arXiv subjects

Sandro Villanova

Publications and source records attributed to Sandro Villanova.

At least 37 records · Page 2Linked to original sources

arXiv2022

Is Terzan 5 the remnant of a building block of the Galactic bulge? Evidence from APOGEE

It has been proposed that the globular cluster-like system Terzan 5 is the surviving remnant of a primordial building block of the Milky Way bulge, mainly due to the age/metallicity spread and the distribution of its stars in the $α$-Fe plane. We employ Sloan Digital Sky Survey (SDSS-IV) data from the Apache Point Observatory Galactic Evolution Experiment (APOGEE 2) to test this hypothesis. Adopting a random sampling technique, we contrast the abundances of 10 elements in Terzan 5 stars with those of their bulge field counterparts with comparable atmospheric parameters, finding that they differ at statistically significant levels. Abundances between the two groups differ by more than 1$σ$ in Ca, Mn, C, O, and Al, and more than 2$σ$ in Si and Mg. Terzan 5 stars have lower [$α$/Fe] and higher [Mn/Fe] than their bulge counterparts. Given those differences, we conclude that Terzan 5 is not the remnant of a $major$ building block of the bulge. We also estimate the stellar mass of the Terzan 5 progenitor based on predictions by the Evolution and Assembly of GaLaxies and their Environments (EAGLE) suite of cosmological numerical simulations, concluding that it may have been as low as $\sim3\times10^8$ M$_\odot$ so that it was likely unable to significantly influence the mean chemistry of the bulge/inner disk, which is significantly more massive ($\sim10^{10}$ M$_\odot$). We briefly discuss existing scenarios for the nature of Terzan 5 and propose an observational test that may help elucidate its origin.

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arXiv2022

C,T1,T2: A complementary method to detect Multiple Populations with the Washington filter system

In this research we test the ability of a three Washington filter combination, (C-T1)-(T1-T2), compared with that of the traditional C-T1 color to find multiple populations on two globular clusters: NGC 7099 and NGC 1851, types I and II Globular clusters, respectively. Our improved photometry and membership selection, now using Gaia proper motions, finds that the second population stars are more centrally concentrated than first population stars, as expected and contrary to our previous findings for NGC 7099. We find that multiple populations are more easily detected in both clusters using the new (C-T1)-(T1-T2) color, although C-T1 conserves the best width/error ratio. We also search for differences of both colors while splitting the red-RGB and the blue-RGB in NGC 1851, but no significant improvement was found.

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arXiv2021

APOGEE-2S view of the globular cluster Patchick 125 (Gran 3). New metallicity and elemental abundances from high-resolution spectroscopy

We present detailed elemental abundances, radial velocity, and orbital elements for Patchick~125, a recently discovered metal-poor globular cluster (GC) in the direction of the Galactic bulge. Near-infrared high-resolution ($R\sim22,500$) spectra of two members were obtained during the second phase of the Apache Point Observatory Galactic Evolution Experiment at Las Campanas Observatory as part of the sixteenth Data Release (DR 16) of the Sloan Digital Sky Survey. We investigated elemental abundances for four chemical species, including $α$- (Mg, Si), Fe-peak (Fe), and odd-Z (Al) elements. We find a metallicity covering the range from [Fe/H] $= -1.69$ to $-1.72$, suggesting that Patchick~125 likely exhibits a mean metallicity $\langle$[Fe/H]$\rangle \sim -1.7$, which represents a significant increase in metallicity for this cluster compared to previous low-resolution spectroscopic analyses. We also found a mean radial velocity of 95.9 km s$^{-1}$, which is $\sim$21.6 km s$^{-1}$ higher than reported in the literature. The observed stars exhibit an $α$-enrichment ([Mg/Fe]$\lesssim+0.20$, and [Si/Fe]$\lesssim +0.30$) that follows the typical trend of metal-poor GCs. The aluminum abundance ratios for the present two member stars are enhanced in [Al/Fe]$\gtrsim +0.58$, which is a typical enrichment characteristic of the so-called `second-generation' of stars in GCs at similar metallicity. This supports the possible presence of the multiple-population phenomenon in Patchick~125, as well as its genuine GC nature. Further, Patchick~125 shows a low-energy, low-eccentric ($<0.4$) and retrograde orbit captured by the inner Galaxy, near the edge of the bulge. We confirm that Patchick~125 is a genuine metal-poor GC, which is currently trapped in the vicinity of the Milky Way bulge.

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arXiv2021

CAPOS: The bulge Cluster APOgee Survey III. Spectroscopic Tomography of Tonantzintla 2

(ABRIDGED) We have performed the first detailed spectral analysis of red giant members of the relatively high-metallicity globular cluster (GC) Tononzintla~2 (Ton~2) using high-resolution near-infrared spectra collected with the Apache Point Observatory Galactic Evolution Experiment II survey (APOGEE-2), obtained as part of the bulge Cluster APOgee Survey. We investigate chemical abundances for a variety of species including the light-, odd-Z, $α$-, Fe-peak, and neutron-capture elements from high S/N spectra of seven giant members. The derived mean cluster metallicity is [Fe/H]$=-0.70\pm0.05$, with no evidence for an intrinsic metallicity spread. Ton~2 exhibits a typical $α$-enrichment that follows the trend for high-metallicity Galactic GCs, similar to that seen in 47~Tucanae and NGC~6380. We find a significant nitrogen spread ($>0.87$ dex), and a large fraction of nitrogen-enriched stars that populate the cluster. Given the relatively high-metallicity of Ton~2, these nitrogen-enriched stars are well above the typical Galactic levels, indicating the prevalence of the multiple-population phenomenon in this cluster which also contains several stars with typical low, first-generation N abundances. We also identify the presence of [Ce/Fe] abundance spread in Ton~2, which is correlated with the nitrogen enhancement, indicating that the \textit{s}-process enrichment in this cluster has been produced likely by relatively low-mass Asymptotic Giant Branch stars. Furthermore, we find a mean radial velocity of the cluster, $-178.6\pm0.86$ km s$^{-1}$ with a small velocity dispersion, 2.99$\pm$0.61 km s$^{-1}$, which is typical of a GC. We also find a prograde bulge-like orbit for Ton~2 that appears to be radial and highly eccentric.

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arXiv2021

APOGEE Chemical Abundance Patterns of the Massive Milky Way Satellites

The SDSS-IV Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey has obtained high-resolution spectra for thousands of red giant stars distributed among the massive satellite galaxies of the Milky Way (MW): the Large and Small Magellanic Clouds (LMC/SMC), the Sagittarius Dwarf (Sgr), Fornax (Fnx), and the now fully disrupted \emph{Gaia} Sausage/Enceladus (GSE) system. We present and analyze the APOGEE chemical abundance patterns of each galaxy to draw robust conclusions about their star formation histories, by quantifying the relative abundance trends of multiple elements (C, N, O, Mg, Al, Si, Ca, Fe, Ni, and Ce), as well as by fitting chemical evolution models to the [$α$/Fe]-[Fe/H] abundance plane for each galaxy. Results show that the chemical signatures of the starburst in the MCs observed by Nidever et al. in the $α$-element abundances extend to C+N, Al, and Ni, with the major burst in the SMC occurring some 3-4 Gyr before the burst in the LMC. We find that Sgr and Fnx also exhibit chemical abundance patterns suggestive of secondary star formation epochs, but these events were weaker and earlier ($\sim$~5-7 Gyr ago) than those observed in the MCs. There is no chemical evidence of a second starburst in GSE, but this galaxy shows the strongest initial star formation as compared to the other four galaxies. All dwarf galaxies had greater relative contributions of AGB stars to their enrichment than the MW. Comparing and contrasting these chemical patterns highlight the importance of galaxy environment on its chemical evolution.

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arXiv2021

Lithium in the lower red giant branch of 5 Galactic globular clusters

Lithium is one of the few elements produced during the Big Bang Nucleosynthesis in the early universe. Moreover, its fragility makes it useful as a proxy for stellar environmental conditions. As such, the lithium abundance in old systems is at the core of different astrophysical problems. Stars in the lower red giant branch allow studying globular clusters where main sequence stars are too faint to be observed. We use these stars to analyze the initial Li content of the clusters and compare it to cosmological predictions, to measure spreads in Li between different stellar populations, and to study signs of extra depletion in these giants. We use GIRAFFE spectra to measure the lithium and sodium abundances of lower red giant branch stars in 5 globular clusters. These cover an extensive range in metallicity, from [Fe/H]$\sim-0.7$ to [Fe/H]$\sim-2.3$ dex. We find that the lithium abundance in these lower red giant branch stars forms a plateau, with values from $\mathrm{A(Li)_{NLTE}}=0.84$ to $1.03$ dex, showing no clear correlation with metallicity. When using stellar evolutionary models to calculate the primordial abundance of these clusters, we recover values $\mathrm{A(Li)_{NLTE}}=2.1-2.3$ dex, consistent with the constant value observed in warm metal-poor halo stars, the Spite plateau. Additionally, we find no difference in the lithium abundance of first and second population stars in each cluster. We also report the discovery of a Li-rich giant in the cluster NGC3201, with $\mathrm{A(Li)_{NLTE}}=1.63\pm0.18$ dex, where the enrichment mechanism is probably pollution from external sources.

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arXiv2021

APOGEE-2S Discovery of Light- and Heavy-Element Abundance Correlations in the Bulge Globular Cluster NGC 6380

We derive abundance ratios for nine stars in the relatively high-metallicity bulge globular cluster NGC 6380. We find a mean cluster metallicity between [Fe/H]$= -0.80$ and $-0.73$, with no clear evidence for a variation in iron abundances beyond the observational errors. Stars with strongly enhanced in [N/Fe] abundance ratios populate the cluster, and are anti-correlated with [C/Fe], trends that are considered a signal of the multiple-population phenomenon in this cluster. We detect an apparent intrinsic star-to-star spread ($\gtrsim 0.27$ dex) in the slow neutron-capture process element (s-element) Ce II. Moreover, the [Ce/Fe] abundance ratio exhibits a likely correlation with [N/Fe], and a somewhat weaker correlation with [Al/Fe]. If confirmed, NGC 6380 could be the first high-metallicity globular cluster where a N-Ce correlation is detected. Furthermore, this correlation suggests that Ce may also be an element involved in the multiple-population phenomenon. Currently, a consensus interpretation for the origin of the this apparent N-Ce correlation in high-metallicity clusters is lacking. We tentatively suggest that it could be reproduced by different channels - low-mass asymptotic giant-branch stars in the high-metallicity regime or fast-rotating massive stars ("spinstars"), due to the rotational mixing. It may also be the cumulative effect of several pollution events including the occurrence of peculiar stars. Our findings should guide stellar nucleosynthesis models, in order to understand the reasons for its apparent exclusivity in relatively high-metallicity globular clusters.

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arXiv2021

APOGEE-2 Discovery of a Large Population of Relatively High-Metallicity Globular Cluster Debris

We report the discovery of a new, chemically distinct population of relatively high-metallicity ([Fe/H] $> -0.7$) red giant stars with super-solar [N/Fe] ($\gtrsim +0.75$) identified within the bulge, disk, and halo of the Milky Way. This sample of stars was observed during the second phase of the Apache Point Observatory Galactic Evolution Experiment (APOGEE-2); the spectra of these stars are part of the seventeenth Data Release (DR 17) of the Sloan Digital Sky Survey. We hypothesize that this newly identified population was formed in a variety of progenitors, and are likely made up of either fully or partially destroyed metal-rich globular clusters, which we refer to as Globular Cluster Debris (GCD), identified by their unusual photospheric nitrogen abundances. It is likely that some of the GCD stars were probable members of the Gaia-Enceladus-Sausage accretion event, along with clusters formed in situ.

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arXiv2021

Intrinsic Metallicity Variation in the Intermediate Mass Type II Globular Cluster NGC 1261

Globular Clusters (GCs) are now well known to almost universally show multiple popu-lations (MPs). The HST UV Legacy Survey of a large number of Galactic GCs in UV filters optimized to explore MPs finds that a small fraction of GCs, termed Type II, also display more complex, anomalous behavior. Several well-studied Type II GCs show intrinsic Fe abundance variations, suggesting that the other, less well-studied, Type II GCs should also exhibit similar behavior. Our aim is to perform the first detailed metallicity analysis of NGC 1261, an intermediate mass Type II GC, in order to determine if this object shows an intrinsic Fe variation. We determined the Fe abundance in eight red giant members using Magellan-MIKE and UVES-FLAMES high-resolution, high S/N spectroscopy. The full range of [Fe/H] for the entire sample from the spectra is from -1.05 to -1.43 dexwith an observed spread sigma_obs=0.133 dex. Compared with the total internal error of Sigma_tot=0.06,this indicates a significant intrinsic metallicity spread of Sigma_int=0.119 dex. We found a very similar variation in [Fe/H] using an independent method to derive the atmospheric parameters based on near-IR photometry. More importantly, the mean metallicity of the five presumed normal metallicity stars is -1.37+/-0.02, while that of the three presumed anomalous/highmetallicity stars is -1.18+/-0.09. This difference is significant at the $\pm$2.4Sigma level. We find indications from existing data of other Type II GCs that several of them presumedto have real metallicity spreads may in fact posses none. The minimum mass required for a GC to acquire an intrinsic Fe spread appears to be $\pm$10^5 Msun. We find no strong correlation betwee nmass and metallicity variation for Type II GCs. The metallicity spread is also independent of the fraction of anomalous stars within the Type II GCs and of GC origin.

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arXiv2021

High-resolution extinction map in the direction of the strongly obscured bulge fossil fragment Liller 1

We used optical images acquired with the Wide Field Camera of the Advanced Camera for Surveys onboard the Hubble Space Telescope and near-infrared data from GeMS/GSAOI to construct a high-resolution extinction map in the direction of the bulge stellar system Liller 1. In spite of its appearance of a globular cluster, Liller 1 has been recently found to harbor two stellar populations with remarkably different ages, and it is the second complex stellar system with similar properties (after Terzan5) discovered in the bulge, thus defining a new class of objects: the Bulge Fossil Fragments. Because of its location in the inner bulge of the Milky Way, very close to the Galactic plane, Liller 1 is strongly affected by large and variable extinction. The simultaneous study of both the optical and the near-infrared color-magnitude diagrams revealed that the extinction coefficient R$_V$ in the direction of Liller 1 has a much smaller value than commonly assumed for diffuse interstellar medium (R$_V=2.5$, instead of 3.1), in agreement with previous findings along different light paths to the Galactic bulge. The derived differential reddening map has a spatial resolution ranging from $1''$ to $3''$ over a field of view of about $90''$X$90''$. We found that the absorption clouds show patchy sub-structures with extinction variations as large as $δ{\rm E}(B-V)\sim0.9$ mag.

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arXiv2021

CAPOS: The bulge Cluster APOgee Survey I. Overview and initial ASPCAP results

Context. Bulge globular clusters (BGCs) are exceptional tracers of the formation and chemodynamical evolution of this oldest Galactic component. However, until now, observational difficulties have prevented us from taking full advantage of these powerful Galactic archeological tools. Aims. CAPOS, the bulge Cluster APOgee Survey, addresses this key topic by observing a large number of BGCs, most of which have only been poorly studied previously. Even their most basic parameters, such as metallicity, [α/Fe], and radial velocity, are generally very uncertain. We aim to obtain accurate mean values for these parameters, as well as abundances for a number of other elements, and explore multiple populations. In this first paper, we describe the CAPOS project and present initial results for seven BGCs. Methods. CAPOS uses the APOGEE-2S spectrograph observing in the H band to penetrate obscuring dust toward the bulge. For this initial paper, we use abundances derived from ASPCAP, the APOGEE pipeline. Results. We derive mean [Fe/H] values of $-$0.85$\pm$0.04 (Terzan 2), $-$1.40$\pm$0.05 (Terzan 4), $-$1.20$\pm$0.10 (HP 1), $-$1.40$\pm$0.07 (Terzan 9), $-$1.07$\pm$0.09 (Djorg 2), $-$1.06$\pm$0.06 (NGC 6540), and $-$1.11$\pm$0.04 (NGC 6642) from three to ten stars per cluster. We determine mean abundances for eleven other elements plus the mean [$α$/Fe] and radial velocity. CAPOS clusters significantly increase the sample of well-studied Main Bulge globular clusters (GCs) and also extend them to lower metallicity. We reinforce the finding that Main Bulge and Main Disk GCs, formed in situ, have [Si/Fe] abundances slightly higher than their accreted counterparts at the same metallicity. We investigate multiple populations and find our clusters generally follow the light-element (anti)correlation trends of previous studies of GCs of similar metallicity. We finally explore the abundances ...

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arXiv2021

CAPOS: the bulge Cluster APOgee Survey II. The Intriguing "Sequoia" Globular Cluster FSR 1758

We present results from a study of fifteen red giant members of the intermediate-metallicity globular cluster (GC) FSR 1758 using high-resolution near-infrared spectra collected with the Apache Point Observatory Galactic Evolution Experiment II survey (APOGEE-2), obtained as part of CAPOS (the bulge Cluster APOgee Survey). Since its very recent discovery as a massive GC in the bulge region, evoking the name Sequoia, this has been an intriguing object with a highly debated origin, and initially led to the suggestion of a purported progenitor dwarf galaxy of the same name. In this work, we use new spectroscopic and astrometric data to provide additional clues to the nature of FSR 1758. Our study confirms the GC nature of FSR 1758, and as such we report for the first time the existence of the characteristic N-C anti-correlation and Al-N correlation, revealing the existence of the multiple-population phenomenon, similar to that observed in virtually all GCs. Furthermore, the presence of a population with strongly enriched aluminium makes it unlikely FSR 1758 is the remnant nucleus of a dwarf galaxy, as Al-enhanced stars are uncommon in dwarf galaxies. We find that FSR 1758 is slightly more metal rich than previously reported in the literature, with a mean metallicity [Fe/H] between $-1.43$ to $-1.36$ (depending on the adopted atmospheric parameters), and with a scatter within observational error, again pointing to its GC nature. Overall, the $α$-enrichment ($\gtrsim+0.3$ dex), Fe-peak (Fe, Ni), light- (C, N), and odd-Z (Al) elements follow the trend of intermediate-metallicity GCs. ... A new examination of its dynamical properties with the \texttt{GravPot16} model favors an association with the Gaia-Enceladus-Sausage accretion event. Thus, paradoxically, the cluster that gave rise to the name of the Sequoia dwarf galaxy does not appear to belong to this specific merging event.

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arXiv2021

Searching for Multiple Populations in Ruprecht 106

More than a decade has passed since the definition of Globular Cluster (GC) changed, and now we know that they host Multiple Populations (MPs). But few GCs do not share that behaviour and Ruprecht 106 is one of these clusters. We analyzed thirteen member red giant branch stars using spectra in the wavelength range 6120-6405 Angstroms obtained through the GIRAFFE Spectrograph, mounted at UT2 telescope at Paranal, as well as the whole cluster using C, V, R and I photometry obtained through the Swope telescope at Las Campanas. Atmospheric parameters were determined from the photometry to determine Fe and Na abundances. A photometric analysis searching for MPs was also carried out. Both analyses confirm that Ruprecht 106 is indeed one on the few GCs to host Simple Stellar Population, in agreement with previous studies. Finally, a dynamical study concerning its orbits was carried out to analyze the possible extra galactic origin of the Cluster. The orbital integration indicates that this GC belongs to the inner halo, while an Energy plane shows that it cannot be accurately associated with any known extragalactic progenitor.

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arXiv2021

APOGEE spectroscopic evidence for chemical anomalies in dwarf galaxies: The case of M~54 and Sagittarius

We present evidence for globular cluster stellar debris in a dwarf galaxy system (Sagittarius: Sgr) based on an analysis of high-resolution \textit{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] abundances are derived for a large number of stars in M~54. Our study reveals the existence of a significant population of nitrogen- (with a large spread, $\gtrsim1$ dex) and aluminum-enriched stars with moderate Mg depletion in the core of the M~54$+$Sagittarius system, which shares the light element anomalies characteristic of second-generation globular cluster stars (GCs), thus tracing the typical phenomenon of 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 different 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 chemical patterns for which both chemical and kinematic evidence is commensurate with the hypothesis that the star has been ejected from M~54. Our findings support the existence of chemical anomalies associated with likely tidally shredded GCs in dwarf galaxies in the Local Group and motivate future searches for such bonafide stars along other known Milky Way streams.

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arXiv2021

VVV CL001: Likely the Most Metal-Poor Surviving Globular Cluster in the Inner Galaxy

We present the first high-resolution abundance analysis of the globular cluster VVV~CL001, which resides in a region dominated by high interstellar reddening towards the Galactic Bulge. Using \textit{H}-band spectra acquired by the Apache Point Observatory Galactic Evolution Experiment (APOGEE), we identified two potential members of the cluster, and estimate from their Fe I lines that the cluster has an average metallicity of [Fe/H] = $-2.45$ with an uncertainty due to systematics of 0.24 dex. We find that the light-(N), $α$-(O, Mg, Si), and Odd-Z (Al) elemental abundances of the stars in VVV~CL001 follow the same trend as other Galactic metal-poor globular clusters. This makes VVV~CL001 possibly the most metal-poor globular cluster identified so far within the Sun's galactocentric distance and likely one of the most metal-deficient clusters in the Galaxy after ESO280-SC06. Applying statistical isochrone fitting, we derive self-consistent age, distance, and reddening values, yielding an estimated age of $11.9^{+3.12}_{-4.05}$ Gyr at a distance of $8.22^{+1.84}_{-1.93}$ kpc, revealing that VVV~CL001 is also an old GC in the inner Galaxy. The Galactic orbit of VVV~CL001 indicates that this cluster lies on a halo-like orbit that appears to be highly eccentric. Both chemistry and dynamics support the hypothesis that VVV~CL001 could be an ancient fossil relic left behind by a massive merger event during the early evolution of the Galaxy, likely associated with either the Sequoia or the \textit{Gaia}-Enceladus-Sausage structures.

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arXiv2021

APOGEE discovery of a chemically atypical star disrupted from NGC 6723 and captured by the Milky Way bulge

The central (`bulge') region of the Milky Way is teeming with a significant fraction of mildly metal-deficient stars with atmospheres that are strongly enriched in cyanogen ($^{12}$C$^{14}$N). Some of these objects, which are also known as nitrogen-enhanced stars, are hypothesised to be relics of the ancient assembly history of the Milky Way. Although the chemical similarity of nitrogen-enhanced stars to the unique chemical patterns observed in globular clusters has been observed, a direct connection between field stars and globular clusters has not yet been proven. In this work, we report on high-resolution, near-infrared spectroscopic observations of the bulge globular cluster NGC 6723, and the serendipitous discovery of a star, 2M18594405$-$3651518, located outside the cluster (near the tidal radius) but moving on a similar orbit, providing the first clear piece of evidence of a star that was very likely once a cluster member and has recently been ejected. Its nitrogen abundance ratio ([N/Fe]$\gtrsim + 0.94$) is well above the typical Galactic field-star levels, and it exhibits noticeable enrichment in the heavy $s$-process elements (Ce, Nd, and Yb), along with moderate carbon enrichment; all characteristics are known examples in globular clusters. This result suggests that some of the nitrogen-enhanced stars in the bulge likely originated from the tidal disruption of globular clusters.

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arXiv2020

A distance determination to the Small Magellanic Cloud with an accuracy of better than 2 percent based on late-type eclipsing binary stars

We present a new study of late-type eclipsing binary stars in the Small Magellanic Cloud (SMC) undertaken with the aim of improving the distance determination to this important galaxy. A sample of 10 new detached, double-lined eclipsing binaries indentified from the OGLE variable star catalogues and consisting of F- and G-type giant components has been analysed. The absolute physical parameters of the individual components have been measured with a typical accuracy of better than 3%. All but one of the systems consist of young and intermediate population stars with masses in the range of 1.4 to 3.8 M_Sun. This new sample has been combined with five SMC eclipsing binaries previously published by our team. Distances to the binary systems were calculated using a surface brightness - color calibration. The targets form an elongated structure, highly inclined to the plane of the sky. The distance difference between the nearest and most-distant system amounts to 10 kpc with the line of sight depth reaching 7 kpc. We find tentative evidence of the existence of a spherical stellar sub-structure (core) in the SMC coinciding with its stellar center, containing about 40% of the young and intermediate age stars in the galaxy. The radial extension of this sub-structure is ~1.5 kpc. We derive a distance to the SMC center of D_SMC=62.44 +/- 0.47 (stat.) +/- 0.81 (syst.) kpc corresponding to a distance modulus (m-M)_SMC=18.977 +/- 0.016 +/- 0.028 mag, representing an accuracy of better than 2%.

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arXiv2020

Aluminum Enhanced Metal-Poor Stars buried in the Inner Galaxy

Stars with higher aluminum and nitrogen enrichment are often the key pieces for the chemical makeup of multiple populations in almost all globular clusters (GCs). There is also compelling observational evidence that some Galactic components could be partially built from dissipated GCs. Thus, the identification of such kinds of stars among metal-poor field stars may provide insights on the composite nature of the Milky Way (MW) bulge and inner stellar halo, as well as reveal other chemical peculiarities. Here, based on APOGEE spectra, we report the discovery of 29 mildly metal-poor ([Fe/H]$\lesssim-0.7$) stars with stellar atmospheres strongly enriched in aluminum (Al-rich stars: [Al/Fe]$\gtrsim+0.5$), well above the typical Galactic levels, located within the Solar radius toward the bulge region, which lies in highly eccentric orbits ($e\gtrsim0.6$). We find many similarities for almost all of the chemical species measured in this work with the chemical patterns of GCs, so we conjecture that they have likely been dynamically ejected into the bulge and inner halo from GCs formed in situ and/or GC formed in different progenitors of known merger events experienced by the MW, such as the \textit{Gaia}-Sausage-Enceladus and/or Sequoia.

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