A new Extended Main Sequence Turnoff star cluster in the Large Magellanic Cloud

1.1 MB·English

by  Andrés E. Piatti

Mon.Not.R.Astron.Soc.000,000–000 (0000) Printed9January2013 (MNLATEXstylefilev2.2)
A new Extended Main Sequence Turnoff star cluster in the
Large Magellanic Cloud
3 Andr´es E. Piatti⋆
1
0 Instituto de Astronom´ıa y F´ısica del Espacio, CC67, Suc. 28, 1428, Ciudad de Buenos Aires, Argentina
2

n
a
J 9January2013

8

ABSTRACT
]
A We present results on the age and metallicity estimates of the poorly studied LMC
clusterSL529,fromCCDSDSSgrphotometryobtainedattheGeminiSouthtelescope
G
with the GMOSattached.The cluster MSTO regionpossessesanextended structure,
h. with an age spread (∼ 0.5 Gyr) bigger than the mean age width of known EMSTO
p LMC clusters.We reportfor the firsttime ameancluster ageof2.25Gyranda mean
- clustermetallicityofZ=0.004,whichplaceitasthemostmetal-poorandoldestcluster
o
in the EMSTO LMC cluster group. In addition, the cluster RC appears to be formed
r
t by two concentrations of stars - although it is not clear whether this feature can be
s
a caused, in part, by binary interactions and mergers -, whereas the cluster core radius
[ of4.2 pc is in excellentagreementwith those determined for the previously12 known

EMSTO LMC clusters.
1
v Key words: techniques: photometric – galaxies: individual: LMC – Magellanic
7 Clouds – galaxies: star clusters.
2
6
1
.
1 1 INTRODUCTION Bica, 2010), thus representing ∼ 3% of the estimated LMC
0
population of clusters formed at thebursting epoch.
3 Up to date, only 12 Large Magellanic Cloud (LMC) star
1 clustershavebeen uncoveredtopossess ExtendedMain Se- In this paper, we report for the first time age and
v: quenceTurnoffs(EMSTOs),namely:NGC1751,1783,1806, metallicity estimates for SL529 (R.A.= 5h31m06s, Dec.=
i 1846, 1852, 1917, 1987, 2108, 2154, 2173, Hodge7, and -63◦32′23′′, J2000), a poorly studied LMC star cluster lo-
X
SL862 (see, e.g., Bertelli et al. 2003, Milone et al. 2009). cated in theouterdisk of thegalaxy. Theresults showthat
r The age range of this cluster sample is ∼ 1-2 Gyr, while thisclusterbelongstothehandfulofEMSTOclustersinthe
a
a roughly constant metal content of [Fe/H] = -0.4 dex has LMC.Theimpactofthisfindingwouldappeartobetwofold:
been reported for them Mackey & Broby Nielsen (2007), first, we actually found a new LMC cluster formed at the
Mackeyetal.(2008),Goudfrooijetal.(2009),Girardietal. beginingoftheburstingformationepochwhichexhibitsthe
(2009).Theseageandmetallicityrangescoincidewiththose EMSTOphenomenon.Tothisrespect,wedoshownotonly
of an important burstingclusterformation epoch that took evidence about the age spread in the cluster MSTO, but
placeintheLMC∼2Gyrago,possibleduethetidalinter- also that the magnitude of the age spread exhibited by its
action between both Magellanic Clouds and, perhaps, also stellarpopulationsiscorrelated withtheclustercoreradius
the Milky Way. Particularly, Piatti (2011) found that the (Keller et al. 2011). Additionally, we identified two concen-
number of studied clusters with ages between 1 and 3 Gyr trationsofstarsinRedClump(RC),althoughitisnotclear
nowdoublesthatofthepopulationofclustersformedatthe whetherthisfeaturecanbecaused,inpart,bybinaryinter-
bursting epoch, thus increasing the probability that more actions and mergers (Yanget al. 2011).
EMSTOstarclustersbeidentified.However,fromatotalof
some70starclusterswithageestimateswithinouragerange Second, SL529 resulted to be the oldest (age ∼ 2.25
ofinterest,thelist ofknownEMSTOclustershasremained Gyr)andthemostmetal-poorEMSTOLMCcluster([Fe/H]
unchanged. The relatively small numberof known EMSTO ∼-0.7dex).Theclusterageisclosetothelimitestimatedby
LMC star clusters is even more noticeable when consider- Kelleretal.(2011,age∼2.3Gyr)fromwhichtheagespread
ingthe2268starclusterscataloged intheLMC(Bonatto& of the cluster stellar populations represents a diminishing
fraction of thecluster age, so that themultiplepopulations
become increasingly harder to resolve photometrically. On
theotherhand,theresultingclustermetallicityislowerthan
⋆ E-mail:[email protected] anyknown1-3GyroldLMCcluster(Piatti & Geisler 2012),
2 A.E. Piatti
makingitainterestingcasetoconstrainEMSTOmodelsat
different overall chemical abundancelevels.
This paper is organized as follows. In Section 2 we
describe the data collected, the reduction procedures per-
formed, and the subsequent photometry standardization.
Section 3 deals with the analysis of the data, from which
clusterage,metallicity,reddeningandstructuralparameters
havebeen derived.InthisSection weprovidewith observa-
tional evidence about the status of SL529 as an EMSTO
LMC cluster. Finally, we summarize our results in Section
4.
2 DATA HANDLING
BasedondataobtainedfromtheGeminiScienceArchive,we
collected CCD SDSSgr (Fukugitaet al., 1996) images cen-
tredon26LMCclusters(GS-2010B-Q-74,PI:Pessev)along
with observations of standard fields and calibration frames
(zero, sky-flat, dome-flat). The data were obtained at the
GeminiSouthtelescopewiththeGeminiMulti-ObjectSpec-
−1
trograph (GMOS) attached (scale = 0.146 pixel ). The
data consist in 2 exposures of 30 s in g and 2 exposures Figure 1. Magnitude and colour errors as a function of g for
of 15 s in r for each cluster under seeing conditions better starswhichpassedalltherequiredphotometricrestrictions.
′′
than 0 .6 and with airmass of 1.25-1.40. Nine Gemini Ob-
servatory standard fields were observed along the 5 cluster
observing nights,for which 2 exposures of 5s perfilter and
airmass in therange ∼ 1.0-2.0 were obtained.
The data reduction followed the procedures docu-
1
mented by the Gemini Observatory webpage and the
IRAF2.gmos package. We processed a total of 118 images
starfinding and point-spread-function (PSF) fitting rou-
(standard star and cluster fields) by performing overscan,
tines in the daophot/allstar suite of programs
trimming corrections, bias subtraction, flattened all data
(Stetson et al. 1990). For each image, a quadratically vary-
images, etc., once the calibration frames (zeros, sky- and
ing PSF was derived by fitting ∼ 60 stars, once the neigh-
dome-flats, etc.) were properly combined. The final field of
′ ′ bourswereeliminatedusingapreliminaryPSFderivedfrom
view of the images resulted to be∼ 5.6 × 5.6.
thebrightest,leastcontaminated∼20stars.Bothgroupsof
Dependingonthenight,nearly30-50independentmag-
PSFstarswereinteractivelyselected.Wethenusedtheall-
nitudemeasuresofstandardstarswerederivedperfilterus-
star program to apply the resulting PSF to the identified
ing the apphot task within IRAF, in order to secure the
stellar objects and to create a subtracted image which was
transformation from theinstrumental totheSDSSgr stan-
used to find and measure magnitudes of additional fainter
dard system. The relationships between instrumental and
stars. This procedure was repeated three times for each
standard magnitudes were obtained by fittingthe following
frame.Afterderivingthephotometryforalldetectedobjects
equations:
ineachfilter,acutwasmadeonthebasisoftheparameters
g=g1+gstd+0.18×Xg+g2×(g−r)std (1) returnedbydaophot.Onlyobjectswithχ<2,photometric
error less than 2σ above the mean error at a given magni-
r=r1+rstd+0.10×Xr+r2×(g−r)std (2) tude,and |SHARP|< 0.5 were keptin each filter.
where gi, and ri (i=1,2) are the fitted coefficients, and X Finally, we standardized the resulting instrumental
represents the effective airmass. We solved the transforma- magnitudes and combined all the independent measure-
tionequationswiththefitparamstaskinIRAFandfound ments using the stand-alone daomatch and daomaster
mean colour terms of -0.058 in g, -0.049 in r. The rms er- programs,whichwerekindlyprovidedbyPeterStetson.The
rors from the transformation to the standard system were finalinformationconsistsofarunningnumberperstar,itsx
0.015 mag for g and 0.023 for r, respectively, indicating an andycoordinates,themeangmagnitudesandg−rcolours,
excellent photometric quality. the standard errors σ(g) and σ(g−r), and the number n
The stellar photometry was performed using the of averaged magnitudes and colours. A portion of Table 1,
which gives this information for a total of 5454 stars in the
fieldofSL529,isshownhereforguidanceregardingitsform
1 http://www.gemini.edu/sciops/instruments/?q=sciops/instruments andcontent,whileFig.1showstheobtainedmagnitudeand
2 IRAF is distributed by the National Optical Astronomy Ob- colour errors. The whole content of Table 1 is available in
servatories, which is operated by the Association of Universities theonlineversion of thejournal. The photometricinforma-
forResearchinAstronomy,Inc.,undercontractwiththeNational tiongatheredfortheremaining25clusterswillbepresented
ScienceFoundation. in a forthcoming paper.
LMC star clusters 3
Figure 2. Left panel: 15 s exposure r image centred on SL529. Figure3.Colour-MagnitudediagramsofSL529(left)andofits
The circles corresponding to rc and rcls are also drawn. Right surroundingfield(right)foranequalclusterarea.Themagnitude
panel: The cluster stellar radial profile (filled box) with the de- andcolourerrorbarsarealsoindicated.
rived errorbars, and the best-fitted King (1962) model superim-
posed.Theverticallinesrepresentrc andrcls.
andAr.Finally,weusedE(g−r)/AV =0.341fortheselec-
tiveextinction in theSDSS system.
When we chose subsets of isochrones for different Z
3 DATA ANALYSIS
metallcity values to evaluate the metallicity effect in the
The cluster centre [(xc,yc)=(1520±10,1100±10) pixels] cluster age, we preferred to follow the general rule of start-
was estimated using the ngaussfit task within the ing without assuming any prearranged metallicity. Instead,
IRAF.stsdas package. We then constructed the cluster ra- we adopted chemical compositions of Z = 0.002, 0.004 and
dial profile depicted in Fig. 2 (right panel), which served 0.008 ([FeH] = -1.0, -0.7, and -0.4, respectively) for the
ustoadopttheclusterradius(r )-definedasthedistance isochrone sets which cover the metallicity range of most of
cls
fromtheclustercentrewherethestellardensityprofileinter- the intermediate-age LMC clusters studied in detail so far
sects the background level- to perform circular extractions (Piatti&Geisler2012).Wethenselectedasetofisochrones
in theColour-Magnitude Diagram (CMD). Wealso fitteda ofMarigoetal.(2008)andsuperimposedthemonthecluster
King (1962) model to the normalized radial stellar density CMD,oncetheywereproperlyshiftedbythecorresponding
profile using theexpression: E(B−V) colour excesses and by the adopted LMC appar-
entdistancemodulus.Finally,weadoptedastheclusterage
1 1 2 the one corresponding to the isochrone of age = 2.25 Gyr
N/No =( − ) +bkg (3)
p1+(r/rc)2 p1+(rt/rc)2 and Z = 0.004 which best reproduces the cluster main fea-
tures in the CMD. Fig. 4 shows the result of the fitting,
whereNo isthecentralstellardensity,andrc andrt arethe whereinweplottedtheisochroneoftheadoptedclusterage
core and tidal radii, respectively. bkg represents the back- and two additional isochrones bracketing the derived age.
ground level. The values for rc, rcls, and rt turned out to Thebracketingisochrones(age=2.0and2.5Gyr)werefit-
be(110±10)pixels,(450±50)pixels,and(1300±100)pixels, tedbytakingintoaccounttheobservedspreadinthecluster
respectively.Circlescentredontheclusterwithradiirc and MSTO.Wewouldliketonotethattheestimatedclusterage
rcls are drawn in the left panel of Fig. 2, and they are also and metallicity place it as the most metal-poor and oldest
indicated with verticallines in theright panel ofthefigure. EMSTO LMC cluster.
We built two CMDs centred on the cluster, one for a In order to quantify the MSTO extension we counted
circularextractionofradiusrcls,andanotherwith aninner thenumberofstars (N)located within therectangle drawn
radius2×rclsandequalareaasforthecluster.Theresulting in Fig. 5, using as independent variable the coordinate (X)
clusterandfieldCMDsareshowninFig.3.Ascanbeseen, along the long axis, according to the precepts outlined by
the cluster MSTO appears noticeably extended, the clus- Goudfrooijetal.(2011,seetheirFig.14).Theresultingfield
ter RC shows a dual structure, and the cluster Red Giant subtracted histogram is depicted in Fig. 6, which clearly
Branch (RGB) resembles that of a single overall metallic- exhibits an extended distribution. When building the his-
ity.Allthesefeaturessuggest thatSL529couldbeincluded togramwehavetakenintoaccounttheeffectscausedbyus-
from now on in thelist of known EMSTO LMC clusters. ingdifferentbinningaswellasthepointerrors,whichcause
In order to estimate the mean cluster age, we adopted the points have the chance to fall outside their own bin if
a distance modulus of (m − M)o = 18.50 ± 0.10 they do not fall in the bin centre (Piatti & Geisler 2012);
(Glatt et al. 2010).Thereddeningvaluewastakenfromthe thus producing an intrinsic distribution. Fig. 6 also com-
Burstein&Heiles(1982),andSchlegel,Finkbeiner,&Davis paresthisresultingdistribution with thoseofGoudfrooij et
(1998) extinction maps, adopting a weighted average value al. (2011) for the known EMSTO LMC clusters NGC1751
of E(B−V) = 0.04 ± 0.01, since both the small redden- (blue line), NGC1783 (red line), NGC1806 (yellow line),
ing values as well as their errors suggest that no differen- NGC1987 (magenta line), and NGC2108 (cyan line), pre-
tial reddening would affect the cluster. We adopted R = viously normalized to the unity and shifted in order to su-
AV/E(B−V)= 3.1 toconvert thecolour excesstotheex- perimpose theyoungest ends. Ascan be seen,themost im-
tinction,andusedtheequationsAg/AV =1.199andAr/AV portant result is that we confirm the existence of an EM-
= 0.858 (Fan 1999) to evaluate the total extinctions in Ag STO(∼0.5Gyr)forSL529, whichisbiggerthanthemean
4 A.E. Piatti
coloursthantheclusterMSTO.Miloneetal.(2009),among
others,alsoclaimedthattakingintoaccountthepresenceof
binary stars alone in intermediate-age LMC clusters is not
enoughtoreproducetheirobservedEMSTOs(∆(age) > 0.3
∼
Gyr).Accordingtotheseresultsandbearinginmindtheage
spreadfoundfortheSL529’sMSTO,wealsoinferthatmore
than one stellar population would appear to be required to
explainthemorphologyoftheMSTOregion.Recently,Liet
al.(2012) haveshownthatmostCMDs,includingextended
ormultipleTOs, can beexplained usingsimplestellar pop-
ulations including both binary and stellar rotation effects,
or composite populations with two components.
Dualredclumps(RCs)havealsobeenobservedinsome
starclusterswithorwithoutEMSTOs(Girardietal.2010).
However,while Girardi et al. (2009) suggested that theori-
gin for such a structure is related to an age spread, Yang
et al. (2011) found that part of theobserved dualRCsmay
be the result of binary interactions and mergers. In order
tohighlight theRCstructureofSL529weplottedinFig. 4
anenlargementoftheRCregion,wheretwogroupsofstars
havebeen represented with blue and red circles. The error-
bar represents the star photometric errors. Notice that all
Figure4.gversusg−rclusterCMDwithisochronesofMarigo the stars are located within rcls (see Fig. 1), and a slightly
more populated picture would be obtained if we included
etal.(2008)fort=2.0,2.25,and2.5GyrandZ=0.004superim-
posed.TheinnerpaneldepictsanenlargementoftheRCregion, cluster red giants spread within 1.5rcls. The additional RC
whereintwogroupsofRCstarshavebeenrepresentedwithblue stars would have reached the outer cluster region due to
andredcircles. thedynamicalrelaxation(Santiago2009).Particularly,some
four stars would be added around (g−r, g) ∼ (0.6, 19.8)
mag. We think that this handful of stars does not consti-
age width of known EMSTO clusters (Milone et al. 2009; tute a clear evidence of the existence of a secondary RC as
Goudfrooij et al. 2011). suggested by Girardi et al. (2009). On the other hand, the
Themagnitudeoftheage spread exhibitedbythestel- RC interacting binaries modeled by Yang et al. (2011, see
lar population of an EMSTO cluster is correlated with the redpointsintheirFig.1)wouldappeartocovertheRCre-
cluster core radius (Keller et al. 2011). The 12 known EM- gionaswellastoreachfaintermagnitudes(∆V <0.5mag)
STO clusters do havecore radii bigger than rc ∼ 3.7 pc. In andbluercolours(∆(V−I)<0.1mag).Unfortunately,they
thecaseofSL529,weusedthederivedclustercoreradius(in didnotpresentCMDsforageslargerthan1.8Gyr,although
pixels)andtherecentlyestimated LMCdistanceof(53.5 ± they mentioned that interacting binaries appear mainly in
0.4) kpc (Haschkeet al. 2012) to obtain rc = (4.20 ± 0.35) clusterswith1.2Gyr<age<3.0Gyr.Despitethelackofa
pc. When entering this core radius and the cluster age in directtransformation ofour(g−r,g)CMD totheir(V −I,
their Fig. 5, SL529 falls into the visibility window of EM- V) one, the overall appearance of both RCs would seem
STO clusters, for which different authors have argued that to be similar. Further spectroscopic studies are still needed
they are witnesses of different/prolonged stellar formation in order to obtain more reliable membership status and to
epochs (Keller et al., 2012, and references therein). confirm whether the cluster RC actually contains a binary
Indeed, Mackey et al. (2008) built synthetic CMDs to star population. We would like to point out, however, that
investigatetheroleplayedbyunresolvedbinarystarsaround the age sequence in the single stellar population models of
the MSTO region of three LMC clusters (NGC1783, 1806, Yangetal.runsperpendiculartothatoftheMarigoetal.’s
and1846).TheyfoundthattheresultingCMDsforasingle isochrones.
population-correspondingtoanisochronewiththeageand
metallicity oftherespectivecluster-donotreproducealone
theEMSTOsseeninthestudiedclusters,althoughtheyin- 4 SUMMARY
cludedasignificantfractionofunresolvedbinaries.However,
when considering two stellar populations, beside a fraction In this study we present for the first time CCD SDSS
of unresolved binaries, the resulting CMDs reproduce bet- gr photometry of stars in the field of the poorly studied
tertheobservedones.Theagespreadofthesethreeclusters LMCclusterSL529.ThedatawereobtainedattheGemini
turned out to be ∼ 0.3-0.4 Gyr. Similarly, Goudfrooij et South telescope with the GMOS attached under high qual-
al. (2011) argued that the impact of interacting binaries as ity photometric conditions. We are confident that the pho-
suggested byYangetal.(2011) in theCMDsof7EMSTOs tometricdatayieldaccuratemorphologyandpositionofthe
LMC clusters studied by them (∆(ageMSTO) = 0.35±0.10 main clusterfeaturesin theCMD. Theanalysisof theclus-
Gyr)is insignificant. Theyconcluded that theplacement in ter MSTO region shows that SL529 possesses an extended
theirCMDsofthesebinariesstarsmostlikelycorrespondto MSTOof∼0.5Gyr.WethusreportthatSL529belongsto
thatof LMC fieldstars. They arealow stellar densitypop- the group of the EMSTO LMC clusters. We estimated the
ulation in theCMD towardsbrightermagnitudesand bluer cluster age and metallicity by fitting theoretical isochrones
LMC star clusters 5
toitsCMD,previouslyshiftedbyitsE(B−V)colourexcess
and apparent distance modulus. The isochrone which best
reproducestheclusterfeaturesturnsoutotbethatof 2.25
Gyr and Z=0.004, with an observed age spread of ± 0.25
Gyr. The estimated cluster age and metallicity place it as
themost metal-poor and oldest EMSTO LMC cluster.
In addition, we also found that thecluster RCappears
to be formed by two concentrations of stars, although it is
not clear whether this feature can be caused, in part, by
binary interactions and mergers. Finally, we fitted a King
profile to the cluster stellar density radial distribution and
obtainedacoreradiusof(4.20±0.35)pc,ifaLMCdistance
of (53.5 ± 0.4) kpc is adopted. This value reinforces the
statusofSL529asaEMSTOcluster,sinceitisinexcellent
agreementwiththosedeterminedforthe12knownEMSTO
LMC clusters.
ACKNOWLEDGEMENTS
Wegreatly appreciatethecommentsandsuggestionsraised
bythereviewerwhichhelpedmetoimprovethemanuscript.
This work was partially supported by the Argentinian in-
Figure 5. Enlargement of the CMD of SL529, focusing on the
stitutions CONICET and Agencia Nacional de Promoci´on
EMSTO region, with isochrones of Marigoet al. (2008) for ages
Cient´ıfica y Tecnol´ogica (ANPCyT).
of 2.0, 2.25, and 2.5 Gyr, overplotted. A rectangle scanning the
MSTOregionisalsosuperimposed.
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6 A.E. Piatti
Table 1.CCDSDSSgr dataofstarsinthefieldofSL529.
ID x y g σ(g) g−r σ(g−r) n
(pixel) (pixel) (mag) (mag) (mag) (mag)
... ... ... ... ... ... ... ...
19 787.031 1523.905 18.004 0.003 0.772 0.004 2
20 1198.930 767.067 18.007 0.007 -0.324 0.009 2
21 1651.733 1353.525 17.968 0.003 0.866 0.004 2
... ... ... ... ... ... ... ...
Yang W., MengX., Bi1Sh., TianZ., LiT., LiuK. 2011, ApJL,
731,L37