arXiv subjects

Andrés E. Piatti

Publications and source records attributed to Andrés E. Piatti.

At least 19 recordsLinked to original sources

arXiv2026

Misaligned tidal tails in open clusters: a signature of the local spiral arm resonance

Tidal tails of open clusters are traditionally expected to align with their orbital motion due to Galactic shear. However, recent observations suggest that a significant population of clusters exhibits misaligned tails, the origin of which remains poorly understood. I investigate the structural properties of tidal tails for a sample of 155 nearby open clusters within 1 kpc of the Sun to characterize the influence of non-axisymmetric Galactic perturbers on stellar stripping geometry.Using high-precision Gaia DR3 astrometry and the probabilistic membership, I trace the primary orientation and physical length of tidal tails. I distinguish between local and global perturbations by analyzing the spatial distribution of misalignment (DeltaPA) relative to the boundaries of the Local Arm and comparing results with recent test-particle simulations of barred and spiral potentials. I identify three distinct dynamical regimes. While the majority of the population is shear-aligned, a significant subset exhibits a spatially indifferent background misalignment (10deg < DeltaPA < 30deg) consistent with global Galactic bar torques. In contrast, a population of nascent tails (< 0.3 kpc) exhibits larger misalignments (DeltaPA > 40deg) with a unique spatial specificity, occurring exclusively within the inter-arm gaps (|d_arm| > 0.2 kpc). These significant misalignments could represent a transient dynamical phase occurring within the corotation resonance's zero-shear zone, where the radial gravitational forcing of the spiral potential dominates stellar exit trajectories by rotating the eigenvectors of the tidal tensor. This could identify nascent tidal tails as uniquely sensitive high-resolution tracers of the non-axisymmetric Galactic potential.

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arXiv2026

Star clusters as tracers of the perturbed southern outer disk of the Large Magellanic Cloud

The complex interaction history between the Large and Small Magellanic Clouds (L/SMC) has left significant imprints on their structural and chemical evolution. In this work, we use a sample of 70 star clusters located in the southern outer disk of the LMC ($R > 5^{\circ}$) to trace the magnitude of these tidal perturbations. Using deep $g,i$ photometry from the SMASH DR2 survey and the Automated Stellar Cluster Analysis (ASTeCA) code, we provide a homogeneous determination of cluster ages, metallicities, and heliocentric distances. Our results show a population with ages ranging from 1.02 to 3.34 Gyr and metallicities between -0.70 and -0.07 dex. We find that clusters at greater heliocentric distances are predominantly located toward the western regions, facing the SMC. Analysis of the vertical distribution reveals an oscillatory variation in the mean height ($\langle Z \rangle$) relative to the LMC mid-plane and an increase in intrinsic vertical dispersion ($W$) toward the West. These geometric features are consistent with a corrugated disk structure induced by recent tidal encounters with the SMC. Chemically, the clusters follow the age-metallicity relationship typical of the LMC and are well-represented by a bursting-type formation model. The lack of chemically distinct clusters suggests that while the interaction was sufficient to dynamically deform and corrugate the outer disk, it did not lead to significant cluster capture from the SMC. We conclude that the southern outer disk remains structurally and chemically part of the LMC, despite being heavily perturbed by tidal forces.

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arXiv2026

Neighbors, Not Kin: Kinematic evidence for tidal tails from NGC 7492 along the Sagittarius stream

The formation, extension, and morphology of extra-tidal stars around globular clusters depend on the internal kinematics of the host cluster and the Galactic potential. Tracing the kinematics of faint tidal tails sheds light on their formation and contribution to the Milky Way halo. NGC 7492 is an outer halo globular cluster with conflicting evidence regarding the presence of tidal tails. If present, the tails are expected to be faint and overlap on the sky with the Sagittarius stream, located at a similar heliocentric distance but with distinct kinematics. We carried out a GIRAFFE spectroscopic follow-up of ten fields covering the expected tidal tails of NGC 7492. Gaia parallaxes were used to remove foreground contaminants, while only loose proper-motion constraints were applied in the target selection. Radial velocities and metallicities were derived for more than 700 stars, from the red giant branch to the upper main sequence. Cluster and extra-tidal stars were identified from their proper motions, radial velocities, and metallicities. This population extends at least 1.8 deg from the cluster center, confirming the tidal tails previously detected only photometrically, with positions consistent with particle-spray models of the cluster disruption. The Sagittarius stream is clearly identified through its distinct proper motions, radial velocities, and more metal-rich population. Despite the low spatial density of the extra-tidal stars, their kinematic signature is clearly detected, demonstrating that the tidal tails overlap on the sky with the Sagittarius stream but are physically unrelated. This spectroscopic dataset provides a robust basis for future studies of the tails at larger angular extents and fainter magnitudes.

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arXiv2026

Spatially resolved star formation history of Sextans dSph

We present a spatially resolved archaeological reconstruction of the Sextans dwarf spheroidal (dSph) galaxy using deep DECam wide-field photometry and the PANCAKE CMD-fitting code. By analyzing the star formation history (SFH) and age-metallicity relationship (AMR) across four radial zones, namely: core, ring, outer body, and outskirts, we find that Sextans is a composite system formed through a minor merger approximately 13 Gyr ago. Our results reveal an inverse metallicity gradient: a primitive, metal-poor host (the current core) surrounded by a more massive, chemically evolved envelope ($Δ$[Fe/H] $\approx$ -0.5 dex) introduced by the accreted satellite. We identify a distinct delayed onset of star formation in the ring at $\sim$ 13 Gyr, marking the merger event. While the core quenched early, star formation in the outer body and ring persisted until $\sim$9 Gyr, suggesting that the final cessation of activity was driven by environmental stripping during infall into the Milky Way halo. We propose a plausible scenario to reconcile the derived inverse metallicity gradient with the observed horizontal-branch (HB) morphology and reported Mg deficits. We suggest that the red HB dominance in the core reflects its ancient, $α$-rich nature, while the blue HB in the outskirts represents an $α$-poor, accreted component. However, we note that our CMD-derived [Fe/H] values are model-dependent inferences based on the total metal content $Z$. These findings suggest a non-monolithic assembly for Sextans, posing a testable prediction of a strong radial gradient in [$α$/Fe].

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arXiv2026

The apparent Large Magellanic Cloud star cluster age gap

In the Large Magellanic Cloud (LMC), there have been very few clusters observed with ages between 4 and 11 Gyr. This phenomenon is sometimes referred to as the `LMC age gap'. We constructed a model of the cluster age distribution aimed at reproducing this scenario. We linked the star formation history to the cluster initial mass function via a power-law relation between maximum initial cluster mass and global star formation rate. Using a constant cluster-forming efficiency of 5%, we obtained the cluster formation history. Applying a model of cluster mass loss calibrated using N-body simulations and an observational completeness limit, we computed the observable fraction of initially formed clusters. We were then able to model the cluster age distribution. For a maximum initial cluster mass below $10^5$M$_\odot$ at a star formation rate of 1 M$_\odot$pc$^{-2}$Gyr$^{-1}$, our model reproduced the observed lack of clusters with ages between 4 and 11 Gyr. However, our model required a maximum initial mass at 1 M$_\odot$pc$^{-2}$Gyr$^{-1}$ of at least $2\cdot 10^5$M$_\odot$ in order to reproduce the population of ancient globular clusters. A linear change between maximum initial cluster mass relations from 8 to 12 Gyr reproduced the age gap to a satisfactory extent. In our model, the age gap is a consequence of the star-forming history and current observational limits. The age gap corresponds to a period characterised by a lower star formation rate, whereby no clusters with an initial mass above approximately 2 to 5$\cdot 10^5$M$_\odot$ were formed. In the present day, these clusters have become so faint that only few of them have been detected. The pattern of both young-and-bright and old-and-massive clusters being more easily detectable than clusters of intermediate ages might reflect a more general phenomenon and not necessarily one specific to the LMC.

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arXiv2026

3D kinematics of SMC star clusters: residual velocities disentangle kinematically perturbed clusters

Understanding the kinematic behaviour of the Small Magellanic Cloud (SMC) remains a challenge addressed by many authors using diverse approaches. Over time, increasing observational evidence has accumulated for tidal perturbations induced by the Large Magellanic Cloud (LMC) on the SMC, especially in its outer regions. In this study, we adopt star clusters as kinematic tracers of the SMC. We analyse 36 clusters distributed across the galaxy's structural regions (Northern Bridge, Southern Bridge, Wing/Bridge, West Halo, Main Body and Counter-Bridge). From each cluster's proper motions, radial velocity and heliocentric distance we estimate Cartesian velocities $(V_x,\,V_y,\,V_z)$ in the SMC reference frame. We also compute the same velocity components under the assumption that the SMC behaves as a rotating disc. We then define the residual velocity $ΔV$ for each cluster as the difference between the two velocities derived. Additionally, we perform a kinematic anisotropy analysis to characterise the distribution of kinetic energy across the SMC. We find that increasing values of $ΔV$ correlate with increasing cluster distance from the SMC center, and that $ΔV \approx 60\ \mathrm{km\,s^{-1}}$ it appears to be a lower limit that separates, in kinematic terms, the areas of tidal origin from those with the best behavior.

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arXiv2026

Kinematics of young star clusters in the outer north-eastern region of the Small Magellanic Cloud

It has been suggested since recent time that the magnitude of the interaction between galaxies could be measured from the level of kinematic disturbance of their outer regions with respect to the innermost ones. Here, I proved that the outer north-eastern region of the Small Magellanic Cloud (SMC), a relatively recent stellar structure with a tidal origin from the interaction with the Large Magellanic Cloud, is imprinted by a residual velocity pattern. I obtained from GEMINI GMOS spectra mean radial velocities of star clusters formed in situ, which added to derived mean proper motions and heliocentric distances, allowed to compute their 3D space velocity components. These space velocities differentiate from those that the clusters would have if they instead orderly rotated with the galaxy, i.e., their residual velocities are larger than the upper limit for an object pertaining to the SMC main body rotation disk. The level of kinematic disturbance depends on the SMC rotation disk adopted; galaxy rotation disks traced using relatively old objects are discouraged.The resulting kinematic disturbance arises in younger and older stellar populations, so that the epoch of close interaction between both Magellanic Clouds cannot be uncovered on the basis of the kinematics behavior of stellar populations populating the outer SMC

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arXiv2025

A possible trigger of the multiple population phenomenon in star clusters

Multiple populations (MPs) is a intra-star cluster phenomenon consisting in star-to-star variation of the abundance of some light chemical elements. They have been observed in many star clusters, most of them old globular clusters, populating the Milky Way and other satellite galaxies. Since the study of MPs became more systematic, different astrophysical parameters have been claimed to be the main responsible for its occurrence. However, at the present time, no attempt would seem to have solved this conundrum. This work deals with a potential trigger of the MPs phenomenon, based on the gathered observational evidence of the existence of MPs in some star clusters and the absence of its in others. We found that star clusters with MPs mostly formed during time intervals of intense star formation activity in a galaxy, for instance during the galaxy formation epoch, a close galaxy encounter, etc. At those time intervals where relative peaks in the galaxy star formation rate occur, star clusters with masses above a lower mass limit harbour MPs. This lower star cluster mass limit would marginally depend on the star cluster age.

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arXiv2025

A hidden link in the LMC star cluster formation history

The age distribution function of star clusters in the Large Magellanic Cloud (LMC) is known to present a feature called the cluster age gap, a period of time from ~ 4 to 11 Gyr ago with a remarkable small number of clusters identified. In this work we performed an in-depth analysis of three recently catalogued age gap cluster candidates with the aim of confirming their physical nature. We used GEMINI@GMOS g,i photometry centred on the objects to build colour-magnitude diagrams from which we recognised their main features. We also performed an automatic search of stellar overdensities using Gaussian mixture model techniques and analysed the corresponding colour-magnitude diagrams similarly as performed for the aforementioned candidates. The present GEMINI data sets would seem to discard any strong evidence of these object being real clusters, and rather support the possibility of the result of stellar density variations of the LMC field star distribution. These three objects, in addition to other 17 new identified candidates, are placed in the LMC outermost disc in a limited region toward the southwest from the LMC centre. They had embraced the possibility to answer the long-time conundrum about the absence of LMC age gap clusters. From an statistical approach, combined with the knowledge of the expected LMC age-metallicity relationship, and recent simulations of the interaction between the LMC and the Milky Way, we provide not only evidence against the physical nature of the studied objects, but also an interpretation on the lack of identification of more LMC age gap clusters.

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arXiv2025

Chemical tagging of tidal tail star candidates of NGC 6362

The inner Milky Way disk globular cluster NGC~6362 appears to exhibit tidal tails composed of stars that have proper motions and positions in the color-magnitude diagram similar to those of cluster stars. Because recent results seem also to show that these stars are distributed across the regions least affected by interstellar absorption and reproduce the observed composite star field density map, we carried out a detailed spectroscopic analysis of a number of chemical element abundances of tidal tail star candidates in order to investigate the relationship of them with NGC~6362. From European Southern Observatory's VLT@FLAMES spectra we found that the red giant branch stars selected as cluster's tidal tail stars do not have overall metallicities nor abundances of Mg, Ca, Sc, Ti, Cr, Ni and Ba similar to the cluster's ones. Moreover, they are mainly alike to stars that belong to the Milky Way thick disk, some of them could be part of the thin disk and a minor percentage could belong to the Milky Way halo star population. On the other hand, since the resulting radial velocities do not exhibit a distribution function similar to that of cluster's stars, we concluded that looking for kinematic properties similar to those of the cluster would not seem to be an approach for selecting cluster's tidal tail stars as suitable as previously thought.

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arXiv2025

On the dark matter formation scenario of NGC 4147

We report results on the radial velocity dispersion profile built out to the outskirts of NGC 4147, a Milky Way globular cluster with detected strong tidal tails. The cluster was chosen to probe, from an observational point of view, recent simulations that suggest that rising velocity dispersion profiles at large distance from the clusters' centers would be seen in globular clusters without tidal tails. From GEMINI@GMOS spectra, centered in the infrared CaII triplet region, of selected stars located along the onset of NGC 4147's tidal tails, we measured their radial velocities and overall metallicities. The derived metallicities were used to ultimately assessing on the highly-ranked cluster candidates of 9 stars, located between ~ 7 and 33 pc from the cluster's center, suitable for testing the aforementioned simulations. We complemented the present radial velocities with others available in the literature for cluster's members, and built a cluster velocity dispersion profile which suggests a mostly flat or slightly rising profile at large distances from the cluster's center. This outcome confirms that kinematically hot outermost cluster's stars are seen in NGC 4147, which disproves the recent model predictions. Nevertheless, the mean velocity dispersion of the outermost cluster's stars agrees with NGC 4147 being formed in a 10^8-10^9Mo dwarf galaxy with a cored dark matter profile that later was accreted on to the Milky Way.

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arXiv2025

Revisiting physical properties of mock globular cluster tidal tails

In this work we present results of the first in-depth analysis of extra-tidal mock stars of Milky Way globular clusters recently generated by Grondin et al. (2024). Particularly, we selected a sample of globular clusters with a general consensus of being formed in the bulge or in the disk of the Milky Way. From the catalog we estimated the width and the dispersion in the z-component of the angular momentum and in the line-of-sight and tangential velocities of their tidal tails, and compared the results with those predicted by cosmological simulations of Malhan et al. (2021) and observations. We found that the resulting values of these four quantities are not in agreement with an in-situ formation of the associated globular clusters. On average, the resulting widths agree with an in-situ origin, while the dispersion in the z-component of the angular momentum, and the dispersion in the line-of-sight and in the tangential velocities fail in matching this formation scenario. The four quantities derived for globular clusters formed in the bulge or in the disk show similar correlations with the stream length, namely: the width and the dispersion in the z-component of the angular momentum increase with the stream length, while the bulk of dispersion values in the line-of-sight and in the tangential velocities is around 12 km s$^{\rm -1}$ along the mock stream.

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arXiv2025

Astrophysical properties of star clusters projected toward tidally perturbed SMC regions

We report on the astrophysical properties of a sample of star clusters in the Small Magellanic Cloud (SMC). They have been selected with the aim of looking for the connection between their ages, heliocentric distances and metallicities with the existence of tidally perturbed/induced outermost SMC regions. We derived the star cluster fundamental parameters from relatively deep Survey of the Magellanic Stellar History (SMASH) DR2 color magnitude diagrams, cleaned from field star contamination, and compared to thousand synthetic CMDs covering a wide range of heliocentric distances, ages and metal content. Heliocentric distances for 15 star clusters are derived for the first time, which represents an increase of 50 per cent of SMC clusters with estimated heliocentric distances. The analysis of the age-metallicity relationships (AMRs) of cluster located in outermost regions distributed around the SMC and in the SMC Main Body reveals that they have followed the overall galaxy chemical enrichment history. However, since half of the studied clusters are placed in front of or behind the SMC Main Body, we concluded that they formed in the SMC and have traveled outward because of the tidal effects from the interaction with the Large Magellanic Cloud (LMC). Furthermore, metal rich clusters formed recently in some of these outermost regions from gas that was also dragged by tidal effects from the inner SMC. This outcome leads to consider the SMC as a galaxy scarred by the LMC tidal interaction with distance-perturbed and newly induced outermost stellar substructures.

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arXiv2025

The outer structure of old star clusters in the Small Magellanic Cloud

We report results on the internal dynamical evolution of old star clusters located in the outer regions of the Small Magellanic Cloud (SMC). Because the SMC has been imprinted with evidence of tidal interaction with the Large Magellanic Cloud (LMC), we investigated at what extend such an interaction has produced extra tidal structures or excess of stars beyond the clusters' tidal radii. For that purpose, we used the Survey of the Magellanic Stellar History (SMASH) DR2 data sets to build number density radial profiles of suitable star clusters, and derived their structural and internal dynamics parameters. The analysed stellar density profiles do not show any evidence of tidal effects caused by the LMC. On the contrary, the Jacobi volume of the selected SMC star clusters would seem underfilled, with a clear trend toward a smaller percentage of underfilled volume as their deprojected distance to the SMC centre increases. Moreover, the internal dynamical evolution of SMC star clusters would seem to be influenced by the SMC gravitational field, being star clusters located closer to the SMC centre in a more advanced evolutionary stage. We compared the internal dynamical evolution of SMC old star clusters with those of LMC and Milky Way globular clusters, and found that Milky Way globular clusters have dynamical evolutionary paths similar to LMC/SMC old star clusters located closer to their respective galaxy's centres. Finally, we speculate with the possibility that globular clusters belonging to Magellanic Clouds like-mass galaxies have lived a couple of times their median relaxation times.

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arXiv2024

Open cluster BSS dynamical clock dependence on the Milly Way gravitational field

Since recent years, mass segregation driven by two-body relaxation in star clusters has been proposed to be measured by the so-called dynamical clock, $A^+$, a measure of the area enclosed between the cumulative radial distribution of blue straggler stars and that of a reference population. Since star clusters spend their lifetime immersed in the gravitational potential of their host galaxy, they are also subject to the effects of galactic tides. In this work, I show that the $A^+$ index of a star cluster depends on both, its internal dynamics as it were in isolation and on the effects of galactic tides. Particularly, I focused on the largest sample of open clusters harboring blue straggler stars with robust cluster membership. I found that these open clusters exhibit an overall dispersion of the $A^+$ index in diagnostic diagrams where Milky Way globular clusters show a clear linear trend. However, as also experienced by globular clusters, $A^+$ values of open clusters show some dependence on their galactocentric distances, in the sense that clusters located closer or farther that $\sim$ 11 kpc from the Galactic center have larger and smaller $A^+$ values, respectively. This different response to two-body relaxation and galactic tides in globular and open clusters, which happen concurrently, can be due to their different masses. More massive clusters can somehow protect their innermost regions from galactic tides more effectively.

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arXiv2024

Surviving tidal tails around the Milky Way bulge globular cluster NGC 6355

We present results of the analysis of a set of images obtained in the field of the Milky Way bulge globular cluster NGC 6355 using the Dark Energy Camera, which is attached to the 4m Blanco telescope of the Cerro-Tololo Interamerican Observatory. We dealt with a heavy differential absorption across the observed field, a crowded field star population, and the superposition of field stars on to the cluster color-magnitude diagram main features to produce an intrinsic cluster stars density map. The resulting stellar density map reveals the presence of an extended envelope, a tidal tail, and scattered debris; the tidal tails pointing toward the Milky Way center. Such extra-tidal overdensities, detected above the mean star field density, resulted to be between four and six times larger that the local star field density fluctuation. They have also been recently generated by two independent studies which performed numerical simulations of synthetic tidal tails of Milky Way globular clusters. These results contrast with previous theoretical speculations about the possibility to detect tidal tails of globular clusters with chaotic orbits because they would be washed out after they were generated.

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arXiv2024

First deep search of tidal tails in the Milky Way globular cluster NGC 6362

I present results of the analysis of a set of images obtained in the field of the Milky Way globular cluster NGC 6362 using the Dark Energy Camera, which is mounted in the 4.0m Victor Blanco telescope of the Cerro-Tololo Interamerican observatory. The cluster was selected as a science case for deep high-quality photometry because of the controversial observational findings and theoretical predictions on the existence of cluster tidal tails. The collected data allowed us to build an unprecedented deep cluster field color-magnitude diagram, from which I filtered stars to produce a stellar density map, to trace the stellar density variation as a function of the position angle for different concentric annulii centered on the cluster, and to construct a cluster stellar density radial profile. I also built a stellar density map from a synthetic color-magnitude diagram generated from a model of the stellar population distribution in the Milky Way. All the analysis approached converge toward a relatively smooth stellar density between 1 and $\sim$ 3.8 cluster Jacobi radii, with a slightly difference smaller than 2 times the background stellar density fluctuation between the mean stellar density of the south-eastern and that of north-western hemispheres, the latter being higher. Moreover, the spatial distribution of the recently claimed tidal tail stars agrees well not only with the observed composite star field distribution, but also with the region least affected by interstellar absorption. Nevertheless, I detected a low stellar density excess around the cluster Jacobi radius, from which I conclude that NGC 6362 present a thin extra tidal halo.

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arXiv2023

An in-depth view of the metallicity distribution of the Small Magellanic Cloud

The spatial metallicity distribution of star clusters in the Small Magellanic Cloud (SMC) has recently been found to correlate as a V-shaped function with the semi-major axis of an elliptical framework proposed to assume a projected galaxy flattening. We report results on the impact that the use of such a framework can produce on our understanding of the SMC formation and its chemical enrichment. We show that clusters with similar semi-major axes are placed at a very different distances from the SMC centre. The recently claimed bimodal metallicity distribution of clusters projected on the innermost SMC regions and the V-shaped metallicity gradient fade away when actual distances are used. Although a large dispersion prevails, clusters older than $\sim$ 1 Gyr exhibit a shallow metallicity gradient, caused by slightly different spatial distributions of clusters younger and older than $\sim$ 4 Gyr; the former being more centrally concentrated and having a mean metallicity ([Fe/H]) $\sim$ 0.15 dex more metal-rich than that of older clusters. This metallicity gradient does not show any dependence with the position angle, except for clusters placed beyond 11 kpc, which are located in the eastern side of the galaxy.

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