We present some results of numerical simulations of a globular cluster orbiting in the central region of a triaxial galaxy on a set of ''loop'' orbits. Tails start forming after about a quarter of the globular cluster orbital period and develop, in most cases, along the cluster orbit, showing clumpy substructures as observed, for example, in Palomar 5. If completely detectable, clumps can contain about 7000 M each, i.e., about 10% of the cluster mass at that epoch. The morphology of tails and clumps and the kinematical properties of stars in the tails are studied and compared with available observational data. Our finding is that the stellar velocity dispersion tends to level off at large radii, in agreement with that found for M15 and ! Cen.

The main topic of this paper is the investigation of the modes of interaction of globular clusters (GCs) moving in the inner part of a galaxy. This is tackled by means of high-resolution N -body simulations, whose first results are presented in this article. Our simulations dealt with primordial very massive (order of 10 7 M ⊙ ) GCs that were able to decay, because of dynamical friction, into the inner regions of triaxial galaxies on a time much shorter than their internal relaxation time. To check the disruptive roles of both tidal forces and GC-GC collisions, their effects were maximised by considering clusters on quasi-radial orbits and choosing the initial conditions so as to give head-on collisions at each passage through the center.The available CPU resources allowed us to simulate clusters with different structural parameters and to follow them on quasi-radial orbits during 8 passages across the center. The main findings are: i) clusters with an initial high enough King concentration parameter (c ≥ 1.2), preserve up to 50% of their initial mass; ii) the inner density distribution of the survived clusters keep a King model profile; iii) GC-GC collisions have a negligible effect with respect to that caused by the passage through the galactic center; iv) the orbital energy dissipation due to the tidal interaction is of the same order of that caused by dynamical friction; v) complex sub-structures like "ripples" and "clumps" formed, as observed around real clusters. These findings support the validity of the hypothesis of merging of GCs in the galactic central region, with modes that deserve further careful investigations.

Context. Detecting and characterising exoworlds around very young stars (age ≤10 Myr) are key aspects of exoplanet demographic studies, especially for understanding the mechanisms and timescales of planet formation and migration. Any reliable theory for such physical phenomena requires a robust observational database to be tested. However, detection using the radial velocity method alone can be very challenging because the amplitude of the signals caused by the magnetic activity of such stars can be orders of magnitude larger than those induced even by massive planets. Aims. We observed the very young (~2 Myr) and very active star V830 Tau with the HARPS-N spectrograph between October 2017 and March 2020 to independently confirm and characterise the previously reported hot Jupiter V830 Tau b (Kb = 68 ± 11 m s−1; mb sin ib = 0.57 ± 0.10 MJup; Pb = 4.927 ± 0.008 d). Methods. Because of the observed ~1 km s−1 radial velocity scatter that can clearly be attributed to the magnetic activity of V830 Tau, we analysed radial velocities extracted with different pipelines and modelled them using several state-of-the-art tools. We devised injection-recovery simulations to support our results and characterise our detection limits. The analysis of the radial velocities was aided by a characterisation of the stellar activity using simultaneous photometric and spectroscopic diagnostics. Results. Despite the high quality of our HARPS-N data and the diversity of tests we performed, we were unable to detect the planet V830 Tau b in our data and cannot confirm its existence. Our simulations show that a statistically significant detection of the claimed planetary Doppler signal is very challenging. Conclusions. It is important to continue Doppler searches for planets around young stars, but utmost care must be taken in the attempt to overcome the technical difficulties to be faced in order to achieve their detection and characterisation. This point must be kept in mind when assessing their occurrence rate, formation mechanisms, and migration pathways, especially without evidence of their existence from photometric transits.

PACS. 05.20.-y -Classical statistical mechanics. PACS. 45.50.Jf -Few-and many-body systems. PACS. 45.50.-j -Dynamics and kinematics of a particle and a system of particles.Abstract. -We study gravitational clustering of mass points in three dimensions with random initial positions and periodic boundary conditions (no expansion) by numerical simulations. Correlation properties are well defined in the system and a sort of thermodynamic limit can be defined for the transient regime of clustering. Structure formation proceeds along two paths: (i) fluid-like evolution of density perturbations at large scales and (ii) shift of the granular (non fluid) properties from small to large scales. The latter mechanism finally dominates at all scales and it is responsible for the self-similar characteristics of the clustering.One of the fundamental challenges of modern cosmology is the understanding of the formation of the structures in the Universe. These structures consist of clusters of galaxies and show complex properties extended to very large scales [1]. Usually the simulations and the models aimed at the understanding of these structures are based on three essential elements: (i) the dynamics under the effect of the gravitational forces; (ii) some particular type of initial conditions; (iii) a model for the Hubble expansion [2,3]. In addition simulations are usually run up to a time which is supposed to represent the present state.Here we would like to take inspiration from these studies and formulate the problem of clustering by gravity in the perspective of the statistical physics of dynamical systems. So we will single out the role of each individual effect at the expense of a loss in realism. We try therefore to identify simple fundamental mechanisms which can be studied in great detail and followed up to their asymptotic state. As a first problem we consider the simple, basic question: how does a random distribution of point masses evolve under gravity? The comparison with the expanding case may then allow us to identify the specific role of this effect. Simulations similar to ours, but in a cosmological context, were performed long ago, e.g., by Itoh et al. [4]. However, we are going to see that the general problematic we consider and the final interpretation will be substantially different.The main results are: (i) the existence of a well defined thermodynamic limit for the transient regime; (ii) the nature of the clustering process arising from the shift of the granular (non fluid-like) characteristics from small to large scales. (iii) the evolution of correlations shows self similar characteristics.c EDP Sciences

Synthetic and projected properties of models of globular clusters have been computed on the basis of stellar evolution and time changes of the dynamical cluster structure. Clusters with five and eight stellar groups (each group consisting of stars with the same mass) were studied. Mass loss from evolved stars was taken into account. Observational features were obtained at ages of 10-19×109 yr. The basic importance of the horizontal- and asymptotic-branch stars was pointed out. A comparison of the results with observed data of M3 is discussed with the purpose of obtaining general indications rather than a specific fit. © 1980 D. Reidel Publishing Co