Multiple Stellar Populations in Galactic GCs: Observational Evidence

0.81 MB·English

by  Giampaolo Piotto

Mem.S.A.It.Vol.79,3
(cid:13)c SAIt 2008 Memoriedella
Multiple Stellar Populations in Gala
ti
GCs:
Observational Eviden
e
GiampaoloPiotto1
8
Dipartimento di Astronomia, Universita` di Padova, Vicolo dell’Osservatorio, 3, I-35122
0
Padova,Italia–e-mail:[email protected]
0
2

n
Abstract. Anincreasingnumber ofphotometricobservations ofmultiplestellarpopula-
a
tions in Galactic globular clusters is seriously challenging the paradigm of GCs hosting
J
single,simplestellarpopulations.Thesemultiplepopulationsmanifestthemselvesinasplit
1 ofsomeevolutionarysequencesoftheclustercolor-magnitudediagrams.Inthispaperwe
2 willsummarizetheobservationalscenario.

] Keywords.Stars:PopulationII–Galaxy:globularclusters
h
p
-
o
1. Introduction in C and N abundances expected from typi-
r
t cal evolutionof low mass stars (Gratton et al.
s Globular clusters (GC) have generally been
2000, Sweigart & Mengel 1979, Charbonnel
a
thoughconsistingofcoevalandchemicallyho-
[ 1994). This abundance pattern is primordial,
mogeneous stars. Color-magnitude diagrams
since itis observedin starsatallevolutionary
1 (CMD) of GCs like NGC 6397 (King et al.
phases(Grattonetal.2001).Finally,thewhole
v 1998, Richer et al. 2007) fully support this
starsareinterested(Cohenetal.2002).
5 paradigm of GCs hosting simple stellar pop-
7 ulations. However,thereis a growingbodyof Inaddition,sincethesixtieswe knowthat
1
observationalfactswhichchallengethistradi- thehorizontalbranches(HB)ofsomeGCscan
3
tional view. Since the eighties we know that be rather peculiar. In some GCs the HB can
.
1 GCs showa peculiarpatternin theirchemical be extended to very hot temperatures, imply-
0 abundances(Grattonelal.2004forareview). ing the loss of most of the stellar envelope
8
While they are generally homogeneous inso- (seecompilationbyRecio-Blancoetal.2006).
0
far Fe-peak elements are considered, they of- ThedistributionofthestarsalongtheHBcan
:
v ten exhibit large anticorrelations between the be clumpy, with the presence of one or more
i abundances of C and N, Na and O, Mg and gaps (Ferraro et al. 1998, Piotto et al. 1999).
X
Al.Theseanticorrelationsareattributedtothe This problem, usually known as the the sec-
r
a presenceatthestellarsurfacesofafractionof ond parameter problem, still lacks of a com-
theGCstarsofmaterialwhichhaveundergone prehensive understanding:many mechanisms,
Hburningattemperaturesofafewtenmillions and many parameters have been proposed to
K (Prantzosetal. 2007).Thispatternispecu- explain the HB peculiarities, but none appar-
liartoGCstars.Fieldstarsonlyshowchanges entlyisabletoexplaintheentireobservational
scenario.Itiswellpossiblethatacombination
Sendoffprintrequeststo:G.Piotto of parameters is responsible for the HB mor-
4 Piotto:MultiplePopulationsinGCs
phology (Fusi Pecci et al. 1993). Surely, the Insummary,anumberofapparentlyinde-
totalclustermassseemstohavearelevantrole pendent observational facts seems to suggest
(Recio-Blancoetal.2006). that,atleastinsomeGCs,therearestarswhich
have formed from material which must have
been processed by a previous generation of
stars.Thequestionsis:dowehavesomedirect,
observationalevidenceofthepresenceofmul-
tiple populationsin GCs? Very recentdiscov-
eries,madepossiblebyhighaccuracyphotom-
etryondeepHST images,allowedustoposi-
tivelyanswertothisquestion.Inthispaper,we
willsummarizethesenewobservationalfacts,
and briefly discuss their link to the complex
abundancepatternandtotheanomalousHBs.
2. DirectObservationalEvidenceof
MultiplePopulationsinGCs
Fig.1. The double MS of ω Centauri. The bluest
MS is more metal rich, and it can be reproduced
by models only assuming a very high He content
0.35<Y<0.40(fromPiottoetal.2005).
Itistemptingtorelatethesecondparame-
ter problemtothe complexabundancepattern
ofGCs.SincehighNaandlowOabundances
are signatures of material processed through
hotH-burning,theyshouldbeaccompaniedby
highHe-contents(D’Antona&Caloi2004).In
most cases, smallHe excesses up to dY∼0.04
(thatisY∼0.28,assumingtheoriginalHecon-
tentwastheBigBangone)areexpected.While
this should have small impact on colors and
magnitudesof stars up to the tip of the RGB,
alargeimpactisexpectedonthecolorsofthe Fig.2.ThemultipleSGBinOmegaCentauri.There
HB stars, since He-rich stars should be less areatleast4distinctSGBs,plusasmallfractionof
stars spreaded between SGB-C and SGB-D (from
massive. E.g., in the case of GCs of interme-
Villanovaetal.2007)
diate metallicity ([Fe/H] -1.5), the progenyof
He-rich,Na-rich,O-poorRGBstarsshouldre-
side on the blue part of the HB, while that of The first, direct observational evidence of
the ”normal” He-poor, Na-poor, O-rich stars the presence of more than one stellar popula-
shouldbe withinthe instabilitystrip orredder tion in a GCs was published by Bedin et al.
than it. However, within a single GC a corre- (2004).Bedinetal.foundthat,forafewmag-
lation is expected between the distribution of nitudes below the turn-off (TO), the main se-
masses (i.e.colors)oftheHB-stars andofNa quence(MS) ofω Centaurisplitsin two (Fig.
andOabundances. 1).Indeed,thesuspectofaMSsplitinωCen
Piotto:MultiplePopulationsinGCs 5
ple sub-giant branch (SGB) with at least four
distinct components (Fig. 2) characterized by
different metallicities and ages (Sollima et al.
2005, Villanova et al. 2007; the latter has a
detailed discussion.) A fifth, more dispersed
componentis spread between the SGB-C and
SGB-DofFig.2.
Theseresultsreinforcedthe suspicionthat
the multiple MS of ω Cen could just be an
additionalpeculiarityofanalreadyanomalous
object, which might not even be a GC, but a
remnant of a dwarf galaxy instead. In order
toshedmorelightonthepossiblepresenceof
multiple MSs in Galactic GCs, we undertook
an observational campaign with HST, prop-
erlydevisedtosearchmultiplesequencesatthe
leveloftheupper-MS,turn-off(TO),andSGB.
Fig.3. The spectacular triple MS of NGC 2808. Thenewdataallowedustoshowthatthemul-
Note the narrowness of the turnoff. The arrow in- tipleevolutionarysequencephenomenonisnot
dicatesthereddeningline. apeculiarityofωCentaurionly.
AsshowninFig.3,alsotheCMDofNGC
2808 is splitted into three MSs (Piotto et al.
wasalreadyraisedbyJayAndersoninhisPhD 2007).Because ofthenegligibledispersionin
thesis,buttheresultwasbasedononlyoneex- Fe peak elements(Carretta et al 2006),Piotto
ternalWFPC2field,andthisfindingwassoun- et al. (2007) proposed the presence of three
expectedthathedecidedtowaitformoredata groupsofstarsinNGC2808,withthreediffer-
andmoreaccuratephotometrytobesureofits ent He contents, in order to explain the triple
reality. Indeed, Bedin et al. (2004) confirmed MSofFig.3.Thesegroupsmaybeassociated
theMSsplitinJayAndersonfieldandinanad- tothethreegroupswithdifferentOxygencon-
ditionalACSfieldlocated17arcminfromthe tentdiscoveredbyCarrettaetal.(2006).These
clustercenter.Now,weknowthatthemultiple results are also consistent with the presence
MS is present all over the cluster, though the of a multiple HB, as discussed in D’Antona
ratioofbluetoredMSstarsdiminishesgoing and Coloi (2004)and D’Antonaet al. (2006).
from the cluster core to its envelope (Sollima Finally,wenotethatthenarrownessoftheTO
etal.2007,Bellinietal.2008,inpreparation). region displayed by Fig. 3 suggests that the 3
Themoreshockingdiscoveryonthemulti- stellar populationsof NGC 2808 must have a
plepopulationsinωCen,however,camefrom smallagedispersion,muchlessthan1Gyr.
afollow-upspectroscopicanalysisthatshowed At the meeting, someone raised the sus-
that the blue MS has twice the metal abun- pectthatthemultiplesequencesinNGC2808
dance of the dominant red branch of the MS may be an artifact of differential reddening.
(Piotto et al. 2005). The only isochrones that Indeed, it is well known since Walker (1999)
wouldfitthiscombinationofcolorandmetal- thatNGC 2808suffersa small amountofdif-
licity(cf.Fig.1)areextremelyenrichedinhe- ferential reddening. Bedin et al. (2000) cal-
lium (Y ∼ 0.38) relative to the dominantold- culated a differential reddening of the order
populationcomponent,whichpresumablyhas of 0.02 magnitudes in E(B-V) from the clus-
primordialhelium. tercenterto∼400arcsecfromthecenter.The
Indeed,thescenarioinωCenisevenmore split of the sequences in Fig. 3 is more than
complex.As it is alreadyevidentin the CMD an order of magnitude larger than this value.
of Bedin et al. (2004), this object has at least In any case, being aware of the presence of
three MSs, which spread into a highly multi- differentialreddening,wecorrectedforit.The
6 Piotto:MultiplePopulationsinGCs
Fig.4. Original,notcorrectedfordifferentialreddeningCMDofNGC2808inthefourquadrantsofthe
ACSfield.ThetripleMSiswellvisible,andwithexactlythesameshapeinallquadrants.Thesmalloffsets
ofthefiduciallineofthereddestMSindicatesthelevelofdifferentialreddeningwithintheACSfield.
CMD of NGC 2808 published in Piotto et al. MSs of Fig. 3 tend to divergefrom the TO to
(2007)andreproducedinFig.3hasbeencor- fainter magnitudes, while differential redden-
rected for differential reddening, adopting the ingwouldsimplycauseashiftofthesequences
proceduredescribedinSarajedinietal.(2007), paralleltothereddeningline.Finally,differen-
whichallowsalsotocorrectforsmall,residual tial reddening tends to randomly broaden the
spatialvariationofthe photometriczeropoint CMD sequences, not to create coherent fea-
which affects the ACS data (see Sarajedini et tures,which,wenote,areexactlythesameev-
al. 2007 for more details). There are addi- erywhere within the ACS field, as shown in
tional arguments against the differential red- Fig.4.
deningargument.Firstofall,theTO-SGBre-
AlsoNGC1851musthaveatleasttwo,dis-
gionsoftheCMD areverynarrow(thisisthe
tinct stellar populations. In this case the ob-
regionoftheCMDwhichshouldbemostlyaf-
servational evidence comes from the split of
fected by differential reddening, as shown by
the SGB in the CMD (Fig. 5) of this clus-
thereddeninglineplottedinFig.3).Thethree
ter (Milone et al. 2007). Would the magni-
Piotto:MultiplePopulationsinGCs 7
tude differencebetweenthe two SGBs be due
only to an age difference, the two star forma-
tion episodes should have been separated by
at least 1 Gyr. However, as shown by Cassisi
et al. (2007), the presence in NGC 1851 of
two stellar populations, one with a normal α-
enhancedchemicalcomposition,andonechar-
acterized by a strong CNONa anticorrelation
pattern could reproduce the observed CMD
split. In this case, the age spread between the
two populations could be much smaller, pos-
sibly consistentwith thesmall agespreadim-
pliedbythenarrowTOofNGC2808.Inother
terms,theSGBsplitwouldbemainlyaconse-
quence of the metallicity difference, and only
negligiblyaffectedby(asmall)agedispersion.
Cassisietal.(2007)hypothesisissupportedby
the presence of a group of CN-strong and a
Fig.6. Afterthecorrectionfordifferentialredden-
groupofCN-weakstarsdiscoveredbyHesser
ing,alsoNGC6388showsadoubleSGB(Piottoet
etal.1982,andbyarecentworkbyYongand al.2008,inpreparation).
Grundahl(2007) who find a NaO anticorrela-
tionamongNGC1851giants.
toassociatethebrighterSGBstarstotheCN-
normal,s-processelementnormalstarsandto
the red HB, while the fainter SGB should be
populated by CN-strong, s-process element-
enhanced stars which should evolve into the
blue HB. In this scenario, the faint SGB stars
should be slightly younger (by a few 107 to
a few 108 years) and should come from pro-
cessed material which might also be moder-
ately He enriched, a fact that would help ex-
plaining why they evolve into the blue HB.
By studying the cluster MS, Milone et al.
(2007) exclude an He enrichment larger than
∆Y=0.03), as expected also by the models of
Cassisi et al. (2007). Nevertheless, this small
He enrichment, coupled with an enhanced
mass loss, would be sufficient to move stars
from the red to the blue side of the RR Lyrae
instabilitystrip.Directspectroscopicmeasure-
Fig.5. The double SGB in NGC 1851. The two mentsofthe SGBandHBstarsinNGC1851
SGBs are separated by about 0.12 magnitudes in
arebadlyneeded.
F606W(fromMiloneetal.2007)
Thereisat leastanotherclusterwhich un-
doubtedlyshowsasplitintheSGB:NGC6388
NGC1851isconsideredasortofprototype (Piotto et al. 2008, in preparation). Figure 6
of bimodal HB clusters. Milone et al. (2007) shows that, even after correction for differen-
note that the fraction of fainter/brighter SGB tial reddening,the SGB ofNGC 6388closely
stars is remarkably similar to the fraction of resemblestheSGBofNGC1851.NGC6388,
bluer/redderHBstars.Therefore,itistempting as well as its twin cluster NGC 6441, are
8 Piotto:MultiplePopulationsinGCs
two extremely peculiar objects. Since Rich et amongLMCclusters.Vallenarietal.(1994)al-
al. (1997), we know that, despite their high readysuggestedthepossibilityofthepresence
metalcontent,higherthanin47Tucanae,they of two stellar populations in the LMC clus-
have a bimodal HB, which extends to ex- terNGC1850.A quickanalysisoftheCMDs
tremely hot temperatures (Busso et al. 2007), of about 50 clusters from ACS/HST images
totally un-expectedfor this metal rich cluster. showsthatabout10%ofthemmightshowev-
NGC 6388 stars also display a NaO anticor- idence of multiple generations (Milone et al.
relation (Carretta et al. 2007). Unfortunately, 2008,inpreparation).
available data do not allow us to study the
MS of this cluster, searching for a MS split.
3. Discussion
Hopefully,newdatacomingfromtheHSTpro-
gram GO11233 should help to constrain the
Sofar,wehaveidentifiedfourGalacticglobu-
MS width, and therefore the He dispersion.
larclustersforwhichwehaveadirectevidence
In this context, it is worth noting that Caloi
ofmultiplestellarpopulations,andtheyareall
and D’Antona (2007), in order to reproduce quitedifferent:
the HB of NGC 6441, propose the presence 1) In ω Centauri (∼ 4×106 M ), the dif-
⊙
of three populations, with three different He
ferentpopulationsmanifestthemselvesbothin
contents,onewithanextremeHeenhancement
a MS split (interpreted as a split in He and
ofY=0.40.Suchastrongenhancementshould
metallicityabundances)andinaSGBsplit(in-
be visible in a MS split, as in the case of ω
terpreted in terms of He, metallicity, and age
CenandNGC2808.AstrongHeenhancement variations>1Gyr),whichimpliesatleastfour
and a consequent MS split may also apply to differentstellargroupswithinthesamecluster,
NGC 6388, because of the many similarities
which formed in a time interval greater than
withNGC6441.
1 Gyr. Omega Centauri has also a very ex-
One more cluster, M54, shows a complex tended HB (EHB), which extends far beyond
CMD(see,e.g.,Siegeletal.2007).Thiscluster 30.000K.
hasbeenshown,however,intoo manypapers 2) In NGC 2808 (∼ 1.6 × 106 M ), the
⊙
tocitehere,tobeapartoftheSagittariusdwarf multiple generation of stars is inferred from
galaxy that is in process of merging into the the presence of three MSs (also in this case
Milky Way, and very possibly the nucleus of interpreted in terms of three groups of stars,
thatgalaxy.Actually,itisstillmatterofdebate with different He content), possibly linked
whichpartsoftheCMDofM54representthe to three stellar groups with different oxygen
cluster population and which ones are due to abundances, and possibly to the multiple HB.
theSagittariusstars.M54mayhaveacomplex Theagedifferencebetweenthe3groupsissig-
stellar populations as ω Cen, though this fact nificantlysmallerthan1Gyr.AlsoNGC2808
willbemuchhardertodemonstrate. hasanEHB,extendedasmuchastheHBofω
Finally, we note that the multiple popu- Cen.ItshowsanextendedNaOanticorrelation.
lation phenomenon in star clusters may not 3) In the case of NGC 1851 (∼ 1.0×106
be confined only to Galactic GCs. Mackey & M ), we have evidence of two stellar groups
⊙
Broby Nielsen (2007)suggestthe presenceof from the SGB split. It is difficult to estab-
twopopulationswithanagedifferenceof∼300 lish theage differencebetweenthe twostellar
Myrinthe2GyroldclusterNGC1846ofthe populationswithoutadetailedchemicalabun-
Large Magellanic Cloud (LMC). In this case, dance analysis. However, available observa-
thepresenceofthetwopopulationsisinferred tional evidence seems to imply that the SGB
bythepresenceoftwoTOsintheCMD.These splitmaybeduetoadifferenceinCN,Na,O,
twopopulationsmayeitherbetheconsequence and s-process elements, while the age differ-
ofatidalcaptureoftwoclustersorNGC1846 encecouldbesmall(e.g.assmallasinthecase
may be showing something analogous to the ofNGC2808).Ontheotherhand,iftheSGB
multiple populationsidentifiedin the Galactic splitwouldbedueonlytoage,thetwostarfor-
GCs. NGC 1846 might not be an exception mationepisodesshouldhave happenedwith a
Piotto:MultiplePopulationsinGCs 9
timeseparationof∼ 1Gyr.Fromtheanalysis two requisites: (i) temperature should be high
of the cluster CMD, thereseems to be no MS enough;and(ii)thestarswheretheburningoc-
split,whichwouldimplyasmallHespread,if cursshouldbeabletogivebacktheprocessed
any(∆Y < 0.03).TheclusterhasnoEHB,but material to the intracluster matter at a veloc-
it shows a bimodalHB. It shows a NaO anti- ity low enough that it can be kept within the
correlation. GC itself (a few tensof km/s).Candidatesin-
4)InNGC6388(∼1.6×106 M⊙)wehave clude: (i) Massive (M > 10M⊙) rotating stars
evidence of two stellar groups from a SGB (Decressin et al. 2007); (ii) the most massive
split. With the available observational data it amongtheintermediatemassstarsundergoing
is not possible to establish whether there is a hot bottom burning during their AGB phase
split in the MS of this GC. NGC 6388has an (Venturaetal.2001).Thetwomechanismsact
EHB, possibly as extended as in the cases of ondifferenttimescales(107and108yr,respec-
NGC 2808 and ω Cen. It shows an extended tively),andbothsolutionshavetheirprosand
NaOanticorrelation. cons. The massive star scenario should avoid
Another massive (∼ 2.0 × 106 M ) GC, mixture of O-poor, Na-rich material with that
⊙
M54,issuspectedtohostmultiplepopulations, richinheavyelementsfromSNe,whileitisnot
though the analysis is strongly hampered by clear how the chemically processed material
the contamination of the Sagittarius galaxy. could be retained by the proto-clusterin spite
Also M54 has an EHB, similar to the HB of ofthefastwindsandSNexplosionsalwaysas-
NGC2808andωCen(Rosenbergetal.2004). sociated to massive stars. Producing the right
AtleastoneLMCintermediateagecluster patternofabundancesfrommassiveAGBstars
showsapopulationsplitattheleveloftheTO: seems to require considerable fine tuning. In
NGC 1846.This is a massive clusters, among addition,bothscenariosrequirethateitherthe
the most massive LMC clusters according to IMFofGCswasveryheavilyweightedtoward
Chrysovergis et al. (1999), though probably massive stars, or that some GCs should have
not as massive as the above Galactic GCs (a lost a major fraction of their original popula-
moreaccuratemassestimateforthisclusteris tion (Bekki and Norris 2006), and then may
needed).Other LMCclustersare suspectedto evenbetheremnantsoftidallydisrupteddwarf
showsasimilarTOsplitting. galaxies,assuggestedbythecomplexityinthe
Many GCs are clearly not simple, single- CMDofωCenandM54.
stellar-population objects. The emerging ev- Also in view of these problems, it will be
idence is that the star-formation history can importanttounderstandtheroleofthecapture
vary strongly from GC to GC, and that, GCs offieldstarsbymassiveformingglobularclus-
ters, as suggested by Kroupa (1998), see also
areabletoproduceveryunusualobjects,asno
such He-rich MS stars have ever been found Fellhaueretal.2006).
The observational scenario is becoming
elsewhere. At the moment, we can note that
morecomplex,but,thenewresultsmighthave
the three GCs in which multiple generations
of stars have been clearly identified (Omega indicated the right track for a comprehensive
understandingofthe formationandearlyevo-
Cen,NGC2808,andNGC1851),andthetwo
other GCs expected to contain more than one lutionofGCs.Weareperhapsforthefirsttime
stellargeneration(NGC6388andNGC6441: close to compose what has been for decades
and still is a broken puzzle, which includes
Fig 6, Caloi & D’Antona 2007, Busso et al.
2007)areamongthetenmostmassiveGCsin theclusterstarchemicalanomalies,theHBpe-
ourGalaxy.Thisevidencesuggeststhatcluster culiarities,and,now,themultipleevolutionary
sequences.
mass might have a role in the star-formation
historyofGCs.
Acknowledgements. I wish to warmly thank J.
Reconstruction of this star-formation his-
Anderson, AndreaBellini,LuigiR.Bedin, IvanR.
tory requires a a better understanding of the King, Antonino P. Milone, without whom most of
chemicalenrichmentmechanisms,butthesite theresultspresentedinthisreviewwouldnothave
of hot H-burning remains unclear. There are beenpossible.IwishalsotothankSandroVillanova
10 Piotto:MultiplePopulationsinGCs
for his help in the spectroscopic investigation of Gratton,R.etal.2001,A&A,369,87
Omega Centauri main sequence and SGB stars. A Gratton, R.; Sneden, C.; Carretta, E. 2004,
specialthankstoAlvioRenziniandRaffaeleGratton ARAA,42,385
forthemanyenthusiasticdiscussionsonthesubject Hesser, J. E., Bell, R. A., Harris, G. L. H., &
ofmultipopulationsinglobularclusters. Cannon,R.D.1982,AJ,87,1470
IwillforeverbegratefultoVittorio,who,more
Kroupa,P.1998,MNRAS,300,200
thananyoneelse,gavemetheopportunitytoappre-
King, I. R., Anderson, J., Cool, A., & Piotto,
ciatetheluckwehavetodoresearchinAstronomy.
G.1998,ApJ,492,L37
CiaoVittorio.
Mackey, A. D. & Broby Nielsen, P. 2007,
MNRAS,379,151
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