Jose G. Fernandez-Trincado

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.

Interaction in massive binary stars is expected to produce a large population of intermediate-mass (2 − 8 M _⊙ ) envelope-stripped stars, yet such objects have remained elusive in the Milky Way. We report the identification of an unambiguous Galactic example in a short-period ( P = 5.94 d), double-lined spectroscopic binary, discovered in the SDSS-V Milky Way Mapper survey. The system consists of a rapidly rotating O-type star and a hotter, lower-mass companion, which shows He II and N IV emission lines with large radial velocity variations, revealing its binary nature. Combined orbital constraints and joint spectroscopic and photometric modelling show that the companion is a hot ( T _* ≈ 60 kK), helium-rich star with a mass of 3.2 − 5.8 M _⊙ , placing it squarely in the intermediate-mass regime and below values typically inferred for classical Wolf–Rayet (WR) stars. Its spectral properties are inconsistent with accretion-powered emission and instead indicate a stripped star that formed through binary mass transfer. The system’s short period, negligible eccentricity, and rapidly rotating O-star point to a post-interaction configuration following efficient mass transfer and spin-up of the accretor. Comparison with binary evolution models suggests that the stripped star is observed in a brief inflated phase following mass transfer, which increases its optical flux contribution and facilitates its detection. The inferred mass-loss rate log Ṁ /(M _⊙ yr ⁻¹ ) = −6.3 ± 0.1 is in line with mass-loss rates observed for classical WR stars in the Milky Way and exceeds those measured for intermediate-mass stripped stars in the Magellanic Clouds, with the caveat that our target selection is biased towards systems with stronger emission features. As an unambiguous and well-characterised intermediate-mass stripped star, this system provides a key benchmark for models of binary evolution at solar metallicity, stripped-envelope supernova progenitors, and the formation of compact-object binaries.

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. O2 stars define the hottest end of the normal O-star sequence, but their identification requires blue-optical nitrogen diagnostics, which are absent in many surveys. Aims. We reassess three luminous Large Magellanic Cloud (LMC) sources with multi-epoch Sloan Digital Sky Survey V spectroscopy obtained with the Baryon Oscillation Spectroscopic Survey (BOSS) spectrographs: [WSI2008] 778, MGSD LH 117-43A/140, and Sk −69 135. Methods. We measured visit-resolved radial velocities and H, He, and N equivalent widths (EWs); compared robust combinations with LMC O2 templates at a common effective resolution; and used OSTAR2002 spectral energy distribution fits and a Gaia colour–magnitude diagram (CMD) as broadband checks. Results. The weighted log|EW(N IV λ 4058)/EW(N III λ 4640)| values are 0.64 ± 0.29, 0.48 ± 0.23, and 0.34 ± 0.44 for [WSI2008] 778, MGSD LH 117-43A/140, and Sk −69 135. The detection of N V λλ 4604, 4620 supports the O2 If ^* and O2 V–III classifications adopted for [WSI2008] 778 and MGSD LH 117-43A/140, respectively. For Sk −69 135, the uncertain N IV/N III ratio and non-detection of N V motivate O2–3 V–III. Weak He I λ 4471 may indicate unresolved later-type contamination in [WSI2008] 778 and Sk −69 135. The radial velocities of Sk −69 135 show suggestive but non-significant variability. OSTAR2002 fits over 45–55 kK and the Gaia CMD place all three sources in the luminous very early O regime; [WSI2008] 778 shows a strong mid-infrared excess, MGSD LH 117-43A/140 is nearly photospheric, and Sk −69 135 shows only a modest 3–8 μm excess. Conclusions. The BOSS spectra place [WSI2008] 778 and MGSD LH 117-43A/140 in the LMC very early O population and identify Sk −69 135 as an additional O2–3 source. These cases show how incomplete blue coverage and ambiguous catalogue associations can conceal the hottest massive stars.

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.

The Sloan Digital Sky Survey (SDSS)-V Local Volume Mapper (LVM) is a wide-field integral field spectroscopic survey of the southern Milky Way plane, the Magellanic Clouds, and nearby Local Group galaxies. We used early science observations of the whole body of the nearest nuclear cluster, Omega Centauri, to extend the LVM beyond its primary interstellar-medium science case. The wide LVM field allowed us to precisely map ω Cen’s line-of-sight rotation out to ~3 r _HL or 15′, reaching a maximum value of (8.4 ± 0.8) km s ⁻¹ at r ≈ 4.7′. Within the central region, comparisons with existing VLT MUSE oMEGACat data explicitly show that the unresolved light signal is dominated by a small number of bright stars, with an effective sample size of only ~12 per resolution element. Using Gaia DR3 as an external reference, we verified that the SDSS-V’s LVM reduction pipeline recovers integrated stellar fluxes to 1–4% across six magnitudes of surface brightness. Our resulting total spectrum of ω Cen is one of the highest S/N integrated spectra for any globular or nuclear star cluster. We used it to test four widely used SSP template libraries against the resolved age-metallicity ground truth from oMEGACat. All templates recovered an old, metal-poor population. However, even at signal to noise ratio of ~1300, the inferred mean ages and mean [Fe/H] vary by ~7 Gyr and ~0.4 dex, respectively, across libraries and wavelength ranges, reflecting a systematic floor for integrated-light studies of old multi-population systems.

Recent JWST observations have revealed galaxies with unusually high N/O ratios, suggesting that nitrogen enrichment may be common in intense star-forming environments in the early Universe. In the Milky Way, nitrogen-rich (N-rich) stars in the Galactic field have long served as probes of early Galaxy formation and globular cluster (GC) enrichment. However, the identification of binaries among these stars raises the possibility that binary mass transfer could contribute to their origin. In this work, we utilize multiepoch radial velocities (RVs) and element abundances from APOGEE DR17 to constrain their formation sites. Among 266 N-rich field stars, 33 exhibit radial velocity (RV) variations of ΔRV > 1 km s ⁻¹ , including 10 robust spectroscopic binaries identified using the F ₂ statistic within a well-sampled subset of 46 stars. The resulting close-binary fraction (21.7% ± 6.1%) is statistically indistinguishable from that of chemically normal field stars (18.1% ± 0.6%), showing no evidence of the excess expected from asymptotic giant branch (AGB) binary pollution. This is further supported by the absence of correlation between [N/Fe] and [Ce/Fe] and the lack of [C/Fe] enhancement. Crucially, we detect an anticorrelation between binary fraction and [Al/Fe], with strongly Al-enhanced stars ([Al/Fe] ≳ 0.5) exhibiting a reduced binary fraction (<10%). This trend serves as a dynamical fingerprint of high-density environments, consistent with the efficient disruption of binaries via three-body interactions in GC cores. Our results do not support binary mass transfer as the dominant formation channel for N-rich field stars; they are predominantly GC escapees that retain the dynamical memory of their dense birth sites.

The distinct stellar populations of the Galactic bulge can be disentangled through detailed analysis of their chemical abundances and kinematical properties. Recent studies have suggested that globular clusters (GCs) located in the Galactic bulge with metallicities around [Fe/H] ≈ −0.7 may represent some of the oldest systems in the Milky Way, potentially tracing the early spheroidal bulge. The coincidence of a metallicity peak at [Fe/H] ≈ −0.7 in both field stars and GCs, together with the presence of phosphorus-rich (P-rich) stars, may provide important clues to the nature of the first generations of stars formed in the Galaxy. In this work, we investigate the odd- Z elements Na, Al, and particularly P in the bulge GCs NGC 6539 ([Fe/H] ∼ −0.75) and NGC 6569 ([Fe/H] ∼ −0.85) using APOGEE spectra. We also examine the clusters Tonantzintla-1 and NGC 6316, which exhibit evidence of phosphorus enhancement. Our analysis confirms that NGC 6539 is a cluster of interest, with one clearly P-rich star, whereas NGC 6569 shows a lower level of P enhancement. This again suggests that there might have been an early bulge building block with the metallicity of [Fe/H] ∼ −0.75, which NGC 6539 would be part of. The observed abundance patterns indicate that the production of Na and Al is consistent with nucleosynthesis in massive stars. However, the origin of the phosphorus enrichment remains uncertain, suggesting that additional nucleosynthetic channels may be required to explain the observed abundances. These findings provide new constraints on the chemical evolution of the Galactic bulge and the nature of its earliest stellar populations.