Chemical Abundances of Two Stars in the Large Magellanic Cloud Globular Cluster NGC~1718

1.7 MB·

by  Charli Sakari

Mon.Not.R.Astron.Soc.000,000–000 (0000) Printed12February2017 (MNLATEXstylefilev2.2)
Chemical Abundances of Two Stars in the Large
Magellanic Cloud Globular Cluster NGC 1718
7 Charli M. Sakari1⋆, Andrew McWilliam2, and George Wallerstein1
1
1 Department of Astronomy, Universityof Washington, Seattle, WA 98195-1580, USA
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2 Observatoriesof the Carnegie Institute of Washington, Pasadena, CA, USA
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n
a
J
12February2017

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ABSTRACT
]
A Detailed chemical abundances of two stars in the intermediate-age Large Magellanic
Cloud(LMC)globularclusterNGC1718arepresented,basedonhighresolutionspec-
G
troscopicobservationswiththeMIKEspectrograph.Thedetailedabundancesconfirm
. NGC 1718 to be a fairly metal-rich cluster, with an average [Fe/H] ∼ −0.55±0.01.
h
p The two red giants appear to have primordial O, Na, Mg, and Al abundances, with
- no convincing signs of a composition difference between the two stars—hence, based
o on these two stars, NGC 1718 shows no evidence for hosting multiple populations.
r
The Mg abundance is lower than Milky Way field stars, but is similar to LMC field
t
s stars at the same metallicity. The previous claims of very low [Mg/Fe] in NGC 1718
a
are therefore not supported in this study. Other abundances (Si, Ca, Ti, V, Mn, Ni,
[
Cu, Rb, Y, Zr, La, and Eu) all follow the LMC field star trend, demonstrating yet

1 again that (for most elements) globular clusters trace the abundances of their host
v galaxy’sfieldstars.Similartothefieldstars,NGC1718isfoundtobemildlydeficient
2
in explosive α-elements, but moderately to strongly deficient in O, Na, Mg, Al, and
0
Cu, elements which form during hydrostatic burning in massive stars. NGC 1718 is
8
also enhanced in La, suggesting that it was enriched in ejecta from metal-poor AGB
3
stars.
0
1. Key words: galaxies: individual(LMC) — galaxies: abundances — galaxies: star
0 clusters: individual(NGC 1718) — globular clusters: general — galaxies: evolution
7
1
:
v
i 1 INTRODUCTION dwarfgalaxies,includingtheLargeMagellanicCloud(LMC;
X
Johnson et al. 2006; Mucciarelli et al. 2009, 2010). These
r Detailed abundancesof stars in globular clusters (GCs) are
a variations seem to be present in M31 GCs as well, and do
essentialfortwoprimarygoals:1)understandingthenature
affect the integrated abundances from distant, unresolved
of GC formation (e.g., Gratton et al. 2012) and 2) tracing
GCs (Colucci et al. 2014, Sakari et al. 2013, 2016). Despite
thepropertiesoffieldstarpopulationsindistant,unresolved
theprevalenceofGCmultiplepopulations,thecauseofthese
galaxies (e.g., Colucci et al. 2013, Sakari et al. 2015). For
abundance variations is not yet well-understood. Observa-
most elements, the abundances of GC stars trace those of
tions of GCs outside of the MW, particularly ones that are
thefieldstarsintheirbirthenvironment(Pritzl et al.2005;
unlikestandard MW GCs, are necessary to understand GC
Hendrickset al. 2016), providing probes of a galaxy’s star
formation. Without a more complete understanding of the
formation history,abundancegradients,chemicalevolution,
multiple populations in GCs, interpreting integrated abun-
and assembly history. However, for a handful of other ele-
dances of unresolved clusters remains difficult.
ments, from light elements like C, N, O, and Na to heavy
neutroncaptureelementslikeBaandEu,MilkyWay(MW) One cluster that is particularly intriguing for detailed
GCs host star-to-star variations that are unique to GCs abundance studies is the intermediate-age LMC cluster
and are not seen in most field stars (detections of field NGC1718.HubbleSpaceTelescopephotometryhasrevealed
starswiththeseabundancevariationsarethoughttobeac- that the cluster is of intermediate age and moderately high
cretedfromdissolvedGCs;e.g.,Martell et al.2016).Similar metallicity, with an age ∼ 2 Gyr and [Fe/H] ∼ −0.4
variations have also been observed in classical, old GCs in (Brocato et al. 2001; Kerberet al. 2007). Calcium triplet
spectroscopy of three cluster stars suggests a slightly lower
valueof [Fe/H]∼−0.8 (Grocholski et al. 2006), while com-
⋆ E-mail:[email protected] parisons with other LMC clusters of a similar age suggest
(cid:13)c 0000RAS
2 Sakari et al.
thatNGC1718shouldhaveametallicityof[Fe/H]∼−0.42 differentiallywithrespecttothewellstudiedgiantArcturus,
(Mackey & Gilmore 2003). NGC 1718 is far too massive andthereforehaverelatively low systematicerrors. Thede-
(Baumgardt et al. 2013) to be an open cluster—it is there- tailedabundancesofthetwostarsarethenexaminedinlight
fore distinctly different from the classical, metal-rich MW of NGC 1718’s context as a young GC and as a member of
GCs,whichareallolderthan10Gyr.1Intermediate-ageGCs theLMC.
like NGC 1718 are therefore excellent targets for studying
the nature of GC multiple populations and for examining
relatively recent star formation in the LMC.
2 OBSERVATIONS AND DATA REDUCTION
The first detailed abundances for NGC 1718 were de-
rived by Colucci et al. (2011, 2012) from integrated light Probable RGB members of NGC 1718 were selected using
(IL)spectroscopy,whereasinglespectrumisobtainedfrom photometryfrom theTwo Micron All-SkySurvey(2MASS;
the entire stellar population. With a spectrum that only Skrutskieet al.2006).Figure1showsa2MASSK-bandim-
covered ∼ 23% of the cluster, they found NGC 1718 to be ageoftheclusterandtheK versusJ−K colour-magnitude
a moderate-metallicity ([Fe/H] = −0.7), intermediate-age diagram (CMD) for stars within 60′′ of the cluster centre.
(1.25-2Gyr),solar[Ca/Fe]clusterwithaverylowMgabun- Thetwotargetsinthispaperareidentified.Followingthe∼2
dance ([Mg/Fe] = −0.9). While Mg is not always expected GyrageestimatedbyElson & Fall(1988)andKerber et al.
totracetheheavierα-elementslikeCaandTiinalowmass (2007),a2Gyr,z=0.004BaSTIisochrone(Pietrinferni et al.
galaxy, it is difficult to explain such a low Mg abundance 2004) is also displayed in Figure 1 to distinguish the RGB
through canonical chemical evolution scenarios. Colucci et from the brighter AGB stars. The star identification num-
al. suggested that NGC 1718’s low integrated Mg abun- bers used here and shown in Table 1 and Figure 1 were
danceindicatedthattheclusterwasenrichedsolelythrough assigned ad-hoc (by distance from input cluster centre in
ejecta from a Type Ia supernova, a scenario which would the2MASS catalog).
increase Fe significantly without appreciably changing Mg High-resolution (R∼45,000) spectra of the two target
(Tsujimoto & Bekki 2012). However, similarly low-[Mg/Fe] stars were obtained on 26 and 27 February 2012, using the
′′
field stars havenot been found in theLMC (Lapenna et al. MIKE echelle spectrograph, with a 0.5 slit, on the Mag-
2012),as would beexpected if NGC1718 formed in aenvi- ellan/Clay telescope. The average atmospheric seeing was
ronment enriched only by a TypeIa supernova. 0.6′′,FWHM,onbothnights.Theusablewavelengthcover-
Becauseofitsageandmass,NGC1718isalsointerest- age is from ∼5300 to 9000 ˚A.
ingasaGC.ThesourceofmultiplepopulationswithinGCs Extraction of the spectra from the CCD data was
continues to be debated; a particularly contentious point is performed using the MIKE pipeline software from Kelson
whetherthemultiplepopulationsareactually multiplegen- (2003). However, subsequent analysis employed thesuite of
erations with a very small (∼ 100 Myr) age spread. Under routines from the Image Reduction and Analysis Facility
many multiple population formation scenarios, the ratios program (IRAF).2 S/N ratios at the peak of the Hα order
of “primordial,” first generation stars (with normal abun- are estimated at 49 and 52 for star #9 and #26, respec-
dance ratios) to “extreme” second generation stars (with tively, per extracted wavelength pixel. Typical weak stellar
enhanced [Na/Fe] and deficient [O/Fe]) requires a signifi- lines haveFWHM ∼5 pixels.
cant amount of mass loss from the first generation prior In order to facilitate continuum placement and EW
to the formation of the second generation. Convincing age measurement, the spectral modulation resulting from the
spreads have not yet been detected in any of the younger echelle blaze was removed by dividing by a high S/N blaze
LMCGCs,includingNGC1718.(Despiteitsbroadmainse- function spectrum, which was found by fitting the contin-
quence,whichsuggeststhattheclusterhostsanagespread, uum flux of the bright, extremely metal-poor, RGB star
its red clump implies that NGC 1718 does host a popula- HD126587.Radialvelocitiesweredeterminedthroughcross-
tion with a single age; Niederhofer et al. 2016.) However, correlationswithahighresolution,highS/NArcturusspec-
although multiple populations have been spectroscopically trum from Hinkleet al. (2003).3 The final, heliocentric ra-
confirmed in LMC and Small Magellanic Cloud GCs older dial velocities are shown in Table 1 and are in agreement
than ∼ 8 Gyr (Johnson et al. 2006; Mucciarelli et al. 2009, withtheradialvelocitiesofotherconfirmedclustermembers
2010; Hollyhead et al. 2016) there is not yet any convinc- from Grocholski et al.(2006).Spectrainthe6270−6370 ˚A
ingevidencefortheexistenceofmultiplepopulationsinGCs range are shown in Figure 2.
youngerthan∼8Gyr(Mucciarelli et al.2008,2011,2014a).
ThisdifferenceimpliesthattheLMCGCsthatformedmore
recently may be fundamentally different from the old GCs
3 ATMOSPHERIC PARAMETERS
thatformedearlyintheuniverse.Theintermediate-ageand
youngLMCGCsaretherefore importanttargetsfor under- Kuruczatmospheres4 (Castelli & Kurucz2004)areadopted
standing thenatureof multiple populations within GCs. for this analysis, with an interpolation scheme to select
Thispaperpresentsthefirstabundanceanalysesofindi- Teff and logg values that fall between the grid points.
vidualstarsinNGC1718,fromhighresolutionspectroscopy
of two cluster members. These abundances are calculated
2 IRAFisdistributedbytheNationalOpticalAstronomyObser-
vatory, which is operated by the Association of Universities for
Research in Astronomy, Inc., under cooperative agreement with
1 Though there are metal-rich, intermediate-age MW clusters theNationalScienceFoundation.
(e.g. Palomar 1; Sakarietal. 2011), these clusters are much less 3 ftp://ftp.noao.edu/catalogs/arcturusatlas/
massivethanNGC1718,andarenotobviouslyclassicalGCs. 4 http://kurucz.harvard.edu/grids.html
(cid:13)c 0000RAS,MNRAS000,000–000
Abundances of NGC 1718 stars 3
Table 1.Targetinformation.
2MASSIDs RA(hms) Dec(dms) J K Observation texp Seeing S/N vhelio
J2000 J2000 Dates (sec) (arcsec) a (kms−1)
NGC1718-9 04522589-6702590 4:52:26.0 −67:02:59.1 13.52 12.56 26Feb,2012 11,400 0.65 49 282.1±1.0
NGC1718-26 04521682-6703242 4:52:16.8 −67:03:24.3 13.95 12.90 27Feb,2012 10,800 0.60 52 283.4±1.0
a S/Nisperfinalextracted wavelength pixelatthepeakoftheHαorder;weaklinestypicallyhaveFWHM∼5pixels.
(a) 2MASSK-bandImage (b) Colour-magnitudediagram
Figure 1. 2MASS data for NGC 1718. Left: K-band image with targets circled. North is up and east is to the right. Right: K vs.
(J−K)colour-magnitudediagram,utilizingobservedcolours.Thetwotargetsfortheabundanceanalysisarecircled.A2Gyr,z=0.004,
solar-scaledBaSTIisochrone(Pietrinfernietal.2004)isalsoshowntohighlighttheRGB(solidthickcyanline)andtheAGB(magenta
dashed line;theisochronehas anextended AGB andamass lossparameter ofη=−0.2).Tworeddeningvectors areshown: onewith
theKerberetal.(2007)valueofE(B−V)=0.1,theotherwiththeSchlafly&Finkbeiner(2011)valueofE(B−V)=0.598;thehigher
valuepredictstemperatures thatareincompatiblewiththespectroscopic temperatures.
Figure 2.Samplespectrainthe6270−6370˚A range.
(cid:13)c 0000RAS,MNRAS000,000–000
4 Sakari et al.
Solar-scaled (ODFNEW) atmospheres are adopted for the
Table 3.EWLinelist.
NGC1718 stars, since the[α/Fe] ratios in thesetargets are
Wavelength Element EP EW(m˚A)
low (see Section 5).
(˚A) (eV) NGC1718-9 NGC1718-26
Photometric effective temperatures were de-
5522.450 26.0 4.210 90.10 72.70
rived utilizing the observed 2MASS (J − K) colours
5525.540 26.0 4.230 98.90 90.00
(see Figure 1(b)) and the empirical relation of
5543.940 26.0 4.220 - 91.00
Gonz´alez Hern´andez& Bonifacio (2009). The redden- 5560.210 26.0 4.430 71.80 78.80
ing and distance modulus from Kerberet al. (2007), 5562.710 26.0 4.430 95.80 100.00
E(B −V) = 0.10 and (m−M) = 18.73, are adopted,
V
and the reddening relations from McCall (2004) are used Notes: Table3ispublishedinitsentiretyintheelectronic
to deredden the observed 2MASS colours. These photo- editionofMonthly Noticesof the Royal Astronomical Society.A
metric temperatures are shown in Table 2. Note that the portionisshownhereforguidanceregardingitsformand
Schlafly & Finkbeiner (2011) map implies a significantly content.
higherreddeningofE(B−V) = 0.598 ± 0.121. However, References:Lineswereselectedfromthelinelistsof
Fulbrightetal.(2006,2007)andKoch&McWilliam(2008);the
a reddening this high would increase the photometric
solarandArcturusEWsarealsotaken fromthosepapers.
temperatures by at least 500 K, which is not in agreement
with the spectroscopic temperatures (see below) or the
temperaturespredictedbyisochrones. Thepredictedstellar were adopted for this analysis. The lines in these lists were
temperatures therefore support the lower reddening from selected to be clean, relatively free of blends, and suitable
Kerberet al. (2007), and indicate that the higher value for high precision, differential analyses. All Arcturus abun-
from Schlafly & Finkbeiner (2011) is likely due to 100 dances are calculated relative to solar abundances derived
µm emission behind the two target stars. Bolometric with the same lines; the EWs from the above sources were
corrections in the K band were then derived given the used to derive these abundances. The LMC stellar [X/H]
photometric temperatures and the empirical relation abundances from each spectral line are then calculated
from Buzzoni et al. (2010). NGC 1718 is estimated to relative to the Arcturus [X/H] abundances from the same
be ∼ 2 Gyr old (Kerberet al. 2007); BaSTI isochrones line.Theaverage∆[X/H]offsetsoftheNGC1718 stars are
(Pietrinferni et al. 2004) with [Fe/H] = −0.6 and an age then applied relative to theaverage [X/H] Arcturusratios.
of 2 Gyr indicate that the mass of red giants in such All abundances are determined with the July 2014
a GC is about 1.4 M⊙. The photometric temperatures, version of the Local Thermodynamic Equilibrium (LTE)
bolometriccorrections, andturnoffmassesthenyieldinitial line analysis code MOOG (Sneden 1973). The abundances
photometric surface gravities. of Fe, Ca, etc. are determined via EWs, which are mea-
Spectroscopic temperatures and microturbulent veloc- suredwiththeautomatedcodeDAOSPEC(Stetson & Pancino
ities (ξ, in km s−1) were derived by flattening slopes 2008) for the NGC 1718 stars. (Recall that the EWs of
in Fe I abundance with wavelength, reduced equivalent Fulbright et al. 2006, 2007 and Koch & McWilliam 2008
width (REW)5, and excitation potential (EP, in eV). As were used for the Sun and Arcturus; these EWs were not
in Fulbright et al. (2006), Koch & McWilliam (2008), and altered.) A moderately-high order polynomial (order 33)
McWilliam et al. (2013),these[FeI/H]abundancesarecal- was fit to the continuum levels of the normalized spec-
culateddifferentially,linebyline,withrespecttothe[Fe/H] tra. Since DAOSPEC can have difficulty accurately measur-
ratios of thecool giant star, Arcturus.Thelarge uncertain- ing the strongest lines (see Sakariet al. 2013), lines with
tiesinthedistancemodulusandreddeningmakethephoto- EWs stronger than 100 m˚A were checked manually with
metricgravitiesveryuncertain.Forthisreason,scaledsolar IRAF’s splot routine. For elements with a few lines (e.g.,
BaSTI isochrones were utilized for the final surface gravi- Mg) all EWs were checked with IRAF’s splot routine. For
ties, using the spectroscopic temperatures. The stars were most elements, lines stronger than REW > −4.7 were not
assumed to be RGB stars; if instead they are AGB stars, considered because they are on the (relatively) flat part of
the gravities would change by <0.05 dex. The values from the curve of growth and are therefore sensitive to uncer-
the z = 0.004, [Fe/H] = − 0.66 and the z = 0.008, tain damping constants and microturbulent velocities (see
[Fe/H] = − 0.35 models were averaged, since the two thediscussion inMcWilliam et al.1995).Forelementswith
NGC1718starshave[Fe/H] ∼ −0.5.Thefinalatmospheric onlystronglinesandHFScomponents,thislimitwaspushed
parametersarelistedinTable2.Star9’sspectroscopictem- to REW = −4.5. Although this may introduce uncertain-
peratureisslightlylowerthanitsphotometrictemperatures, ties ∼ 0.1 dex, tests were performed to ensure that the
whichmaybeduetouncertaintiesintheforegroundredden- lineswerenotsaturated.RandomerrorsinEW-basedabun-
ing. dances were determined as in Shetrone et al. (2003). The
lines utilized for EWs are shown in Table 3, along with the
valuesmeasured in NGC 1718-9 and -26.
Other abundances are derived via spectrum syntheses
4 LINE LISTS AND ANALYSIS TECHNIQUES
(SS);inthiscase,spectrallinesinagivenwavelengthrange
The line lists of Fulbright et al. (2006, 2007), were selected from the Kurucz database.6 Molecular lines
Koch & McWilliam (2008), and McWilliam et al. (2013) were included in regions where they are noted in the Arc-
turus atlas (Hinkleet al. 2003). Syntheses of the sun and
5 REW = log(EW/λ), where λ is the wavelength of the transi-
tion. 6 http://kurucz.harvard.edu/linelists.html
(cid:13)c 0000RAS,MNRAS000,000–000
Abundances of NGC 1718 stars 5
Table 2.Atmosphericparameters.
Photometric Spectroscopic Isochrone
Teff (K)a logg Teff (K) ξ (kms−1) logg
NGC1718-9 4049±30 0.70±0.2 3820±50 1.90±0.10 0.52±0.10
NGC1718-26 3858±30 0.80±0.2 3890±50 1.80±0.10 0.67±0.10
a TheerrorsinthephotometrictemperatureonlyconsidertheKerberetal.(2007)reddening.
(a) NGC1718-9
(b) NGC1718-26
Figure 3.Trends in∆[Fe/H](relativeto Arcturus)forNGC 1718-9(top) andNGC 1718-26 (bottom). ThesolidcirclesareFeIlines.
The dashed red lineshows the average offset, whilethe solidbluelineshows the linear least squares fit. The slopes are quoted ineach
panel.
ArcturuswerefirstperformedtoidentifythesolarandArc- are calculated using Fe I. Many analyses utilize Fe II for
turus synthesis-based abundances. The uncertainty in SS- singly ionized species (and OI), because the systematic er-
based abundances is determined from the range of abun- rors are expected to be similar. However, Table A3 demon-
dances that can fit a given spectral line profile. The lines strates that for the NGC 1718 stars, the uncertainties are
usedforSSareshowninTable4,alongwiththeabundances smaller when the[X/Fe]ratios are calculated with FeI (see
for the sun, Arcturus, and the NGC 1718 stars. Figure 4 AppendixA).
shows examples of the fits to the 6318/6319 ˚A Mg I lines,
which are near a CaI autoionization feature.
5.1 Iron
Appendix A presents a detailed analysis of the abun-
dance sensitivity to the adopted atmospheric parameters, The Fe I abundances in the two NGC 1718 stars are de-
following the procedure of McWilliam et al. (1995, 2013). rived from EWs of ∼ 100 unblended lines, while only two
The finalerrors in [X/Fe] ratios are shown in Table A3. FeII lines are measurable. The greater numberof FeI lines
means that Fe I has a lower random error than Fe II; still,
FeI and FeII are in good agreement in all cases. In partic-
ular, any offsets that may be expected from NLTE effects
5 ABUNDANCES
(e.g., Kraft & Ivans2003) are minimized with this differen-
FinalabundancesareshowninTable5.AsdiscussedinSec- tialabundanceapproach(sinceArcturusisexpectedtohave
tions 3 and 4, all abundances are calculated differentially thesame NLTE corrections as the NGC1718 stars).
with respect to Arcturus and the Sun. All [X/Fe] ratios The abundances from the two NGC 1718 stars
(cid:13)c 0000RAS,MNRAS000,000–000
6 Sakari et al.
Table 4.SSLinelist.
Wavelength Element EP logǫ(X)
(˚A) (eV) Sun Arcturus NGC1718-9 NGC1718-26
6300.304 8.0 0.00 8.84 8.69 8.24±0.07 8.29±0.05
6154.222 11.0 2.10 6.28 5.87 5.54±0.10 5.64±0.10
6160.746 11.0 2.10 6.33 5.94 5.64±0.10 5.44±0.10
6318.705 12.0 5.10 7.60 7.43 7.15±0.10 7.10±0.10
6319.232 12.0 5.10 7.60 7.45 7.25±0.10 7.20±0.10
6696.015 13.0 3.14 6.30 6.20 5.68±0.10 5.75±0.05
6698.665 13.0 3.14 6.32 6.13 5.65±0.10 5.75±0.10
5782.110a 29.0 1.64 4.22 3.94 3.04±0.10 3.14±0.10
6645.130 63.1 1.37 0.42 0.24 0.13±0.05 0.19±0.05
a HFScomponents wereincludedinthesyntheses.
Figure 4. Syntheses of the 6318/6319 ˚A MgI lines in Arcturus (top), NGC 1718-9 (middle), and NGC 1718-26 (bottom). The solid
linesshowthebest-fitabundances tothe6318˚A line;dashedlinesshowNGC1718’sintegratedlightvaluefromColuccietal.(2012).
(cid:13)c 0000RAS,MNRAS000,000–000
Abundances of NGC 1718 stars 7
Table 5.Derivedabundances andrandomerrors;total errorsaregiveninTableA3.
Arcturus NGC1718-9 NGC1718-26
[X/Fe] N Method [X/Fe] N Method [X/Fe] N Method
FeI −0.53±0.02 152 EW −0.55±0.01 99 EW −0.54±0.01 103 EW
FeII −0.45±0.03 5 EW −0.54±0.01 2 EW −0.57±0.03 2 EW
OI 0.30±0.05 1 SS −0.13±0.07 1 SS −0.11±0.05 1 SS
NaI 0.13±0.03 2 SS −0.13±0.07 2 SS −0.18±0.09 2 SS
MgI 0.36±0.06 11 EW/SS 0.11±0.04 7 EW/SS 0.11±0.03 7 EW/SS
AlI 0.41±0.05 5 EW/SS 0.01±0.07 4 EW/SS 0.04±0.03 4 EW/SS
SiI 0.30±0.02 19 EW 0.11±0.03 9 EW 0.13±0.04 12 EW
CaI 0.20±0.02 14 EW 0.09±0.10 2 EW 0.11±0.07 2 EW
TiI 0.27±0.02 25 EW 0.09±0.03 7 EW 0.06±0.03 12 EW
TiII 0.20±0.02 6 EW −0.10±0.10 2 EW −0.06±0.02 2 EW
VI 0.09±0.03 2 EW −0.09±0.08 3a EW −0.06±0.04 3a EW
MnI −0.12±0.04 5 EW −0.19±0.12 3a EW −0.22±0.08 3a EW
NiI 0.11±0.02 17 EW −0.02±0.05 15 EW −0.02±0.05 14 EW
CuI 0.25±0.10 1 SS −0.63±0.10 1 SS −0.49±0.10 1 SS
RbI 0.03±0.02 2 EW −0.24±0.09 2 EW −0.25±0.13 2 EW
YII −0.09±0.07 3 EW −0.04±0.08 2 EW −0.06±0.08 1 EW
ZrI −0.25±0.04 4 EW −0.18±0.06 3a EW −0.05±0.06 3a EW
LaII −0.05±0.04 5 EW 0.27±0.07 4 EW 0.30±0.10 3 EW
EuII 0.27±0.05 1 SS 0.22±0.05 1 SS 0.26±0.05 1 SS
Notes: a Lineshave−4.7<REW<−4.5;inallcasestheselineshaveHFScomponents.
indicate an average cluster metallicity of [FeI/H] = database7 confirmsthatanycorrectionsshouldbesmall.At
−0.55±0.01. This value roughly agrees with the isochrone most, non-LTE effects would introduce offsets ∼0.1 dex in
fits by Kerber et al. (2007), and is consistent with the thesegiant stars (Mashonkina et al. 2000).
age/metallicity relations of other LMC GCs (see Fig- Mg and Al are derived with EWs and SSs. Most of
ure 5 and Mackey & Gilmore 2003) and field stars (e.g., the Mg and Al lines are sufficiently clean for EW analy-
Piatti et al. 2012). However, this value is higher than the ses, with two exceptions: the 6318 and 6319 ˚A Mg I lines
integratedlightmetallicityfromColucci et al.(2011,2012), and the 6696 and 6698 ˚A Al I lines. In the latter case the
who find [FeI/H] = − 0.70 ± 0.05 (though note lines are weak enough to make EW measurements difficult.
that their [FeII/H] = − 0.26 ± 0.18) and the cal- The6318and6319˚A MgIlinesarefairlystrong—however,
cium triplet measurements from Grocholski et al. (2006), they are located on top of a broad Ca I autoionization fea-
who find [Fe/H] = −0.80±0.03. Integrated abundances ture,which makescontinuumidentification difficult. Figure
can be extremely sensitive to the properties of the adopted 4showsthesesynthesesinArcturusandthetwoNGC1718
isochrone (e.g., Sakariet al. 2014; Colucci et al. 2016). In stars.
fact, for NGC 1718, Colucci et al. find two appropriate The only previous detailed abundances for NGC 1718
isochronesthatreproducetheirFeIlinestrengths:onewith are from the integrated light analysis of Colucci et al.
an age of 1 Gyr and [Fe/H] = −0.39, the other with age (2012). The integrated [Na/Fe] and [Al/Fe] ratios are
= 2.5 Gyr and [Fe/H]= −0.89. Because their analysis was slightly higher than the individual stars, but are in agree-
unable to distinguish between the solutions, Colucci et al. ment within the errors. The integrated [Mg/Fe]= −0.90±
averagedtheageandmetallicity,derivingafinalabundance 0.30 is considerably lower than the individual abundances.
of[FeI/H] = −0.70 ± 0.05. Uncertaintiesin theageand IntheiranalysisofMWGCs,Colucci et al.(2016)findthat
metallicity may introduce uncertainties in the integrated their IL [Mg/Fe] ratios are generally ∼ 0.2−0.3 dex lower
[Fe I/H] ratio & 0.1 dex (Sakari et al. 2014; Colucci et al. than the values from individual values. This offset may be
2016). The disagreement with the CaT metallicities could due to systematic effects (e.g., NLTE effects, as proposed
be due to NGC 1718’s lower [α/Fe], such that the CaT in- byColuccietal.)oritmayreflectrealabundancevariations
dicates a low [Z/H] rather than [Fe/H]. withinthecluster.Thispossibilitywillbediscussedinmore
detail in Section 6.1.
5.3 α-Elements
The Si, Ca, and Ti abundances are determined from EWs.
5.2 Light Elements: O, Na, Mg, and Al
In the NGC 1718 stars there are 8-12 Si I lines, but only
TheOandNaabundancesarederivedsolely withSSs.The twoCaIlines(therearemanyCaIlines inthiswavelength
forbiddenlineat6300˚A isusedtodeterminetheOIabun- range, but most are too strong for this analysis; see Sec-
dances.CNlinesareincludedinthesesyntheses,andtheO tion4).Thereare7-12TiIlinesavailable in theNGC1718
abundances are therefore mildly sensitive to the adopted C
abundance. The Na abundances are from the 6154/6160 ˚A
doublet, which should be least sensitive to non-LTE effects 7 Data obtained from the INSPECT database, version 1.0:
in this metallicity range (Lind et al. 2011); the INSPECT http://www.inspect-stars.com/
(cid:13)c 0000RAS,MNRAS000,000–000
8 Sakari et al.
Figure5.Theage-metallicityrelationforLMCGCs.TheagesarefromBaumgardtetal.(2013).Theabundancesofthetwoindividual
NGC 1718 stars are shown as solid red stars, while NGC 1718’s IL abundance from Coluccietal. (2012) is shown as an open star.
Abundances of other LMC GCs are also shown. The individual stars analyzed by Mucciarellietal. (2008, 2010, 2011, 2014a) and
Johnsonetal.(2006)areaveraged together foreach cluster andareshownas small,open circles.TheILvalues forother clusters from
Coluccietal.(2012)areshownaslargeopencircles.
spectra, but only 2 Ti II lines. In NGC 1718-9, the Ti II are from McWilliam et al. (2013). A solar isotopic ratio of
abundance is lower than Ti I. Si, Ca, and Ti I are all in 63Cu/65Cu= 2.24 is adopted (Asplundet al. 2009). In this
excellent agreement,suggesting thatNGC1718 is amoder- metallicity range, non-LTE effects are not expected to be
ately α-enhanced cluster. significant for these Cu lines (Yan et al. 2015).
From the integrated light spectrum, Colucci et al. The abundances of the two stars indicate that
(2012) derive [Ca/Fe] = −0.14±0.14, which is lower than NGC 1718 has solar [Ni/Fe], mildly subsolar [V/Fe] and
the individual stars analyzed in this work. Colucci et al.’s [Mn/Fe], and very deficient [Cu/Fe]. Colucci et al.’s inte-
[TiI/Fe] = 0.7 is much higher than NGC 1718-9 and -26; gratedlightanalysissuggestedthattheabundancesofiron-
this high abundance suggests that the integrated Ti is sys- peak elements Mn and Ni are roughly solar in NGC 1718.
tematically offset in some way. Theoptical IL TiI lines are Althoughtheindividualstarsshowalower[Mn/Fe],there-
very sensitive to stochastic sampling effects (Sakari et al. sults are in agreement within theerrors.
2014); similarly, Colucci et al. (2016) find a large scatter in
theIL TiI abundancesof their MW GCs.
5.5 Neutron-Capture Elements
Neutron-captureelementsformwhenfreeneutronsarecap-
5.4 Iron-peak Elements turedbyseednuclei.Thebuild-upofneutronsinthenucleus
leads to heavier isotopes, while the subsequent decay into
EWs are utilized for abundances of V, Mn, and Ni, though
protonsgradually forceselementstohigheratomicnumber.
[Mn/Fe]wasalsoverifiedwithSSs.Cuwasdeterminedsolely
Thetypesof elements and isotopes that form from neutron
with SSs.V,Mn,andCurequireHFScomponentstoprop-
capturesdependontheincomingneutronflux,i.e.,theslow
erlyaccountforthestrengthsofthelines.TheHFSlinelists
(s-) neutron-capture process has different nucleosynthetic
andtheArcturusEWmeasurementsfromMcWilliam et al.
yields than the rapid (r-) process. Although the heavy el-
(2013) are adopted here. Note that two of the three V I
ements typically form in both processes, certain elements
lines and all three of the Mn I lines have REWs larger
than−4.7.However,theselinesallhaveHFSsplittingwhich form primarily in only oneof theprocesses.
de-saturates the lines; the EWs of such features are there-
fore still sensitive to the abundance. Tests of these fea-
5.5.1 s-Process Elements
tures show that the lines are formed throughout the atmo-
spheres and are not saturated in the top few layers, where In the sun, Y, Zr, and La form primarily in the s- process,
the models are least reliable. The Cu abundance is deter- while ∼ 50% of Rb forms via the s-process (Burris et al.
mined from the 5782 ˚A line. HFS components and iso- 2000). Abundances of all four elements were determined
topicsplitswereincludedinthesyntheses—again,theselists fromEWs,utilizingtheHFSlinelistsfromMcWilliam et al.
(cid:13)c 0000RAS,MNRAS000,000–000
Abundances of NGC 1718 stars 9
(2013). Two Rb I lines are utilized, at 7800 and 7947 ˚A; as an intermediate-age, sparse GC (which will be discussed
these lines are of moderate strength in both stars. Two in Section 6.1), and 2) its presence in the LMC, which can
Y II lines (5728 and 7450 ˚A) are detectable in NGC 1718- be used to probe the chemical evolution of the LMC (see
9, while only the bluer one is detectable in NGC 1718-26. Section 6.2).
Zr abundances are determined from three strong lines, all
with −4.7 < REW < −4.5; again, these lines have HFS
components,andtestsweredonetoensurethattheindivid- 6.1 NGC 1718 as a Globular Cluster
ualcomponentswerenotsaturated.Threetofourmoderate
Section 1 described the presence of multiple populations in
strength LaII lines were utilized.
GCs. To summarize, in the Milky Way, all classical GCs
Colucci et al.(2012)determineanintegrated[Y/Fe]for
show star-to-star abundance variations in Na and O; some
NGC1718, whichtheyfindtoberoughlysolar.Thisagrees
also show variations in Mg and Al (e.g., Carretta et al.
with the abundances from the two individual stars in this
2009).ThoughMgandAlvariationsaretypicallyonlyseen
work. Though they do not detect La lines in NGC 1718,
inmassive,metal-poorGCs(e.g.,M15;Sneden et al.1997),
Colucci et al. do present an integrated Ba abundance of
recent observations in the H-band suggest that the Mg/Al
[Ba/Fe] = 0.20±0.30.Bariumisalsoprimarilyans-process
anticorrelation may be present in metal-rich GCs as well
element in this metallicity range, and the lines are easily
(Meszaros et al. 2015). No convincing signs of similar mul-
detectable; unfortunately, in this analysis the barium lines
tiple populations have yet been detected in intermediate-
are far too strong in both of the target stars for a reliable
age or young LMC GCs (e.g., Mucciarelli et al. 2008, 2011,
abundanceanalysis(seeSection4).Giventhattheyareboth
2014a) though they have been detected in old LMC GCs
secondpeak,s-processelements,BaisexpectedtotrackLa.
(Johnson et al. 2006; Mucciarelli et al. 2009). Abundance
Though this analysis finds slightly higher [La/Fe], the two
variations within distant GCs can be inferred from inte-
are in agreement within theerrors.
grated spectra, e.g., through high integrated [Na/Fe] ratios
(Sakariet al.2013,2015;Colucci et al.2014).Colucci et al.
5.5.2 r-Process Elements (2012)findthattheintermediate-ageGCsdonothavehigh
integrated [Na/Fe] ratios, while the older GCs do. Though
Eu is primarily an r-process element, with only 3% form-
they did find low integrated [Mg/Fe] in NGC 1718, they
ing from the s-process in the sun (Burris et al. 2000). Eu
attributedthistolowprimordialMgratherthantostar-to-
wasdeterminedfromspectrumsynthesesofthe6645˚A line.
star variations within the cluster.
NearbyCNfeatureswereincludedinthesyntheses.TheHFS
Withonlytwostarsinthissampleitisdifficulttoassess
andisotopicinformationfromMcWilliam et al.(2013)were
the presence of any abundance spreads within NGC 1718.
included, and the solar 153Eu/151Eu = 1.09 ratio was as-
Though the two target stars do appear to have slightly dif-
sumed(Asplundet al.2009).NotethatColucci et al.(2012)
ferent O and Na abundances (with one more O-deficient
donotprovideaEuabundanceforNGC1718.Inthiswork,
and Na-enhanced than the other) theabundancesare iden-
NGC 1718 is found to show moderate r-process enhance-
tical within random errors. The Na abundance differences
ment.
are driven solely by the 6160 ˚A line (the 6154 ˚A line gives
similar Na abundances). Thus, there is no convincing evi-
5.6 Lithium dencefor a spread in Na and O between thetwo stars. Ad-
ditionally,neitherofthestarshaveenhancedNaordeficient
LithiumisdetectableinthetwoNGC1718stars,asshownin
O relative to the LMC field stars (see Figure 7). Similarly,
Figure6.Synthesesweredoneutilizinglines andHFScom-
theMgandAlabundancesinthetwoNGC1718starstrack
ponentsfromtheKuruczdatabase8andconsideringonly7Li
theLMCfieldstardistribution andareidenticalwithin the
lines.Thederivedabundancesarelogǫ(Li) = 0.05±0.05
LTE errors (see Figure 8(a)).
and logǫ(Li) = 0.15 ± 0.05 for NGC 1718 - 9 and
LTE The O, Na, Mg, and Al abundances of these stars
NGC 1718 - 26, respectively. Note that NLTE corrections
therefore appear to follow the “primordial” abundance sig-
are expected to be ∼ +0.4 dex for these stars, based
nature of the cloud from which NGC 1718 formed. This
on estimates from the INSPECT database9 (Lind et al.
lack of significant abundance differences between these two
2009, though note that the atmospheric parameters in the
starsdoesnotruleoutthepresenceofmultiplepopulations
database do not extend to sufficiently cool temperatures
in NGC 1718—indeed, Colucci et al.’s higher [Na/Fe] and
and low surface gravities). These rough Li abundances
[Al/Fe]andlower[Mg/Fe]integratedratiossuggestthatob-
seem appropriate for normal, evolved RGB stars in GCs
servationsofmoreclusterstarsarenecessarytoresolvethis
(e.g., Mucciarelli et al. 2014b; D’Orazi et al. 2014, 2015;
issue. It does imply, however, that Colucci et al.’s unex-
Kirby et al. 2016).
pectedintegratedabundancesarenotlikelytobetheresult
of unusualchemical evolution in the LMC.
6 DISCUSSION
6.2 The Chemical Evolution of NGC 1718, the
AsdiscussedinSection1,NGC1718isavaluabletargetfor
LMC, and its GC System
chemical abundance analyses, for two reasons: 1) its status
The agreement between the abundances of the NGC 1718
8 http://kurucz.harvard.edu/linelists/ stars and the LMC field stars suggests that NGC 1718 is a
9 Data obtained from the INSPECT database, version 1.0: valuable probe for examining the chemical evolution of the
http://www.inspect-stars.com/ LMC,particularlyforelementslikeMnandRb,whichhave
(cid:13)c 0000RAS,MNRAS000,000–000
10 Sakari et al.
Figure6.Synthesesofthe6707˚A LiIlineintheNGC1718stars.Thesolidlinesshowthebest-fitabundances,whilethedashedlines
showthe±1σ uncertainties.
Figure 7. [Na/Fe] (left) and [O/Fe] (right) ratios in NGC 1718 compared to MW and LMC field stars and other LMC GCs. Red
stars show the two NGC 1718 stars from this analysis, along with the random errors. The large maroon open star shows NGC 1718’s
integrated light abundance from Coluccietal. (2012). The large black square shows the Arcturus value derived in this paper. Grey
points areMWfieldstarsfromVennetal.(2004), withsupplements fromReddyetal.(2006)andBensbyetal.(2005)—notethat the
Oabundances fromReddyetal.arenotshown,becausetheyrequireNLTEcorrections.SmallbluecrossesaretheLMCbarstarsfrom
VanderSwaelmenetal.(2013),whilesmallfilledbluecirclesarethediskstarsfromPomp´eiaetal.(2008)whichhavebeenreanalyzed
by VanderSwaelmenetal. (2013). LMC GCs are shown as open circles: large circles are integrated light values from Coluccietal.
(2012)whilesmallcirclesareaverages ofindividualstarsfromJohnsonetal.(2006)andMucciarellietal.(2008,2010,2011,2014a).
(cid:13)c 0000RAS,MNRAS000,000–000