Mario Cadelano

Context . Globular clusters are efficient factories of recycled pulsars, but searches towards high dispersion-measure (DM) clusters can be strongly limited around 1 GHz – which is where most pulsar surveys are typically conducted – due to dispersive smearing and interstellar scattering. Observations at higher radio frequencies provide a complementary route to mitigate these effects and improve sensitivity. Aims . We present the first results from a MeerKAT S-band ( ν ≃ 2.4 GHz) pulsar survey of 14 globular clusters. Our aims were to discover new pulsars, re-detect known pulsars with a uniform high-frequency dataset, and assess practical S-band sensitivity and DM-search requirements for future globular cluster pulsar surveys. Methods . High time and frequency resolution S-band observations were searched in the Fourier domain using segmented acceleration and jerk techniques to maintain sensitivity to pulsars in compact binaries, followed by candidate identification and folding. For selected detections and non-detections, we additionally folded the S-band data together with archival MeerKAT L-band observations using available timing ephemerides to derive flux-density measurements and spectral-index constraints. Results . We re-detected 39 previously known pulsars across 14 clusters and discovered four new millisecond pulsars in Glimpse-C01 (J1848–0129C, D, E, and F). Multi-epoch follow-up enabled preliminary Keplerian orbital fits for two of the new systems based on their observed barycentric spin-period modulation. J1848–0129C is an eclipsing millisecond pulsar with P _b ∼ 5 d, placing it among the long-period eclipsing systems known as ‘huntsman’ binaries. J1848–0129D is in a P _b ∼ 3.4 d nearly circular orbit with a ∼1 M _⊙ massive white dwarf companion. Two Glimpse-C01 pulsars exhibit substantial DM offsets relative to the average cluster value. Comparison with other Galactic globular clusters showed that large intracluster DM spreads become increasingly common at high foreground DMs, implying that searches confined to narrow DM windows may be sub-optimal for high-DM clusters. From detections and non-detections, we inferred a practical single-epoch S-band tied-array detectability scale of ∼10–20 μJy, which serves as an operational benchmark rather than a strict completeness limit. Conclusions . MeerKAT S-band observations are highly effective for probing globular clusters affected by strong dispersion and scattering, enabling both reliable re-detections and the discovery of new millisecond pulsars in high-DM environments such as Glimpse-C01. The results of our work highlight the importance of high-frequency surveys for expanding the known pulsar population in obscured clusters.

We report on the identification of the optical counterpart to the recently discovered millisecond pulsar (MSP) NGC 362D in the Galactic globular cluster NGC 362 based on deep, multi-band, and multi-epoch Hubble Space Telescope observations. Our analysis robustly shows that this object is a very low-mass (∼0.18 M _⊙ ) He white dwarf (WD) still in the pre-cooling phase. Interestingly, a detailed comparison with updated binary evolution models indicates that this object completed the mass-transfer phase only recently (∼0.6 Gyr ago), thus strongly suggesting that COM-NGC 362D is the youngest WD companion to a MSP identified to date. Remarkably, in this respect, the photometric properties of NGC 362D show, for the first time in this class of objects, significant wavelength-dependent variations that are consistent with the presence of residual circumstellar material. This system therefore provides a valuable test case to directly probe the immediate aftermath of the MSP recycling process and to constrain the early evolutionary stages of proto-WD companions and their radio and optical properties. Our results suggest that residual material can produce radio and optical signatures that mimic those of systems with non-degenerate companions, potentially leading to the misclassification of young MSPs.

We present the first homogeneous study of the radial distribution of the binary fraction across the full extent of six Galactic globular clusters (GCs) spanning a wide range of dynamical ages, from dynamically young systems to core-collapsed clusters. We measured the radial variation of the binary fraction using a combination of deep optical HST observations and wide-field ground-based data. For the first time, we provide evidence that the binary fraction in GCs does not decrease monotonically with radius, as commonly assumed, but instead exhibits a bimodal distribution characterized by an excess in the outer regions. Specifically, the binary fraction displays a central peak, followed by a minimum at intermediate radii and a rising branch beyond approximately 1–2 half-light radii. The position of this minimum correlates with the cluster relaxation timescale, indicating that it is shaped by long-term dynamical effects of two-body relaxation driving binary evolution, segregation, and disruption. The minimum radius also correlates with the A ⁺ _rh parameter derived from the radial distribution of blue straggler stars, an empirical indicator of a cluster’s dynamical age, further supporting the interpretation that this feature is due to internal dynamical processes. Numerical simulations presented in a companion paper show that such bimodal distributions naturally arise from the combined effects of binary disruption and mass segregation of the surviving binaries in clusters.

The distinct dynamical environments occupied by multiple stellar populations in globular clusters play a significant role in many aspects of cluster evolution, including the dynamics of binary stars. Recent observational analyses of Galactic globular clusters have revealed a bimodal radial trend in the binary fraction: the fraction of binaries is enhanced in both the central and outer regions of clusters with a noticeable minimum in the cluster’s intermediate regions. In this work, we investigated the dynamical origin of this feature and the mechanisms responsible for preserving this bimodality for an extended portion of a cluster’s lifetime. We utilized a suite of Monte Carlo simulations that follow the long-term dynamical evolution of both single-population and multiple-population globular clusters. We investigated how mass segregation and binary disruption function cohesively to produce a bimodal profile and show that although single-population clusters can briefly generate a similar profile, the effect is weak and transient. Conversely, we show that the structural properties associated with multiple stellar populations significantly strengthen and preserve the bimodality. We also show that the effects of long-term dynamical evolution drive a broad relationship between a cluster’s dynamical age and the radial location of the binary fraction minimum, which migrates outward over time. Overall, our results strongly indicate that the typical multi-scale structure of multiple-population globular clusters (initially characterized by a dense and centrally concentrated subsystem of second-population stars embedded in a more extended first-population system) plays a key role in the origin of the observed bimodal profiles. Our results further demonstrate the analytical power of binary stars as dynamical tracers of globular cluster formation and dynamical evolution.

Binary millisecond pulsars (MSPs) in globular clusters (GCs) are key for binary and stellar evolution studies under extreme conditions. The identification of their optical companion stars is instrumental in order to characterise these systems and to constrain the possible recycling mechanisms. For this work, we searched for the optical counterpart to PSR J1717+4308A (hereafter M92A) in the GC M92. To this end, we exploited a multi-epoch, multi-wavelength dataset obtained with the Hubble Space Telescope. We constructed colour–magnitude diagrams, investigated proper motions to assess cluster membership, and modelled the observed light curves. We identified an object located at only 0.02 arcsec from the nominal radio position as the likely optical companion to M92A. The star is significantly bluer than the main sequence at the same luminosity level and exhibits clear photometric variability with a periodicity in agreement with the orbital motion of the binary. The light curve displays two maxima and two minima, indicative of strong tidal distortion and only mild irradiation. Such mild irradiation is consistent with the ratio of the pulsar spin-down to the companion flux ( f _sd ), which for M92A lies close to the boundary between ellipsoidal- and irradiation-dominated regimes ( f _sd ≈ 2.71). From the light curve modelling we inferred the main physical properties of the companion star. The best-fit model indicates a high-inclination system with a relatively low-mass companion and a massive neutron star. With a base temperature of ~7200 K, the companion ranks among the hottest redbacks known to date. This object therefore adds additional pieces to the puzzle of MSP companion properties and contributes to outlining the characteristics of redbacks across the different classes.

In this paper, we present the chemical screening of the complex stellar population discovered in the Bulge Fossil Fragment Liller 1. This study is part of the Bulge Cluster Origin (BulCO) survey based on a Large Program at the ESO-Very Large Telescope with the high-resolution spectrograph CRIRES+. The survey is aimed at performing an unprecedented chemical screening of 17 stellar systems orbiting the Milky Way bulge, with the ultimate goal of unveiling their origin and true nature. We measured precise chemical abundances of iron, CNO, iron-peak, α-elements, a few other light elements, and neutron-capture elements for a sample of 30 red giant branch stars, kinematic members of Liller 1. The presented analysis provides the high-resolution spectroscopic proof of the complex chemistry of this massive stellar system, with multimetallicity subpopulations of different ages that nicely fit into a self-enrichment scenario. We find no evidence for the Na–O anticorrelation associated with genuine globular clusters; rather, the overall abundance trends are similar to those seen in the bulge field and in Terzan 5, providing definitive evidence of an in situ formation of Liller 1 within the Galactic bulge.

We present a ‘multiple-pass’ data-reduction tool designed for Euclid , based on software developed for the Hubble Space Telescope (HST), which improves the astrometric and photometric precision for faint sources and in crowded fields. In this work, we apply it to Euclid Early Release Observations of the Galactic globular cluster NGC 6397. By combining our new catalogue with archival HST data, separated by a time span of approximately 20 years, we were able to measure high-precision proper motions and investigate the radial variations in the energy equipartition and velocity anisotropy of the cluster. The combination of deep and wide-field observations also allowed us to derive the present-day local mass function of NGC 6397 and to study the radial dependence of mass segregation and binary fraction. Finally, we report the discovery of a subtle under-density of stars in the colour-magnitude diagram of NGC 6397 around a stellar mass of 0.35 M _⊙ with a more than 5 σ confidence level. This feature is consistent with the Gaia M-dwarf gap discovered in Galactic field stars, but it has never previously been observed in a globular cluster. The gap is caused by the onset of full convection in stellar interiors. We demonstrate that the properties of the gap provide tight constraints on the distance to NGC 6397 and its intrinsic metallicity dispersion, offering a new benchmark for stellar evolution models.

PSR J1905+0154A is a binary millisecond pulsar (MSP) located in the globular cluster (GC) NGC 6749. It was discovered in 2004 in a search for pulsars in GCs carried out with the Arecibo 305-m radio telescope. The pulsar has a spin period of 3.2 ms, an orbital period of 0.81 days, and is in a low-eccentricity orbit with a low-mass white dwarf (WD) companion. By combining timing data from the early Arecibo observations of NGC 6749 with timing data from observations of this GC made with the Five-hundred-meter Aperture Spherical Telescope (FAST), we derived a phase-coherent timing solution for this pulsar, which now spans 20 years. This solution includes a precise measurement of the astrometric, spin, and orbital parameters of the system. The small range of predicted accelerations expected from the gravitational field of this GC allows us to give an estimate of the intrinsic spin-down: the inferred magnetic field at the surface (2.2 - 2.4 times 10^8, G) and characteristic age (2.8 - 3.5 Gyr) are typical of what one finds among MSPs in the Galactic field. The position of this pulsar, the only confirmed to date in this GC, coincides with the position of one of the very few candidate WDs in the whole HST dataset on this GC. The position of the companion in the colour-magnitude diagram is consistent with a helium WD with a mass of 0.17 - 0.19, (which implies an orbital inclination between 28 and 40 degrees), a cooling age of 0.4 - 0.7 Gyr, and a surface temperature of 11,600 - 14,800 K. A comparison with the characteristic age of the pulsar indicates that at the time of Roche lobe detachment, the spin period was between 1.98 and 2.62 ms. The relatively large proper motion difference relative to the motion of the GC, which is 4.5-σ significant and an order of magnitude larger than the escape velocity, raises the possibility that, despite its location close to the centre of the GC, the pulsar might not be associated with it. Finally, our effort to confirm a second pulsar candidate in this GC did not yield a positive confirmation, nor the discovery of any additional pulsar in this GC.

In this paper, we present the chemical screening of the stellar population belonging to the globular cluster NGC 6553 in the Galactic bulge. This study has been conducted in the contest of the Bulge Cluster Origin (BulCO) survey, an ESO-VLT Large Program that is currently ongoing with CRIRES in the near-infrared (NIR) domain. This survey is performing an unprecedented chemical screening of 17 stellar systems orbiting the Milky Way bulge, with the aim of unveiling their origin and true nature. Here we present and discuss the abundances of 18 elements produced via distinct nucleosynthetic channels for 14 red giant branch stars belonging to NGC 6553. We found a mean [Fe/H] = −0.20 ± 0.01 deX, and about solar-scaled iron-peak elements, confirming that this is one of the most metal-rich globular clusters in the Milky Way. We also found [X/Fe] enhancement of α and several other light elements. Furthermore, we assessed the presence of multiple populations typical of genuine globular clusters from the significant spreads in Na, N, and C, along with an almost vertical Na-O anticorrelation. Finally, by using classical ([α/Fe] versus [Fe/H]) and newly-defined ([V/Fe] and [Zn/Fe] versus [Fe/H]) “chemical DNA tests”, we have proven its in situ formation within the Galactic bulge.

In this paper, we present the results of a detailed photometric analysis of Glimpse-C02, one of the most extincted globular clusters of the Milky Way. We built a deep color magnitude diagram spanning ≈10 magnitudes and enabling the very first identification of the main sequence turnoff of the cluster. Due to the extreme reddening conditions of the region where the stellar system is located, a differential reddening correction was necessary. The resulting reddening map shows color excess variations up to δE ( B − V ) ≈ 2.5 mag in the direction of the target. From isochrone-fitting of the differential reddening corrected color-magnitude diagram, we obtained a new estimate of the mean color excess, E(B−V) = 6.33 _−0.04 ^+0.05 , and a distance modulus (m−M) ₀ = 14.00 _−0.11 ^+0.26 , corresponding to a distance of d = 6.3 _−0.3 ^+0.8 kpc from the Sun, and a Galactocentric distance of 2.6 _−0.7 ^+0.6 kpc. This distance value, within the associated uncertainties, suggests that the cluster may be located closer to the Galactic Center compared to previous estimates, possibly supporting its classification as a bulge globular cluster. Furthermore, we obtained a photometric metallicity estimate of [Fe/H] = −0.30 _−0.08 ^+0.10 and the first absolute age determination for Glimpse-C02, resulting in t = 11.9 _−0.6 ^+0.7 Gyr, as typically measured for Galactic globular clusters at this metallicity. We also obtained a new estimate of the center of gravity of the cluster and determined its projected density profile from resolved star counts, finding a high King concentration parameter ( c = 1.97 _−0.67 ^+0.51 ) and a core radius r _c = 8.72 _−0.35 ^+0.40 arcsec. Finally, from the surface brightness profile of the system, we derived an integrated H -band magnitude M _H = −7.9, corresponding to a mass of M = 3.57 _−0.19 ^+0.22 × 10 ⁴ M _⊙ . Thus, our work classifies Glimpse-C02 as an old and metal-rich globular cluster that is in an advanced stage of its dynamical evolution.