Luis Aguilar

On the evolution of globular cluster systems. I - Present characteristics and rate of destruction in our Galaxy

The current dynamic state and rate of destruction for the system of globular clusters in the Galaxy are modeled. It is found that gravitational shocks due to the central bulge of the Galaxy are very efficient in destroying clusters on highly radial orbits. This effect, extrapolated at the present unweighted destruction rate over 10 to the 10th yr, can account for the observed difference between the velocity distribution of globular clusters and other tracers of the halo population. 44 references.

Circumstellar and circumbinary discs in eccentric stellar binaries

We study the existence of stable trajectories, where gas could accumulate to form accretion discs, around stars that form binary systems in eccentric orbits. Since the potential is time dependent, no fixed, periodic, close orbits exist. Instead, we search for invariant loops: closed curves that change shape in synchronism with the binary orbital phase. Non-intersecting loops can provide the scaffolding for circumstellar and circumbinary discs in these systems. We investigate the range of regions in phase space where these non-intersecting loops can exist and find this to depend on both, the mass ratio of the stars and their orbital eccentricity, with a strong dependence on the latter. The recent discovery of planets within close binary systems makes this work very relevant.

The structure and dynamics of galaxies formed by cold dissipationless collapse

The observable characteristics and phase-space structure of systems formed by dissipationless collapse are investigated. An extensive set of N-body simulations is described; and three-dimensional and spherical collapses, their shapes, and dynamics are examined. It is demonstrated that collapses started from cold initial conditions are quantitatively different from warm collapses, in the sense that their final shapes are nearly uncorrelated with their initial shapes. It is suggested that this phenomenon is due to an instability similar to the radial-orbit instability in anisotropic equilibrium models. The results show that clumpy initial conditions are not required to produce a realistic final state via collapse, if the initial state is sufficiently cold; and that if elliptical galaxies collapsed from cold and smooth initial configurations (and did not evolve significantly thereafter), they should be triaxial or prolate, with intrinsic flattening of approximately 2:1. 24 refs.

IDENTIFICATION OF MOVING GROUPS AND MEMBER SELECTION USING HIPPARCOS DATA

A new method to identify coherent structures in velocity space – moving groups – in astrometric catalogues is presented: the Spaghetti method. It relies on positions, parallaxes and proper motions, and is ideally suited to searching for moving groups in the Hipparcos Catalogue. No radial-velocity information is required. The method has been tested extensively on synthetic data, and applied to the Hipparcos measurements for the Hyades and IC 2602 open clusters. The resulting lists of members agree very well with those of Perryman et al. for the Hyades and those of Whiteoak & Braes for IC 2602.

Geometrical and physical properties of circumbinary discs in eccentric stellar binaries

In a previous work (Pichardo et al. 2005), we studied stable configurations for circumstellar discs in eccentric binary systems. We searched for “invariant loops”: closed curves (analogous to stable periodic orbits in time-independent potentials) that change shape with the binary orbital phase, as test particles in them move under the influence of the binary potential. This approach allows us to identify stable configurations when pressure forces are unimportant, and dissipation acts only to prevent gas clouds from colliding with one another. We now extend this work to study the main geometrical properties of circumbinary discs. We have studied more than 100 cases with a range in eccentricity 0 6 e 6 0.9, and mass ratio 0.1 6 q 6 0.9. Although gas dynamics may impose further restrictions, our study sets lower stable bounds for the size of the central hole in a simple and computationally cheap way, with a relation that depends on the eccentricity and mass ratio of the central binary. We extend our previous studies and focus on an important component of these systems: circumbinary discs. The radii for stable orbits that can host gas in circumbinary discs are sharply constrained as a function of the binary’s eccentricity. The circumbinary disc configurations are almost circular, with eccentricity ed < 0.15, but if the mass ratio is unequal the disk is offset from the center of mass of the system. We compare our results with other models, and with observations of specific systems like GG Tauri A, UY Aurigae, HD 98800 B, and Fomalhaut, restricting the plausible parameters for the binary.

The analysis of realistic stellar Gaia mock catalogues – I. Red clump stars as tracers of the central bar

In this first paper we simulate the population of disc Red Clump stars to be observed by Gaia. We generate a set of test particles and we evolve it in a 3D barred Milky Way like galactic potential. We assign physical properties of the Red Clump trace population and a realistic 3D interstellar extinction model. We add Gaia observational constraints and an error model according to the pre-commissioning scientific performance assessments. We present and analyse two mock catalogues, offered to the community, that are an excellent test bed for testing tools being developed for the future scientific exploitation of Gaia data. The first catalogue contains stars up to Gaia G 20, while the second is the subset containing Gaia radial velocity data with a maximum error of sigmaVr=10 kms. Here we present first attempts to characterise the density structure of the Galactic bar in the Gaia space of observables. The Gaia large errors in parallax and the high interstellar extinction in the inner parts of the Galactic disc prevent us to model the bar overdensity. This result suggests the need to combine Gaia and IR data to undertake such studies. We find that IR photometric distances for this Gaia sample allow us to recover the Galactic bar orientation angle with an accuracy of approximately 5 degrees.

Characterizing the Galactic warp with Gaia – I. The tilted ring model with a twist

We explore the possibility of detecting and characterizing the warp of the stellar disc of our Galaxy using synthetic Gaia data. The availability of proper motions and, for the brightest stars radial velocities, adds a new dimension to this study. A family of Great Circle Cell Counts (GC3) methods is used. They are ideally suited to find the tilt and twist of a collection of rings, which allow us to detect and measure the warp parameters. To test them, we use random realizations of test particles which evolve in a realistic Galactic potential warped adiabatically to various final configurations. In some cases a twist is introduced additionally. The Gaia selection function, its errors model and a realistic 3D extinction map are applied to mimic three tracer populations: OB, A and Red Clump stars. We show how the use of kinematics improves the accuracy in the recovery of the warp parameters. The OB stars are demonstrated to be the best tracers determining the tilt angle with accuracy better than

Regions of dynamical stability for discs and planets in binary stars of the solar neighbourhood

\sim0.5

Detection of Satellite Remnants in the Galactic Halo with Gaia - II. A modified Great Circle Cell Method

up to Galactocentric distance of

Habitable zones with stable orbits for planets around binary systems

\sim16