Discovery of a large population of Nitrogen-Enhanced stars in the Magellanic Clouds

We report the APOGEE-2S$+$ discovery of a unique collection of nitrogen-enhanced mildly metal-poor giant stars, peaking at [Fe/H]$\sim -0.89$ with no carbon enrichment, toward the Small and Large Magellanic Clouds (MCs), with abundances of light- (C, N), odd-Z (Al, K) and $\alpha-$elements (O, Mg, Si) that are typically found in Galactic globular clusters (GCs). Here we present 44 stars in the MCs that exhibit significantly enhanced [N/Fe] abundance ratios, well above ([N/Fe]$>+0.6$) typical Galactic levels at similar metallicity, and a star that is very nitrogen-enhanced ([N/Fe]$>+2.45$). Our sample consists of luminous evolved stars on the asymptotic giant branch (AGB), eight of which are classified as bonafide semi-regular (SR) variables, as well as low-luminosity stars similar to that of stars on the tip of the red giant branch of stellar clusters in the MCs. It seems likely that whatever nucleosynthetic process is responsible for these anomalous MC stars it is similar to that which caused the common stellar populations in GCs. We interpret these distinctive C-N patterns as the observational evidence of the result of tidally shredded GCs in the MCs. These findings might explain some previous conflicting results over bulge N-rich stars, and broadly help to understand GC formation and evolution. Furthermore, the discovery of such a large population of N-rich AGB stars in the MCs suggests that multiple stellar populations might not only be exotic events from the past but can also form at lower redshift.

Galactic GCs are expected to lose mass through processes like evaporation and tidal stripping (e.g., Baumgardt & Makino 2003), and the favored hypothesis for the origin of the N-rich stars is that they were once part of a GC. This conjecture appears to be well-supported by the chemo-kinematics similarity between a number of nitrogen-enriched MW field stars and its possible GC progenitors (see, e.g., Martell et al. 2016;Tang et al. 2020;Fernández-Trincado et al. 2019a. Alternatively, it has been suggested that such a N-rich population could include the oldest stars in the MW, which could have been born in high-density environments (Chiappini et al. 2011;Bekki 2019).
The N-rich population is often typified by larger N over-abundances, accompanied by decreased abundances of carbon ([C/Fe] +0.15) and α−elements (O, and Mg). Sometimes they exhibit atmospheres extremely enriched in aluminum ([Al/Fe]> +0.5) and s-process elements, suggesting that some of them could be objects in the AGB evolutionary stages that have undergone the hot bottom burning (see e.g., Fernández-Trincado et al. 2016;Pereira et al. 2017;Fernández-Trincado et al. 2017, 2019a, including a few cases which became strongly enriched in phosphorus (see, e.g., Masseron et al. 2020), which could be biased towards red giant branch (RGB) stars in previous studies , or could be objects chemically enriched by an AGB companion (e.g., Cordero et al. 2015;Fernández-Trincado et al. 2019c).
The existence of N-rich stars in specific environments has proven to have important implications in the chemical makeup of multiple populations (MPs) in the context of GC evolution across a wide range of metallicity (see, e.g., Renzini et al. 2015;Fernández-Trincado et al. 2019d;Mészáros et al. 2020). Establishing whether Nrich stars in the MW and/or nearby Local Group systems formed in a GC could either reveal that MP populations can also form, to some extent, in lower-density environments (e.g., Savino & Posti 2019), as well as provide important insights on the assembly history of their host systems, in particular on the role of GC disruption.
those of MW field stars) light-element abundances (He, C, N, O, Na, Al, and Mg).
While the abundance properties of N-rich stars have been limited to GC stars and the inner/outer (up to 100 kpc) stellar halo (Martell et al. 2016;Fernández-Trincado et al. 2017, 2019a and widely explored, little is known of these stars in nearby dwarf galaxies that surround the MW, even though some attempts to search for dissolved GCs in these systems via the investigation of chemical anomalies have been tried (e.g., Lardo et al. 2016).
The Apache Point Observatory Galactic Evolution Environment (APOGEE: Majewski et al. 2017), is currently obtaining near-IR spectra for stars in the Small and Large Magellanic (SMC/LMC) Clouds (Nidever et al. 2020), providing us with an excellent window to examine the presence of disrupted GCs in nearby Local Group galaxies. In this Letter, we report the discovery of a large population of N-rich stars likely associated with GC dissolution and/or evaporation in the SMC/LMC. To our knowledge, none of the large spectroscopic surveys of the SMC/LMC system have so far included measurements of nitrogen abundances.
This work is organized as follows. In Section 2, we discuss the data and selection criteria employed to create the parent stellar sample used throughout the paper. We present our analysis and conclusions in Section 3.
We identified potential GC debris over ∼3,535 mildly metal-poor (−2.0 < ∼ [Fe/H] < ∼ − 0.7) stars toward the MCs in the [N/Fe]-[Fe/H], plane following the same methodology as described in Fernández-Trincado et al. (2019a). This yielded the serendipitous discovery of 44 stars toward the MCs with stellar atmosphere strongly  Bica & Schmitt (1995), Palma et al. (2016), and Milone et al. (2020) are marked as gray squares, gray triangles, and orange diamond symbols, respectively. The large blue and green circles mark the tidal radius of NGC 362 and NGC 104, respectively.  Here, we perform a spectral synthesis analysis, independently of the ASPCAP pipeline, to disentangle the underlying C, N, and O abundances from the 12 C 16 O, 12 C 14 N, and 16 OH band strengths. To this purpose, we performed an LTE analysis with a MARCS grid of spherical models with the BACCHUS code (Masseron et al. 2016), adopting the same methodology as described in Fernández-Trincado et al. (2016, 2019a. It is important to keep in mind that ASPCAP uses a global fit to the continuum in three detector chips independently, while we place the pseudo-continuum in a region around the lines of interest. We believe that our manual method is more reliable, since it avoids possible shifts Figure 3. Distributions of various elemental-abundance ratios from the APOGEE-2S+ survey, represented by iso-abundance contours for the MW stars (orange), SMC (black), and LMC (red) stars, and Galactic GCs (blue crosses) from Mészáros et al. (2020). Bright N-rich RGB stars in the SMC and LMC are marked as lime circles and squares, respectively, while the green unfilled circles indicate N-rich AGB stars in the SMC. The SR variables in the SMC and LMC are highlighted with black circles and squares, respectively.
in the continuum location due to imperfections in the spectral subtraction along the full spectral range.
In order to provide a consistent chemical analysis, we re-determine the chemical abundances by means of a careful line selection, and measure abundances based on a line-by-line basis with the BACCHUS code, and by adopting the line selection for the various elements as in Fernández-Trincado et al. (2019a). Finally, we rederived chemical abundances adopting as input the uncalibrated effective temperatures (T unc eff ), surface gravities log g unc and the overall metallicity ([M/Fe]) from the ASPCAP pipeline. We do not calculate chemical abundances based on the photometric atmospheric parameters, as they become error dominated for stars toward the MCs, due the difficulty of calculating accurate reddenings in these regions (see, e.g., Nidever et al. 2020), making them unsuitable to estimate precise chemical abundances for stars in the inner MCs.

RESULTS AND ANALYSIS
We find that the newly identified N-rich stars span a wide range of metallicities (−1.4 <[Fe/H]< −0.7), peaking at [Fe/H]∼ −0.89, and that they exhibit nitrogen abundances well above typical Galactic levels over a range of metallicities, which is 3-4σ above the typical MW [N/Fe]. It seems likely that whatever nucleosynthesis process is responsible for these nitrogen overabundances in the field of the MCs is similar to that which caused the unusual stellar populations in Galactic GCs at similar metallicity.  reveal that the radial-velocity distributions differ from those of the field GCs, while the Color-Magnitude Diagram (CMD) using Gaia bands (see Figures 2c;f) also rules out the possibility these N-rich stars are stellar debris of these two Galactic GCs. This is also supported by inspection of [Fe/H]; the N-rich stars exhibit a larger metallicity scatter, on average being more metal rich than NGC 362 and more metal poor than NGC 104.
Based on these properties, we conclude that these are bona fide N-rich stars in the MCs, which are chemically identical to those identified toward the bulge and halo of the MW (see, e.g., Fernández-Trincado et al. 2017, 2019a. Figure 3 presents some of the light-, odd-Z, and αelement patterns, and demonstrates a very distinct separation in nitrogen, with a star-to-star scatter ∆ [N/Fe] +0.28 dex and ∆ [C/Fe] +0.25 dex, which is moderately anti-correlated with [C/Fe] (Mészáros et al. 2020) at similar metallicity. There is also a star with [N/Fe] +2.42 that exceeds the extreme abundance patterns seen in Galactic GCs, highlighting the uniqueness of these stars. Overall, we can see clearly from Figure 3 that our sample of stars in the MCs behave in a similar way as MW GC stars, supporting the idea that most of the newly identified stars could be related to MC GCs.
The newly identified N-rich population is separated relatively cleanly from MW stars and MC stars in the [C/Fe] vs. [N/Fe] and [N/Fe] vs. [Al/Fe] planes. In general, these N-rich stars exhibit slightly higher abundance ratios in Al, Si, Ti, and Ni compared to the SMC and LMC populations, but they exhibit lower abundance ratios in O, Mg, Al, Si, K, and C compared to MW field stars, with the α−elements (O, Mg, Si, and Ca) at comparable levels (∼ +0.1) to low-α halo MW field stars (e.g., Hayes et al. 2018).
The star-to-star scatter is between 0.1 -0.3 dex for the different chemical species, being slightly lower for the α−elements. Therefore, the star-to-star scatter in iron and other chemical species could be attributed to different progenitors, which could explain the observed chemical anomalies toward the MCs, in a similar manner as observed in and around the MW halo metal-poor stars today. The mildly metal-poor N-rich stars toward the MCs may have formed following minor merger events in the early history of the MCs. However, there are other observational features in our sample that allow us to invoke other possible scenarios to explain the observed abundance patterns toward the MCs.
Our sample includes five SR variable stars reported in the ASAS-SN Catalog of Variable Stars (Jayasinghe et al. 2020). Four of them are located toward the SMC, with one been selected as a possible O-rich AGB star based on its position in the (J-Ks, H) diagram (see Nidever et al. 2020). We also identified a SR variable in our sample toward the LMC, selected as a bright RGB in Nidever et al. (2020). These SR stars have periods between ∼85 and 757 days and variability amplitudes between 0.17 and 0.51 mag in the V-band. In conclusion, we find evidence that the SR stars are neither carbon rich nor oxygen rich, but exhibit lower carbon and oxygen abundance ratios, [C/Fe]< +0.15 and [O/Fe] +0.23. It is worthing to mention that no bias or uncertainties are introduced in our spectroscopic analysis, as is the case for, e.g., shorter-period Cepheids or RR Lyrae stars (Pancino et al. 2015).
Thus, the observed nitrogen over-abundances and the modest enhancements of the s-process elements, coupled with the apparent variability of these SR stars, suggest that some of the evolved objects could be likely intermediate-mass (∼3-5M ) AGB stars (one of the likely agent that self-enriches the GCs) that have undergone hot bottom burning and are becoming N-rich, according to chemical evolution models (Karakas et al. 2018), but without production of significant amounts of aluminum, as envisioned by Ventura et al. (2016). These SR N-rich stars have remarkably stronger 12 C 14 N lines (see Figure 4) compared to other stars with similar relevant parameters; it can be asserted that these have much higher nitrogen abundance. The presence of such young, mildly metal-poor stellar populations in the MCs has important implications. Thus, the interpretation of our results depends crucially on establishing the evolutionary stage of the stars under analysis. It is also important to note that there are no known MC GCs within an angular separation of approximately one arcmin of these stars.
In this context, one can immediately notice that two sub-samples occupy different loci in the CMD displayed in Figure 2c;f. N-rich stars with G 15.0 (LMC) and G 15.5 (SMC) occupy the same locus as MC AGB stars (referred to as "N-rich AGB" stars henceforth), while the N-rich stars with fainter G magnitudes roughly occupy the same locus as the bright RGB stars in the MC stellar clusters. These stars are tagged as genuine migrants from MC GCs (hereafter N-rich BrRGB stars), and are among the oldest objects in the MCs. The existence of N-rich AGB stars in our sample can be also further assessed by the possible presence of circumstellar dust (Habing 1996), as the N-rich AGB stars occupy a locus towards colors that are redder than those N-rich BrRGB stars in the CMD diagram. In particular, 55% of the N-rich stars in our sample inhabit the AGB part of the diagram, which provides further evidence for an important contribution of AGB stars to our N-rich sample.
Although these stars have elemental abundances consistent with each other, we find that in the N-rich AGB stars the oxygen abundance ratios -generally lower than the [O/Fe] of N-rich BrRGB stars-this lends further support to the notion that the two populations do not share the same origin.
It is also interesting to note that all the N-rich AGB stars in our sample were identified toward the SMC system, while the N-rich BrRGB stars are present in both, and likely could be part of the oldest stars in the MCs. We also conclude that there is significant evidence for a large contribution of possible AGB stars to our sample toward the MCs, suggesting that the detection of N-rich stars toward the MW has been biased towards RGB stars (e.g., Schiavon et al. 2017), which should result in a substantial difference in AGB contribution to the N-rich sample and the rest of the field, as already noted in Fernández-Trincado et al. (2019a).
Our finding can be understood in terms of different scenarios. Here, we conjecture that there may be at least two possible channels for the production of N-rich stars in the MCs: (i ) the N-rich BrRGB stars could be former members of a population of GCs that was previously dissolved and/or evaporated in the LMC/SMC, and were later incorporated into the field of the MCs themselves-'smoking gun' evidence that they have been accreted along with their now-disrupted host GCs. Such a scenario could potentially explain the predominance of N-rich BrRGB stars that are currently not gravitationaly bound to any MC clusters. The chemical patterns of these stars are identical or comparable to those seen in old MW GC stars (e.g., Mészáros et al. 2020), and possibly associated with MC GCs, with ages between ∼2 and 10 Gyr (Hollyhead et al. 2018;Lagioia et al. 2019;Milone et al. 2020). In support of this scenario, one would expect to find N-rich BrRGB stars in the same environments as GCs today, as at least some of them would have been formed in the same molecular clouds as the GCs themselves. Thus, this observational finding would suggest that either some old GCs in the MCs have possibly experienced significant stellar massloss (e.g., Mackey et al. 2007;Dalessandro et al. 2016). This would suggest a common, single pathway for the formation and evolution of old ( 2 Gyr) GCs within the Local Group (Martocchia et al. 2017); (ii ) On the other hand, the discovery of a significant population of N-rich stars in the AGB evolutionary stage (and possibly of intermediate-mass), further supports the idea that AGB stars are possibly one of the key players in the pollution of the intracluster medium (e.g., Ventura et al. 2016) proposed to explain the formation of MPs in GCs.
The presence of such a significant young and moderately metal-poor stellar populations in the MCs would have interesting consequences for the understanding of the formation and evolution of GC systems in the Local Universe, i.e., the presence of star-to-star abundance spreads in this possible "young" N-rich AGB population appears to be at odds with the apparent near-exclusivity of this population within old GCs.

CONCLUDING REMARKS
We report the serendipitous discovery of a large population of mildly metal-poor N-rich stars toward the LMC/SMC. Our sample is composed mainly of stars in the bright RGB (45%) and AGB (55%) evolutionary stage. This sample adds to the literature nitrogen measurements for several new stars; to our knowledge, none of the large spectroscopic surveys of the LMC/SMC have so far included measurements of significant nitrogen abundances.
The discovery of two sub-populations of N-rich stars suggests that the occurrence of chemical anomalies (also crucial in the chemical makeup of MPs in all GCs) might not be exotic events from the past, but can also form at lower redshift, as also envisaged by Bekki (2019), and not limited to stars with masses less than ∼1.6 M (see, e.g., Bastian & Lardo 2018).
Our findings motivate the future search for N-rich stars in Local Group dwarf galaxies that could have once hosted GCs, now in the form of disrupted remnants. As light-element variations have, however, not been found amongst the dwarf's field stars (Geisler et al. 2007;Villanova et al. 2017).
We thank the referee for insightful comments that improved the paper. J.G.F-T. is supported by FONDE-CYT No. 3180210. T.C.B. and V.M.P. acknowledge partial support for this work from grant PHY 14-30152; Physics Frontier Center / JINA Center for the Evolution of the Elements (JINA-CEE), awarded by the US National Science Foundation. S.V. gratefully acknowledges the support provided by Fondecyt regular n. 1170518. AR-L acknowledges financial support provided in Chile by Comisión Nacional de Investigación Científica y Tecnológica (CONICYT) through the FONDECYT project 1170476 and by the QUIMAL project 130001. B.B. acknowledge partial financial support from the brazilian agencies CAPES -Financial code 001, CNPq and FAPESP. L.C. thanks Gloria Delgado-Inglada for useful discussions. We also acknowledge the support of Universidad de Concepción for providing HPC resources on the Supercomputer TITAN.
This work has made use of data from the European Space Agency (ESA) mission Gaia (http://www.cosmos. esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, http://www.cosmos.