
presentthestellarnumber-densityprofiles. servedfeaturesdonotdependontheclusterorfieldregionsselected:
ForNGC1783,wesynthesizedthefieldregionscolour-magnitude seeExtendedDataFigures3and4,whereweuseNGC1783asbench-
diagram based on a combination of our observations of the clusters mark. InExtendedDataFigure3, weshowthefield-decontaminated
peripheryontheimagecontainingthecluster(‘field1’)andthefield colour-magnitude diagrams pertaining to three different samples of
5
Figure2|Spatialdistributionsofclusterandfieldstars.Redandbluepointsrepresentclusterstarsandtheadoptedfieldstars,respectively.a.NGC1783;b.
NGC1696;c.NGC411.
17.5 a: R ≥30” 17.5 b: R ≥60” 17.5 c: R ≥90”
18 18 18
18.5 18.5 18.5
19 19 19
g)19.5 g)19.5 g)19.5
a a a
m m m
B ( 20 B( 20 B ( 20
20.5 20.5 20.5
21 21 21
21.5 21.5 21.5
22 22 22
0 0.5 1 0 0.5 1 0 0.5 1
B–I(mag) B–I(mag) B–I (mag)
Figure3|Field-stardecontaminatedcolour-magnitudediagramsforsamplesofstarsatdifferentradiiinNGC1783.a.R≥30(cid:48)(cid:48);b.R≥60(cid:48)(cid:48);c.R≥90(cid:48)(cid:48).
6
ters, NGC 1783, NGC 1696 and NGC 411 display extended main-
sequence turn-off regions26,36, which renders determination of their
main-sequenceagesdifficult.However,ithasbeenreportedthatagesof
−65.96
18 suchintermediate-agestarclusterscanbeconstrainedbyconsideration
ag)20 eg.)−65.98 ovfietwh)e,irwthigichht sruebpgreisaennttbtrhaencshteelsl3a7r,3e8v(obluuttiosenearryefs.ta3g9efsotarrsanenotpeproosnincge
m22 d −66
B ( RA ( they have exhausted the hydrogen in their cores through nuclear fu-
24 −66.02 sion.Indeed,allofoursampleclustersexhibittightsubgiantbranches.
a b Thefinalagedeterminationyieldslog(tyr−1)=9.15, 9.17and9.14
26
−66.04
forNGC1783,NGC1696andNGC411,respectively.
0 1 2 3 74.7 74.75 74.8 74.85
B–I (mag) DEC (deg.) We next determined the ages of each of the blue sequences. For
NGC1783,sequencesAandBarecharacterizedbyagesoflog(tyr−1)
−65.95
= 8.65 and 8.95, respectively. The NGC 1783 main-sequence stars,
18
as well as those in sequences A and B, share the same metal abun-
g)20 g.) −66 dance,Z=0.008(40%ofsolarmetallicity),andvisualextinction,AV
ma22 de = 0.06 mag. We adopted a true distance modulus for NGC 1783 of
B (24 RA(−66.05 (m−M)0 =18.46mag(correspondingtoadistanceof49.2kilopar-
secs).TheyoungsequenceinNGC1696isadequatelycharacterizedby
c d
26 −66.1 alog(tyr−1)=8.70isochronewiththesamemetallicityastheclusters
0 1 2 3 74.75 74.8 74.85 74.9 mainsequence,Z =0.004(20%solar). However,theyoungsequence
B–I (mag) DEC (deg.)
appears0.06magbluerthanthezero-agemainsequenceoftheclusters
bulkstellarpopulation. Anenhancedheliumabundance(Y =0.330)
−65.95
18 canexplainthecolouroffset40,whereY =0.256fortheclustersmain
−66 sequence.TheadoptedextinctionanddistancemodulusforNGC1696
ag)20 eg.) areAV =0.10magand(m−M)0=18.50mag(50.1kiloparsecs),re-
B (m22 RA (d−66.05 =sp8ec.5ti0v.elIyt.hTashethyeosuanmgesemqeuteanllcieciitnyNasGtChe4c1l1ushtaesrsanmaaginesoefqluoegn(ctey,rZ−1=)
24
−66.1
e f 0.002. Itsyoungsequenceisalsoveryblue,whichcanagainonlybe
26
explainedifitischaracterizedbyanenhancedheliumabundanceofY
−66.15
0 1 2 3 74.75 74.8 74.85 74.9 =0.400comparedwithY =0.252fortheclustersmainsequence.Fore-
B–I (mag) DEC (deg.)
groundextinctionofA =0.25magisappropriateandouradoptedtrue
V
distancemodulusis(m−M) =18.90mag(60.3kiloparsecs).
0
2 Blue Straggler Stars as possible origin of the
Figure 4 | Field-star decontaminated colour-magnitude diagrams of NGC
youngersequences?
1783 for three different adopted reference fields. a, b. Resulting colour-
magnitudediagram(a)basedonfield-starsampledrawnfromtheimagecontain- One possible explanation for these bright sequences is that they
ingthecluster(b). c,d. Aspanelsaandb,butforarepresentativefieldregion arecomposedofbluestragglerstars. Therefore,weinvestigatedtheir
takenfromaseparateimage.e,f.Aspanelscandd,butforadifferentfieldregion
relativeradialconcentrationwithrespecttostarsthathavesimilarlu-
takenfromthesame,separateimage.
minosities. In NGC 1783, NGC 1696 and NGC 411, the latter stars
are mostly red-giant-branch and red-clump stars: see Extended Data
Figure7.
However, as shown in Figure 2, the stars defining the bright se-
cluster members, at radiiR ≥ 30(cid:48)(cid:48), R ≥ 60(cid:48)(cid:48) and R ≥ 90(cid:48)(cid:48). They
quences are all less centrally concentrated than red-giant stars with
allexhibitdistinctyoungersequences. InExtendedDataFigure4,we
similar luminosities. If they are genuine blue stragglers, irrespective
select representative field regions from different images (two from a
of their origin, they are expected to be more centrally concentrated
separateimageandonefromtheimagewhichalsocontainstheclus-
thansimilar-luminosityred-giantstars,becausebluestragglersareex-
teritself).Theobservedyoungsequencesremainclearlyvisibleforall
pectedtobemoremassivethanredgiants. Dynamicalinteractionsare
threebackgroundregionsadopted.InExtendedDataFigure5,weshow
also unlikely to have redistributed all blue stragglers to the outskirts
the decontaminated colour-magnitude diagrams resulting from adop-
of our sample clusters, since the typical dynamical timescales41 are
tionofdifferentgridsizes.Thisfigureshowsthattheobservedfeatures
muchlongerthantheclusterscurrentages. Inaddition, theflatclus-
arealmostindependentofgridsize. Extensivetestsalsoshowedthat
ter cores observed for NGC 1783, NGC 1696 and NGC 411 render
thesequencesfoundintheNGC1696andNGC419colour-magnitude
theprobabilityofcore-collapseeventshavingoccurredveryunlikely.
diagrams are similarly well-defined. This confirms that the observed
Corecollapsewouldproduceacuspyradialdensityprofile42andsuch
sequencesarephysicallyrealratherthancausedbystatisticalsampling
aprocessisaprerequisiteforthepresenceofacoevalcollisionalblue
effects. We also found that, for the adopted cell sizes, the reduced
stragglerpopulation.Themoreextendedradialdistributionsofthestars
colour-magnitude distributions are similar in appearance to the real
intheyoungersequencescomparedwiththedominantclusterpopula-
background colour-magnitude diagrams. Our adopted method there-
tionsargueagainststellarcollisionsintheclustercoreshavingplayed
foreperformsadequately. FromExtendedDataFigure6(panelsa, e
animportantrole.
andi),onecandeducethatthesebrightsequencesareindeedalready
embeddedintheunreducedcolour-magnitudediagrams.
3 GasAccretionfromtheInterstellarMedium
Weobtainedbestfitstoallobservedsequences,includingtheclus-
tersmainsequences,basedonmatchingtheobservationswiththeoreti- Wefollowedthemethodofref.23toestimatetheregionsofparam-
cal stellar isochrones9. Similarly to other intermediate-age star clus- eterspacewhereanNGC1783-likestarclusterwithamassof1.8×105
7
17.5 17.5 17.5
a:0.30 ×0.15mag 2 b:0.40 ×0.20mag 2 c:0.50 ×0.25mag 2
18 18 18
18.5 18.5 18.5
19 19 19
ag)19.5 ag)19.5 ag)19.5
m m m
B ( 20 B ( 20 B ( 20
20.5 20.5 20.5
21 21 21
21.5 21.5 21.5
22 22 22
0 0.5 1 0 0.5 1 0 0.5 1
B–I(mag) B–I (mag) B–I (mag)
Figure5|Field-stardecontaminatedcolour-magnitudediagramsofNGC1783forthreedifferentgridsizes. a. 0.30×0.15mag2; b. 0.40×0.20mag2; c.
0.50×0.25mag2.
16 16 16 16
18 18 18 18
20 20 20 20
22 a 22 b 22 c 22 d
0 1 2 0 1 2 0 1 2 0 1 2
18 18 18 18
)
g
a
m 20 20 20 20
(
B
22 e 22 f 22 g 22 h
0 0.5 1 1.5 0 0.5 1 1.5 0 0.5 1 1.5 0 0.5 1 1.5
18 18 18 18
20 20 20 20
22 i 22 j 22 k 22 l
−0.5 0 0.5 1 1.5 −0.5 0 0.5 1 1.5 −0.5 0 0.5 1 1.5 −0.5 0 0.5 1 1.5
B –I (mag)
Figure6|Colour-magnitudeanalysis.Firstcolumn:rawcolour-magnitudediagrams.Secondcolumn:field-decontaminatedcolour-magnitudediagrams(alsoshown
inFigure1). Thirdcolumn:colour-magnitudediagramsoftherepresentativefieldregions. Fourthcolumn:stellarcolour-magnitudedistributionsthatwereremoved
fromtherawcatalogues.Firstrow(a-d):NGC1783;secondrow(e-h):NGC1696;thirdrow(i-l):NGC411.
8
17 17 17
18
18 18
19
19 19
g) g) g)
a a a
m m m20
B( B ( B (
20 20
21
21 21
22
a b c
22 22 23
0 1 2 3 0 1 2 3 4 0 0.5 1 1.5 2
B–I (mag) B–I (mag) B–I (mag)
Figure7|Colour-magnitudediagramshighlightingspecificfeatures.a,Purpleanddarkgreensquares:starsinNGC1783sequencesAandB,respectively.Dark
greencircles:correspondingred-giant-branchandred-clumpstars,usedforcomparisonwithsequenceB.Thecombinationofdarkgreenandpurplecirclesrepresents
thesampleusedforcomparisonwithsequenceA.b.Purplesquares:NGC1696young-sequencestars;redcircles:correspondingred-giant-branchandred-clumpstars
usedforcomparison.c.Asthemiddlepanel,butforNGC411.
solarmassesandahalf-massradiusof11.4parsecs43couldaccretethe
requiredmasstoformtwoadditionalgenerationsofstars:onecontain-
ing 250 solar masses in stars some 520 million years after the initial
star-formation event, and a subsequent generation composed of 370
solar masses of stars 440 million years later. The key equations in
ref. 23areequations(3),(5)and(8). Forallcalculationsweassumed
a star-formation efficiency of 10% (which implies that only 10% of
theavailablegasisconvertedintostars). Weexploredtwoprocesses
throughwhichaclustercanaccretegasfromtheinterstellarmedium23,
oneowingtogravity(‘Bondiaccretion’),andoneduetosweepingup
materialastheclusterorbitsarounditshostgalaxyscentre. Thelatter
processinvolvessomeinitialintraclustergaswhichinteractscollision-
ally with the interstellar gas. The accretion rates from both of these
processesdependonboththegasdensity,n,andtherelativevelocity
ofclusterandgas,V. Ram-pressurestrippingcanlimittheaccretion,
andthisprocessalsodependsonnandV.
ExtendedDataFigure8showstheallowed(shaded)regionsofpa-
rameterspacein(n,V)thatwouldenableaclustertoaccretethede-
siredamountofgasintheperiodbetweenstar-formationevents. This
figure assumes accretion from a volume-filling interstellar medium
overtheentiredurationbetweenrespectivegenerations(Bondiaccre-
tion).TheBondilineiscurved,becausethisaccretionratealsodepends
onthegassoundspeed,whichwehaveassumedtobe10kms−1(i.e.,
gas at a temperature of ∼104 K). Anything to the upper right of the
Figure8|Gas-accretiondiagnosticdiagram. Theshadedregionsindicatethe
‘Ram’linewillstripthegasfromthecluster.Belowthatline,andfora
parameterspacewhereanNGC1783-likeclustercanaccretetherequiredmassto
givendensity,theBondilinedefinestheupperlimittotheclustersbulk formthetwoadditionalgenerationsofstars,namelyoneof250solarmassesover
velocity that would be allowed for the cluster to accrete this amount 520millionyears(blue),andasecondof370solarmassesover440millionyears
ofgas. Inotherwordsatavelocitybelowthecurve,theclusterwould (green). Wehaveassumedastar-formationefficiencyof10%forallcalculations
accretegasmorequickly, and, forinstance, ifthestar-formationeffi- inthisfigure.Theregionstotherightofthe‘Gravity’curvescorrespondtowhere
Bondiaccretioncanaccumulateatleasttherequiredmassandtheregionstothe
ciencywerelessthan10%, itcouldstillformthesamemassofstars
rightofthe‘Sweep’curvescorrespondtowhereaccretionbycollisionalsweeping
overthesameperiod. Conversely,atagivendensity,sweepingupof
upofambientinterstellargasbyseedintraclustergascanaccumulateatleastthe
gasismoreefficientatlargervelocities,sothedashedlineshowsthe
requiredmass.Theparameterspaceabovethe‘Ram’curvesareexcludedbecause
lowerlimittothevelocitythatwouldberequiredtoreachthedesired rampressurestripsclustersoftheirgasinthoseregions.
amountofgas.
ExtendedDataFigure9showstwoexamplesofhowtheaccreted
mass could accumulate over time for the Bondi regime (solid lines)
and the sweeping regime (dashed lines). For the Bondi regime, we
usedarelativevelocityof4kms−1 andadensityof0.3cm−3. For
9
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fromcarbonstarkinematics.Astron.J.124,2639–2663(2002).
Figure9|Gasmassaccretedfromtheinterstellarmediumasafunctionof
time. Wehaveadoptedastar-formationefficiencyof10%andcalculatedrepre-
sentativeinterstellargas-accretionframeworksthatcanexplainthestellarmasses
inthesecondarysequencesAandBinNGC1783. FortheBondiregime(solid
lines),weusedarelativevelocityof4kms−1andadensityof0.3cm−3.Forthe
sweepingregime(dashedlines),weusedavelocityof50kms−1andadensity
of0.05cm−3.Theblueandgreendatapointsindicatethestellarmassesandage
offsetsofNGC1783sequencesAandB,respectively. Theerrorbarsrepresent
uncertaintiesinthestar-formationefficiencyof10%.-
thesweepingregime,weusedavelocityof50kms−1andadensityof
0.05cm−3 (whichissimilartotherotationvelocityatthepositionof
NGC1783withrespecttotheLargeMagellanicCloudscentre44).The
datapointsshowthesemasses,with150solar-masserrorbarsdivided
bythestar-formationefficiencyof10%.
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10