The strange case of NGC2419: stellar populations, chemical composition, dynamics

0.39 MB·English

by  M. Bellazzini

Mem.S.A.It.Vol.00,0
(cid:13)c SAIt 2008 Memoriedella
The strange case of NGC2419: stellar
populations, chemical composition, dynamics.
3 M. Bellazzini1,A. Mucciarelli2,R.A. Ibata3,A. Sollima4,1,E. Dalessandro2,S.
1
Chapman5,C. Nipoti2,T. Merle6,G. Beccari7,C. Lardo2,A. Bragaglia1,E.
0
2 Carretta1,andE. Pancino1

n
a
1 IstitutoNazionalediAstrofisica–OsservatorioAstronomicodiBologna,ViaRanzani1,
J
I-40127Bologna,Italy
5 2 Dipartimento di Fisica e Astronomia, Universita` di Bologna, Viale Berti Pichat 6/2, I-
1
40127Bologna,Italy
3 Observatoire Astronomique, Universite´ de Strasbourg, CNRS, 11 Rue de l’Universite´,
]
A F-6700Strasbourg,France
4 Istituto Nazionale di Astrofisica – Osservatorio Astronomico di Padova, Vicolo
G dell’Osservatorio5,I-35122Padova,Italy
. 5 InstituteofAstronomy,MadingleyRoad,CambridgeCB30HA,UK
h 6 Universite´deNiceSophia-antipolis,CNRS(UMR7293),ObservatoiredelaCoˆtedAzur,
p
LaboratoireLagrange,BP4229,06304Nice,France
-
o 7 European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei
r Mu¨nchen,Germany
t e-mail:[email protected]
s
a
[

Abstract. A few years ago we started an observational campaign aimed at the thorough
1
study of the massive and remote globular cluster NGC2419. We have used the collected
v
data, e.g., to test alternative theories of gravitation, to constrain the stellar M/L ratio by
8
9 directanalysisoftheobserved luminosityfunction,andto searchforDarkMatterwithin
2 thecluster.Herewepresent some recent resultsabout (a)thepeculiar abundance pattern
3 thatweobservedinasampleofclustergiants,and(b)newlyfoundphotometricevidence
. for the presence of multiple populations in the cluster. In particular, from new deep and
1 accurate uVI LBT photometry, wefind that the color spread on theRed Giant Branch is
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significantlylargerthantheobservationalerrorsbothinV-Iandu-V,andthatthestarslying
3
totheblueoftheRGBridgelinearemoreconcentratedtowardthecenteroftheclusterthan
1
thoselyingtotheredoftheridgeline.
:
v
i Keywords.Stars:abundances–Galaxy:globularclusters–Globularclusters:individual:
X NGC2419
r
a
1. Introduction
Themassiveandremoteglobularcluster(GC)
Sendoffprintrequeststo:M.Bellazzini NGC2419 is a very interesting stellar system
Bellazzinietal.:ThestrangecaseofNGC2419 1
under several aspects (see Bellazzini et al.
2012;DiCriscienzoetal. 2011,andreferences
therein).Fiveyearsago,westartedanobserva-
tionalcampaignaimedatthethoroughstudyof
this cluster. We have used the collected data,
e.g., to test alternative theories of gravitation
(Ibata et al. 2011a,b), to constrain the stellar
M/Lratiobydirectanalysisoftheobservedlu-
minosityfunction(Bellazzinietal. 2012),also
providingafurtherobservationalconfirmation
oftheun-relaxeddynamicalstatusoftheclus-
ter(Dalessandroetal. 2008).
Recently we have investigated the mass
distributionwithintheclusterusingbothpara-
metric and non-parametric methods, finding
thattheavailabledataarenotcompatiblewith
thepresenceofa“cosmological”DarkMatter
halosurroundingthecluster(Ibataetal. 2012). Fig.1. Potassium - Magnesium anti-correlation
in NGC2419 Red Giants, from the analysis by
In the following we briefly report on the
Mucciarellietal. (redcircles2012)andbyCohen
chemical composition of cluster stars and on
&Kirby (2012,yellowtriangles).
newly found photometric evidences of the
presenceofmultiplepopulationsinNGC2419.
2. Chemicalcomposition
InMucciarellietal. (2012)wehavepresented
Fe, Ca, Ti,Mg,andK abundancesfor49Red
Giant Branch (RGB) stars of NGC2419, de-
rivedfromasubsetofthehighsignal-to-noise
medium-resolution spectra presented in Ibata
etal. (2011a).
We found that cluster stars show no de-
tectablespreadintheabundanceofFe,Ti,and
Ca, while they display a huge spread in both
MgandKabundances.InFig.1weshowthat
Mg and K abundancesare anti-correlatedand
the distributions of both elements are clearly
bimodal. Fig.2. The distribution of [Mg/Fe] vs. [Fe/H] for
NGC2419 stars (Mucciarelli et al. 2012) is com-
Fig. 2, on the other hand, show that (a)
paredwith(a)RGBstarsfor15GalacticGCsfrom
thespreadin[Mg/Fe]detectedinNGC2419is
Carrettaetal. (2009,×symbols;inbluestarsfrom
larger than what found in any other GCs ob-
NGC7078andinredstarsfromNGC2808),(b)stars
servedbefore,and(b)thattheself-enrichment
from three dwarf spheroidal galaxies (small dots,
mechanism at work in the cluster is of com- see labels), from different sources (Kirby et al.
pletely different nature with respect to the 2011a,b;Letarteetal. 2006;Bonifacioetal. 2006).
one driving the chemical evolution of dwarf
spheroidal galaxies. Our results have been ants1.Wesuggestthattheobservedabundance
nicely confirmed by Cohen & Kirby (2012),
from high resolution spectra of 13 bright gi- 1 TheonlydifferenceintheresultsbyMucciarelli
et al. (2012) and Cohen &Kirby (2012) isabout
2 Bellazzinietal.:ThestrangecaseofNGC2419
patternmay emergeas an extremecase ofthe
multiplepopulationsphenomenoninGCs(see
Gratton, Carretta & Bragaglia 2012, for re-
view and references). Within this framework,
(Venturaetal. 2012)recentlyproposedanin-
terestingtheoreticalscenariotointerpretthese
results.
3. ColorspreadintheRGB
Weacquiredalargenumberofu ,V,Iwide-
SDSS
field images of NGC2419, using simultane-
ously the two LBC cameras (Giallongo et al.
2008) mounted at the binocular telescope
LBT2. We derived accurate V, u-V and V, V-
I CMDs to search for color spreads on the
RGB (especially in the NUV-optical colors),
that are now generally recognized as a typi- Fig.3. Upper panels: V, u-V CMDs of observed
cal photometric signature of the presence of and artificial stars from our LBT photometry of
multiple populations (see Lardo et al. 2011; NGC2419.TheRGBstarsintherange19.8 ≤V ≤
Gratton,Carretta&Bragaglia 2012;Sbordone 22.0aredividedintwosamples,accordingtotheir
positiontotheblue(RGB-B)ortothered(RGB-R)
et al. 2011; Milone et al. 2012). The origi-
oftheridgeline(ingrey).Lowerpanels:comparison
nalideawastofollowuptheresultsbyLardo
betweentheradialdistributionofRGB-B(continu-
et al. (2011), that detected significant spread
ousblueline)andRGB-R(dotted-dashedredline)
ontheRGBofsevenclusters,butwereunable
forobservedandartificialstars.Theprobabilitythat
todrawfirmconclusionsonNGC2419because
RGB-R and RGB-B samples are drawn from the
the photometrythey consideredwas not suffi- sameradialdistribution,accordingtoaKolmgorov-
cientlyaccuratetostudythisverydistantclus- Smirnovtest,isreportedinthepanels.Starswithin
ter. 50arcsecfromtheclustercenterhavebeenexcluded
We found that the observed color spread fromtheanalysis.
is significantly larger than what expected
from observational errors (accurately traced
enrichment occurred in this cluster , second
by means of extensive artificial stars exper-
generation stars lie to the blue of the RGB
iments), thus confirming that the effects of
ridge line, at variance with the majority of
the presence of multiple populations can be
otherGCs(Lardoetal. 2011;Gratton,Carretta
detected on the RGB also in the case of
&Bragaglia 2012).
NGC2419. While the spread is larger in u-V,
Inparticular,inFig.3wedemonstrate,for
it is clearly significant also in V-I, confirming
the first time, that second generation stars (in
earlier results by Di Criscienzo et al. (2011).
theinterpretationbyDiCriscienzoetal. 2011)
These authors concluded that, given the low
aremorecentrallyconcentratedthanfirstgen-
overall metallicity and the high degree of He
eration stars. The average fraction of second
generation stars within the cluster, as derived
thespreadintheCaabundance:theformerconclude
that“...thespreadincalciumabundanceisabsentor fromRGBstars,is∼ 40%.Athoroughanaly-
verysmall...”,whileCohen&Kirby (2012)claims sisofthisnewphotometricdatasetwillbepre-
that asmall but real spread isactually there, espe- sentedinBeccarietal. (2013).
cially among Mg-poor stars. From their data, us-
ingtheMaximumLikelihoodtechniqueadoptedby Acknowledgements. Weacknowledge the financial
Mucciarellietal. (2012),wefindthattheintrinsic support of INAF through the PRIN-INAF 2009
Caspreadisσ =0.05±0.13. grant assigned totheproject Formationand evolu-
[Ca/Fe]
2 http://lbt.inaf.it/index.html tionofmassivestarclusters,P.I.R.G.Gratton.
Bellazzinietal.:ThestrangecaseofNGC2419 3
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