Matias Gomez

Context. Recent optical and near-IR photometric databases such as Gaia Data Release 3 (DR3) and the VISTA Variables in the Vía Láctea eXtended (VVVX) ESO public survey allow for the exploration of candidate unresolved multiple systems (binaries, triples, quadruples, etc.) in nearby clusters, which need spectroscopic confirmation. Aims. NGC 2437 (M 46) is a benchmark open cluster with well-determined physical parameters. Its relative proximity ( D = 1.6 ± 0.1 kpc) makes it an ideal laboratory for measuring the fractions of unresolved multiple systems. We aim here to identify unresolved triple and higher-order systems that are reliable members of the open cluster NGC 2437, and provide the spectroscopic confirmation using radial velocities. Methods. The selection of cluster members was made using Gaia DR3 astrometry and optical photometry in combination with near-IR photometry from the VVVX survey. We made optical and near-IR color-magnitude diagrams to select stars located well above the cluster main sequence, which should include suitable unresolved multiple systems. Results. We confirm that there is a sequence of unresolved binaries in NGC 2437, and report the discovery of a sequence of unresolved triple systems in this cluster. We corroborate membership for 13 binary systems and 8 triple systems using radial velocities measured by the ESO KMOS VVVX-GalCen spectroscopic survey. We further select stars with colors consistent with K-dwarfs, in order to have a controlled sample, comparable to the solar neighborhood. We also find that the fraction of unresolved triples and quadruples is higher than that of the older open cluster Trumpler 19, studied using a similar setup, suggesting that these systems merge to form blue stragglers or get dissolved with time. Conclusions. We report the spectroscopic confirmation of 95% of the selected VVVX targets, thereby securing the presence of a sequence of unresolved triple systems in NGC 2437, located above the sequence of equal-mass binaries. The single stars constitute 60% of the mass of the cluster, while the rest of the mass (40%) is lodged in the unresolved multiple systems.

We report Garro 04, a previously unidentified stellar cluster toward the Galactic disk, which was discovered as part of the ESO/KMOS VVVX-GalCen Spectroscopic Survey. This object was serendipitously confirmed as a stellar overdensity in the VISTA Variables in the Vía Láctea eXtended (VVVX) survey footprint and subsequently confirmed through medium-resolution K -band spectroscopy obtained with the K -band Multi Object Spectrograph (KMOS) at the ESO Very Large Telescope. The combined photometric and spectroscopic analysis reveals a coherent stellar population with common radial velocities and chemical properties, supporting its classification as a genuine star cluster rather than a statistical fluctuation of the disk field. We derived the fundamental properties of Garro 04, including its structural parameters, distance, and kinematics, and computed its orbital properties, finding that its motion remains confined to the Galactic disk region. Finally, our analysis suggests that Garro 04 is likely an old open cluster or an object straddling the boundary between the canonical globular and open cluster classifications, with an estimated age of 5 ± 2 Gyr and a metallicity of [Fe/H] = −0.5 ± 0.2. Our photometric and spectroscopic determinations are in excellent agreement.

Context . Deep photometry of Galactic star clusters provides one of the most powerful tools for determining their physical properties. In particular, in low Galactic latitude regions that suffer from heavy extinction and crowding, it is essential to have accurate optical and near-IR photometry for benchmark clusters. Aims . NGC 2437 is the most extended star cluster in the near-IR footprint of the VISTA Variables in the Via Lactea Extended Survey (VVVX), covering more than one degree on the sky. In this paper, we aim to characterize the physical properties of NGC 2437 using Gaia data release 3 (DR3) databases in the optical and the VVVX in the near-IR. Methods . We use Gaia DR3 proper motions (PMs) to select NGC 2437 members in order to make optical and near-IR color–magnitude and color–color diagrams. We further exploited the newly constructed VVVX deep stack images to obtain the deepest near-IR color–magnitude diagram currently available for this cluster. Results . Using the selected cluster members, we estimate the main physical parameters for NGC 2437, including the mean parallax ϖ = 0.608 mas and PMs ( µ _{α ∗} , µ _δ ) = (−3.85, 0.41) mas/yr. We also estimate a mean reddening of E ( J − K _s ) = 0.059 mag and an extinction of A k _s = 0.034 mag for the NGC 2437 field, with no significant differential reddening spread throughout the field. A distance modulus of ( m − M ) = 11.08 mag is estimated, equivalent to a distance of D = 1644 pc. This places NGC 2437 at z = 115 pc above the Galactic plane and at a galactocentric distance of R _G = 9.24 kpc. We measure the structural parameters of the cluster by obtaining the core radius r _c = 10.79 arcmin. We estimate the total absolute magnitude in the near-IR of M _K = −4.91 mag, equivalent to an optical absolute magnitude of M _V = −3.70 mag. We measure a cluster mean age of t = 350 Myr, using the PARSEC-COLIBRI isochrones for solar metallicity. We also measure the binary fraction using the optical and near-IR color–magnitude diagrams. Based on Gaia DR3 data and VVVX deep stacks, we conclude that the NGC 2437 binary fraction is 28.6%. We also discuss the implications of the revised cluster parameters for the nearby open cluster NGC 2425, the planetary nebula NGC 2438, and the evolved OH/IR source OH 231.8+04.2. Conclusions . The VVVX deep stacks increase the photometric depth by approximately 1.6 mag in the K _s band, nearly doubling the number of detected point sources and enabling significantly improved studies of stellar populations throughout the southern Galactic plane.

Context . Understanding how the Milky Way disc assembled and evolved requires tracing the coupled evolution of stellar chemistry and orbital structure over time. Open clusters, as coeval stellar populations with well-constrained ages, distances, and chemical properties, provide powerful benchmarks for this purpose. Aims . We aim to characterise the age-dependent chemo-dynamical structure of the Galactic disc and to investigate how open clusters populate dynamical phase space as a function of age. Methods . We analysed a sample of Galactic open clusters using homogeneous chemical abundances from the Gaia –ESO Survey together with Gaia DR3 phase-space information. Combining cluster ages with orbital parameters and actions, we examined how clusters have populated dynamical phase space over the last ∼4 Gyr. Results . We find that clusters with similar chemical properties occupy coherent regions of action space, while their orbital structure shows a clear dependence on age. Radial excursions and orbital eccentricities broaden systematically towards older ages, indicating increasingly diverse orbital configurations in the older cluster population. The dispersion in the [Fe/H]– R _g relation also increases with age, which is consistent with a combination of secular redistribution, survival bias, and dynamically distinct outer-disc populations. In addition, older clusters reach larger vertical amplitudes and vertical actions, revealing a coupled but anisotropic evolution between in-plane and vertical orbital structure. Conclusions . Open clusters provide precise, age-resolved tracers of the chemo-dynamical structure of the Galactic disc. Our results reveal systematic age-dependent variations in the orbital and chemical properties of the cluster population, consistent with the combined effects of secular evolution, orbital redistribution, and environmentally dependent cluster survival over the last few gigayears.

Context . It is difficult to identify unresolved triple and higher-order systems beyond the solar vicinity. Star clusters yield controlled environments with known physical parameters, providing a way to explore homogeneous stellar samples. Aims . Combining modern optical and near-IR surveys, our main goal is to explore the region of the color–magnitude diagrams located above the sequence of equal-mass binaries, which is inhabited by unresolved triple and quadruple systems. Specifically, here we aim to discover and characterize the presence of multiple systems in the aged open star cluster Trumpler 19. Methods . We selected cluster members using Gaia astrometry and photometry from the VVVX and DECAPS2 surveys. We made optical and near-IR color–magnitude diagrams to select stars located > 0.75 mag above the cluster main sequence, including suitable unresolved triple and quadruple systems. Results . We confirm previous results that there is a sizable sequence of unresolved binaries in Trumpler 19. We also report the discovery of a sequence of unresolved multiple systems in this cluster, and confirm some of these multiple systems via radial velocities measured by the ESO KMOS VVVX-GalCen spectroscopic survey. The fraction of multiple stars in this cluster is comparable to that of the solar neighborhood, with the caveat that the local sample probes deeper down the main sequence. The fraction of unresolved triples and quadruples is also three times higher than that of the open cluster M 67, which is of a similar age and chemical composition. Conclusions . The discovery of unresolved triple systems in Trumpler 19 leads to our main conclusion that these systems were able to survive within the cluster for nearly 4 Gyr. In addition, this cluster contains more multiple systems than M 67, which has a similar total present mass. Further investigations are needed to unveil if this is a primordial feature or the result of dynamical evolution.

A new distance was established to the Cepheid SV Vul ( P ≃ 45 ^d ) and its host cluster Alicante 13, and is tied in part to deeper UKIDSS-DR6 photometry and Gaia DR3 observations ( d = 2.30 ± 0.13 kpc). SV Vul and Alicante 13 possibly belong to a broader coeval stellar complex, which could include Liu-Pang 1738. The clusters and Cepheid share comparable astrometry (e.g., σ _π ≃ 0.02 mas, σ μ δ ≃ 0.06 mas yr ⁻¹ ), and similar cluster turnoffs (ages) exist as indicated by ultraviolet UVEX color–color analyses, Gaia XP spectroscopically differentially dereddened color–magnitude diagrams, and Padova isochrones. Yet SV Vul may be comparatively overluminous. Robust radial velocities for both clusters could substantiate certain hypotheses.

Context . The recent discovery of free-floating planets (FFPs) in nearby young stellar associations suggests that these objects might be common in the Galaxy. Our search for FFPs in the young Lower Centaurus-Crux (LCC) association using the VISTA Variables in the Vía Láctea VVV and VVVX surveys revealed several candidates with distances d < 200 pc. Aims . The main goal of the paper is to identify binary FFPs among this sample. The presence of such binaries is useful to contrast two different main formation scenarios: the formation in the circumstellar disk of the parent star with subsequent ejection by dynamical interactions and the in situ formation by gravitational collapse of a protostellar cloud. Methods . We used the Gaia , VVV, VVVX, and DECaPS databases to identify pairs of low-mass objects in the LCC association sharing common proper motions. We examined the optical and near-IR color-magnitude and color-color diagrams, and visually confirmed the detections in the available optical and near-IR images. Results . We find 17 young low-mass binaries in the LCC association, with distances starting from 68 pc and projected separations ranging from 88 to 6742 au. A couple of candidates have additional faint companions that need confirmation to secure them as triple systems. Adopting an age of 17 Myr for the LCC association, we find that 14 of the components are faint enough to have planetary masses. Conclusions . Our results indicate that binaries represent ≳2% of the population of FFPs in the LCC association, and this suggests that their preferred formation mechanism is the gravitational collapse independent of a star. Also, many of the recently discovered FFPs in LCC may be unresolved giant binary planets. The wide range of colors and flux reversals observed suggests that the existence of clouds in their atmospheres is important and points to continuity with the BD populations.

Context . While large samples of nearby M dwarfs are becoming available, it is usually difficult to estimate some intrinsic physical parameters for isolated field M dwarfs, especially their ages, radii, and masses. These parameters can be estimated through a comparison with model stellar isochrones and/or by studying eclipsing binary systems. Aims . Our goal is to gather and analyze a large sample of variable M dwarfs that are members of nearby young open clusters and have known reddenings, distances, ages, and metallicities. This distilled sample would be useful for the comparison with theoretical models and also for the identification of eclipsing binaries with substellar companions (brown dwarfs or giant planets) for future follow-up. Methods . We selected two dozen benchmark young and nearby open clusters (mostly with 40 < age < 132 Myr and 400 < D < 700 pc, respectively). We then combined the optical data from Gaia Data Release 3 (DR3) with near-IR JHK _s −band photometry from the VISTA Variables in the Vía Láctea eXtended survey (VVVX) for these target clusters using the proper motions in order to select members within the cluster radii. Gaia and VVVX both provide a wide areal coverage that is ideally suited for mapping these nearby clusters, which are extended on the sky. We then produced optical and near-IR color–magnitude and color-color diagrams and applied appropriate color cuts to choose the M-type dwarfs at the end of the main sequence. The reddening and extinction for all clusters were estimated using the J − K _s color distributions of the cluster M dwarfs. We also compared them with the PARSEC theoretical isochrones adjusted to the respective cluster metallicities, ages, extinctions, and distances. Results . Finally, we selected variable M stars according to Gaia DR3 (using phot_variable_flag=VARIABLE) and present a catalog of 318 variable M-dwarf star members of two dozen young nearby Galactic open clusters. The cross-validation with the Gaia DR3 parallaxes supports the cluster membership for all these sources. We also present a few examples to validate the sample of variable cluster M dwarfs. Conclusions . This well-characterized catalog of M-dwarf members of benchmark open clusters not only constitutes a prime sample to search for transiting substellar companions, but also offers a training dataset for machine-learning applications aimed at selecting future similar targets.

Context. The Circinus galaxy is the nearest type-2 Seyfert galaxy, located at a distance of 4.2 Mpc. Its environment is challenging to explore because of the low Galactic latitudes of its location, behind the disc of the Milky Way. Aims. The long-term goal of this work is to characterise the Circinus galaxy halo. We are also interested in determining the possible presence of dwarf satellites using near-infrared (NIR) data. Methods. We selected 1542 galaxies from the VISTA Variables in the Vía Láctea (VVV) NIR galaxy catalogue within a 2-degree radius around the Circinus galaxy, representing roughly 2/3 of the virial radius. We used structural parameters, such as the half-light radii and colours, and correlations were examined. A neural network was trained with 486 galaxies with known spectroscopic redshifts to estimate photometric redshifts for all of these galaxies. The potential satellites of the Circinus galaxy were defined on the basis of half-light radii that were compatible with the typical sizes of dwarf satellites in galaxies and combined with photometric redshifts. Results. The galaxy properties have been reliably characterised down to K _s ∼ 15.5 mag, representing about 90% completeness of the detections. At the distance of the Circinus galaxy, this limiting magnitude corresponds to a K _s absolute magnitude of −12.6 mag, allowing us to locate dwarf galaxies. In total, there are 20 galaxies with half-light radii larger than 2.45 arcsec, but only 8 of them have photometric redshifts below 0.04. None of these galaxies are close to the Circinus galaxy, which has a redshift of 0.0015 and shows no evidence of associated clustering. The ANNz model exhibited a high degree of accuracy in the range of 0.001 < z _phot < 0.023, thereby validating this method in these extinct regions. Conclusions. The presence of dwarf satellites associated with the Circinus galaxy could not be confirmed with the available data in the studied region. The apparent lack of satellites could be genuine, possibly related to AGN feedback effects. Nevertheless, this work demonstrates the effectiveness of combining near-infrared data and machine learning techniques to estimate photometric redshifts at low Galactic latitudes, thereby providing useful information for future spectroscopic follow-up campaigns.

Context. Globular clusters (GCs) are fundamental tools to unveil the interaction and merger history of their host galaxies. Aims. Our goal is to perform the photometric analysis of the globular cluster system (GCS) of the elliptical galaxy NGC 4709, which is the brightest galaxy of the Cen 45 spiral-rich galaxy group, and to highlight its interaction history with NGC 4696, the giant elliptical galaxy of the Cen 30 subcluster. Methods. We obtained deep Magellan 6.5 m/MegaCam (g′, r′, i′) photometry, with which we identified a sample of 556 GC candidates around NGC 4709 that were analyzed in the context of the interaction history with the giant elliptical NGC 4696 and other galaxies of the Cen 45 group. After modeling and subtracting the galaxy light, we used criteria based on shape, colors and magnitude to select GC candidates. Results. Our results point toward a complex interaction history that shaped the GCS of NGC 4709. Inside a galactocentric radius r < 5 × r_eff, two populations were found with mean colors of (g′−i′)₀ = 0.905 ± 0.009 mag and (g′−i′)₀ = 1.170 ± 0.008 mag. The azimuthal distribution of the GCs show peaks at the position angles PA₁ = 92° and PA₂ = 293°, with PA₂ coinciding with the direction toward NGC 4696, confirming that the interaction between these galaxies shaped the GCS of NGC 4709. From the GC luminosity function we derived a distance of 29.9 ± 2.1 Mpc, which is much closer than the other galaxies of the Centaurus cluster, and a specific frequency of S​_N = 3.7 ± 0.5, in good agreement with previously estimated values. From the GCs density maps, we identified overdensities corresponding to the positions of five other galaxies of the Centaurus cluster, and we found a bridge of GCs between NGC 4709 and NGC 4696 with a distance of d = 34.69 ± 2.21 Mpc, which is between that of the two galaxies. Conclusions. All of these findings point toward a complex GCS for NGC 4709, strongly influenced by the interaction with NGC 4696, and confirm previous findings that the galaxy’s apparent distance is smaller than that of the main cluster galaxy NGC 4696 by ∼8.5 Mpc and that it is smaller than the distance of the other Centaurus’ galaxies, making it an outlier in Centaurus and suggesting a past first encounter with Cen 30. In future work, we will combine our data with E-MOSAICS simulations to unravel the trajectory of NGC 4709, from the past interaction to the future new rendezvous with NGC 4696.