Three Parameters for the Horizontal-Branch Morphology in Globular Clusters

0.51 MB·English

by  A. P. Milone

Mem.S.A.It.Vol.
(cid:13)c SAIt 2010 Memoriedella
Three Parameters for the Horizontal-Branch
Morphology in Globular Clusters.
A.P.Milone1,2
3
1
0 1 Research School of Astronomy and Astrophysics, The Australian National University,
2 CotterRoad,Weston,ACT,2611,Australiae-mail:[email protected]
n 2 Instituto de Astrof`ısicade Canarias, and Department of Astrophysics University of La
a Laguna,E-38200LaLaguna,Tenerife,CanaryIslands,Spain
J

0
2 Abstract. Thehorizontal branch(HB)morphology of globularclusters (GCs)ismainly
describedbymetallicity.Thefactthatsomeclusterswithalmostthesamemetallicityexhibit

] different HBdemonstratesthatotherparametersareatwork.Wepresent resultsfromthe
R
analysis of the CMD of 72 GCs obtained with the Advanced Camera for Survey (ACS)
S of the Hubble Space Telescope (HST). We find a significant correlation between the HB
. color extension andthemassof thehosting cluster,whilethecolordistancebetween the
h
HBandthered-giant branch(RGB)depends onmetallicityandage.Wesuggest thatage
p
andmetallicityarethemainglobalparametersoftheHBmorphologyinGCs,whiletheHB
-
o extensionismainlyduetointernalheliumvariation,associatedtomultiplepopulations.
r
st Keywords.Stars:PopulationII-Galaxy:globularclusters–
a
[

1 1. Introduction Recently,Dotteretal.(2010)measuredthe
v median color difference between the HB and
7 Since the early fifties, metallicity has been the RGB (∆(V-I))from ACS/HST photometry
1 consideredthemainparameterthatdetermines of sixty GCs and demonstrated that, after the
6 the HB morphology in GCs. Few years later,
dependence with the metallicity is accurately
4 the evidence that some clusters with similar
removed, ∆(V-I) correlates with cluster age.
1. metallicity exhibit different HBs already sug- Also the total mass of a GC certainly plays a
0 gestedthatatleastonesecondparameterisre- roleonitsHBmorphology.Recio-Blancoetal.
3 quired to properly characterize the HB mor- (2006) discovered that more massive clusters
1 phology of GCs. Since then, the so called
tendtohaveHBsmoreextendedtohighertem-
: second-parameterproblemhasbeenwidelyin-
v perature.FusiPeccietal.(1993)foundthatthe
i vestigated by many authors. Several candi- extensionof the HB and the presence and ex-
X dates have been suggested as possible sec-
tent of blue tails in particular are correlated
r ond parameters but a comprehensive picture with the cluster density and concentrations,
a
is still lacking. We refer the reader to the pa-
withmoreconcentratedordenserclustershav-
pers by Catelan (2009); Dotteretal. (2010);
ingalsobluerandmore-extendedHB.
Grattonetal.(2010)forreviews.
The possibility of self-enrichment in he-
lium as responsible of the HB shape has
Sendoffprintrequeststo:A. P.Milone
A. P. Milone:ThreeParametersfortheHBMorphologyinGCs 283
been investigated by several authors, as mul-
tiple stellar populations with different he-
lium abundance can indeed explain fea-
tures such us tails and multimodalities in
the HBs of GCs (e.g.D’Antonaetal. (2002);
Grattonetal.(2010)).Theideaofaconnection
between multiple populations and HB mor-
phology rises in the early eighties, when pio-
neering papers showed that the cyanogendis-
tribution is closely connect with shape of the
HB(e.g.Norris(1981))andisconfirmedbyre-
centstudiesofHBstars.
InthiscontexttheM4representsastrongcase.
ThisGChoststwostellarpopulationswithdif-
ferent Na and O abundance that define two
RGBs in the U versus U − B CMD. The HB
of M4 is also bimodal and is well-populated
Fig.1. Upper panel: Normalized histogram color
both to the red and the blue side of the RR
distribution of stars in the HB (blue histogram)
Lyrae gap. The bimodality in Na and O is
and RGB sample (red histogram) for NGC5904.
alsopresentamongtheHB.Blue-HBstarsbe-
The twosample of HB and RGB stars arecolored
long to the second generation and are O-poor
blueandred,respectivelyinthelower-panelCMD,
and Na-rich, while red-HB stars are first gen- wherewealsoshowthepoints P , P , P andthe
A B C
eration (Marinoetal. (2008, 2011)). Similar L1andL2segments(seetextfordetails).
analysis of HB stars in other GCs show that
first-generation stars populate the reddest HB
segment, while second-generation HB stars orscorrespondtotheforthandtheninety-sixth
have bluer colors (e.g.Villanovaetal. (2009); percentileofthecolordistributionofHBstars.
Grattonetal.(2011);Lovisietal.(2012)). The color of the third point P is assumed as
C
Inthispaperweusethehomogeneousphotom- the median color of RGB stars. L1 is defined
etryfromACSSurveyofGCs(Sarajedinietal. asthecolordifferencebetweenP ansP ,and
C B
(2007); Dotteretal. (2011) to re-investigate L2asthedistancebetweenP andP .
B A
theHBmorphologyatthelightofthenewfind-
Inthefollowingweinvestigatetherelation
ingsonmultiplepopulationsinGCs.
betweentheL1andL2quantitiesandsomepa-
rametersoftheirhostGCs.Figure2shows L1
2. The L1and L2parametersto against [Fe/H]. An inspection of this plot re-
vealsthatallthemetal-richGCshavesmallL1
describetheHBmorphology
valuesandhenceexhibitthered-HB.Atlower
TodescribetheHB,wedefinedtwoquantities: metallicities,thereareclusterswithalmostthe
i) L1, which is representative of the distance sameironabundanceandyetdifferent L1val-
between the RGB and the coolest part or the ues.Thisreflectsthewell-knownphenomenon
HB, and ii) L2 that indicates the color exten- that while in some GCs the red HB is absent,
sionoftheHB. otherclusterswithalmostthesamemetallicity
The procedure to determine L1 and L2 is il- hostred-HBstars.
lustrated in Fig. 1 for the case of NGC5904. Thefactthatclusterspopulatedistinctregions
Briefly, we selected by eye a sample of HB intheL1versus[Fe/H]plane,allowsustode-
stars, and a sample containing all RGB stars fine three groups of GCs: i) The first group,
thatdifferbylessthan±0.1F606Wmagfrom ‘G1’,includesallthemetal-richGCs([Fe/H]>
the mean level of the HB (F606W , see −1.0); ii) the second one, ‘G2’, is made of
HB
Dotteretal. (2010)). Then, we have defined clusters with [Fe/H]< −1.0 and L1 <0.4; iii)
two pointsontheHB, P and P , whosecol- the remaining GCs with L1 >0.4 belong to
A B
284 A. P. Milone:ThreeParametersfortheHBMorphologyinGCs
the group ‘G3’. Since the second-parameter
phenomenon is more evident among metal-
poorclusters,wefurtherdefineafourthgroup
(‘G2+G3’),includingalltheGCsinthegroups
‘G2’ and ‘G3’. Both ‘G2’ and ‘G3’ clusters
Fig.2.L1vs. clustermetallicity(fromHarris1996,
2010edition)for72GCs.‘G1’,‘G2’,and‘G3’clus-
tersarecoloredred,green,andblue,respectively.
Fig.3. Upper panel: logarithm of the maximum
helium difference between cluster subpopulations
(∆Y)asafunction of L2.Lowerpanel: L2vs. ab-
exhibitsignificantcorrelationbetween L2 and soluteluminosity(fromHarris1996, 2010edition)
theabsoluteclusterluminosity(mass).Thisis fortheGCsstudiedinthispaper.
shown in the lowerpanel of Fig. 3, where we
plot L1 as a function of M . The Spearman’s
V
rank correlation coefficient is r =−0.86, and L1isplottedasafunctionofclusteragein
G2
r =−0.72forthe ‘G2’and ‘G3’ sample, and thelowerpanelofFig.4,whilethehistograms
G2
r =−0.78 for ‘G2+G3’ GCs. There is no of the age distributions for ‘G1’, ‘G2’, and
G1+G2
significantcorrelationbetweenL1andM . ‘G3’GCsareshownintheupperpanel.L1and
V
Recent papers, have shown that the CMDs of age are significantly correlated for ‘G2+G3’
GCsaretypicallymadeofmultiplesequences clusters (rG1+G2=0.75), with ‘G3’ GCs being,
that can be followed continuously from the on average,older than ‘G2’ ones. There is no
MS up to the RGB, and that correspond to significantcorrelationbetweenL2andage.
stellar populations with different helium con-
tent(Miloneetal.(2012a)).Themaximumhe-
3. Discussion
liumvariationchangesfromoneclustertoeach
other and ranges from ∆Y∼0.14 for the mas- Freeman&Norris(1981)suggestedthat,apart
sive NGC2808 and ωCen (e.g. Piottoetal. from metallicity, at least two parameters are
(2007); Kingetal. (2012)) to ∆Y∼0.01 for needed to explain the HB morphology. One
NGC6397 (Miloneetal. (2012b)). The upper of these should be a global parameter that
panel of Fig. 3 shows that L2 is correlated varies from cluster to cluster, and the other a
with∆Y,inthesmallnumberof‘G2’and‘G3’ non-global parameter that varies within the
wherethismeasureisavailable. cluster.
A. P. Milone:ThreeParametersfortheHBMorphologyinGCs 285
multiplepopulations,arethemainresponsible
oftheHBextension.
Acknowledgements. Results of this paper come
from the work of several persons. I thank
J.Anderson, A.Aparicio, L.R.Bedin, A.Dotter,
A.F. Marino, M.Monelli, G.Piotto, A.Recio
Blanco, A.Sarajedini and the ACS GCs treasury
group.
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