Detailed Chemical Composition and Orbit of the New Globular Cluster

Home , FSR 1758, Milky Way (mythology)

arXiv:1906.05653v1 [astro-ph.GA] 11 Jun 2019 [email protected] orsodn uhr adoVillanova Sandro author: Corresponding ob ihyeccentric. highly be to iprin 4.9 dispersion, o h ea-orcutrmmes h enrda eoiyo th of velocity radial mean The members. cluster cluster globular Halo metal-poor Way the Milky mas for with this consistent suggested are have results studies fo our other event, estimates While confir photometric spread. (R and dex, previous 0.08 resolution motions with small, proper consistent High metallicity DR2 find spectrogra Gaia Bulge. We using echelle Galactic determined MIKE the was with membership of instrumented direction telescope the Magellan/Clay in cluster star ∗ 1 2 5 4 3 atao Chile Santiago, eatmnod srnma ail 6-,Uiesddde Universidad 160-C, Casilla Astronomia, de Departamento eatmnod inisFscs autdd inisExa Ciencias de Facultad Fisicas, Ciencias de Departamento eatmnod iiayAtooi,Uiesddd aSere La de Universidad Astronomia, y Fisica Italy de State, Departamento City Vatican Mackenna V00120 Vicuna Observatory, Av. Vatican Astrophysics, of Institute Millennium ae nosrain are u t...... udrpr under ...... at out carried observations on Based yee sn L using 2019 Typeset 14, June version Draft epeetdtie hmclaudne,rda eoiisadorb and velocities radial abundances, chemical detailed present We MLCTOSFRTEACEINO H EUI WR AAYO GALAXY DWARF SEQUOIA THE OF ACCRETION THE FOR IMPLICATIONS EALDCEIA OPSTO N RI FTENWGLOBULAR NEW THE OF ORBIT AND COMPOSITION CHEMICAL DETAILED Keywords: ± A T . ms km 1.2 E X pia:sas-oe lsesadascain:gnrl-stars: - general associations: and clusters open - stars optical: twocolumn − 1 stpclfrgoua lses eas ofimartord Galac retrograde a confirm also We clusters. globular for typical is , at Minniti, Dante Rcie ..... eie ..... cetdJn 1 June Accepted ...... ; Revised ...... ; (Received tl nAASTeX61 in style adoVillanova Sandro ui O’Connell Julia ,3 4 3, 2, aln Assmann, Paulina gas...... ograms umte oApJ to Submitted ABSTRACT ts nvria nrsBlo v enne oca70 L 700, Concha Fernandez Av. Bello, Andres Universidad ctas, 80 8-46 atao Chile Santiago, 782-0436, 4860, ocpin Cooncepción,Concepcion, Chile a vnd unCsena10,L eea Chile. Serena, La 1200, Cisternaa Juan Avenida na, 1 1 n oez Monaco Lorenzo and — — — ∗ n ogGeisler Doug and 1 ieojc a etersl fapeiu accretion previous a of result the be may object sive lse,+226.8 cluster, e tlprmtr o S 78 eetydiscovered recently a 1758, FSR for parameters ital n oof Barb Rodolfo and ihcaatrsi aOat-orltosfound anti-correlations Na-O characteristic with s hscutr F/]=-1.58 = [Fe/H] cluster, this r h aeeghrange wavelength gh, e ihorrda eoiymeasurements. velocity radial our with med ∼ 200 pcr eeotie sn the using obtained were spectra 42,000) 1 ,2019) 4, 2 abundances ± . ms km 1.6 a ´ 5 T H IK WAY MILKY THE NTO LSE FSR1758: CLUSTER − ∼ 1 4900-8700 i ri htappears that orbit tic ihasalvelocity small a with ± .3dx iha with dex, 0.03 .Cluster A. ˚ sCondes, as 2 Villanova et al.

1. INTRODUCTION The census of Galactic globular clusters (GCs) appears to be incomplete in the direction of the central regions of the Milky Way due to high interstellar ab- sorption and source crowding ( e.g. Ivanov et al. 2005, 2017), as illustrated by several recent discoveries (e.g. Minniti et al. 2017a,b; Ryu et al. 2018; Borissova et al. 2018; Camargo 2018; Camargo & Minniti 2019; Palma et al. 2019). While most of the new GC candidates have low luminosity, one of the most unexpected discoveries is FSR 1758, that is a large metal-poor GC recently found in the direction of the Galactic bulge (Barba et al. 2019), centered at coordinates RA = 17 : 31 : 12, and DEC = −39 : 48 : 30 (J2000), and Galactic coordinates l = 349.217 deg, b = −3.292 deg. They use data from Gaia, the DECaPS optical survey (Schlafly et al. 2018) and the VVVX survey (Minniti et al. 2010, 2018) to Figure 1. Proper motions in the field of FSR 1758. Clus- estimate the GC parameters. In particular, Barba et al. ter members are the indicated with cyan circles, while our (2019) determined a metallicity [Fe/H] = −1.5 ± 0.3 targets with red points. dex based on the optical and near-IR CMDs from Gaia and VVV, respectively. They also measure a distance the Sequoia and Sausage events. Much remains to be ± − ± D = 11.5 1.0 kpc, reddening E(J Ks)=0.20 0.03 understood in terms of the past Milky Way accretion mag, radii Rc = 10 pc, and Rt = 150 pc, total mag- events, and globular clusters with well determined phys- ∼ − ± nitude Mi 8.6 1.0 mag, and proper motions ical parameters, like is now FSR 1758, are key elements − µα = 2.95 mas/yr, and µδ =2.55 mas/yr. of evidence. The determination of the fundamental parameters of We have obtained high dispersion spectra for 9 stars this new GC is important because it has been recently that are confirmed members of the cluster. The deep proposed that FSR 1758 belongs to the past accretion photometry and accurate proper motions measured by of a dwarf galaxy (named Sequoia) by the Milky Way, Gaia and VVV are complemented by our high disper- along with other GCs (Myeong et al. 2019). These au- sion spectroscopy to fully characterize this GC, defining thors argue that the Sequoia event along with the so its parameters. The spectroscopy presented in this pa- called Sausage event are the two most significant accre- per allows us to measure accurate radial velocities and tion events that occurred in the Milky Way. detailed chemical abundances for the target stars. In Barba et al. (2019) argues that FSR 1758 may be particular, the Gaia proper motions in combination with part of an extended dwarf galaxy disrupted by the our accurate radial velocities are employed to improve Milky Way, along with possibly Ton2 and NGC 6380, the cluster’s orbit. that are located within 1 deg of FSR 1758. However, In this paper we aim to answer the following questions: − the RVs of these clusters (RVT on2 = 182 km/s and What is the detailed chemical composition of the new − RVNGC6380 = 4 km/s) are very different from that of GC FSR 1758? Does it have a measurable dispersion in FSR 1758 (RVFSR1758 = +227km/s), making the as- chemical elements, suggesting the presence of multiple sociation very unlikely, based on the Gaia RV measure- populations? How massive is this cluster? Is this a ment of Simpson (2019), who concludes that this cluster real GC or the nucleus of an accreted dwarf? Does this has a retrograde orbit. cluster belong to Milky Way GCS or to an external dwarf On the other hand, based on the strongly retrograde that has been accreted? What is the cluster orbit? Is orbit, Myeong et al. (2019) argues that it belongs to the this a halo, bulge, or disk GC? Sequoia dwarf galaxy, accreted by the Milky Way along with the GCs WCen, NGC3201, NGC6101, NGC6535, 2. OBSERVATIONS AND DATA REDUCTION NGC6388, and NGC6401. They argue that these clus- Our data set consists of high-resolution spectra col- ters share common properties in the age-metallicity lected in 2018 October-November. The spectra come space for example. from single 300s exposures, obtained with the MIKE As a caveat, according to Myeong et al. (2019), the slit echelle spectrograph at the Magellan/Clay telescope. Gaia-Enceladus event (Helmi et al. 2018) includes both Weather conditions were excellent with a typical seeing Abundances in FSR 1758 3

tions of the members are:

pmRA=-2.7911±0.0097 mas/year

pmDEC =+2.6042±0.0090 mas/year

Gaia photometry was cross-correlated with 2MASS and we present multi-band CMDs of the cluster in Fig. 2. The cluster presents well populated RGB and HB and shows to be affected by differential reddening. The HB is not visible in the infrared bands since HB stars emit most of their flux in the blue and ultraviolet. Radial velocities were measured with the fxcor package in IRAF, using a synthetic spectrum as a template. The mean radial velocity we obtained is 226.8±1.6 km/s. There are no clear outliers in the radial velocity distribution. If we consider also that all our targets have the same proper Figure 2. CMDs of the member stars (cyan circles). Field motions (see Table 1) we conclude that they are all clus- stars are indicated with black crosses, while our targets are ter members. We found a radial velocity dispersion of the red circles. 4.9±1.2 km/s Table 1 lists the basic parameters of the 9 observed targets: ID, J2000.0 coordinates, proper mo- of 0.7 arcsec at the zenith and a clear sky. We selected 9 tions and G, B , R magnitudes from Gaia Data release isolated stars around the RGB tip. The MIKE spectra P P 2, and J, H, K magnitudes from 2MASS. have a spectral coverage from 4900 to 8700 A˚ with a res- s olution of ∼42,000 and signal-to-noise ratio (S/N)∼40 at 6300 A.˚ Data were reduced using MIKE dedicated pipeline (Kelson et al. 2000, 2003) 1, including bias sub- 3. ABUNDANCE ANALYSIS traction, flat-field correction, and wavelength calibra- Figure 2 shows that the cluster is affected by dif- tion, scattered-light and sky subtraction. Single orders ferential reddening. For this reason an estimation of were rectified and combined using IRAF 2 . stellar parameters bases on photometry would require In order to select our targets we took advantage a differential reddening correction. Therefore as ini- of the proper motions published in the Gaia DR2 tial temperature, gravity and microturbulence for our (Gaia collaboration 2018) since the cluster suffers of stars we just assumed tipical values for RGB stars close a severe foreground contamination. Proper motions to the tip, that is Teff =4000 K, log(g)=0.5 dex, and of the cluster field stars are reported in Fig. 1, where vt=1.90 km/s. Atmospheric models were calculated us- cluster members are indicated with cyan symbols. As ing ATLAS9 code (Kurucz 1970), assuming our estima- members we selected those stars within a radius of 0.7 tions of Teff , log(g), and vt, and the [Fe/H] value from mas/year around the center of the cluster in the proper Barba et al. (2019). motion space. The value of the radius was determined Then Teff , log(g), and vt were re-adjusted and new at- empirically assuming 1 mas/year as initial guess, and mospheric models calculated in an interactive way in or- reducing it as much as possible in order to remove field der to remove trends in excitation potential and reduced contamination as shown from the CMD. We found that equivalent width (EQW) versus abundance for Teff and reducing the radius below 0.7 mas/year did not improve vt, respectively, and to satisfy the ionization equilibrium the CMD significantly. Among these members we se- for log(g). 60-70 FeI lines and 6-7 FeII lines (depending lected our targets, indicated by red circles. The targets on the S/N of the spectrum) were used for the latter are RGB stars close to the tip. The mean proper mo- purpose. The [Fe/H] value of the model was changed at each iteration according to the output of the abundance analysis. The Local Thermodynamic Equilibrium 1 https://code.obs.carnegiescience.edu/mike 2 (LTE) program MOOG (Sneden 1973) was used for the IRAF is distributed by the National Optical Astronomy Ob- servatory, which is operated by the Association of Universities for abundance analysis. Research in Astronomy, Inc., under cooperative agreement with CaI, TiI, TiII, CrI, FeI, FeII, and NiI abundances the National Science Foundation. were estimated using the EQW method. For this purpose we measured EQW using the automatic program 4 Villanova et al.

Table 1. ID, coordinates, proper motions and magnitudes of the observed stars. See text for details.

ID RA(2000.0) DEC(2000.0) pmRA pmDEC G BP RP J H K [hours] [deg] [mas/yr] [mas/yr] mag mag mag mag mag mag 01 17:31:08.7 -39:49:42.2 -2.99056 2.35811 13.688 15.069 12.529 10.587 9.570 9.299 02 17:31:49.3 -39:56:16.5 -2.87837 2.64815 13.797 15.227 12.629 10.604 9.577 9.315 03 17:31:17.3 -39:50:30.5 -3.16035 2.51542 13.831 15.159 12.688 10.814 9.828 9.586 04 17:31:20.8 -39:45:48.1 -3.04149 2.58301 13.866 15.161 12.736 10.821 9.873 9.617 05 17:30:56.0 -39:50:38.5 -2.59973 2.61964 13.914 15.176 12.719 10.829 9.968 9.702 06 17:31:10.7 -39:50:30.6 -2.75174 2.51828 14.016 15.296 12.888 11.057 10.111 9.878 045 17:31:10.8 -39:47:02.8 -3.13759 2.31699 13.616 14.971 12.456 10.525 9.551 9.289 087 17:31:21.6 -39:50:12.2 -3.07022 2.52644 13.971 15.278 12.822 10.896 9.948 9.700 097 17:31:28.7 -39:54:06.2 -3.09229 2.51471 13.735 15.059 12.587 10.665 9.735 9.451

Table 2. Parameters and abundances of the observed stars. The last rows give the mean abundances of the cluster and the relative error from the mean.

ID Teﬀ log(g) vt RVHELIO [Fe/H] [O/Fe] [Na/Fe] [Mg/Fe] [Al/Fe] [Ca/Fe] [TiI/Fe] [TiII/Fe] [Cr/Fe] [Ni/Fe] [Ba/Fe] [Eu/Fe] oK [km/s] [km/s] dex dex dex dex dex dex dex dex dex dex dex dex 01 3880 0.00 1.83 229.6 -1.52 0.15 0.25 0.62 0.45 0.40 0.39 0.44 -0.18 -0.16 0.12 0.25 02 3880 0.00 1.91 223.8 -1.55 -0.06 0.61 0.74 0.77 0.42 0.34 - -0.33 -0.05 0.07 0.28 03 4020 0.25 1.95 229.7 -1.62 0.29 0.05 0.69 0.41 0.37 0.33 0.42 -0.27 -0.04 -0.17 0.36 04 4010 0.28 1.88 226.2 -1.54 0.13 0.49 0.66 0.40 0.37 0.37 0.28 -0.22 -0.08 -0.13 0.19 05 4170 0.09 2.30 217.5 -1.77 0.27 0.01 0.74 - 0.17 0.08 0.38 -0.29 -0.14 0.08 0.19 06 4080 0.43 1.72 234.7 -1.57 0.32 -0.02 0.60 0.49 0.34 0.34 0.31 -0.18 -0.13 0.10 0.46 045 3870 0.15 1.93 228.5 -1.64 0.31 0.22 0.82 0.17 0.19 0.24 - -0.31 -0.05 -0.05 0.39 087 4035 0.58 1.92 222.9 -1.52 0.25 0.24 0.55 0.46 0.34 0.39 0.25 -0.20 -0.03 -0.15 0.48 097 3950 0.10 1.85 228.1 -1.52 0.16 0.33 0.79 0.21 0.31 0.38 0.29 -0.23 -0.13 -0.05 0.24 Mean 226.8 -1.58 0.20 0.24 0.69 0.42 0.32 0.32 0.34 -0.25 -0.09 -0.02 0.32 Error 1.6 0.03 0.04 0.07 0.03 0.07 0.03 0.03 0.03 0.02 0.02 0.04 0.04

DAOSPEC (Stetson et al. 2008) 3. OI, NaI, MgI, AlI, splitting help to remove the line-core saturation produc- BaII, and EuII abundances were obtained using the ing a change in the final abundance as estimated by the spectro-synthesis method. For this purpose 5 synthetic spectro-synthesis method up to 0.1 dex. Also Eu has spectra were generated for each line with 0.25 dex abun- odd-isotopes affected by hyperfine splitting, but their dance step and compared with the observed spectrum. lines are weak and the line-core saturation is not at work. The line-list and the methodology we used are the same So hyperfine splitting corrections are negligible. used in previous papers (e.g. Villanova et al. 2013), so The abundances we obtained are reported in Tab. 2together we refer to those articles for a detailed discussion about with the mean values for the cluster and the error on this point. Here we just underline that we took hy- the mean. For Ti we reported the mean values of TiI perfine splitting into account for Ba as in our previous and TiII abundances. Na is an element affected by studies. This is particularly important because Ba lines NLTE effets. For this reason we looked in the INSPEC are very strong even in metal-poor stars and hyperfine 4 database for suitable NLTE corrections. We found the they are very small (∼-0.05 dex) with no significant

3 DAOSPEC is freely distributed by http://www.cadc-ccda.hia- iha.nrc-cnrc.gc.ca/en/community/STETSON/daospec/ 4 version 1.0 (http://inspect.coolstars19.com/index.php?n=Main.HomePage) Abundances in FSR 1758 5 variation (less then 0.02 dex) in our temperature range. For this reason we decided not to apply them to our Na abundances. As a cross check of our abundance analysis, we plot in Fig. 3 for each element the difference between the abundance of each star and the mean abundance of the cluster. We see that there is no evident trend of the abundances with temperature. This indicates that the methodology used to obtain chemical abundances is consistent over the entire temperature range. A detailed internal error analysis was performed using the method described in Marino et al. (2008). It gives us σ(Teff )=40 K, σ(log(g))=0.15, and σ(vt)=0.05 km/s. The error on [Fe/H] due to the S/N is 0.03 dex. Then we choose star #097 as representative of the sample, varied its Teff , log(g), [Fe/H], and vt according to the atmospheric errors we just obtained, and redetermined the abundances. Results are shown in Tab. 3, includ- Figure 3. Difference between the abundance of each star ing the error due to the noise of the spectra. This er- and the mean abundance of the cluster. Different elements are reported with different symbols. BLACK: [Fe/H] (Filled ror was estimated for elements whose abundance was circles), [Ca/Fe] (empty circles), [Ti/Fe] (filled squares), obtained by EQWs, as the errors on the mean given [Cr/Fe] (empty squares), [Ni/Fe] (empty stars). RED: by MOOG, and for elements whose abundance was ob- [Ba/Fe] (filled circles), [Eu/Fe] (empty circles), [O/Fe] (filled tained by spectrum-synthesis, as the error given by the squares), [Na/Fe] (empty squares), [Mg/Fe] (empry stars). fitting procedure. ∆tot is the squared sum of the indi- BLUE: [Al/Fe] (filled circles). vidual errors. For each element we report the observed spread of the sample (RMSobs) with its error and in the on these elements we derive for the cluster a mean α final column the significance (in units of σ) calculated element abundance of: as the absolute value of the difference between RMSobs ± and ∆tot divided by the error on RMSobs. This tells [α/Fe]=+0.32 0.01 us if the observed dispersion RMSobs is intrinsic or due As far as heavy elements are concerned Ba is slightly to observational errors. Values larger than 3σ imply an sub-solar, while Eu is super-solar. intrinsic dispersion in the species chemical abundance Figure 4 shows the star by star (black filled cir- among the cluster stars. cles) α-element abundances of the cluster compared 4. RESULTS with a variety of galactic and extra-galactic objects. We have included values from GGCs (Carretta et al. 4.1. Iron-peak, α elements and heavy elements 2009, 2010, 2014a,b, 2015a,b; Villanova et al. 2010, The mean iron content we obtained is: 2011, 2013; Munoz et al. 2013; San Roman et al. 2006, red filled squares), Disk and Halo stars (Fulbright [Fe/H]= −1.58 ± 0.03 2000; Reddy et al. 2003, 2006; Cayrel et al. 2004; Simmerer 2004; Barklem et al. 2005; Francois et al. with a dispersion of: 2007; Johnson et al. 2012, 2014, gray filled squares), the Bulge (Gonzalez et al. 2011, purple stars), and extra- σ =0.08 ± 0.02 [F e/H] galactic objects such as Magellanic clouds (Pompeia et al. Reported errors are errors on the mean. The measured 2008; Johnson et al. 2006; Mucciarelli et al. 2008, 2009, iron dispersion in Tab. 3 well agrees with the dispersion blue filled squares), Draco, Sextans, Ursa Minor and due to measurement errors so we no evidence for an Sagittarius dwarf galaxy and the ultra-faint dwarf intrinsic Fe abundance spread. As far as other iron-peak spheroidals Boötes I and Hercules (Monaco et al. 2005; elements are concerned, both Cr and Ni are syb-solar. Sbordone et al. 2007; Shetrone et al. 2001; Ishigaki et al. The α elements Ca and Ti are overabundant com- 2014; Koch et al. 2008, green filled squares). pared to the Sun. This is a feature common to almost The α elements in FSR 1758 follows the same trend all Galactic GC and Halo field stars as well as to very as Galactic GCs and are fully compatible with Halo and metal-poor stars ([Fe/H]<-1.5) in outer galaxies. Based Thick Disk field stars while it falls in a region scarcely 6 Villanova et al.

Table 3. Estimated errors on abundances due to errors on atmospheric parameters and to spectral noise for star #097 (column 2 to 6). Column 7 gives the total error calculated as the root squared of the sum of the squared of columns 2 to 6. This total error must be compared with the observed dispersion (RMS) of the data with its error (column 8). The last column gives the signiﬁcance of the diﬀerence between the total error and the observed dispersion, in units of σ. See text for more details.

El. ∆Teff =40 K ∆log(g)=0.15 ∆vt=0.05 km/s ∆[Fe/H]=0.05 S/N ∆tot RMSobs Significance (σ) ∆([O/Fe]) 0.02 0.05 0.01 0.01 0.10 0.11 0.12±0.03 0.3 ∆([Na/Fe]) 0.02 0.01 0.00 0.00 0.05 0.05 0.21±0.05 3.2 ∆([Mg/Fe]) 0.01 0.01 0.01 0.00 0.07 0.07 0.09±0.02 1.0 ∆([Al/Fe]) 0.01 0.04 0.01 0.01 0.09 0.10 0.18±0.05 1.6 ∆([Ca/Fe]) 0.05 0.01 0.00 0.01 0.07 0.09 0.09±0.02 0.0 ∆([TiI/Fe]) 0.07 0.02 0.00 0.01 0.05 0.09 0.10±0.02 0.5 ∆([Cr/Fe]) 0.06 0.01 0.01 0.01 0.04 0.07 0.06±0.01 1.0 ∆([Fe/H]) 0.03 0.02 0.01 0.00 0.02 0.04 0.08±0.03 1.2 ∆([Ni/Fe]) 0.00 0.02 0.01 0.01 0.04 0.05 0.05±0.01 0.0 ∆([Ba/Fe]) 0.04 0.02 0.03 0.00 0.08 0.10 0.11±0.03 0.3 ∆([Eu/Fe]) 0.03 0.06 0.01 0.01 0.12 0.14 0.11±0.03 1.0 populated by extragalactic objects. So, according to its α-element content, FSR 1758 is likely a genuine Galactic cluster. The chemical abundances for the iron-peak elements Cr and Ni and for the heavy-elements Ba and Eu are reported in Fig. 5 and 6 respectively. Around FSR 1758 metallicity, Galactic and extragalactic environments share the same abundances. We underline the low Cr content of the cluster compared with the general trend, at the very lower boundary. Finally for all α, iron-peak and heavy elements Tab. 3 shows that the observed dispersion agrees well with the measurement errors so we can rule out any intrinsic abundance spread. We check also for possible correlations of these elements with lights elements such Na and Al, but we did not find evidence for significant trends.

4.2. Light elements Figure 4. Ca and Ti abundances for FSR 1758 (black The light element Na has an observed spread that ex- filled circles) compared with a variety of Galactic and extra- ceeds the observational uncertainties (see Tab. 3). O, galactic environments: Galactic Globular Clusters (red filled Mg and Al instead show homogeneity, at least as far as squares), Disk and Halo stars (gray filled squares), Bulge stars (purple stars), Magellanic clouds (blue filled squares), observed dispersions are concerned. and other dwarf and ultra-faint dwarf galaxies (Draco, Sex- In Fig. 7 we compare the O and Na abundances of the tans, Ursa Minor, Sagittarius, Bootes I and Hercules, green targets with different environments. A clear Na-O an- filled squares). See text for more details. ticorrelation appears. The content of the two elements for the FSR 1758 first generation stars (the targets with light elements available from our study. Al shows the ∼ ∼ [O/Fe] 0.3, [Na/Fe] 0.0) well matches the mean O and second largest spread among the elements (0.18 dex), Na abundances of the Milky Way Halo and the mean O however it is not significant according to our error anal- and Na content of the other GC first generation stars. ysis. There is also no apparent relation between Na and On the other hand the cluster shows depletion in O Mg or Al at odd with wha found in other GCs (i.e. M28, and Na enhancement as far as the subsequent genera- Villanova et al. 2017), or a relation between Mg and Al. tion targets are concerned (the targets with [O/Fe]<0.3, [Na/Fe]>0.1). Fig. 8 shows the relations between all the Abundances in FSR 1758 7

Figure 5. Cr and Ni abundances for FSR 1758 (black filled circles) compared with a variety of Galactic and extragalactic environments: Galactic Globular Clusters (red filled Figure 7. Na-O anticorrelations for FSR 1758 (black filled squares), Disk and Halo stars (gray filled squares), Bulge circles) compared with Galactic Globular Clusters (red filled stars (purple stars), Magellanic clouds (blue filled squares), squares) and Disk plus Halo stars (gray filled squares); See andother dwarf and ultra-faint dwarf galaxies (Draco, Sex- text for more details. tans, Ursa Minor, Sagittarius, Bootes I and Hercules, green filled squares). See text for more details.

Figure 8. Ligh elements correlations for FSR 1758 (black filled circles). See text for more details. Figure 6. Ba and Eu abundances for FSR 1758 (black filled circles) compared with a variety of Galactic and extra- Further studies with larger datasets are required to con- galactic environments: Galactic Globular Clusters (red filled squares), Disk and Halo stars (gray filled squares), Bulge firm or disproof this result. stars (purple stars), Magellanic clouds (blue filled squares), and other dwarf and ultra-faint dwarf galaxies (Draco, Sex- 5. THE ORBIT tans, Ursa Minor, Sagittarius, Bootes I and Hercules, green FSR 1758 is located at 11.5 kpc from the Sun, on filled squares). See text for more details. the other side of the Galactic Bulge. In order to check if it belongs to the Bulge we calculated its orbit using 8 Villanova et al.

pmDEC =+2.6042 mas/year

RVHelio=226.7 km/s

As far as the model is concerned we used: R⊙= 8.2 kpc

U⊙, V ⊙, W ⊙=10.0,225.2,7.2 km/s

We integrated to orbit forward for 2 Gyrs. In Fig. 9 and Fig. 10 we report the projection of the orbit on the X,Y and X,Z Galactic planes. Fig. 9 shows that FSR 1758 orbit does not enter the Bulge region and the cluster minimum distance from the center is about 3.8 kpc while the maximum distance is 13.6 kpc. Fig. 10 on the Figure 9. Projecton on the X,Y galactic plane of the orbit. other hand suggests an orbit with a maximum height of See text for more details. about 7 kpc. The eccentricity and the Z component of the orbital angular momentum (LZ ) are 0.56 and 1.20 kpc×km/s respectively.

We compare our orbit with that published by Simpson (2019) where the author use initial parameters very sim- ilar to ours:

0 RAcluster =262.806

0 DECcluster=-39.822

Distance=11.5 kpc

pmRA=-2.85 mas/year

pmDEC =+2.55 mas/year Figure 10. Projecton on the X,Z galactic plane of the orbit. See text for more details. RVHelio=227 km/s

GravPot16 5 (Fernandez-Trincado 2017). This is a web- +0.9 He find a perincenter at 3.8−0.9 kpc and an apocenter code that allow to calculate orbits based on a Galactic +8 at 16−5 kpc. Both parameters anicely gree with ours. gravitational potential driven by the Besanon Galaxy Also the maximum height agrees with Simpson (2019) Model mass distribution. As input parameters for the value beeing 7÷8 kpc. GravPot16 includes a Bar as cluster we assumed: far as the Bulge potential is concerned, while Simpson (2019) assumes a spherical potential for the Bulge. Since 0 RAcluster=262.810 the cluster does not enter the Bulge region, the gravitational potential of the Bar does not affect its orbits 0 DECcluster=-39.815 significantly as the comparison of the two results shows.

Distance=11.5 kpc We then support the Simpson (2019) classiﬁcation of FSR 1758 as a Halo cluster based on its orbital parame- pmRA=-2.7911 mas/year ters, since both its highly eccentric orbits and the maximum distance from the Galactic plane do not match typical Disk orbits that are more circular and conﬁned

5 https://gravpot.utinam.cnrs.fr/ Abundances in FSR 1758 9 within 1 Kpc from the plane. We confirm also that, due • The orbit is retrograde and highly eccentric. to the LZ value we find, the orbit is retrograde. • Based on the orbit, this cluster does not appear to belong to the Disk or Bulge components of the 6. DISCUSSION AND CONCLUSIONS Milky Way, and is more consistent with that of a Our target stars were carefully selected to be mem- Galactic Halo building block. bers of the cluster based on Gaia proper motions. All • This GC probably belonged to the Sequoia dwarf the observed target stars turned out to be RV members galaxy that contained at least two large GCs of the cluster, giving confidence to our selection proce- (ωCen and FSR1758), and was accreted in the past dure. Based on the available spectra for 9 objects, we by the Milky Way. measure a velocity dispersion σ = 4.9 ± 1.2 km/s. RV • We suggest that the nucleus of this dwarf galaxy This velocity dispersion is relatively small, typical for a was ωCen, and not FSR1758 that appears to have globular cluster (e.g. Pryor & Meylan 1993). homogeneous composition typical for a normal We also confirm the mean RV measurement of GC. Simpson (2019), and therefore his conclusion that this cluster has a retrograde orbit. The orbital parameters • We predict that the Sequoia dwarf galaxy may we find agree nicely with theirs. have had more GCs. Simpson (2019) and Myeong et al. (2019) find that this GC has a strongly retrograde orbit with high eccentricity (e ∼ 0.6). This is important because the orbit of Simpson (2019) favors the interpretation of Myeong et al. (2019) that this GC belongs to an early SV gratefully acknowledges the support provided by accretion event in the Milky Way. Myeong et al. (2019) Fondecyt reg. n. 1170518. D.G. gratefully acknowl- studies the action space and the GCs analyzed by edges support from the Chilean BASAL Centro de Ex- Vasiliev (2019), concluding that among the different celencia en Astrofisica y Tecnologias Afines (CATA) substructures present, there was a clear signature of an grant PFB-06/2007. This work has made use of data individual event of accretion that was retrograde, corre- from the European Space Agency (ESA) mission Gaia sponding to the Sequoia dwaf galaxy. (https://www.cosmos.esa.int/gaia), processed by The orbit that we compute places this GC in a singu- the Gaia Data Processing and Analysis Consortium lar position with common properties with ω Centauri. (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). It then appears that both clusters are the defining mem- Funding for the DPAC has been provided by national bers of this Sequoia dwarf galaxy. institutions, in particular the institutions participat- In summary, our main conclusions are: ing in the Gaia Multilateral Agreement. DM is sup- ported by the BASAL Center for Astrophysics and • FSR1758 is a bonafide GC, and not the nucleus of Associated Technologies (CATA) through grant AFB a dwarf galaxy. 170002, by the Programa Iniciativa Cientfica Milenio grant IC120009, awarded to the Millennium Institute of Astrophysics (MAS), and by Proyecto FONDECYT • FSR1758 shows the typical Na-O anticorrelation No. 1170121. common to almost all the GCs in the Galaxy.

Facilities: Magellan-Clay:Mike • Its velocity dispersion is small, typical for a globular cluster. Software: MOOG (Sneden 1973)

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