NGC5694: another extra-galactic globular cluster

0.49 MB·English

by  A. Mucciarelli

Mem.S.A.It.Vol.75,282
(cid:13)c SAIt 2008 Memoriedella
NGC 5694: another extra-galactic globular
cluster
A.Mucciarelli1,M.Bellazzini2,M.Catelan3,4,P.Amigo3,4,M.Correnti5,C.Corte´s6,
3
1 E.Dalessandro1 andV.D’Orazi7.8
0
2
1 Dipartimento di Fisica & Astronomia, Universita` degli Studi di Bologna, Viale Berti
n
Pichat,6/2-40127,Bologna,Italy
a 2 INAF-OsservatorioAstronomicodiBologna,ViaRanzani,1-40127,Bologna,Italy
J
3 Pontificia Universidad Cato´lica de Chile, Facultad de F´ısica, Departamento de
6 Astronom´ıayAstrof´ısica,Av.Vicun˜aMackena4860,782-0436Macul,Santiago,Chile
1 4 TheMilkyWayMillenniumNucleus,Santiago,Chile
5 INAF-Istituto di Astrofisica Spaziale e Fisica Cosmica, Via Gobetti 101, I-40129,
]
Bologna,Italy
A
6 Departamento de F´ısica, Facultad de Ciencias Ba´sicas, Universidad Metropolitana de
G Ciencias de la Educacio´n, Av. Jose´ Pedro Alessandri 774, 776-0197 N˜un˜oa, Santiago,
. Chile
h 7 DepartmentofPhysicsandAstronomy,MacquarieUniversity,Balaclavard,NorthRyde,
p
NSW2109,Australia
- 8 MonashCentreforAstrophysics,SchoolofMathematicalSciences,Building28,Monash
o
r University,VIC3800,Australia
t
s
a
[ Abstract. WediscussthechemicalcompositionofsixgiantstarsoftheouterHaloglobular

clusterNGC5694,[email protected]
1
clusterhasanaverageironcontent[Fe/H]=–1.83±0.01,solar-scaled[α/Fe]ratiosandavery
v
lowBaabundance([Ba/Fe]=–0.71±0.06).Theseanomalousabundancepatternsarediffer-
6
entfromthoseobservedinotherHaloglobularclustersbutsimilartothoseofthemetal-poor
0
starsintypicaldwarfspheroidalgalaxies.Thesefindingssuggestanextra-galacticoriginfor
6
NGC5694,likelyfromadwarfspheroidalgalaxy.
3
.
1 Keywords.Stars:abundances–Stars:atmospheres–Stars:PopulationII–Galaxy:glob-
0 ularclusters–
3
1
:
v 1. Introduction to the study of its chemical and kinematical
i properties,pointingouttwopeculiarities:(i)a
X
NGC5694isanold,metal-poorglobularclus-
largeradialvelocity,V =–140.7km/s,ases-
rad
r ter (GC) locatedin the outerHalo, at a galac-
timated byGeisleretal. (1995),and(ii)a pe-
a
tocentric distanceof 30 kpc(Lee et al. 2006).
culiar chemical composition in one giant star
Thisclusterhasnotreceivedagreatdealofat-
discussedbyLeeetal.(2006).
tention and only a few of works are devoted
Theunusualabundancepatternoftheonlystar
studiedsofar(withsolar-scaledabundancera-
Sendoffprintrequeststo:A.Mucciarelli
Mucciarelli:NGC5694:anotherextra-galacticglobularcluster 283
tios for the α-elements and sub-solar abun- becauseoftheoccurrenceofNLTEeffects.
dance ratios for neutron-capture elements) is Noevidencesofintrinsicspreadintheiron
uniqueamongthe haloGC, suggestinga pos- contentoftheclusterhavebeenfound.The
sible extra-galactic origin for this star cluster, derived metallicity well agrees with that
as suggested for other (more metal-rich) GCs providedbyLeeetal.(2006);
withanomalousabundanceratios,asRuprecht – NGC 5694 is remarkably deficient in α-
106 (Brown et al. 1997), Palomar 12 (Cohen elements. [O/Fe], [Mg/Fe], [Ca/Fe] and
2004)andTerzan7(Sbordoneetal.2007). [Ti/Fe] abundance ratios are solar-scaled,
while only the [Si/Fe] ratio is enhanced
with respect to the solar value. These val-
2. Chemicalanalysis
ues are lower than those usually observed
Inordertoconfirmthehypothesisofanextra- in Galactic GCs of similar metallicity.
galacticoriginforNGC5694proposedbyLee Fig. 1 shows the position of NGC 5694
etal.(2006)andtoprovideacompletedescrip- in the [α/Fe] vs [Fe/H] diagram, in com-
tion of the chemical composition of this GC, parison with Galatic GCs (green points),
we analysed high-resolution spectra acquired Halo field stars (grey points) and dwarf
with the spectrograph UVES@FLAMES spheroidalgalaxiesstars (bluepoints);the
(VLT) of six giant stars located in the bright position of NGC 6397 as derived by our
portionoftheRedGiantBranchofthecluster. differentialanalysisisshownasastarsym-
The targets have been selected among cluster bol.Also, the positionof three anomalous
members already confirmed by Geisler et al. GCs (namely, Ruprecht 106, Palomar 12
(1995). Atmospheric parameters have been andTerzan7)isexplicitlyshown;
derivedfromthephotometrybyCorrentietal. – NGC 5694 exhibits a peculiar under-
(2011). abundance of the [Ba/Fe] ratio, with an
As a sanity check, we analysed average value [Ba/Fe]=–0.70±0.06. As
UVES@FLAMES spectra retrieved from shown in Fig. 2, [Ba/Fe] of the cluster
the ESO archive of 13 giant stars in the is ∼0.5-0.7 dex lower than in the Milky
Galactic GC NGC 6397, taken as prototype Waystars;inparticular,theHaloGCswith
of a genuine Halo cluster with the same metallicities similar to that of NGC 5694
metallicity of NGC 5694. The analysis of have solar-scaled [Ba/Fe] ratios (D’Orazi
NGC 6397 has been performed by using etal.2010);
the same linelist and the same methodology – Fig. 3 shows the position of the six indi-
adoptedforNGC5694. vidualstars of NGC 5694 in the [Na/Fe]–
[O/Fe]plane,incomparisonwiththeindi-
vidual stars analysed in 19 Galactic GCs
3. Chemicalcompositionof by Carretta et al. (2009a). One star of
NGC5694 NGC 5694 out of six exhibit a lower
[Na/Fe]abundanceratiowithrespecttoall
Themainresultsconcerningthechemicalcom-
the others. Instead, O abundances do not
positionof NGC5694are summarisedas fol-
showhintsofintrinsicspread.ThelowNa
lows:
abundance (coupled with a low Al abun-
dance) in this star is compatible with the
– the six stars studied do not show any de-
occurrenceofself-enrichmentprocessesin
tectablespreadintheabundancesofFe,O,
theearlystagesoftheclusterlife.
Mg, Si, Ca, Ti and Ba. Forthisreason we
will refer to the average abundance of the
sixstarsastotheclusterabundance;
– theclusterhasanironcontentof[Fe/H]=–
4. Conclusions
1.83±0.01 from the analysis of single
ionised lines. Neutral iron lines provide The analysis of six giant stars of NGC 5694
a lower abundance ([Fe/H]= –1.97±0.03) confirms the first results provided by Lee et
284 Mucciarelli:NGC5694:anotherextra-galacticglobularcluster
Fig.1. <[α/Fe]>abundanceratiosasafunctionof Fig.2. [Ba/Fe] abundance ratios as a function of
[Fe/H]: NGC 5694 is marked as a red pentagon. [Fe/H]. Same symbols of Fig. 1. Data for the
Grey points are Galactic field stars (Edvardsson GalacticGCsarefromD’Orazietal.(2010).
et al. 1993; Reddy et al. 2003, 2006); green cir-
cles are Galactic GCs (Brown et al. 1997; Cohen
2004; Sbordone et al.2007; Carrettaetal.2009b),
blue points are stars of dwarf spheroidal galaxies
(Shetrone, Cote´ & Sargent 2001; Shetrone et al.
2003; Sbordone et al. 2007; Letarte et al. 2010;
Lemasleetal.2012).NGC6397isshownasaempty
starsymbol.
al. (2006) based on a single star. The clus-
ter shows unusual low [α/Fe] abundance ra-
tios, lower than those observed in Galactic
(both field and GCs) stars of similarmetallic-
ity. Also, NGC 5604 is remarkably deficient
of Ba abundance, with a [Ba/Fe] ratio lower
then the Galactic stars by ∼0.7 dex. In light
of these findings, NGC 5694 formed from a
gas already enriched by Type Ia Supernovae Fig.3. [Na/Fe] abundance ratios as a function of
and likely in a galactic environment charac- [O/Fe]. Red circles are the six individual stars of
terisedbyastarformationrateslowerthanthat NGC 5694. Grey points are individual stars of
typicaloftheGalacticHalo.Theseanomalous GalacticGCsbyCarrettaetal.(2009a).
chemicalpatternsareincompatiblewith those
observed in other halo globular clusters, but
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