Dhanraj Risbud

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.

Context. 47 Tucanae is one of the largest, brightest, and closest globular clusters to Earth. It hosts an exotic stellar population with stellar dynamics that indicate a complex evolution history. The cluster contains a large number of X-ray binaries and millisecond pulsars. Aims. Searching for pulsars in this cluster has been an ongoing effort over more than two decades with fruitful results. However, given its large distance relative to the known pulsar population, previous surveys have found only the very brightest sources. Therefore, surveys with increased sensitivity should find many additional pulsars. Increasing the number of pulsars is crucial to investigate the dynamics of this globular cluster and could also lead to the discovery of unusual types of system. Methods. With a significantly increased sensitivity compared to earlier telescopes, MeerKAT is the natural choice to perform new surveys. We carried out two campaigns with different observational cadences to account for the high scintillation along the line of sight to this cluster. All observations were carried out using multi-beam filter-banking user-supplied equipment to increase efficiency and localisation. Results. Here we report the discovery of fifteen new pulsars in 47 Tucanae with MeerKAT. These discoveries bring the total number of known pulsars in this globular cluster to 42, and the MeerKAT discoveries in this cluster to 17. We discuss some of their characteristics, which include preliminary localisations and estimates of orbits for most systems. Highlights include the discovery of 47 Tuc af, a ‘black widow’ pulsar with a short orbital period that was identified optically in 2002 as a candidate binary pulsar, and 47 Tuc ai, an eccentric binary pulsar with a massive companion, a unique system in 47 Tuc to date. Apart from the new systems, we also re-detect and localise 47 Tuc P and V, two elusive, seldom-detected systems that had no precise localisation from a phase-connected timing solution. The localisation of 47 Tuc V places it in a position consistent with a continuum source detected earlier in MeerKAT imaging data.

In this paper we present PSR J0024−7204ai, a 13.026-ms binary pulsar recently discovered in the globular cluster 47 Tucanae by the MeerKAT radio telescope. This is the slowest-spinning pulsar known in this globular cluster. It has a ∼1.67-day orbit with an eccentricity of e ≈ 0.18. Although it is not yet possible to derive an unambiguous phase-connected timing solution, by combining detections obtained from MeerKAT and archival Parkes data, we were able to measure the rate of advance of periastron to a high significance: ω ˙ = 0.1601 ± 0.0046 deg yr ⁻¹ . This value implies a total system mass of 2.41 ± 0.11 M _⊙ (68.3% C.L.), which, when combined with the binary mass function, gives a maximum pulsar mass of ∼1.7 M _⊙ and a minimum companion mass of ∼0.7 M _⊙ . Apart from being the slowest pulsar in 47 Tucanae, its orbit is by far the most eccentric, and its companion is the most massive among all known binary pulsars in this globular cluster. It is possible that the system is an old millisecond pulsar–carbon-oxygen white dwarf binary whose orbit was perturbed by stellar dynamical interactions in the cluster core. Further follow-up observations of this system will be essential to obtaining a more detailed characterisation of this system and its evolution.

Context . Recent studies using Gaia data have reported tidal tail detections for tens to hundreds of open clusters. However, a comprehensive assessment of the reliability and completeness of these detections is lacking. Aims . This work aims to summarise the expected properties of tidal tails based on N -body simulations, review the reliability of tidal tail detections in the literature, and grade them according to a set of diagnostic tests. We also provide an overview of the general characteristics of tidal tails available in the literature. Methods . We used a grid of 68–20 000 M _☉ simulated clusters and analysed the formation and evolution of the tidal tails. We compiled 122 catalogues from the recent literature, encompassing 58 unique clusters within 500 pc of the Sun. We employed various tests based on photometric, morphological, and dynamical signatures and comparisons with simulated clusters to grade the tidal tails as gold, silver, and bronze. One of the primary tests was to measure apparent torsion in the Galactocentric XY plane. Results . Based on the simulations, we analysed the complex morphology of the tidal tails and their properties (such as their size, span, stellar types, number density, and mass function) at various cluster masses and ages. During the first 100−200 Myr of evolution, the tails typically form a characteristic Ƨ shape, with an amplitude that scales with cluster mass. The tail span increases at a rate of ≈ 4 times the initial velocity dispersion, and the near-tail (within 100 pc of the cluster) is predominantly populated by recent escapees. In evaluating 122 published tidal tail catalogues, we found that 15 gold-quality catalogues and 55 silver-quality catalogues passed the majority of the tests. The remaining 51 catalogues were graded as bronze; care should be taken before using these catalogues for further analysis. The age, metallicity, binary fraction, and mass function of stars in the tails were generally consistent with those of their parent clusters. Conclusions . The simulations presented here provide first-order approximations of the structure and evolution of the tidal tails. The gold and silver-grade catalogues (69 catalogues of 40 clusters) represent reliable samples for detailed analyses of tidal tails. Future spectroscopic and astrometric data from large-scale surveys will be essential for further validation and for leveraging tidal tails as tracers of cluster dissolution and the Galactic potential.

Context: Recent studies using Gaia data have reported tidal tail detections for tens to hundreds of open clusters. However, a comprehensive assessment of the reliability & completeness of these detections is lacking. Aims: This work aims to summarise the expected properties of tidal tails based on N-body simulations, review the reliability of tidal tail detections in the literature, and grade them according to a set of diagnostic tests. Methods: We used a grid of 68-20000 Msun simulated clusters & analysed the formation & evolution of the tails. We compiled 122 catalogues (58 unique clusters) from literature, within 500 pc of the Sun. We employed tests based on photometric, morphological & dynamical signatures, & comparisons with simulations, to grade their tails. Results: Based on the simulations, we analysed the complex morphology of the tails & their properties (e.g., size, span, stellar types, number density, & mass function) at various cluster masses & ages. During the first 200 Myr of evolution, the tails typically form a characteristic S shape, with an amplitude that scales with cluster mass. The tail span increases at a rate of 4 times the initial velocity dispersion, & the near-tail is predominantly populated by recent escapees. Conclusions: In the 122 published tail catalogues, we found that 15 gold & 55 silver catalogues passed the majority of the tests. The remaining 51 were graded as bronze; care should be taken before using these for further analysis. The age, metallicity, binary fraction, & mass function of stars in the tails were generally consistent with those of their parent clusters. The gold/silver catalogues (69 catalogues of 40 clusters) represent reliable samples for detailed analyses of tails. Future data will be essential for further validation & for leveraging tidal tails as tracers of cluster dissolution & the Galactic potential.

Context. Tidal tails of open clusters are the result of stellar evaporation from the cluster through the Galactic potential and internal dynamics. With the recent availability of high-precision data, tidal tails are being detected for most of the nearby open clusters. Aims. We identify the tidal tail members for all open clusters within a distance of 400 pc that are older than 100 Myr and have >100 members. To do this, we use model-independent methods. Methods. We used the convergent-point (CP) method to identify the co-moving stars near the open clusters using Gaia DR3 data. A new method called the self-compact convergent-point method was proposed and applied to some of the clusters. It performed better overall in tracing the tails. We also analysed the colour-magnitude diagrams and orbital energy to diagnose possible contamination. Results. Nineteen out of 21 clusters have tidal tails. Five of them were discovered for the first time through this work. The typical span of the tidal tails is 20–200 pc, and 30–700 member stars lie in the region inside the tidal radius and the tidal tails. Four out of 19 tidal tails are tilted away from direction of the Galactic centre. This contradicts the known theory of the tidal-tail formation. The luminosity functions of the tails and clusters are consistent with each other and with the canonical stellar interstellar mass function, but systematically higher radial velocities for the trailing tail than for the leading tail were observed for the first time. Conclusions. The CP method is useful for detecting tidal tails on a scale of ≈100 pc for clusters closer than 400 pc. A further analysis of theoretical N-body models is required to understand the incompleteness and biases in the current sample of tidal tails.

Context: Tidal tails of open clusters are the result of stellar evaporation from the cluster through the Galactic potential and internal dynamics. With the recent availability of high-precision data, tidal tails are being detected for most of the nearby open clusters. Aims: We identify the tidal tail members for all open clusters within a distance of 400 pc that are older than 100 Myr and have $>$100 members. To do this, we use model-independent methods. Methods: We used the convergent-point (CP) method to identify the co-moving stars near the open clusters using Gaia DR3 data. A new method called the self-compact convergent-point (SCCP) method was proposed and applied to some of the clusters. It performed better overall in tracing the tails. We also analysed the colour-magnitude diagrams and orbital energy to diagnose possible contamination. Results: Nineteen out of 21 clusters have tidal tails. Five of them were discovered for the first time through this work. The typical span of the tidal tails is 20--200 pc, and 30--700 member stars lie in the region inside the tidal radius and the tidal tails. Four out of 19 tidal tails are tilted away from direction of the Galactic centre. This contradicts the known theory of the tidal-tail formation. The luminosity functions of the tails and clusters are consistent with each other and with the canonical stellar interstellar mass function, but systematically higher radial velocities for the trailing tail than for the leading tail were observed for the first time. Conclusions: The CP method is useful for detecting tidal tails on a scale of approx. 100 pc for clusters closer than 400 pc. A further analysis of theoretical N-body models is required to understand the incompleteness and biases in the current sample of tidal tails.