Scale-free dynamical models for galaxies: flattened densities in spherical potentials

This paper presents two families of phase-space distribution functions (DFs) that generate scale-free spheroidal mass densities in scale-free spherical potentials. The `case I' DFs are anisotropic generalizations of the flattened f(E,L_z) model, which they include as a special case. The `case II' DFs generate flattened constant-anisotropy models, in which the constant ratio of rms tangential to radial motion is characterized by Binney's parameter beta. The models can describe the outer parts of galaxies and the density cusp structure near a central black hole, but also provide general insight into the dynamical properties of flattened systems. The dependence of the intrinsic and projected properties on the model parameters and the inclination is described. The observed ratio of the rms projected line-of-sight velocities on the projected major and minor axes of elliptical galaxies is best fit by the case II models with beta > 0. These models also predict non-Gaussian velocity profile shapes consistent with existing observations. The distribution functions are used to model the galaxies NGC 2434 (E1) and NGC 3706 (E4), for which stellar kinematical measurements out to two effective radii indicate the presence of dark halos (Carollo et al.). The velocity profile shapes of both galaxies can be well fit by radially anisotropic case II models with a spherical logarithmic potential. This contrasts with the f(E,L_z) models studied previously, which require flattened dark halos to fit the data.Comment: LaTeX file, uses standard macros mn.sty, epsf.sty. 17 pages including 7 figure

A refurbished convergent point method for finding moving groups in the Hipparcos Catalogue

The Hipparcos data allow a major step forward in the research of `moving groups' in the Solar neighbourhood, as the common motion of group members causes converging proper motions. Previous knowledge on these coherent structures in velocity space has always been limited by the availability, reliability, and accuracy of ground-based proper motion measurements. A refurbishment of Jones' convergent point method is presented which takes full advantage of the quality of the Hipparcos data. The original implementation of this method determines the maximum likelihood convergent point on a grid on the sky and simultaneously selects group members from a given set of stars with positions and proper motions. The refurbished procedure takes into account the full covariance matrix of the Hipparcos measurements instead of standard errors only, allows for internal motions of the stars, and replaces the grid-based approach by a direct minimization. The method is tested on Monte Carlo simulations of moving groups, and applied to the Hyades. Despite the limited amount of data used by the convergent point method, the results for stars in and around the cluster- centre region agree very well with those of the recent comprehensive study by Perryman et al. (1998).Comment: 14 pages, 7 Postscript figures, LaTeX using mn.sty and psfig.sty; accepted for publication in MNRA

Fine structure in the luminosity function in young stellar populations with

Context. A pioneering study showed that the fine structure in the luminosity function (LF) of young star clusters contains information about the evolutionary stage (age) and composition of the stellar population. The notable features include the H-peak, which is the result of the onset of hydrogen burning turning pre-main sequence stars into main sequence stars. The feature moves toward the faint end of the LF, and eventually disappears as the population evolves. Another detectable feature is the Wielen dip, a dip at MV ≃ 7 mag in the LF first identified in 1974 for stars in the solar environment. Later studies also identified this feature in the LF of star clusters. The Wielen dip is caused by the increased importance of H− opacity in a certain range of low-mass stars. Aims. We studied the detailed structure in the luminosity function using the data from Gaia DR2 and PARSEC stellar evolution models with the aim to further our understanding of young stellar populations. Methods. We analyzed the astrometric properties of stars in the solar neighborhood (< 20 pc) and in various relatively nearby (< 400 pc) young (< 50 Myr) open clusters and OB associations, and compare the features in the luminosity function with those generated by PARSEC models. Results. The Wielen dip is confirmed in the LF of all the populations, including the solar neighborhood, at MG ≃ 7 mag. The H-peak is present in the LF of the field stars in the solar neighborhood. It likely signals that the population is mixed with a significant number of stars younger than 100 Myr. The H-peak is found in the LF of young open clusters and OB associations, and its location varies with age. The PARSEC evolutionary models predict that the H-peak moves from ∼ − 1 mag towards ∼ + 6 mag in MG for populations with ages increasing from 1 to ∼70 Myr. Our observations with Gaia DR2 confirm the evolution of the H-peak from ∼5 Myr up to ∼47 Myr. We provide a calibration function between MG and age that works in the age domain between 1 and 30 Myr. Conclusions. The fine structure in the luminosity function in young stellar populations can be used to estimate their age

The 800 pc long tidal tails of the Hyades star cluster

The tidal tails of stellar clusters provide an important tool for studying the birth conditions of the clusters and their evolution, coupling, and interaction with the Galactic potential. The Gaia satellite, with its high-quality astrometric data, opened this field of study, allowing us to observe large-scale tidal tails. Theoretical models of tidal-tail formation and evolution are available. However, the exact appearance of tidal features as seen in the Gaia catalogue has not yet been studied. Here we present the N-body evolution of a Hyades-like stellar cluster with backward-integrated initial conditions on a realistic 3D orbit in the Milky Way galaxy computed within the AMUSE framework. For the first time, we explore the effect of the initial cluster rotation and the presence of lumps in the Galactic potential on the formation and evolution of tidal tails. For all of our simulations we present Gaia observables and derived parameters in the convergent point (CP) diagram. We show that the tidal tails are not naturally clustered in any coordinate system and that they can span up to 40 km s−1 relative to the cluster centre in proper motions for a cluster age of 600–700 Myr. Models with initial rotation result in significant differences in the cluster mass loss and follow different angular momentum time evolution. Thus the orientation of the tidal tails relative to the motion vector of the cluster and the current cluster angular momentum constrain the initial rotation of the cluster. We highlight the use of the standard CP method in searches for co-moving groups and introduce a new compact CP (CCP) method that accounts for internal kinematics based on an assumed model. Using the CCP method, we are able to recover candidate members of the Hyades tidal tails in the Gaia Data Release 2 and early Data Release 3 (eDR3) reaching a total extent of almost 1 kpc. We confirm the previously noted asymmetry in the detected tidal tails. In the eDR3 data we recovered spatial overdensities in the leading and trailing tails that are kinematically consistent with being epicyclic overdensities and thus would present candidates for the first such detection in an open star cluster. We show that the epicyclic overdensities are able to provide constraints not only on the cluster properties, but also on the Galactic potential. Finally, based on N-body simulations, a close encounter with a massive Galactic lump can explain the observed asymmetry in the tidal tails of the Hyades