The enigmatic globular cluster UKS~1 obscured by the bulge: \textit{H}-band discovery of nitrogen-enhanced stars
José G. Fernández-Trincado, Dante Minniti, Timothy C. Beers, Sandro Villanova, Doug Geisler, Stefano O. Souza, Leigh C. Smith, Vinicius M. Placco, Katherine Vieira, Angeles Pérez-Villegas, Beatriz Barbuy, Alan Alves-Brito, Christian Moni Bidin, Javier Alonso-García, Baitian Tang, Tali Palma
AAstronomy & Astrophysics manuscript no. UKS1 c (cid:13)
ESO 2020October 2, 2020
The enigmatic globular cluster UKS 1 obscured by the bulge: H -band discovery of nitrogen-enhanced stars José G. Fernández-Trincado , (cid:63) , Dante Minniti , , Timothy C. Beers , Sandro Villanova , Doug Geisler , , , StefanoO. Souza , Leigh C. Smith , Vinicius M. Placco , Katherine Vieira , Angeles Pérez-Villegas , Beatriz Barbuy , AlanAlves-Brito , Christian Moni Bidin , Javier Alonso-García , , Baitian Tang and Tali Palma , Instituto de Astronomía y Ciencias Planetarias, Universidad de Atacama, Copayapu 485, Copiapó, Chile Institut Utinam, CNRS-UMR 6213, Université Bourgogne-Franche-Compté, OSU THETA Franche-Compté, Observatoire de Be-sançon, BP 1615, 251010 Besançon Cedex, France Depto. de Cs. Físicas, Facultad de Ciencias Exactas, Universidad Andrés Bello, Av. Fernández Concha 700, Las Condes, Santiago,Chile Vatican Observatory, V00120 Vatican City State, Italy Department of Physics and JINA Center for the Evolution of the Elements, University of Notre Dame, Notre Dame, IN 46556,USA Departamento de Astronomía, Casilla 160-C, Universidad de Concepción, Concepción, Chile Departamento de Astronomía, Universidad de La Serena, Avenida Juan Cisternas 1200, La Serena, Chile Instituto de Investigación Multidisciplinario en Ciencia y Tecnología, Universidad de La Serena. Benavente 980, La Serena, Chile Universidade de São Paulo, IAG, Rua do Matão 1226, Cidade Universitária, São Paulo 05508-900, Brazil Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK NSF’s Optical-Infrared Astronomy Research Laboratory, Tucson, AZ 85719, USA Universidade Federal do Rio Grande do Sul, Instituto de Física, Av. Bento Gonçalves 9500, Porto Alegre, RS, Brazil Instituto de Astronomía, Universidad Católica del Norte, Av. Angamos 0610, Antofagasta, Chile Centro de Astronomía (CITEVA), Universidad de Antofagasta, Av. Angamos 601, Antofagasta, Chile Millennium Institute of Astrophysics, Santiago, Chile School of Physics and Astronomy, Sun Yat-sen University, Zhuhai 519082, China Universidad Nacional de Córdoba, Observatorio Astronómico de Córdoba, Laprida 854, 5000 Córdoba, Argentina Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Godoy Cruz 2290, Ciudad Autónoma de Buenos Aires,ArgentinaReceived: 02 / / / / ABSTRACT
The presence of nitrogen-enriched stars in globular clusters provides key evidence for multiple stellar populations (MPs), as has beendemonstrated with globular cluster spectroscopic data towards the bulge, disk, and halo. In this work, we employ the VVV InfraredAstrometric Catalogue (VIRAC) and the DR16 SDSS-IV release of the APOGEE survey to provide the first detailed spectroscopicstudy of the bulge globular cluster UKS 1. Based on these data, a sample of six selected cluster members was studied. We find themean metallicity of UKS 1 to be [Fe / H] = − . ± .
11, considerably more metal-poor than previously reported, and a negligiblemetallicity scatter, typical of that observed by APOGEE in other Galactic globular clusters. In addition, we find a mean radial velocityof 66 . ± . − , which is in good agreement with literature values, within 1 σ . By selecting stars in the VIRAC cataloguetowards UKS 1, we also measure a mean proper motion of ( µ α cos( δ ), µ δ ) = ( − . ± . − . ± .
16) mas yr − . We find strongevidence for the presence of MPs in UKS 1, since four out of the six giants analysed in this work have strong enrichment in nitrogen([N / Fe] (cid:38) + .
95) accompanied by lower carbon abundances ([C / Fe] (cid:46) − . α - (O, Mg, Si, Ca, Ti), Fe-peak(Fe, Ni), Odd-Z (Al, K), and the s -process (Ce, Nd, Yb) elemental abundances of our member candidates are consistent with thoseobserved in globular clusters at similar metallicity. Furthermore, the overall star-to-star abundance scatter of elements exhibiting themultiple-population phenomenon in UKS 1 is typical of that found in other global clusters (GCs), and larger than the typical errors ofsome [X / Fe] abundances. Results from statistical isochrone fits in the VVV colour-magnitude diagrams indicate an age of 13.10 + . − . Gyr, suggesting that UKS 1 is a fossil relic in the Galactic bulge.
Key words. stars: abundances – stars: chemically peculiar – Galaxy: globular clusters: UKS 1 – techniques: spectroscopic
1. Introduction
Globular clusters (GCs) are generally considered one of the keyprobes for revealing vital information about the mass-assemblyhistory of the Milky Way (Khoperskov et al. 2018; Minnitiet al. 2018; Massari et al. 2019; Fernández-Trincado et al. 2020; (cid:63)
Corresponding author: [email protected]
Hanke et al. 2020). In the dawning era of the
Gaia mission (GaiaCollaboration et al. 2018), it has been possible to provide use-ful information on the fundamental parameters (see, e.g. Baum-gardt et al. 2019) of almost all ( ∼ Article number, page 1 of 16 a r X i v : . [ a s t r o - ph . GA ] S e p & A proofs: manuscript no. UKS1 census of GCs in the central part of the Milky Way appears tobe incomplete due to high interstellar extinction and crowding(Alonso-García et al. 2017, 2018). A large number of GC can-didates have been reported in the VVV survey (Borissova et al.2014; Minniti et al. 2017), and subsequent studies combining as-trometric data from the
Gaia satellite (Gaia Collaboration et al.2018), the VVV Infrared Astrometric Catalogue (VIRAC: Smithet al. 2018), and chemistry from massive high-resolution spec-troscopic surveys are helping to properly characterize these can-didates (see, e.g. Contreras Ramos et al. 2018; Villanova et al.2019).In this context, near-IR high-resolution spectroscopy surveyssuch as the Apache Point Observatory Galactic Evolution Ex-periment (APOGEE Majewski et al. 2017) have helped mini-mize the e ff ect of extinction, allowing for the detailed study ofthe Galactic bulge region (see also Rojas-Arriagada et al. 2020;Queiroz et al. 2020). Detailed abundances for a number of chem-ical species with a variety of nucleosynthetic origins have pro-vided deeper insight into the multiple-population phenomenonCarretta et al. (2003); Bastian & Lardo (2018); Mészáros et al.(2020) found in virtually all bulge GCs (Carretta et al. 2009; Pan-cino et al. 2017; Mészáros et al. 2020). Identifying and / or test-ing the mechanism responsible for this puzzling phenomenon inGCs (see, e.g. Decressin et al. 2007; de Mink et al. 2009; Bastian& Lardo 2018) helps us understand not only GC formation andevolution, but also the chemical evolution of galaxies (Pipinoet al. 2009; Mauro et al. 2013; Barbuy et al. 2018).It is now firmly established that stars showing light-elementabundance variations are almost ubiquitous within GCs (see, e.g.Gratton et al. 2004; Carretta et al. 2009; Mészáros et al. 2020).The most commonly measured signatures are the Na-O anti-correlation (see, e.g. Carretta et al. 2009, and references therein),the Al-Mg anti-correlation (Pancino et al. 2017), and the N-Canti-correlation (Masseron et al. 2019; Mészáros et al. 2020).Stars within a cluster often exhibit higher [Na,N / Fe] and lower[C,O / Fe] values, with the former well above the typical Galacticlevels at the same [Fe / H] as the cluster. In this context, Schiavonet al. (2017) and Fernández-Trincado et al. (2019d) have demon-strated that the identification of stars in bulge GCs with stellaratmospheres strongly enriched in nitrogen is a reliable tracer ofthe multiple-population phenomenon at all metallicities, and thatit prevails in bulge GCs as metal-rich as [Fe / H] ∼ − . ff orts being carriedout towards the bulge region, e.g. CAPOS (the bulge ClusterAPOgee Survey – Geisler et al. 2020, in preparation), a num-ber of GCs have not been thoroughly investigated due to highforeground extinction. This also strongly limits observations inthe optical regime (see, e.g. Cohen et al. 2018), pending inde-pendent constraints from ground-based spectroscopic observa-tions. Among them is the case of UKS 1, a bulge GC discoveredby Malkan et al. (1980) and neighbour of VVV CL001 (Min-niti et al. 2011), which lies in a region of the Galactic bulgewhere interstellar extinction is very high, with E(B-V) ∼ ∼ Fig. 1.
Spatial distribution of stars (grey dots) in the APOGEE-2 sur-vey towards the bulge GC UKS 1. The highest likelihood members ofUKS 1 are marked with black open squares. The black circle is the tidalradius of UKS 1 ( ∼ ∼ ∼ ∼ smaller heliocentric distance of ∼ ∼ . × M (cid:12) by comparing the cluster density profile from themost recent Gaia
DR2 data (Gaia Collaboration et al. 2018) toa large suite of direct N-body star cluster simulations. There isalso a discrepancy regarding the cluster metallicity, that is to say,integrated infrared photometry indicates metallicities of [Fe / H] ∼ − . − .
18 (Bica et al. 1998), while echelle spectra cover-ing the range 1.5 – 1.8 µ m estimate an intermediate metallicity of[Fe / H] = − .
78 (Origlia et al. 2005). In addition, there is no ageestimate available for UKS 1. Therefore, a detailed and reliablephysical characterization of this cluster is still lacking (Cohenet al. 2018).The main purpose of the present paper is to explore thenear-IR high-resolution spectroscopic observations from theAPOGEE survey towards UKS 1, combined with an updatedversion of the VVV Infrared Astrometric Catalogue (VIRAC).These help us obtain a better view of the properties of the stellarpopulation in the inner regions of UKS 1, as well as to producea much improved CMD, helping to roughly estimate the clusterage by adopting a Bayesian statistical approach (see, e.g. Souzaet al. 2020). In Section 2, we briefly describe the observations.In Section 3, the data and selection of the potential members ofUKS 1 are described. In Section 4, we describe the adopted at-mospheric parameters, followed by a discussion of the chemicalabundances of our candidate stars in Section 5. We present anorbital analysis of UKS 1 in Section 6. In Section 7 we presentresults for the estimated age of UKS 1 based in a statisticalisochrone fitting method following a Bayesian approach. Finally,in Section 8, we summarize the results and draw our conclusions.The proper motion computation from VIRAC data and the di ff er-ential reddening correction are described in Appendices A andB. Article number, page 2 of 16osé G. Fernández-Trincado et al.: Nitrogen-enhanced stars in UKS 1
2. UKS 1 in the APOGEE DR16
UKS 1 was observed by one of the APOGEE twin spectrographs(Wilson et al. 2012; Eisenstein et al. 2011; Wilson et al. 2019)mounted on the Irénée du Pont 2.5m telescope at Las CampanasObservatory (Bowen & Vaughan 1973) in Chile. These obser-vations were part of the APOGEE bulge programme with fieldcentre in ( l , b ) ∼ (5 ◦ , 0 ◦ ), collecting high-resolution ( R ∼ H -band spectral information for 448 sources. Cluster targetswere positioned towards the north-west direction of the field in( l , b ) ∼ ( 5.13 ◦ , 0.76 ◦ ), and four visits were needed in order toarchive a minimal S / N >
50 at K s , (cid:46) . / N between 96 to 192. This work makes use of publicspectra collected in the sixteenth data release (APOGEE DR16:Ahumada et al. 2020), as part of the Sloan Digital Sky SurveyIV (Blanton et al. 2017).
3. Membership
We identified a sample of potential stellar members of UKS 1in the APOGEE DR16 database. Table 1 summarizes the mainproperties of the likely members of UKS 1. Radial velocities aretaken from the APOGEE catalogue (Ahumada et al. 2020), whilethe absolute proper motions and near-IR magnitudes (J, K s ) areretrieved from VIRAC and 2MASS, respectively.We selected probable cluster members based on their radialvelocities, absolute proper motions, metallicity, and position inthe near-IR CMD. First, we restrict our sample to stars within6.32 (cid:48) from the GC centre, that is, inside the nominal GC tidalradius given in the new catalogue of fundamental properties ofGalactic GCs (Baumgardt & Hilker 2018; Baumgardt et al. 2019;Hilker et al. 2020).In addition, for the stars selected above, we manually val-idated their metallicities ([Fe / H]) from the strength of selectediron (Fe I) lines, by adopting the same methodology as describedin Fernández-Trincado et al. (2019b), i.e. the [Fe / H] ratios havebeen derived by using the
BACCHUS code (Masseron et al. 2016),and by performing a LTE analysis with a MARCS grid of spher-ical models (Gustafsson et al. 2008). For the remainder of thiswork, metallicity refers to the [Fe / H] determined from Fe I ab-sorption features. We refer the reader to Nataf et al. (2019) andMészáros et al. (2020) for further discussion of di ff erences be-tween metallicities from optical and IR range.Then, we selected objects that clump in the [Fe / H]–radial ve-locity and proper motion (PM) space, located closest to the nom-inal metallicity, radial velocity, and PMs of the cluster. The finalsample contained six potential cluster members. The sky posi-tion of these stars is displayed in Figure 1; their proper motiondistribution, metallicity, and radial velocities are summarized inFigure 2.Figure 3 follows the sky position of 4 out of the 6 stars ina mosaic of multi-band images (FoV 6’ × For further details, we direct the reader to García Pérez et al. (2016) –APOGEE Stellar Parameter and Chemical Abundances pipeline (ASP-CAP), Holtzman et al. (2018) – grid of synthetic spectra and discussionof associated errors, and Nidever et al. (2015) – data reduction pipelinefor APOGEE. The model grids for APOGEE DR16 are based on a com-plete set of
MARCS (Gustafsson et al. 2008) stellar atmospheres, whichare now extended T e ff (cid:46) up in full detail in the near-IR with VVV and Glimpse. It be-comes a conspicuous cluster in Pan-Starrs, but is very contami-nated in 2MASS, Glimpse, and WISE. We note that one of ourstars identified so far in the very innermost region of the clus-ter has an atmosphere strongly enriched in nitrogen, supportingthe existence of multiple stellar populations (MPs) in UKS 1 (assummarized below ).Finally, we examined the location of our six potential clustermembers in the di ff erential-reddening corrected VVV + + ffi cient scatter to accommodate a widerange in [Fe / H], in agreement with other GCs at similar metallic-ity. We find that the selected stars from the APOGEE cataloguelie in the upper part of the RGB, indicated by lime squares inFigure 2.
4. Spectroscopic parameters
To determine the chemical abundances of the selected membersfrom UKS 1, we adopted the uncalibrated stellar atmosphericparameters (T e ff , log g , and [M / H]) computed from the
ASPCAP pipeline (García Pérez et al. 2016). With the fixed T e ff , log g ,and first-guess [M / H], the first step consisted in determining themetallicity from selected Fe I lines, the micro-turbulence veloc-ity ( ξ ), and the convolution parameter with the BACCHUS code(Masseron et al. 2016). Thus, the metallicity provided is the av-erage abundance of selected Fe lines, while the micro-turbulencevelocity is obtained by minimizing the trend of Fe abundancesagainst their reduced equivalent width, and the convolution pa-rameter stands for the total e ff ect of the instrument resolution.We proceed in the same manner as in Hawkins et al. (2016), i.e.we derive a single global convolution value per spectrum, basedon the average broadening of Fe lines, by assuming a Gaus-sian convolution profile. Once [Fe / H], ξ , and the convolution pa-rameters are determined, O, C, and N abundances are estimatedfrom selected OH, C O, and C N molecules (Smith et al.2013). Once those elements are measured, the full process is iter-ated until convergence (see, e.g. Fernández-Trincado et al. 2016,2017, 2019a,b,c,d).Finally, for each chemical species and each line, the abun-dance ratios are determined with the
BACCHUS code follow-ing the procedure as described in Hawkins et al. (2016), andbriefly summarized here for guidance. ( i ) a spectrum synthe-sis, using the full set of lines from the internal APOGEE DR14atomic / molecular linelist (linelist 20150714) to find the localcontinuum level via a linear fit; ( ii ) cosmic and telluric rejec-tions are performed; ( iii ) the local S / N is estimated; ( iv ) a se-ries of flux points contributing to a given absorption line is au-tomatically selected; and ( v ) abundances are then derived bycomparing the observed spectrum with a set of convolved syn-thetic spectra characterized by di ff erent abundances. The code fi-nally proceeds with four di ff erent abundance determinations: ( a )line-profile fitting; ( b ) core line intensity comparison; ( c ) globalgoodness-of-fit estimate ( χ ); and ( d ) equivalent-width compari-son. Each method yields validation flags, and a decision tree thenrejects the line or accepts it, keeping the best-fit abundance. Inthis work, we adopt the χ method, which is most robust (e.g.Hawkins et al. 2016; Fernández-Trincado et al. 2019b). How-ever, the information from the other diagnostics is stored, in-cluding the standard deviation between all four methods.Using the methods outlined above, we re-analyse theAPOGEE spectra of the six cluster member candidates and man- Article number, page 3 of 16 & A proofs: manuscript no. UKS1
Fig. 2. (a) Radial velocity versus [Fe / H] for APOGEE stars towards the UKS 1 field. (b) kernel density estimation (KDE) of the VIRAC propermotions. (c) CMD using VVV + s bands for stars within the half-light cluster radius. The six highest likelihood members of UKS 1analysed in this work are marked with lime square symbols. In (a), the average in radial velocity and metallicity of our sample is marked by theblue cross symbol, while the blue rectangle indicates 3 σ RV and 3 σ [Fe / H] . The black dotted lines in (a) and (b) indicate the nominal [Fe / H], µ α cos δ , µ δ , and radial velocity from Origlia et al. (2005) and Baumgardt et al. (2019), respectively. The blue dotted lines in (b) indicate our estimated µ α cos δ and µ δ using VIRAC proper motions. Error bars are provided in Tables 1 and 2. Fig. 3.
From left to right, and from top to bottom: DSS2, 2MASS, WISE, PanStarrs, VVV, and Glimpse images (size 6’ × + ’ (N-normal stars) signs in each panel mark the position of 4 out of the 6 highest likelihood membersof UKS 1 analysed so far in this work; the magenta plus symbol indicate the centre of UKS 1. ually estimate their chemical abundances. In this work, we focuson the Fe-peak elements (Fe, Ni), the light-elements (C, N), the α − elements (O, Mg, Si, Ca, and Ti), the odd-Z elements (Al, K),and the s -process elements (Ce, Nd, Yb) because their spectrallines are relatively strong and prominent in the APOGEE spectrafor mildly metal-poor stars. Table 1 shows the derived chemicalabundances. The reference Solar photospheric abundances arefrom Asplund et al. (2005), except for Ce II, Nd II, and Yb II,for which we have adopted the Solar abundances from Grevesseet al. (2015). The adopted atmospheric parameters of our stars are consistent within the error range with parameters of UKS 1RGB stars analysed in Origlia et al. (2005).It is important to note that we do not provide abundance de-terminations based on photometric temperatures, as this intro-duces its own set of problems, mostly related to high E(B-V)values (see, e.g. Mészáros et al. 2020), and particularly becauseUKS 1 lies in a region with significant reddening (E(B − V) (cid:38) ASPCAP , as any photometric temperature esti-mation will not be reliable towards UKS 1.
Article number, page 4 of 16osé G. Fernández-Trincado et al.: Nitrogen-enhanced stars in UKS 1
In order to further check the statistical significance of ourmethod in the calculation of chemical abundances, we also com-pute synthetic spectra by assuming uncertainties in the parame-ters in the range of ∆ T e ff =
100 K, ∆ log g = .
3, and ∆ ξ = . − , following the strategy outlined in Fernández-Trincadoet al. (2019b), to investigate the sensitivity of abundances dueto the variations in the adopted atmospheric parameters. Table2 shows the mean sensitivity of abundances according to theatmospheric parameters changes. The variation of atmosphericparameters result in a total uncertainty of about 0.1 to 0.25 dexin abundances; the e ff ective temperature and log g uncertainties,along with line-by-line variation, are the main contribution of theabundance uncertainties, depending on the chemical species.Based on the calculated chemical abundances, we have dis-covered that four of our six likely cluster members are nitrogen-enriched. Figure 4 provides a brief examination of typical H -band spectra for the four nitrogen enriched stars around the C N spectral absorption features. This figure confirms theexistence of a real chemical peculiarity in these objects. Thespectra of these stars are shown in a wavelength range contain-ing several C N lines, which are indicated by the grey andcyan shadow regions. The N-rich stars have remarkably stronger C N lines compared to a star with similar atmospheric param-eters and a normal ([N / Fe] (cid:46) + .
5) nitrogen abundance, which,in view of the fact that the pair of stars have nearly the same at-mospheric parameters, can only mean that the UKS 1 stars musthave much higher nitrogen abundances. The same figure showsan example of the corresponding C N lines modelled with the
BACCHUS code.
5. Chemical properties of UKS 1
The present study adds a substantial contribution to the chemicalcharacterization of UKS 1. In comparison, Origlia et al. (2005)analysed the elemental abundances [Fe / H], [O / Fe], [Ca / Fe],[Si / Fe], [Mg / Fe], [Ti / Fe], [ α / Fe], and [C / Fe] of four likely clus-ter members at a similar (1.5 - 1.8 µ m ) spectral coverage. Oursample of likely cluster members increases this number to six,and to the best of our knowledge, this is the largest sample yetanalysed in UKS 1, allowing us to observe the abundance varia-tions comparatively within the cluster. Table 1 contains the abun-dance values of the fourteen chemical species ([C / Fe], [N / Fe],[O / Fe], [Mg / Fe], [Al / Fe], [Si / Fe], [K / Fe], [Ca / Fe], [Ti / Fe],[Fe / H], [Ni / Fe], [Ce / Fe], [Nd / Fe], and [Yb / Fe]) determined inthis work.We proceed to compare our results with the homogeneoussample from Mészáros et al. (2020), as that data set has beencarefully examined with the same code and similar methodologyas adopted in this paper. We avoid any comparison with GCsonly based in the
ASPCAP
APOGEE pipeline as they may exhibitlarger systematic o ff sets (see, e.g. Nataf et al. 2019). From our overall spectral analysis we find an average [Fe / H] = − .
98, with scatter of σ = / H]spread of 0.3 dex. This metallicity is on average ≈ .
25 dexlower than that reported in Origlia et al. (2005), but with a di ff er-ence not greater than our measured [Fe / H] spread. We also notethat the large star-to-star [Fe / H] spread measured in this work isroughly comparable to the typical errors of [Fe / H] in some starsin our sample. Therefore, the large intrinsic [Fe / H] spread foundin this work could be due to the large line-by-line (Fe I) variationin our spectra (see Table 2). Some stars is our sample are as metal-rich ([Fe / H] ∼ -0.79)as those found by Origlia et al. (2005), but having found moremetal-poor members of UKS 1, this naturally implies that ourmean [Fe / H] for the cluster di ff ers from theirs. Figure 5 showsthat our [Fe / H] spread observed in UKS 1 is consistent with thatseen in NGC 2808, NGC 1851, NGC 362, NGC 6171, and NGC6121 (see Mészáros et al. 2020). We show that UKS 1 hosts starsas metal-poor as [Fe / H] = − .
09 and as metal rich as that ofOriglia et al. (2005). Based on our analysis, UKS 1 is likely aGC with an intermediate [Fe / H] = − . (cid:104) [Ni / Fe] (cid:105) = . ± .
05 (with astar-to-star spread ∼ / H] ∼ − We also measure an average carbon abundance in UKS 1 of (cid:104) [C / Fe] (cid:105) = − . ± .
26, which is slightly higher than that re-ported in Origlia et al. (2005). Figure 5 shows that UKS 1 hasa median distribution in agreement with the result by Mészároset al. (2020) for GCs, with a large star-to-star [C / Fe] spread ( (cid:38) / Fe] spreadseen in UKS 1 is comparable to that commonly seen in high-mass GCs at similar metallicity (see Figure 5), but has a mediandistribution slightly higher than those commonly massive GCs.This [C / Fe]-mass trends could indicate that UKS 1 is as mas-sive as NGC 6171 and NGC 6121, but with a significantly large[C / Fe] spread.As can be seen in Table 1, there are two clear groups ofstars in our sample. Two stars in UKS 1 exhibit low nitrogenabundances ([N / Fe] (cid:46) + .
03) with a slight enrichment in carbon([C / Fe] ( (cid:46) + . / Fe] (cid:38) + .
9) and accompanied by lowerlevels of carbon ([C / Fe] < / Fe] and [C / Fe], suggesting that UKS 1 exhibits a N-C anti-correlation (see Figure 6(b)), which is clearly much larger thanthe typical errors in [C / Fe] and [N / Fe]. UKS 1 displays a sig-nificant [C / Fe] spread and a significant [N / Fe] spread, > (cid:38) . / Fe] has been estab-lished for UKS 1. We conclude that this high [N / Fe] abundanceis indicative of the presence of multiple populations in UKS 1. α -elements: O, Mg, Si, Ca, and Ti We have found mean values (star-to-star spread) for (cid:104) [O / Fe] (cid:105) =+ . ± .
08 ( ∼ .
26 dex), (cid:104) [Mg / Fe] (cid:105) = + . ± .
11 ( ∼ . (cid:104) [Si / Fe] (cid:105) = + . ± .
06 ( ∼ .
16 dex), (cid:104) [Ca / Fe] (cid:105) =+ . ± .
03 ( ∼ .
06 dex), and (cid:104) [Ti / Fe] (cid:105) = + . ± .
12 ( ∼ . α -element abundances are compatiblewith other Galactic GCs (Mészáros et al. 2020), however, smalldi ff erences of the median [O,Mg,Si,Ca,Ti / Fe] distributions canbe drawn from Figure 5 as a function of the cluster mass. Ourobserved [O / Fe], [Mg / Fe], and [Si / Fe] abundances are slightlylower than that of low-mass GCs, but show comparable levels tohigh-mass GCs. On the other hand, [Ca / Fe] is in good agreementwith GCs of similar metallicity (Figure 5), with small star-to-starspread. Overall, UKS 1 displays slightly higher α − element en-hancement, a possible signature of the fast enrichment provided Article number, page 5 of 16 & A proofs: manuscript no. UKS1
Fig. 4.
Left:
APOGEE spectra (in air wavelength) in a region containing several C N lines, indicated by vertical shaded regions. The magentaline show the spectrum of N-normal stars with similar atmospheric parameters as the N-rich stars.
Right:
Example of our spectral synthesis analysis(red squares) around the line 1.5317 µ m (blue light shadow region). The A(N) at 1.5317 µ m is marked. by supernovae (SNe) II events, as expected in old GCs (see, e.g.Crestani et al. 2019).Origlia et al. (2005) successfully measured [O / Fe], [Mg / Fe],[Si / Fe], [Ca / Fe], and [Ti / Fe] abundances for UKS 1. Their mea-sured abundances for these are consistent with the ones deter-mined in this work. However, they find that [Mg / Fe] is enhanced( ∼ + .
3) with small star-to-star spread ( < .
08 dex), while wefind low-Mg stars and slightly sub-solar [Mg / Fe] values with anaverage (cid:104) [Mg / Fe] (cid:105) close to Solar, and a high star-to-star spread( > .
29 dex). We suspect that these two distinctive Mg popu-lations in UKS 1 likely correspond to two di ff erentiated stellarpopulations.From Figure 6(d) we are able to confirm the presence of aweak Si-Mg anti-correlation in USK 1, similar to that seen inthe massive cluster NGC 2808, indicating that leakage from theMgAl chain into Si production is also likely present in UKS 1.Figure 6(c) also shows the presence of a weak Al-Si correlation, confirming the possible existence of Si leakage from the MgAlchain. Interestingly, most of the Si-enriched stars in UKS 1 alsoseem to correspond to the extreme Mg-depleted ([Mg / Fe] < / Fe] (cid:39) − .
1, therefore our finding of low-Mg stars is consistent with our lower metallicity for UKS 1. Toour knowledge, sub-solar Mg abundances have not been foundin GCs more metal rich than [Fe / H] ∼ − . Article number, page 6 of 16osé G. Fernández-Trincado et al.: Nitrogen-enhanced stars in UKS 1 [ C / F e ][ N / F e ][ O / F e ][ M g / F e ][ A l / F e ][ S i / F e ][ K / F e ][ C a / F e ][ T i / F e ][ F e / H ][ N i / F e ][ C e / F e ][ N d / F e ][ Y b / F e ] C h e m i c a l Sp e c i e s (a) NGC 2808 (8.69×10 M ): [Fe/H] = 0.92UKS 1 (0.8×10 M ) [ C / F e ][ N / F e ][ O / F e ][ M g / F e ][ A l / F e ][ S i / F e ][ K / F e ][ C a / F e ][ T i / F e ][ F e / H ][ N i / F e ][ C e / F e ][ N d / F e ][ Y b / F e ] C h e m i c a l Sp e c i e s (b) NGC 362 (3.36×10 M ): [Fe/H] = 1.02UKS 1 (0.8×10 M ) [ C / F e ][ N / F e ][ O / F e ][ M g / F e ][ A l / F e ][ S i / F e ][ K / F e ][ C a / F e ][ T i / F e ][ F e / H ][ N i / F e ][ C e / F e ][ N d / F e ][ Y b / F e ] C h e m i c a l Sp e c i e s (c) NGC 1851 (2.81×10 M ): [Fe/H] = 1.03UKS 1 (0.8×10 M ) [ C / F e ][ N / F e ][ O / F e ][ M g / F e ][ A l / F e ][ S i / F e ][ K / F e ][ C a / F e ][ T i / F e ][ F e / H ][ N i / F e ][ C e / F e ][ N d / F e ][ Y b / F e ] C h e m i c a l Sp e c i e s (d) NGC 6121 (0.929×10 M ) : [Fe/H] = 1.02 UKS 1 (0.8×10 M ) [ C / F e ][ N / F e ][ O / F e ][ M g / F e ][ A l / F e ][ S i / F e ][ K / F e ][ C a / F e ][ T i / F e ][ F e / H ][ N i / F e ][ C e / F e ][ N d / F e ][ Y b / F e ] C h e m i c a l Sp e c i e s (e) NGC 6171 (0.813×10 M ): [Fe/H] = 0.85UKS 1 (0.80×10 M )
Fig. 5. [X / Fe] and [Fe / H] abundance density estimation comparison between UKS 1 (left green symbols) and GCs (right grey symbols), followingthe constraints outlined in Table 5 from Mészáros et al. (2020). Each violin representation indicates with horizontal lines the median and limits ofthe distribution. The corresponding number of stars with available abundances in our sample is marked in the bottom labels. The top label indicatesthe cluster mass from Baumgardt et al. (2019).
Regarding Al and K, we found mean values for (cid:104) [Al / Fe] (cid:105) =+ . ± .
15 and (cid:104) [K / Fe] (cid:105) = + . ± .
09, with a star-to-star spread of ∼ .
40 dex and ∼ .
27 dex, respectively. Fig-ure 5 shows that the median distributions of these two chem-ical species are comparable to GCs at similar metallicity, ex-cept [Al / Fe], which exhibits a lower [Al / Fe] enrichment than thatseen in low-mass GCs, but comparable to that of high-mass GCs.Figure 6(a) shows a clear anti-correlation between Al andMg in UKS 1, which is larger than the typical errors of [Al / Fe]and [Mg / Fe] by a factor of ∼ / Fe]limit at around 0.3 dex, as proposed by Mészáros et al. (2020),it is possible to roughly separate the so-called first (FG) and sec-ond (SG) generation stars. We identified 2 out of 6 stars with[Al / Fe] (cid:38) + . Article number, page 7 of 16 & A proofs: manuscript no. UKS1
For the s -process elements, our abundance analysis yields an av-erage (star-to-star spread) (cid:104) [Ce / Fe] (cid:105) = . ± .
36 ( (cid:38) (cid:104) [Nd / Fe] (cid:105) = . ± .
23 ( (cid:38) (cid:104) [Yb / Fe] (cid:105) = . ± . (cid:38) s -process elements comparable to other GCs at similar metal-licity (Mészáros et al. 2020), but with a large star-to-star scatter.It is possible that the modest enrichment in s -process elements,accompanied by the lower levels in carbon and aluminium andthe high enrichment in nitrogen, could tentatively be producedby intermediate-mass ( ∼ (cid:12) ) AGB stars (Ventura et al. 2016),supporting the pollution of this cluster by such stars. We canclearly see in Figure 2 that the majority of our stars lie in thevery upper part of the RGB of UKS 1, which is in reasonableagreement with the expected behaviour for AGB stars. It is thusbelieved that a significant fraction of the stars in our sample areevolved AGB stars, possibly of intermediate masses.
6. Orbit
We briefly investigate the implications of our PM measurements(see Appendix A) and the discrepancy in the heliocentric dis-tance by calculating some possible orbits for UKS 1. The radialvelocity adopted in this work is 66.13 ± − , which wasobtained from the average of our stars from APOGEE with ac-curate radial velocity measurements.For the orbit computation, we adopt a sophisticated orbitmodelling algorithm– GravPot16 , which considers the Galac-tic perturbations due to a physical ‘boxy / peanut’ bar structure(see, e.g. Fernández-Trincado et al. 2020), and the superpositionof ten disk components (including the ISM contribution) sur-rounded by an oblate Hernquist stellar and spherical dark matterhalo, whose density profiles mimic that of the Besançon Galaxymodel (Robin et al. 2003, 2012, 2014). For a more detailed de-scription regarding the formalism of the GravPot16 model, werefer the reader to a forthcoming paper (Fernández-Trincado etal., in preparation).In this study, we employed the same Galactic configurationand Solar motion defined in Fernández-Trincado et al. (2020),except for the bar pattern speeds, for which we have assumedan angular velocity of the bar of Ω bar =
41 km s − kpc − (seeBovy et al. 2019). Simulations were also obtained by adoptingan uncertainty in Ω bar of ±
10 km s − kpc − . The Galactic po-tential has been rescaled to the Sun’s galactocentric distance, R (cid:12) = Θ = . − , given by Robin et al. (2017). To place error bars on the or-bital elements, we integrate an ensemble of 100,000 orbits, ran-domly selecting values from Gaussian distributions centred onthe mean values of distance, proper motions, radial velocity, andGalactic parameter variations. Figure 7 shows this ensemble oforbits as yellow / green-coloured paths, for two di ff erent helio-centric distances from Minniti et al. (2011) and Baumgardt et al.(2019). Table 3 lists the orbital parameters and their uncertain-ties, which have been estimated as the 50 th (median), 16 th , and84 th percentiles of the distributions resulting from integrationsover a 2 Gyr timespan.Figure 7 shows that UKS 1 is bound to the ‘bar / bulge’ region( r apo (cid:46) . | Z max | (cid:46) . (cid:38) .
97) P-R orbit, assuming a heliocentric distance https://gravpot.utinam.cnrs.fr We call prograde-retrograde (P-R) orbits the ones that flip their sensefrom prograde to retrograde, or vice versa, along their orbits. of ∼ ff erence in proper motions.Therefore, our measured proper motions with VIRAC have nodramatic e ff ect on the orbital configuration.A di ff erent picture is obtained if UKS 1 is initially positionedbehind the Galactic bulge at ∼ e (cid:38) .
94) progradeor P-R orbits (depending on the bar’s angular velocity) confinedto the inner halo ( | Z max | (cid:46) (cid:46) Gaia -Enceladus-Sausage early merger event (Belokurov et al. 2018), with similarchemistry and orbital properties as NGC 2808 (Baumgardt et al.2019; Massari et al. 2019).Figure 8 follows these two possible scenarios. This figureshows the characteristic orbital energy versus the orbital Jacobiconstant as envisioned by Moreno et al. (2015) and Fernández-Trincado et al. (2020), which is conserved in the reference framewhere the bar is at rest. We can clearly see the two possible sce-narios for UKS 1: (i) assuming a distance of 7.8 kpc leads to theconclusion that the cluster belongs to the GC family trapped intothe bulge and possibly in the bulge / bar region; (ii) assuming adistance of 15.9 kpc positions UKS 1 in the region dominatedby the group of GCs thought to belong to the Gaia -Enceladus-Sausage galaxy merger debris, such as NGC 2808.We notice that the properties of the Galatic potential a ff ectthe orbits only to a lesser degree. Variations in the heliocen-tric distance completely dominates the uncertainty in our under-standing of the global dynamical picture of UKS 1.
7. Age
Deriving the age for the UKS 1 GC is not an easy task because,as mentioned before, it is in a region with very high extinction(see, e.g. Minniti et al. 2011). Through isochrone fitting, we tryto provide a rough estimation of the age. To accomplish that, weemploy the SIRIUS code (Souza et al. 2020), which applies astatistical Bayesian Markov Chain Monte Carlo method.For the isochrone fitting, we adopt the Darthmouth StellarEvolutionary Database (DSED; Dotter et al. 2008) with an α -enhancement of + . ∼ .
25) models.The DSED isochrones are available in the 2MASS photometrysystem, and they were converted to the VVV photometry system.Since we do not have the entire CMD available, in particu-lar, the turn-o ff region, we imposed Gaussian distribution priorsfor the metallicity of [Fe / H] = − .
98, based on the mean deter-mination from this work, with a standard deviation of 0.11 dex,and for the distance of 7 . .
78 kpc. In contrast, we also employed aheliocentric distance of 15.9 kpc (Minniti et al. 2011). However,no age distribution converged.Figure 9 presents the best isochrone fitting in the Ks ver-sus (J-Ks) CMD. Our fit provides a reasonable solution both inthe overplotted isochrone (left panel) and the posterior distribu-tions of the corner plot (right panel). As the best determinationto represent the distributions, we adopted the median as the mostprobable value and the uncertainties calculated from 16th and84th percentiles. Based on the DSED isochrones, we found anage of 13 . + . − . Gyr.The 1 − σ region (the red stripe in Figure 9) is mostly a ff ectedby age uncertainties. It is relevant to mention that in the RGB Article number, page 8 of 16osé G. Fernández-Trincado et al.: Nitrogen-enhanced stars in UKS 1
Fig. 6.
Distributions of light- (C, N), α - (Mg, Si) and odd-Z (Al) elements in di ff erent abundance planes: In panels (a), (b), (c), and (d) the planes[Al / Fe] – [Mg / Fe], [N / Fe]–[C / Fe], [Al / Fe]–[Si / Fe], [Si / Fe]–[Mg / Fe] are respectively shown for GCs from Mészáros et al. (2020). The black dottedline at [A / Fe] = + . region of the CMD, an age variation could be seen as a colourdisplacement (see Figure 2 of Souza et al. 2020). Also, we wantto highlight that our probable solutions within 1 − σ fit well thecentral part of the CMD, reinforcing that the age estimation is areasonable determination for UKS 1.
8. Concluding remarks
We have employed the public APOGEE DR16 catalogue in com-bination with the internal release of the VIRAC catalogue to ex-tensively characterize the chemical composition of UKS 1. Wehave performed a high-resolution spectral analysis of six stars in the innermost region of the cluster. Our main conclusions aresummarized as follows: • We find an intermediate metallicity, [Fe / H] = − .
98, witha star-to-star spread of 0.11 dex. The reported metallicityis ∼ • We find a radial velocity of 66.13 km s − , which is in goodagreement with Baumgardt et al. (2019). However, the radialvelocity dispersion is found to be slightly higher, ∼ − , but still typical of some GCs (see, e.g. Baumgardt &Hilker 2018). Due to the observed nitrogen over-abundance(see below) in our sample, the possibility of these stars beingfield stars is very low. Article number, page 9 of 16 & A proofs: manuscript no. UKS1
Fig. 7.
Probability density map for the R - z and x - y projections of a an ensemble of 100,000 orbits for UKS 1 in the reference frame where thebar is at rest. Simulations for three di ff erent value of the bar patterns speed ( Ω bar ) are shown. The white dotted circle indicates the assumed bulgeradius of ∼ ∼ . / peanut bar structureemployed in the GravPot16 model. The white star and square symbols indicate the initial and end position of the cluster after a 2 Gyr backwardsintegration time. The black lines show the orbit of UKS 1 by assuming the central values of the observables, while the yellow colour map showthe more probable regions of the space crossed by the cluster (considering the uncertainty in the observables). The orbit results are presented byadopting two di ff erent heliocentric estimates, i.e. 7.8 kpc ( left panels ) and 15.9 kpc ( right panels ). The white ‘star’ and square symbols indicatethe present and final position of the orbit of the cluster, respectively. • We identified four stars in the innermost region of UKS 1with stellar atmospheres strongly enriched in nitrogen([N / Fe] > + .
95) accompanied by low carbon abundance ra-tios ([C / Fe] (cid:46) − . • Even though our analysis strongly supports the hypothesisthat UKS 1 is a GC native to the bulge region, better con-straints in the distance will help confirm or refute if it is apermanent resident of the Galactic bulge or the inner stellarhalo. • We provide, for the first time, a rough estimate of the age forUKS 1 by adopting a statistical isochrone fitting techniquein the VVV CMDs following a Bayesian approach, whichdetermine the ages, distances, metallicity, and reddening ina self-consistent way (see, e.g. Souza et al. 2020). We as-sumed that UKS 1 currently lies in the bulge, which returns an age estimated of 13.10 + . − . Gyr (see Section 7), suggest-ing that UKS 1 is an old cluster in the Galactic bulge. Weattempted to obtain consistent results by assuming a largedistance ( ∼ Acknowledgements.
The authors thank the expert anonymous referee, whoprovided generous detailed feedback that substantially improved the paper.We gratefully acknowledge data from the ESO Public Survey program ID179.B-2002 taken with the VISTA telescope, and products from the CambridgeAstronomical Survey Unit (CASU) and from the VISTA Science Archive(VSA). This publication makes use of data products from the WISE satellite,which is a joint project of the University of California, Los Angeles, and theJet Propulsion Laboratory / California Institute of Technology, funded by theNational Aeronautics and Space Administration. This research has made useof NASAs Astrophysics Data System Bibliographic Services and the SIMBADdatabase operated at CDS, Strasbourg, France.
Article number, page 10 of 16osé G. Fernández-Trincado et al.: Nitrogen-enhanced stars in UKS 1 km s ]3.02.52.01.51.00.5 C h a r a c t e r i s t i c e n e r g y [ × k m s ] UKS 1 ( d = 15.9 kpc)UKS 1 ( d = 7.8 kpc)NGC 2808 bar = 41 km s kpc M-DM-BL-EH-ESeqG-ESagH99G-E/Seq; L-E/SeqH99/G-E; H99?; H99??
Fig. 8.
Characteristic orbital energy (( E max + E min ) /
2) versus the orbital Jacobi constant ( E J ) in the non-inertial reference frame where the bar is atrest. Square symbols refer to Galactic GCs, colour-coded according to their associations with di ff erent progenitors from Massari et al. (2019). Therevisited dynamics of UKS 1, adopting the input parameters presented in this work, is shown with the black crosses. NGC 2808 (black triangle) ishighlighted for reference. J.G.F-T is supported by FONDECYT No. 3180210. D.M. is supported by theBASAL Center for Astrophysics and Associated Technologies (CATA) throughgrant AFB 170002, and by project FONDECYT Regular No. 1170121. D.G.gratefully acknowledges support from the Chilean Centro de Excelencia enAstrofísica y Tecnologías Afines (CATA) BASAL grant AFB-170002. D.G.also acknowledges financial support from the Dirección de Investigación yDesarrollo de la Universidad de La Serena through the Programa de Incentivoa la Investigación de Académicos (PIA-DIDULS). T.C.B. acknowledges partialsupport for this work from grant PHY 14-30152: Physics Frontier Center / JINA Center for the Evolution of the Elements (JINA-CEE), awarded by theUS National Science Foundation. S.V. gratefully acknowledges the supportprovided by Fondecyt reg. n. 1170518. J.A.-G. acknowledges support fromFondecyt Regular 1201490 and from ANID, Millennium Science InitiativeICN12_009, awarded to the Millennium Institute of Astrophysics (MAS). S.O.Sacknowledges the FAPESP PhD fellowship 2018 / / Article number, page 11 of 16 & A proofs: manuscript no. UKS1
Fig. 9.
Best isochrone fit in the Ks versus ( J − K s ) CMD using DSED models. Left panel:
CMD with the results from the fitting. The red line isthe most probable solution and the red stripe are the solutions within 1 − σ . Right panel:
Posterior distributions. from National Natural Science Foundation of China under grant No. U1931102and support from the hundred-talent project of Sun Yat-sen University.
BACCHUS have been executed on the Supercomputer TITAN from the Departa-mento de Astronomía de la Universidad de Concepción.Funding for the Sloan Digital Sky Survey IV has been provided by the AlfredP. Sloan Foundation, the U.S. Department of Energy O ffi / University of Tokyo,Lawrence Berkeley National Laboratory, Leibniz Institut für AstrophysikPotsdam (AIP), Max-Planck-Institut für Astronomie (MPIA Heidelberg),Max-Planck-Institut für Astrophysik (MPA Garching), Max-Planck-Institutfür Extraterrestrische Physik (MPE), National Astronomical Observatory ofChina, New Mexico State University, New York University, University ofNotre Dame, Observatório Nacional / MCTI, The Ohio State University,Pennsylvania State University, Shanghai Astronomical Observatory, UnitedKingdom Participation Group, Universidad Nacional Autónoma de México,University of Arizona, University of Colorado Boulder, University of Oxford,University of Portsmouth, University of Utah, University of Virginia, Universityof Washington, University of Wisconsin, Vanderbilt University, and YaleUniversity.
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Article number, page 13 of 16 & A proofs: manuscript no. UKS1 T a b l e . B a s i c p a r a m e t e r s o f S t a r s i n UK S . S t a rI d s A P OG EE _ I D [ C / F e ][ N / F e ][ O / F e ][ M g / F e ][ A l / F e ][ S i / F e ][ K / F e ][ C a / F e ][ T i / F e ][ F e / H ][ N i / F e ][ C e / F e ][ N d / F e ][ Y b / F e ] M − − . + . + . + . + . + . − . + . + . − . − . + . + . + . M − − . + . + . + . + . + . + . + . + . − . − . + . + . + . M − − . + . + . − . + . + . + . + . + . − . + . + . + . ... M − − . + . + . + . + . + . + . + . + . − . + . + . + . + . M − + . + . + . + . + . + . + . + . + . − . + . + . + . + . M − + . − . ... − . + . + . + . ... + . − . + . + . ...... S t a rI d s A P OG EE _ I D α δ T e ff l og g [ M / H ] ξ S / N J M A SS K s , M A SS J − K ∗ s K ∗ s R V σ R V µ α c o s ( δ ) µ δ ( hh : mm : ss )( dd : mm : ss )( K )( k m s − )( p i x e l − )( m a g )( m a g )( m a g )( m a g )( k m s − )( k m s − )( m a s y r − )( m a s y r − ) M − : : . − : : . . − . . . . . ± . . ± . . . − . ± . − . ± . M − : : . − : : . . − . . . . . ± . . ± . . . − . ± . − . ± . M − : : . − : : . . − . . . . . ± . . ± . . ... − . ± . − . ± . M − : : . − : : . . − . . . . . ± . . ± . . . − . ± . − . ± . M − : : . − : : . . − . . . . . ± . . ± . . . − . ± . − . ± . M − : : . − : : . . − . . . . . ± . . ± . . . − . ± . − . ± . N o t e s . ( ∗ ) D e no t e s t h e d i ff e r e n ti a l r e dd e n i ng c o rr ec t e d K s a nd J − K s . Article number, page 14 of 16osé G. Fernández-Trincado et al.: Nitrogen-enhanced stars in UKS 1
Table 2.
Sensitivity to typical uncertainties in atmospheric parameters and standard deviation between lines of the same species. Iron lines representthe [Fe I / H] values, while all other species are the [X / Fe] ratios, for elements X = C, N, O, Mg, Al, Si, K, Ca, Ti, Ni, Ce, Nd, and Yb.APOGEE_ID C N O Mg Al Si K Ca Ti Fe Ni Ce Nd Yb(dex) (dex) (dex) (dex) (dex) (dex) (dex) (dex) (dex) (dex) (dex) (dex) (dex) (dex)2M17542167 − σ T e ff σ [X / H] , log g σ ξ t σ mean σ total − σ T e ff σ log g σ ξ t σ mean σ total − σ T e ff σ log g σ ξ t σ mean σ total − σ T e ff σ log g σ ξ t σ mean σ total − σ T e ff σ log g σ ξ t σ mean σ total − σ T e ff σ log g σ ξ t σ mean σ total Table 3.
Orbital elements of UKS 1. d (cid:12) = . Ω bar r peri r apo | Z max | e L minz L maxz Orbit(km s − kpc − ) (kpc) (kpc) (kpc) ( × km s − kpc) ( × km s − kpc)31 0.01 ± ± ± ± − ± ± ± ± ± ± − ± ± ± ± ± ± − ± ± d (cid:12) = . Ω bar r peri r apo | Z max | e L minz L maxz Orbit(km s − kpc − ) (kpc) (kpc) (kpc) ( × km s − kpc) ( × km s − kpc)31 0.17 ± ± ± ± − ± ± ± ± ± ± − ± − ± ± ± ± ± − ± − ± & A proofs: manuscript no. UKS1
Appendix A: The absolute PMs of UKS 1
For the orbit computations, we adopt the proper motions mea-sured by VIRAC (Smith et al. 2018), which were examined indetail to find a good size sample of UKS 1 stars with the leastpossible contamination from field stars, so that a robust determi-nation of the cluster’s proper motion can be made. The followingcuts were performed, yielding a sample of 2297 stars: − . < K s < . J − K s >
2: The brightest cut inmagnitude rejects stars a ff ected by saturation e ff ects whilethe faintest cut avoids excedingly large errors in proper mo-tions and significant contamination from field stars. The cutin colour corresponds to the known locus of UKS 1 stars. − Distance to cluster centre r < . (cid:48) : An examination of µ δ versus r for a bright red sample (11 . < K s < . J − K s >
2) revealed that a cut in radius is necessary to avoidsignificant contamination from the bulge red giants, at thecost of losing the outskirts of the cluster. In the innermostradius there will be some contamination, but it is minimizedby the large number of UKS 1 stars. − Absolute value of normalized proper motion in declinationless than 5: To properly account for the e ff ects of proper mo-tion errors, we estimate, for the sample selected by the twoprevious items, an approximate mean proper motion in dec-lination for the cluster of -2.2 mas yr − , and we use that tocompute: µ δ, norm = µ δ − ( − . (cid:15) µ δ . This quantity e ff ectivelybrings closer to zero many members of UKS 1, despite theirspan in proper motion errors. This allows us to build a largeand robust sample dominated by UKS 1 stars that properlyreflects the distribution of VIRAC proper motion errors. Thisvalue also spreads a substantial number of field outliers tolarge values far from zero. This procedure was only donein µ δ because field and cluster populations only separate farenough in this coordinate for it to work successfully. − | µ α cos( δ ) | <
30 mas yr − : Once all previous steps were ap-plied, we finally cut evident outliers in µ α cos( δ ), so that thedata spans around the observed mean value to an extent sim-ilar to µ δ .The proper motions of the final resulting sample of 2297stars were analysed using a quantile-quantile Q-Q plot, to com-pare their distribution to a normal standard N(0,1) distribution,in order to see if it properly describes the data and to computeits parameters (mean, standard deviation, and its correspond-ing errors). In a Q-Q plot, data that is normally distributed willend up following a straight line, whose zero point is the meanand the slope is the standard deviation of the data. We foundthat the UKS 1 selected sample has heavier tails than a nor-mal distribution, not unexpectedly. On the other hand, the inner-most ∼
80% behaves well, and fitting a straight line to it yieldedthe following results for the mean proper motion of UKS 1:( µ α cos( δ ) , µ δ ) = ( − . , − . ± (0 . , .
16) mas yr − . Theerrors quoted here were computed by dividing the obtained stan-dard deviations (2 . , .
29) mas yr − by the square root of thenumber of data points used in the fit. The formal errors of thisprocedure for the mean value are in fact much smaller, 0.005 masyr − per year, but we believe this value underestimates the realquality of the data, and prefer to adopt the chosen error estimateas more representative of the data. The K s versus J − K s CMDof the above selected sample (2297 stars) looks as expected forUKS 1, and confirms our selection to be reasonable.Our proper motion value is similar but slightly o ff in µ δ fromthe one obtained by Baumgardt et al. (2019) using GAIA DR2: ( − . , − . ± (0 . , . µ δ to lower values, whichactually forced us to limit the sample to the innermost portion ofthe cluster in an e ff ort to minimize it. This strategy was not a fac-tor for GAIA
DR2 because its data are much shallower than thoseof VIRAC. This also confirms, the enormous value (because oftheir depth) and exquisite quality (because of it being consistentwith GAIA DR2) of the VIRAC proper motions.
Appendix B: Differential reddening correction
The di ff erential reddening correction was performed using giantstars, and by adopting the reddening law of Cardelli et al. (1989)and O’Donnell (1994) and a total-to-selective absorption ratio R V = .
1. For this purpose, we selected all RGB stars withina radius of 5’ from the cluster centre and that have proper mo-tions compatible with that of UK1 within 1 mas yr − . First, wedraw a ridge line along the RGB, and for each of the selectedRGB stars we calculated its distance from this line along thereddening vector. The vertical projection of this distance givesthe di ff erencial A K absorption at the position of the star, whilethe horizontal projection gives the di ff erential E(J − K) redden-ing at the position of the star. After this first step, for each starof the field we selected the three nearest RGB stars, calculatedthe mean di ff erential A K absorption and the mean di ff erentialE(J − K) reddening, and finally subtracted these mean valuesfrom its J − K s , VVV colour and K s , VVV magnitude. We underlinethe fact that the number of reference stars used for the reddeningcorrection is a compromise between having a correction a ff ectedas little as possible by photometric random error and the high-est possible spatial resolution. The di ff erential reddening correc-tions for VVV bands is listed in Table 2.erential reddening correc-tions for VVV bands is listed in Table 2.