APOGEE spectroscopic evidence for chemical anomalies in dwarf galaxies: The case of M~54 and Sagittarius
José G. Fernández-Trincado, Timothy C. Beers, Dante Minniti, Christian Moni Bidin, Beatriz Barbuy, Sandro Villanova, Doug Geisler, Richard R. Lane, Alexandre Roman-Lopes, Dmitry Bizyaev
AAstronomy & Astrophysics manuscript no. ms © ESO 2021February 17, 2021
APOGEE spectroscopic evidence for chemical anomalies in dwarfgalaxies: The case of M 54 and Sagittarius
José G. Fernández-Trincado , , (cid:63) , Timothy C. Beers , Dante Minniti , , Christian Moni Bidin , BeatrizBarbuy , Sandro Villanova , Doug Geisler , , , Richard R. Lane , Alexandre Roman-Lopes and DmitryBizyaev , Institut Utinam, CNRS UMR 6213, Université Bourgogne-Franche-Comté, OSU THETA Franche-Comté, Observatoire de Be-sançon,BP 1615, 25010 Besançon Cedex, France Instituto de Astronomía y Ciencias Planetarias, Universidad de Atacama, Copayapu 485, Copiapó, Chile Centro de Investigación en Astronomía, Universidad Bernardo O Higgins, Avenida Viel 1497, Santiago, Chile Department of Physics and JINA Center for the Evolution of the Elements, University of Notre Dame, Notre Dame, IN 46556,USA Depto. de Cs. Físicas, Facultad de Ciencias Exactas, Universidad Andres Bello, Av. Fernández Concha 700, Las Condes, Santiago,Chile Vatican Observatory, V00120 Vatican City State, Italy Instituto de Astronomía, Universidad Católica del Norte, Av. Angamos 0610, Antofagasta, Chile Universidade de São Paulo, IAG, Rua do Matão 1226, Cidade Universitária, São Paulo 05508-900, Brazil Departamento de Astronomía, Casilla 160-C, Universidad de Concepción, Concepción, Chile Departamento de Astronomía, Universidad de La Serena, 1700000 La Serena, Chile Instituto de Investigación Multidisciplinario en Ciencia y Tecnología, Universidad de La Serena. Benavente 980, La Serena, Chile Apache Point Observatory and New Mexico State University, Sunspot, NM, 88349, USA Sternberg Astronomical Institute, Moscow State University, Moscow, RussiaReceived ...; Accepted ...
ABSTRACT
We present evidence for globular cluster stellar debris in a dwarf galaxy system (Sagittarius: Sgr) based on an analysis of high-resolution H -band spectra from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey. We add [N / Fe],[Ti / Fe], and [Ni / Fe] abundance ratios to the existing sample of potential members of M 54; this is the first time that [N / Fe] abundancesare derived for a large number of stars in M 54. Our study reveals the existence of a significant population of nitrogen- (with a largespread, (cid:38) + Sagittarius system, which sharesthe light element anomalies characteristic of second-generation globular cluster stars (GCs), thus tracing the typical phenomenonof multiple stellar populations seen in other Galactic GCs at similar metallicity, confirming earlier results based on the Na-O anti-correlation. We further show that most of the stars in M 54 exhibit di ff erent chemical - patterns evidently not present in Sgr field stars.Furthermore, we report the serendipitous discovery of a nitrogen-enhanced extra-tidal star with GC second-generation-like chemicalpatterns for which both chemical and kinematic evidence is commensurate with the hypothesis that the star has been ejected fromM 54. Our findings support the existence of chemical anomalies associated with likely tidally shredded GCs in dwarf galaxies in theLocal Group and motivate future searches for such bonafide stars along other known Milky Way streams. Key words. stars: abundances – stars: chemically peculiar – globular clusters: individual: M 54 – techniques: spectroscopic
1. Introduction
The Sagittarius (Sgr) dwarf spheroidal (dSph) galaxy is one ofthe closest massive satellites of the Milky Way (MW) (Ibataet al. 1994), and has yielded a wealth of observational evidenceof ongoing accretion by the MW in the form of persistent stel-lar debris and tidal streams discovered by Mateo et al. (1996),and extensively studied with photometric and spectroscopic ob-servations over a huge range of distances ( ∼ ff erent stel-lar tracers–including Carbon stars (Totten & Irwin 1998), thefirst all-sky map of the tails using 2MASS M-giants (Majew- (cid:63) To whom correspondence should be addressed; E-mail:[email protected] and / or [email protected] ski et al. 2003), red clump Stars (Correnti et al. 2010), RR Lyraestars (Newberg et al. 2003; Ramos et al. 2020), and CN-strongstars (Hanke et al. 2020), among other tracers, usually in smallpatches along the stream (see, e.g., Li et al. 2019). These studieshave been followed-up by numerical studies (see, e.g., Law et al.2005; Vasiliev et al. 2020), as well as by using precise astrometryfrom the Gaia second data release (Gaia DR2; Gaia Collabora-tion et al. 2018a), based on proper motions alone (Antoja et al.2020). Its proximity provides a unique laboratory to study accre-tion in detail, through the tidally stripped streams that outflowfrom the Sgr system (Hasselquist et al. 2017, 2019; Hayes et al.2020).As a natural result of such an accretion event, there is a claimin the literature that not only field stars but also GCs have been
Article number, page 1 of 10 a r X i v : . [ a s t r o - ph . GA ] F e b & A proofs: manuscript no. ms
Fig. 1.
Panel (a): Spatial positions of the stars in our sample, with thetidal radius ( r t = (cid:48) ) of M 54 over-plotted with a solid line. The openred symbols designate N-rich stars (the diamond symbol refers to a fieldstar, while the open circle highlights the extra-tidal member of M 54).The lime circles designate the M 54 population analyzed in this work,while the black plus symbols designate the stars analyzed by Nataf et al.(2019). The empty grey ‘star’ symbols designate the potential Sgr pop-ulation from Hayes et al. (2020). The two concentric circles indicate 5 r t and 7 r t for reference. Panel (b): Gaia
EDR3 proper motions of starsthat have been associated with the Sgr stream: blue symbols for the An-toja et al. (2020) stars and open black ‘star’ symbols for Hayes et al.(2020) stars. The orbital path of Sgr is shown by the dotted (backward)and solid (forward) purple line in panels (a) and (b), with the thick andthin lines showing the central orbit, and one hundred ensemble of orbitsthat shows the more probable regions of the space, which are crossedmore frequently by the simulated orbit, respectively. Panel (c): Colormagnitude diagram from
Gaia
EDR3 photometry of our sample. Thesymbols are the same as in panels (a) and (b), except the white circles,which denotes the M 54 members from
Gaia
EDR3, selected on propermotions and within 3 (cid:48) from the cluster center. Panel (d): Radial veloc-ities versus [Fe / H] ratios determined from APOGEE-2 / ASPCAP (blacksymbols) and our [Fe / H] ratio determinations from
BACCHUS (green andred symbols) in the field around M 54. The [Fe / H] APOGEE-2 / ASPCAP determinations have been systematically o ff set by ∼ / H] BACCHUS determinations, as suggested in(Fernández-Trincado et al. 2020c). accreted (see, e.g., Massari et al. 2019). Some have been specu-lated to be lost in the disruption process, and may lie immersedin the Sgr stream. Candidates include: M 54, Terzan 7, Arp 2,Terzan 8, Pal 12, Whiting 1, NGC 2419, NGC 6534, and NGC4147 (e.g., Law & Majewski 2010; Bellazzini et al. 2020), but afirm connection is still under debate (e.g., Villanova et al. 2016;Tang et al. 2018; Huang & Koposov 2020; Yuan et al. 2020).In this context, “chemical tagging" (e.g., Freeman & Bland-Hawthorn 2002), which is based on the principle that the pho-tospheric chemical compositions of stars reflect the site of theirformation, is a promising route for investigation of this question.While the abundances of light and heavy elements for in-dividual stars in GCs have been widely explored (e.g., Pancinoet al. 2017; Mészáros et al. 2020), little is known about these abundances in disrupted GCs likely associated with the closestdwarf galaxies, such as Sgr (Karlsson et al. 2012). Althoughsome evidence for chemical anomalies has been detected to-wards the inner bulge and halo of the MW (see, e.g., Fernández-Trincado et al. 2016; Recio-Blanco et al. 2017; Schiavon et al.2017; Fernández-Trincado et al. 2017) and Local Group dwarfgalaxies (see, e.g., Fernández-Trincado et al. 2020b), suggestingthe presence of GCs in the form of disrupted remnants, alter-native ways to produce these stars have been recently discussed(Bekki 2019).This paper is outlined as follows. The high-resolution spec-troscopic observations are discussed in Section 2. Section 2.1describes the sample associated with M 54, including a compar-ison with data from the literature. Section 3 presents our esti-mated stellar parameters and derived chemical-abundance deter-minations. Section 4 discusses the results, and our concludingremarks are presented in Section 5.
2. Data
We make use of the internal dataset (which includes all datataken through March 2020) of the second-generation ApachePoint Observatory Galactic Evolution Experiment (APOGEE-2; Majewski et al. 2017), which includes the first observationsfrom the Irénée du Pont 2.5-m Telescope at Las Campanas Ob-servatory (APO-2S; Bowen & Vaughan 1973) in the SouthernHemisphere (Chile), and more observations from the Sloan 2.5-m Telescope at Apache Point Observatory (Gunn et al. 2006,APO-2N;) in the Northern Hemisphere (New Mexico). The sur-vey operates with two nearly identical spectrographs (Eisen-stein et al. 2011; Wilson et al. 2012, 2019), collecting high-resolution ( R ∼ , µ m, vacuum wavelengths). This data setprovides stellar parameters, chemical abundances, and radial ve-locity (RV) information for more than 600,000 sources, whichinclude ∼ MARCS stellar atmospheres (Gustafsson et al. 2008), whichnow extend to e ff ective temperatures as low as 3200 K, and spec-tral synthesis using the Turbospectrum code (Plez 2012). TheAPOGEE-2 spectra provide access to more than 26 chemicalspecies, which are described in Smith et al. (2013), Shetroneet al. (2015), Hasselquist et al. (2016), Cunha et al. (2017), andHoltzman et al. (2018).
The APOGEE-2 field toward M 54 was previously examined inMészáros et al. (2020) based on public DR16 spectra. In thatwork, 22 stars were identified as potential members linked toM 54 based in the APOGEE-2 radial velocities (Nidever et al.2015), i.e., stars with RV within 3 σ RV , cluster , metallicity within ± Gaia
Early Data Release 3 (
Gaia
EDR3; Gaia Collaboration
Article number, page 2 of 10osé G. Fernández-Trincado et al.: Globular cluster disruption in dwarf galaxies et al. 2020) within 2.5 σ around the cluster average proper mo-tion, and located inside the cluster tidal radius, r t (cid:46)
10 arcmin,(Harris 1996, 2010 edition) were classified as potential mem-bers of M 54. However, only 7 out of 22 stars were spectroscopi-cally examined with the
BACCHUS code in Masseron et al. (2016),since only these stars achieved a signal-to-noise (S / N >
70) su ffi -cient to provide reliable abundance determinations.The post-APOGEE DR16 dataset provides incremental visitstoward M 54, which has allowed to increase the signal-to-noisefor 20 out of 22 of the potential cluster members. As a result,nitrogen, titanium, and nickel abundances can be now obtainedfrom the stronger absorption features (as shown for C N linesas shown in Figure A.1), and other chemical species can also bestudied.Nataf et al. (2019), using APOGEE-2 DR14 data (Abol-fathi et al. 2018) and abundance determinations from the
Payne pipeline (Ting et al. 2019), have catalogued eight possible mem-bers from M 54. Two of those objects (2M18544275 − − / N ( <
70) spectra, resulting in very uncertain CNOabundance ratios for many chemical species, since the molecu-lar lines ( OH, C O, and C N) are very weak. Secondly, 6out of the 8 objects in Nataf et al. (2019) exhibit [Fe / H] > − . / N > We also report on the serendipitous discovery of two nitrogen-enhanced (N-rich) metal-poor stars beyond the tidal radius ofM 54, as shown in pane (a) of Figure 1. APOGEE-2 stars inthe stream + core Sagittarius (Sgr) system (see, e.g., Hasselquistet al. 2017, 2019; Hayes et al. 2020) are highlighted as blackopen ‘star’ symbols in pane (a) of Figure 1, while potential starmembers (blue symbols) of the stream + core Sgr system fromAntoja et al. (2020) are also displayed in panel (a) of Figure 1.It is important to note that the [Fe / H] abundance of APOGEE-2Sgr stars are provided by the
ASPCAP pipeline (see Hasselquistet al. 2017, 2019; Hayes et al. 2020). In order to compare withour [Fe / H] determinations, an o ff set of ∼ ASPCAP metallicities in panel (d) of Figure 1, as suggested inFernández-Trincado et al. (2020c).Panels (a) to (d) of Figure 1 reveal that one(2M18565969 − − ff set from the M 54 population.In particular, this star is brighter than the typical populationof M 54 (see panel (c) in Figure 1), and both proper motionsand RV di ff er from the nominal proper motion and RV of thecluster as shown in panels (b) and (d) of Figures 1. It is likelythat 2M18533777 −
3. Stellar parameters and chemical-abundancedeterminations
The chemical analysis is very similar to that carried out byFernández-Trincado et al. (2019a,b,c,d, 2020a,b,c,e, 2021b). Thestellar parameters ( T e ff , log g , and first guess on metallicity)for the 20 cluster members with S / N >
70 were extracted fromMészáros et al. (2020), while we adopt the atmospheric param-eters from the uncalibrated post-APOGEE DR16 values for thetwo stars beyond the cluster tidal radius. The elemental abun-dances and final errors in [Fe / H] and [X / Fe], astrometric andkinematic properties of our sample are listed in Tables A.1, A.2,and A.3, respectively.A consistent chemical-abundance analysis was then carriedout with the
BACCHUS code (Masseron et al. 2016), from whichwe obtained the metallicities from Fe I lines, and abundancesfor twelve other chemical species belonging to the light- (C, N), α - (O, Mg, Si, Ca, and Ti), Fe-peak (Ni), odd-Z (Al, K) and s -process (Ce, Nd) elements.
4. Results and discussion
Panel (a) of Figure 2 summarizes the chemical enrichment seenin M 54 stars analyzed in this work, and compares to theMészáros et al. (2020) determinations. The chemical composi-tion of the two newly identified N-rich stars beyond the clustertidal radius is also shown in the same figure. Overall, the chemi-cal abundance of M 54 based on the added cluster stars is withinthe typical errors, and does not a ff ect the science results pre-sented in Mészáros et al. (2020), while the two external N-richstars share chemical patterns similar to the M 54 population.For M 54, we find a mean metallicity (cid:104) [Fe / H] (cid:105) = − . ± .
12, which agrees well with Mészáros et al. (2020) . The spreadin [Fe / H] increased from 0.04 to 0.12 dex, but it is still smallerthan that reported in Carretta et al. (2010). Even if the measuredscatter is larger than that reported by Mészáros et al. (2020), itdoes not seem to indicate the presence of a significant spreadin [Fe / H], and is similar to that observed in Galactic globularclusters (GCs) at similar metallicity, such as M 10 (see, e.g.,Mészáros et al. 2020). Nickel (an element that belongs to the Fe-group), exhibits a flat distribution as a function of [Fe / H], similarto that observed in Carretta et al. (2010), and at odds with thatobserved in Sgr stars.Regarding the other chemical species, we find excellentagreement with the values provided by Mészáros et al. (2020),as can be seen in panel (a) of Figure 2, with the main di ff er-ence that the added stars introduce a larger star-to-star scatterthan previously measured. M 54 exhibits a modest enhance-ment in α -elements, with mean values for [O / Fe], [Mg / Fe],[Si / Fe], [Ca / Fe], and [Ti / Fe] which is similar to what is seenin halo GCs: (cid:104) [O / Fe] (cid:105) = + . ± .
36 (14 stars); (cid:104) [Mg / Fe] (cid:105) =+ . ± .
11 (18 stars); (cid:104) [Si / Fe] (cid:105) = + . ± .
10 (20 stars); (cid:104) [Ca / Fe] (cid:105) = + . ± .
07 (16 stars); and the new measured (cid:104) [Ti / Fe] (cid:105) = + . ± .
21 (16 stars), indicating a fast enrich-ment provided by supernovae (SNe) II events. Mean values are ingood agreement with Mészáros et al. (2020), with the exceptionof oxygen, which displays the larger star-to-star spread expectedin likely second-generation stars.We also find that the [O / Fe], [Mg / Fe], and [Si / Fe] ratios arealmost flat as a function of the metallicity, while [Ca / Fe] and Note that here, and for the abundances described below, the numberfollowing the average abundance represents the one-sigma dispersion,not the error in the mean. Article number, page 3 of 10 & A proofs: manuscript no. ms
Fig. 2.
BACCHUS elemental abundances . Panel (a): The observed[X / H] and [Fe / H] abundance-density estimation (violin representation)of M 54 stars, and the observed abundance ratios of newly identifiedN-rich stars. The extra-tidal star from M 54 and a field star is high-lighted with a black open circle and diamond, respectively. Each violinindicates with horizontal lines the median and limits of the distribution.The lime and dark violet violin representation refer to the abundanceratios of 20 stars (this work) and 7 stars from Mészáros et al. (2020),respectively. Panels (b)–(e): Distributions of light- (C,N), α - (Mg, Si)and odd-Z (Al) elements in di ff erent abundance planes. In each panel,the planes [Al / Fe] – [Mg / Fe], [N / Fe]–[C / Fe], [Al / Fe]–[Si / Fe], [Si / Fe]–[Mg / Fe] are shown, respectively, for GCs from Mészáros et al. (2020).The black dotted line at [Al / Fe] = + . [Ti / Fe] ratios slightly increases as [Fe / H] increases, similar tothe behaviour found by Carretta et al. (2010). On the contrary,the α -element trend observed in Sgr stars (see, e.g., Carrettaet al. 2010; McWilliam et al. 2013; Hasselquist et al. 2017, 2019)di ff er from those seen in the population of M 54. Overall, the α -elements in the cluster are higher than seen in Sgr stars. Inconclusion, the measured α -enrichment in this work support theprevious hypothesis suggesting that the α -element in M 54 starsformed before the typical e -folding time for SN Ia contributingtheir ejecta to the gas pool (e.g., Carretta et al. 2010).We also found that some stars in M 54 appear to be quite Mgpoor, with strong enrichment in aluminum and nitrogen, provid-ing further evidence for the presence of second-generation starsin M 54, and the signature of very high temperatures achievedduring H-burning (e.g., Carretta et al. 2010; Mészáros et al.2020). The odd-Z elements (Al and K) in M 54 exhibit anaverage (cid:104) [Al / Fe] (cid:105) = + . ± .
37 (19 stars) and (cid:104) [K / Fe] (cid:105) =+ . ± .
18 (17 stars), with a clear anti-correlation in Al-Mg, ascan be seen in in panel (b) of Figure 2, with moderate Mg deple-tions related to the enrichment in Al abundances, as the result ofthe conversion of Mg into Al during the Mg-Al cycle (e.g., Car-retta et al. 2010; Denissenkov et al. 2015; Renzini et al. 2015;Pancino et al. 2017). This pattern is evidently not present in theSgr stars, where, on the contrary,
ASPCAP
Mg and Al abundancesare positively correlated with each other (see, e.g., Hasselquistet al. 2017, 2019; Hayes et al. 2020).We derived average abundances for C and N in M 54, of (cid:104) [C / Fe] (cid:105) = − . ± .
25 (13 stars) and (cid:104) [N / Fe] (cid:105) = + . ± .
48 (17 stars). Most of the stars in M 54 are C deficient([C / Fe] (cid:46) + / Fe] > + . Si production from the result of a secondary leakage in themain Mg-Al cycle, which is instead absent in the Sgr stars.For the elements produced by neutron( n )-capture processes(Ce II and Nd II), we find, on average, (cid:104) [Ce / Fe] (cid:105) = + . ± . / Fe] = + .
44 (1 star). Overall, M 54 exhibits amodest enrichment in s -process elements, with a few stars as en-hanced as + .
4, similar to that observed in Galactic GC stars atsimilar metallicity (see, e.g., Mészáros et al. 2020), suggestingthat it is possible that the s -process enrichment has been pro-duced by a di ff erent source than the progenitor of the Mg-Alanti-correlations, possibly by low-mass asymptotic giant branchstars. Lastly, we find that [Ce / Fe] ratios in M 54 are almost flatas a function of metrallicity. Unfortunately, Nd II is measured inonly one star, which has been found to exhibit the modest en-hancement, consistent with a moderate enrichment of s -processelements.Furthermore, we report the serendipitous discovery of twoN-enhanced stars identified within ∼ r t from M 54, as shown inpanel (a) of Figure 1. Panel (a) of Figure 2 show the collection of[X / Fe] and [Fe / H] abundance ratios for the two newly identifiedN-rich stars beyond the tidal radius of M 54. Both stars exhibitvery similar chemical-abundance patterns as those seen in thepopulation of M 54. A plausible explanation is that both starswere previous members of M 54, from which they have beenejected. However, this possibility seems unlikely for one of these
Article number, page 4 of 10osé G. Fernández-Trincado et al.: Globular cluster disruption in dwarf galaxies extra-tidal stars (2M18533777 − − + M 54 stars, nor theorbital path of Sgr . It is also the most luminous star in oursample, making it a likely foreground star. The possibility thatthis star was disrupted from M 54 and deposited in the innerGalaxy seems unlikely, as the perigalacticon of M 54 is locatedwell beyond the solar radius (see, e.g., Baumgardt et al. 2019).We conclude that 2M18533777 − ff erent progenitor than M 54, but with a similarchemical-enrichment history to this cluster.Aside from 2M18533777 − − ∼ × r t from the cluster center, which exhibits a stellar atmospherestrongly enriched in nitrogen ([N / Fe] > + . / Fe] < − . s -process elements (Ce II). Since the [Al / Fe] ratio is > + .
5, whichis a ‘typical’ value for stars in GCs, and unlikely in dwarf galaxypopulations, we conclude that 2M18565969 − − ff erent from Sgr stars. On the other hand, N-rich stars arecommonly observed to be more centrally concentrated in GCs(e.g. Dalessandro et al. 2019) and as a consequence they havesmaller probabilities to be tidally stripped. Thus, it is likely thatthe extra-tidal star could well be just a stripped M 54 star asmany others in its surroundings. Our finding demonstrate thatN-rich stars are a promising route for identifying the unam-biguous chemical signatures of stars formed in GC-like environ-ment which may lie immersed in the M 54 + Sgr core and / or Sgrstream, as well as confirm or discard the possible association ofGCs to the Sgr stream (Bellazzini et al. 2020).Following the same methodology as described in Fernández-Trincado et al. (2021a), we compute the predicted number( N N − rich ) of N-rich field stars observed in APOGEE-2 towardM 54 / Sgr using the smooth halo density relations presented inHorta et al. (2021), and by adopting the same Monte Carlo im-plementation of the Von Neumann Rejection Technique (see e.g.,Press et al. 2002) as in Eq. 7 in (Fernández-Trincado et al. 2015).We find the expected number of observed N-rich halo stars be-yond d (cid:12) (cid:38)
15 kpc over the sky area of 1.5 degree radius cen-tred in M 54, and with both astrometric and kinematic propertiesas M 54 to be N N − rich < . GravPot16 model(Fernández-Trincado et al. 2020d) to explore the expectations for The Sgr orbit was computed with the
GravPot16 model, https://gravpot.utinam.cnrs.fr , by adopting the same model configu-rations as described in Fernández-Trincado et al. (2020c). For the Sgrcentre, we adopt the heliocentric distance d (cid:12) = RV =
142 km s − from Vasiliev & Belokurov(2020), and proper motions from Gaia Collaboration et al. (2018b): µ α cos δ = − .
692 mas yr − and µ δ = − .
359 mas yr − , with uncer-tainties assumed of the order of 10% in d (cid:12) , RV , and proper motions. a "default" Milky Way along the RVs to the Sgr + M˜54 surround-ing field beyond d (cid:12) (cid:38)
15 kpc. The "all" sample is dominated byhalo kinematics with a negligible contribution from the thin andthick disk beyond RV (cid:38)
120 km s − . Thus, our Milky Way sim-ulated sample act to guide us in RV space, confirming that thekinematics of the newly identified extra-tidal N-rich star di ff ersfrom the disk population, with practically low contribution formthe expected halo. ,
5. Concluding remarks
We present a spectroscopic analysis for 20 out 22 red giant starsthat are members of M 54 from the internal APOGEE DR16dataset. This study doubles the sample of stars with spectro-scopic measurements for this cluster, and the new post-APOGEEDR16 spectra achieve high signal-to-noise (S / N > H -band–APOGEE-2footprint.Overall, the chemical species re-examined in M 54 werefound to be consistent with previous studies (Mészáros et al.2020), although most of them exhibit a large star-to-star scat-ter. We find that 15 out of the 20 stars investigated show a high[N / Fe] abundance ratio ([N / Fe] (cid:38) + . / Fe] and [Ti / Fe],not previously examined in Mészáros et al. (2020), were found tobe in good agreement with measurements in the literature. In par-ticular, we confirm the [Ti / Fe] ratio slightly increases as [Fe / H]increases, as has been reported in Carretta et al. (2010). We alsofind a large spread in [Al / Fe], and the presence of a genuinesecond-generation star in M 54, which exhibits Mg deficiency([Mg / Fe] <
0) accompanied with large enhancements in nitrogenand aluminum. In general, all chemical species examined in theM 54 members present distinguishable chemical behaviour com-pared with Sgr stars, suggesting a di ff erent chemical-evolutionhistory that resembles other Galactic halo GCs at similar metal-licity.Furthermore, we report on the serendipitous discovery of apotential extra-tidal star toward the surrounding regions of theM 54 + Sgr core, which exhibits a strong enrichment in nitrogencomparable to that seen in M 54 stars. As far as we know this isthe first study reporting on the unambiguous chemical signaturesof stars formed in GC-like environment into a nearby satellitedwarf galaxy around the Milky Way. Finding out how many ofsuch chemical unusual stars likely originated in GCs are presentin dwarf galaxy systems, help to understand the link betweenGCs and their stellar streams (see e.g., Bellazzini et al. 2020).
Acknowledgements.
The author is grateful for the enlightening feedbackfrom the anonymous referee. J.G.F-T is supported by FONDECYT No.3180210. T.C.B. acknowledges partial support for this work from grant PHY14-30152: Physics Frontier Center / JINA Center for the Evolution of theElements (JINA-CEE), awarded by the US National Science Foundation.D.M. is supported by the BASAL Center for Astrophysics and AssociatedTechnologies (CATA) through grant AFB 170002, and by project FONDECYTRegular No. 1170121. S.V. gratefully acknowledges the support providedby Fondecyt regular No. 1170518. D.G. gratefully acknowledges supportfrom the Chilean Centro de Excelencia en Astrofísica y Tecnologías Afines(CATA) BASAL grant AFB-170002. D.G. also acknowledges financial supportfrom the Dirección de Investigación y Desarrollo de la Universidad de LaSerena through the Programa de Incentivo a la Investigación de Académicos(PIA-DIDULS). A.R.-L. acknowledges financial support provided in Chileby Agencia Nacional de Investigación y Desarrollo (ANID) through theFONDECYT project 1170476. B.B. acknowledge partial financial support fromthe Brazilian agencies CAPES-Financial code 001, CNPq, and FAPESP.This work has made use of data from the European Space Agency(ESA) mission Gaia ( ), pro-cessed by the Gaia Data Processing and Analysis Consortium (DPAC,
Article number, page 5 of 10 & A proofs: manuscript no. ms ). Fundingfor the DPAC has been provided by national institutions, in particular theinstitutions participating in the Gaia Multilateral Agreement.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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Fig. A.1. Detection of C N lines.
Spectral synthesis for the deter-mination of nitrogen abundances for two N-rich stars beyond the tidalradius of M 54. Each panel shows the best-fit syntheses (red lines) from
BACCHUS compared to the observed spectra (black squares) of selected C N lines (grey arrows)
Appendix A: Spectrum of N-rich stars
Figure A.1 shows an example of the local thermodynamic equi-librium (LTE) –
BACCHUS spectral synthesis of selected C Nlines for the two newly identified N-rich stars beyond the tidalradius of M 54. The black squares represent the observed spec-trum, and the solid red line is the best abundance fit.Table A.1 and A.2 list the atmospheric parameters, abun-dance ratios and the final errors in [Fe / H] and [X / Fe], while Ta-ble A.3 list of the main physical properties of our sample.
Article number, page 7 of 10 & A proofs: manuscript no. ms T a b l e A . . E l e m e n t a l a bund a n ce s o f ou r s a m p l e . A P OG EE I D T e ff l og g [ M / H ] ξ t [ 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 ] [ K ][ c g s ] k m s − N - r i c hfi e l d s t a r M − . − . . − . + . + . + . + . + . + . + . + . − . − . + . + . e x t r a -t i d a l N - r i c h s t a r M − . − . . − . + . + . + . + . + . + . + . + . − . − . + . ... M s t a r s M − . − . . ......... + . + . + . ... + . + . − . − . ...... M − . − . . − . + . + . + . − . + . + . + . + . − . + . + . ... M − . − . . ......... + . − . + . + . ...... − . + . ...... M − . − . . + . + . + . + . + . + . + . ... + . − . − . ...... M − . − . . ... + . + . + . + . + . − . + . + . − . − . + . ... M − . − . . ... + . + . + . + . + . + . + . + . − . + . + . ... M − . − . . − . + . + . + . + . + . + . + . + . − . + . + . + . M − . − . . − . ... + . + . − . + . ... + . + . − . − . + . ... M − . − . . ......... + . − . + . + . ...... − . + . ...... M − . − . . ......... + . ... + . ... + . ... − . + . ...... M − . − . . − . + . ...... + . + . + . ... + . − . − . ...... M − . − . . − . + . + . + . + . + . − . + . + . − . + . + . ... M − . − . . − . + . + . + . − . + . + . + . + . − . + . ...... M − . − . . − . + . + . + . + . + . + . + . + . − . + . ...... M − . − . . − . + . + . + . + . + . − . + . + . − . + . + . ... M − . − . . − . + . + . + . − . + . − . + . ... − . − . ...... M − . − . . ......... + . + . + . + . + . + . − . + . ...... M − . − . . ......... + . − . + . ... + . + . − . + . ...... M − . − . . − . + . − . − . + . + . + . + . + . − . − . + . ... M − . − . . − . + . + . + . − . + . + . + . + . − . − . + . ... M − . − . . ... + . ...... + . + . + . ...... − . + . ...... M − . − . . − . + . + . + . − . + . − . + . + . − . − . + . ... Article number, page 8 of 10osé G. Fernández-Trincado et al.: Globular cluster disruption in dwarf galaxies T a b l e A . . F i n a l e rr o r s i n [ F e / H ] a nd [ X / F e ] . A P OG EE I D σ [ C / F e ] σ [ N / F e ] σ [ O / F e ] σ [ M g / F e ] σ [ A l / F e ] σ [ S i / F e ] σ [ K / F e ] σ [ C a / F e ] σ [ T i / F e ] σ [ F e / H ] σ [ N i / F e ] σ [ C e / F e ] σ [ N d / F e ] N - r i c hfi e l d s t a r M − . . . . . . . . . . . . . e x t r a -t i d a l N - r i c h s t a r M − . . . . . . . . . . . . ... M s t a r s M − ......... . . . ... . . . . ...... M − . . . . . . . . . . . . ... M − ......... . . . . ...... . . ...... M − . . . . . . . ... . . . ...... M − ... . . . . . . . . . . . ... M − ... . . . . . . . . . . . ... M − . . . . . . . . . . . . . M − . ... . . . . ... . . . . . ... M − ......... . . . . ...... . . ...... M − ......... . ... . ... . ... . . ...... M − . . ...... . . . ... . . . ...... M − . . . . . . . . . . . . ... M − . . . . . . . . . . . ...... M − . . . . . . . . . . . ...... M − . . . . . . . . . . . . ... M − . . . . . . . . ... . . ...... M − ......... . . . . . . . . ...... M − ......... . . . ... . . . . ...... M − . . . . . . . . . . . . ... M − . . . . . . . . . . . . ... M − ... . ...... . . . ...... . . ...... M − . . . . . . . . . . . . ... Article number, page 9 of 10 & A proofs: manuscript no. ms
Table A.3.
Astrometric and kinematic properties of our sample. The last two columns indicate the typical S / N of the spectra and number ofAPOGEE visits.
APOGEE ID
RUWE µ α cos( δ ) ± ∆ µ δ ± ∆ G G BP G RP RV RV -scatter S / N − mas yr − [mag] [mag] [mag] km s − km s − pixel − N-rich field star pixel − − ± − ± extra-tidal N-rich star − − ± − ± M 54 stars − − ± − ± − − ± − ± − − ± − ± − − ± − ± − − ± − ± − − ± − ± − − ± − ± − − ± − ± − − ± − ± − − ± − ± − − ± − ± − − ± − ± − − ± − ± − − ± − ± − − ± − ± − − ± − ± − − ± − ± − − ± − ± − − ± − ± − − ± − ± − − ± − ± − − ± − ±0.028 15.15 15.96 14.27 135.09 0.38 192 12