Keywords

The occurrence of planetary nebulae (PNe) in globular clusters (GCs) provides an excellent chance to study low-mass stellar evolution in a special (low-metallicity, high stellar density) environment. We report a systematic spectroscopic survey for the [O iii] 5007 Å emission line of PNe in 1469 Virgo GCs and 121 Virgo ultra-compact dwarfs (UCDs), mainly hosted in the giant elliptical galaxies M87, M49, M86, and M84. We detected zero PNe in our UCD sample and discovered one PN (${M}_{5007}=-4.1\,\mathrm{mag}$) associated with an M87 GC. We used the [O iii] detection limit for each GC to estimate the luminosity-specific frequency of PNe, α, and measured α in the Virgo cluster GCs to be $\alpha \sim {3.9}_{-0.7}^{+5.2}\times {10}^{-8}\,\mathrm{PN}/{L}_{\odot }$. The value of α in the Virgo GCs is among the lowest reported in any environment, due in part to the large sample size, and it is 5–6 times lower than that for the Galactic GCs. We suggest that α decreases toward brighter and more massive clusters, sharing a similar trend as the binary fraction, and the discrepancy between the Virgo and Galactic GCs can be explained by the observational bias in extragalactic surveys toward brighter GCs. This low but nonzero efficiency in forming PNe may highlight the important role played by binary interactions in forming PNe in GCs. We argue that a future survey of less massive Virgo GCs will be able to determine whether PN production in the Virgo GCs is governed by an internal process (mass, density, binary fraction) or if it is largely regulated by the external environment.

We report results on the kinematics of Milky Way (MW) globular clusters (GCs) based on updated space velocities for nearly the entire GC population. We found that a 3D space with the semimajor axis, the eccentricity, and the inclination of the orbit with respect to the MW plane as its axes, is helpful in order to dig into the formation of the GC system. We find that GCs formed in situ show a clear correlation between their eccentricities and their orbital inclination in the sense that clusters with large eccentricities also have large inclinations. These GCs also show a correlation between their distance to the MW center and their eccentricity. Accreted GCs do not exhibit a relationship between eccentricity and inclination, but span a wide variety of inclinations at eccentricities larger than ∼0.5. Finally, we computed the velocity anisotropy β of the GC system and found, for GCs formed in situ, that β decreases from ≈0.8 down to 0.3 from the outermost regions toward the MW center, but remains fairly constant (0.7–0.9) for accreted ones. These findings can be explained if GCs formed from gas that collapsed radially in the outskirts, with a preference for relatively high infall angles. As the material reached the rotating forming disk, it became more circular and moved with a lower inclination relative to the disk. Half of the GC population was accreted and deposited in orbits covering the entire range of energies from the outer halo to the bulge.

Tidally disrupted globular cluster (GC) streams are usually observed, and therefore perceived, as narrow, linear, and one-dimensional structures in the 6D phase space. Here, we show that the GD-1 stellar stream, which is the tidal debris of a disrupted GC, possesses a secondary diffuse and extended stellar component (∼100 pc wide) around it, detected at the >5σ confidence level. Similar morphological properties are seen in synthetic streams that are produced from star clusters that are formed within dark matter sub-halos and then accrete onto a massive host galaxy. This lends credence to the idea that the progenitor of the highly retrograde GD-1 stream was originally formed outside of the Milky Way in a now defunct dark satellite galaxy. We deem that in future studies, this newly found cocoon component may serve as a structural hallmark to distinguish between the in situ and ex situ (accreted) formed GC streams.

In a dense stellar environment, such as the core of a globular cluster (GC), dynamical interactions with black holes (BHs) are expected to lead to a variety of astrophysical transients. Here we explore tidal disruption events (TDEs) of stars by stellar-mass BHs through collisions and close encounters. Using state-of-the-art cluster simulations, we show that these TDEs occur at significant rates throughout the evolution of typical GCs and we study how their relative rates relate to cluster parameters such as mass and size. By incorporating a realistic cosmological model of GC formation, we predict a BH–main-sequence-star TDE rate of approximately 3 Gpc⁻³ yr⁻¹ in the local universe (z < 0.1) and a cosmological rate that peaks at roughly 25 Gpc⁻³ yr⁻¹ for redshift 3. Furthermore, we show that the ejected mass associated with these TDEs could produce optical transients of luminosity ∼10⁴¹−10⁴⁴ erg s⁻¹ with timescales of about a day to a month. These should be readily detectable by optical transient surveys such as the Zwicky Transient Facility. Finally, we comment briefly on BH–giant encounters and discuss how these events may contribute to the formation of BH–white-dwarf binaries.

We present results of a search for variable stars in a region of the globular cluster NGC 4147 based on photometric observations with a 4K × 4K CCD imager mounted at the axial port of the recently installed 3.6 m Devasthal optical telescope at Aryabhatta Research Institute of Observational Sciences, Nainital, India. We performed time series photometry of NGC 4147 in the V and R bands, and identified 42 periodic variables in the region of NGC 4147, 28 of which have been detected for the first time. Seventeen variable stars are located within the half-light radius ≲048, of which 10 stars are newly identified variables. Two of the 10 variables are located within the core radius ≲009. Based on their location in the V/(V − R) color–magnitude diagram and variability characteristics, seven, eight, five, and one newly identified probable member variables are classified as RRc, EA/E, EW, and SX Phe, respectively. The metallicity of NGC 4147 estimated from the light curves of RRab and RRc stars with the help of Fourier decomposition is found to be characteristic of Oosterhoff II. The distance derived using the light curves of RRab stars is consistent with that obtained from the observed V/(V − R) color–magnitude diagram.

By adopting empirical estimates of the helium enhancement (ΔY) between consecutive stellar generations for a sample of Galactic globular clusters (GGCs), we uniquely constraint the star formation efficiency () of each stellar generation in these stellar systems. In our approach, the star formation efficiency () is the central factor that links stellar generations as it defines both their stellar mass and the remaining mass available for further star formation, fixing also the amount of matter required to contaminate the next stellar generation. In this way, is here shown to be fully defined by the He enhancement between successive stellar generations in a GC. Our approach also has an impact on the evolution of clusters and thus considers the possible loss of stars through evaporation, tidal interactions and stellar evolution. We focus on the present mass ratio between consecutive stellar generations (M_(j−1)G/M_(j)G) and the present total mass of GGCs (M_GC). Such considerations suffice to determine the relative proportion of stars of consecutive generations that remain today in globular clusters (α_(j−1)G/α_(j)G). The latter is also shown to directly depend on the values of ΔY and thus the He enhancement between consecutive stellar generations in GGC places major constraints on models of star formation and evolution of GC.

Resonant relaxation has been discussed as an efficient process that changes the angular momenta of stars orbiting around a central supermassive black hole due to the fluctuating gravitational field of the stellar cluster. Other spherical stellar systems, such as globular clusters, exhibit a restricted form of this effect where enhanced relaxation rate only occurs in the directions of the angular momentum vectors, but not in their magnitudes; this is called vector resonant relaxation (VRR). To explore this effect, we performed a large set of direct N-body simulations, with up to 512k particles (where k =1024) and ∼500 dynamical times. Contrasting these simulations, which naturally include the collective effects, with Spitzer-style Monte Carlo simulations, which by design only exhibit two-body relaxation, we show that the temporal behavior of the angular momentum vectors in N-body simulations cannot be explained by two-body relaxation alone. VRR operates efficiently in globular clusters with N > 10⁴. The fact that VRR operates in globular clusters may open a way to use powerful tools in statistical physics for their description. In particular, since the distribution of orbital planes relaxes much more rapidly than the distribution of the magnitude of angular momentum and the radial action, the relaxation process reaches an internal statistical equilibrium in the corresponding part of phase space while the whole cluster is generally out of equilibrium, in a state of quenched disorder. We point out the need to include effects of VRR in Monte Carlo simulations of globular clusters.

Using deep, high-resolution optical imaging from the Next Generation Virgo Cluster Survey, we study the properties of nuclear star clusters (NSCs) in a sample of nearly 400 quiescent galaxies in the core of Virgo with stellar masses 10⁵ ≲ ${\text{}}{M}_{* }$/${\text{}}{M}_{\odot }$ ≲ 10¹². The nucleation fraction reaches a peak value f_n ≈ 90% for ${\text{}}{M}_{* }$ ≈ 10⁹ ${\text{}}{M}_{\odot }$ galaxies and declines for both higher and lower masses, but nuclei populate galaxies as small as ${\text{}}{M}_{* }$ ≈ 5 × 10⁵ ${\text{}}{M}_{\odot }$. Comparison with literature data for nearby groups and clusters shows that at the low-mass end nucleation is more frequent in denser environments. The NSC mass function peaks at M_NSC ≈ 7 × 10⁵ ${\text{}}{M}_{\odot }$, a factor 3–4 times larger than the turnover mass for globular clusters (GCs). We find a nonlinear relation between the stellar masses of NSCs and those of their host galaxies, with a mean nucleus-to-galaxy mass ratio that drops to M_NSC/M_⋆ ≈ 3.6 × 10⁻³ for ${\text{}}{M}_{* }$ ≈ 5 × 10⁹ ${\text{}}{M}_{\odot }$ galaxies. Nuclei in both more and less massive galaxies are much more prominent: ${M}_{\mathrm{NSC}}\propto {M}_{* }^{0.46}$ at the low-mass end, where nuclei are nearly 50% as massive as their hosts. We measure an intrinsic scatter in NSC masses at a fixed galaxy stellar mass of 0.4 dex, which we interpret as evidence that the process of NSC growth is significantly stochastic. At low galaxy masses we find a close connection between NSCs and GC systems, including very similar occupation distributions and comparable total masses. We discuss these results in the context of current dissipative and dissipationless models of NSC formation.

Recent investigations of the double red clump in the color–magnitude diagram of the Milky Way bulge cast serious doubts on the structure and formation origin of the outer bulge. Unlike previous interpretation based on an X-shaped bulge, stellar evolution models and CN-band observations have suggested that this feature is another manifestation of the multiple stellar population phenomenon observed in globular clusters (GCs). This new scenario requires a significant fraction of the outer bulge stars with chemical patterns uniquely observed in GCs. Here we show from homogeneous high-quality spectroscopic data that the red giant branch stars in the outer bulge (>55 from the Galactic center) are clearly divided into two groups according to Na abundance in the [Na/Fe]−[Fe/H] plane. The Na-rich stars are also enhanced in Al, while the differences in O and Mg are not observed between the two Na groups. The population ratio and the Na and Al differences between the two groups are also comparable with those observed in metal-rich GCs. The only plausible explanation for these chemical patterns and characteristics appears to be that the outer bulge was mostly assembled from disrupted proto-GCs in the early history of the Milky Way.