Mauricio Reyes-Ruiz

Accretion Disks in Pre-Planetary Nebulae

A number of planetary nebulae (PNs) exhibit collimated, high-velocity outflows or jets. These hydrodynamical structures cannot be easily accommodated within the classical models of the evolution of post-asymptotic giant branch stars, and understanding them has become a topical problem in PN research. One way to explain the existence of jets in PNs has been to invoke the presence of accretion disks, which would presumably set the conditions for the collimation and driving of the outflows. This work investigates in detail the type of binary systems that are likely to lead to Roche-lobe overflow (RLOF) and the formation of accretion disks as a consequence of common-envelope evolution, and explores the expected basic physical structure of such disks. The results of the analysis show substantial restrictions on the composition of binary systems that can form a disk upon accretion onto the primary. Typically, it is found that for a primary asymptotic giant branch (AGB) core of 0.6 M☉ and envelope mass of 2-3 M☉, secondaries with M2 0.08 M☉ and initial separation ai 200 R☉ will not lead to RLOF. For systems that do lead to RLOF, this is achieved at orbital separations <2 R☉. We also find that dynamically stable mass transfer from secondaries with M2 0.08 M☉ does not lead to disk formation, since the circularization radius lies below the surface of the AGB core. Only lower mass companions, after a dynamically unstable mass transfer process, may lead to disk formation. Under reasonable simplifying assumptions, we estimate the resulting accretion disk properties and evolution and discuss their potential role in driving collimated outflows.

Evolution of magnetized protoplanetary disks

We investigate the global evolution of a turbulent protoplanetary disk in its viscous stage, incorporating the effects of Maxwell stress due to a large-scale magnetic field permeating disk. We assume that the viscous stress is given by an alpha model. A magnetic field is produced contemporaneously by an alpha omega dynamo mechanism and the resultant Maxwell stress assists the viscous stress in providing the means for disk evolution. The aim of this work is to compare the evolution of magnetized and nonmagnetized disks driven by turbulent viscosity of the same magnitude and thus assess the effects of a self-generated magnetic field on the structure and dynamical evolution of protoplanetary disks. Two illustrative examples corresponding to two different initial conditions are considered: a high-mass case that starts with a disk of 0.245 solar mass and angular momentum of 5.6 x 10(exp 52)g sq cm/s, and a low-mass that case starts with a disk of 0.11 solar mass and angular momentum of 1.8 x 10(exp 52)g sq cm/s. For each of these two cases the radial development of a disk is calculated numerically assuming a fiducial value of the dimensionless viscosity parameter alpha(sub ss) = 0.01, as well as alpha(sub ss) = 2 x 10(exp -3). In all cases the central star has a mass equal to 1 solar mass. The most striking feature of magnetized disk evolution is the presence of the surface density bulge located in the region of the disk where the dynamo mechanism cannot support a magnetic field. The bulge persists for a time of the order of 10(exp 5)-10(exp 6) yr. The presence and persistence of the surface density bulge may have important implications for the process of planet formation and the overall characteristics of resultant planetary systems.

The Magnetorotational Instability across the Dead Zone of Protoplanetary Disks

We examine the linear stability of a flow threaded by a weak, vertical magnetic field in a disk with a Keplerian rotation profile and a vertical stratification of the ionization degree as that predicted for vast portions of protoplanetary disks. A quasi-global analysis is carried out, in which the form of the perturbations in the vertical direction is determined. Considering the ohmic magnetic diffusivity of the gas, the conditions leading to the magnetorotational instability are analyzed as a function of the diffusivity at the disk surfaces, its vertical profile, and the strength of the unperturbed magnetic field. For typical conditions believed to prevail in protoplanetary disks at radial distances between 0.1 and 10 AU, where the so-called dead zone is proposed to exist, we find that generally the instability is damped. This implies that, if the MRI is considered the only possible source of turbulence in protoplanetary disks, no viscous angular momentum transport occurs at those radii.

CONSTRAINING THE MAGNETIC EFFECTS ON H I ROTATION CURVES AND THE NEED FOR DARK HALOS

The density profiles of dark halos are usually inferred from the rotation curves of disk galaxies based on the assumption that the gas is a good tracer of the gravitational potential of the galaxies. Some authors have suggested that magnetic pinching forces could alter significantly the rotation curves of spiral galaxies. In contrast to other studies that have concentrated on the vertical structure of the disk, here we focus on the problem of magnetic confinement in the radial direction to determine the magnetic effects on the H I rotation curves. It is shown that azimuthal magnetic fields hardly speed up H I disks of galaxies as a whole. In fact, based on virial constraints we show that the contribution of galactic magnetic fields to the rotation curves cannot be larger than ~10 km s-1 at the outermost point of H I detection, if the galaxies did not contain dark matter at all, and is up to 20 km s-1 in the conventional dark halo scenario. The procedure to estimate the maximum effect of magnetic fields is general and applicable to any particular galaxy disk. The inclusion of the surface terms, namely, the intergalactic (thermal, magnetic, or ram) pressure, does not change our conclusions. Other problems related to the magnetic alternative to dark halos are highlighted. The relevance of magnetic fields in the cuspy problem of dark halos is also discussed.

Dynamics of escaping Earth ejecta and their collision probabilities with different Solar System bodies

Abstract It has been suggested that the ejection of terrestrial crustal material to interplanetary space, accelerated in a large impact, may result in the interchange of biological material between Earth and other Solar System bodies. In this paper, we analyze the fate of debris ejected from Earth by means of direct numerical simulations of the dynamics of a large collection of test particles. This allows us to determine the probability and conditions for the collision of Earth ejecta with other planets of the Solar System. We also estimate the amount of particles falling back to Earth and colliding with the Moon as a function of time after being ejected. The Mercury-6 code is used to compute the dynamics of test particles under the gravitational effect of the planets in the Solar System and the Sun. A series of simulations are conducted with different ejection speeds, considering more than 10 5 particles in each case. We find that in general, the collision rates of Earth ejecta with Venus and the Moon, as well as the fall-back rates, are within an order of magnitude of results reported in the literature. By considering a larger number of particles than in all previous calculations we have also determined, on the basis of direct numerical simulations, the collision probability with Mars and, for the first time, computed collision probabilities with Jupiter and Saturn. We find that the collision probability with Mars is greater than values determined from collision cross section estimations previously reported.

STABILITY OF THE OUTER PLANETS IN MULTIRESONANT CONFIGURATIONS WITH A SELF-GRAVITATING PLANETESIMAL DISK

We study the effect of a massive planetesimal disk on the dynamical stability of the outer planets assuming, as has been suggested recently, that these were initially locked in a compact and multiresonant configuration as a result of gas-driven migration in a protoplanetary disk. The gravitational interaction among all bodies in our simulations is included self-consistently using the Mercury6.5 code. Several initial multiresonant configurations and planetesimal disk models are considered. Under such conditions a strong dynamical instability, manifested as a rapid giant planet migration and planetesimal disk dispersal, develops on a timescale of less than 40 Myr in most cases. Dynamical disk heating due to the gravitational interactions among planetesimals leads to more frequent interactions between the planetesimals and the ice giants Uranus and Neptune, in comparison to models in which planetesitmal-planetesimal interactions are neglected. On account of the rapid evolution of the multiresonant configurations obtained with fully self-consistent simulations, our results are inconsistent with the dynamical instability origin of the Late Heavy Bombardment as currently considered by the Nice model for the Solar System.

Numerical simulation of viscous-like flow in and around the plasma tail of a comet

Aims. We model the interaction of the solar wind with the plasma tail of a comet using numerical simulations, taking into account the effects of viscous-like forces. Methods. We developed a 2D hydrodynamical, two species, finite difference code to solve the time-dependent continuity, momentum, and energy conservation equations, and model the interaction of the solar wind with a cometary plasma tail. We compute the evolution of the plasma of cometary origin in the tail as well as the properties of the shocked solar wind plasma around it, as it transfers momentum on its passage by the tail. Velocity, density and temperature profiles across the tail are obtained. Several models with different flow parameters are considered to study the relative importance of viscous-like effects and the coupling between species on the flow dynamics. Results. Assuming a Mach number equal to 2 for the incident solar wind as it flows past the comet’s nucleus, the flow exhibits three transitions with location and properties depending on the Reynolds number of each species and on the ratio of the timescale for interspecies coupling to the crossing time of the free-flowing solar wind. By comparing our results with the measurements taken in situ by the Giotto spacecraft during its flyby of comet Halley, we constrain the flow parameters for both plasmas. Conclusions. In the context of our approximations, we find that our model is qualitatively consistent with the in situ measurements as long as the Reynolds number of both solar wind protons and cometary H2O+ ions is low, less than 100, suggesting that viscous-like momentum transport processes may play an important role in the interaction of the solar wind and the plasma environment of comets.

Chaotic dynamics of Comet 1P/Halley: Lyapunov exponent and survival time expectancy

The orbital elements of comet Halley are known to a very high precision, suggesting that the calculation of its future dynamical evolution is straightforward. In this paper we seek to characterize the chaotic nature of the present day orbit of comet Halley and to quantify the timescale over which its motion can be predicted condently. In addition, we attempt to determine the timescale over which its present day orbit will remain stable. Numerical simulations of the dynamics of test particles in orbits similar to that of comet Halley are carried out with the Mercury 6.2 code. On the basis of these we construct survival time maps to assess the absolute stability of Halley’s orbit, frequency analysis maps, to study the variability of the orbit and we calculate the Lyapunov exponent for the orbit for variations in initial conditions at the level of the present day uncertainties in our knowledge of its orbital parameters. On the basis of our calculations of the Lyapunov exponent for comet Halley, the chaotic nature of its motion is demonstrated. The e-folding timescale for the divergence of initially very similar orbits is approximately 70 years. The sensitivity of the dynamics on initial conditions is also evident in the self-similarity character of the survival time and frequency analysis maps in the vicinity of Halley’s orbit, which indicates that, on average, it is unstable on a timescale of hundreds of thousands of years. The chaotic nature of Halley’s present day orbit implies that a precise determination of its motion, at the level of the present day observational uncertainty, is dicult to predict on a timescale of approximately 100 years. Furthermore, we also nd that the ejection of Halley from the solar system or its collision with another body could occur on a timescale as short as 10,000 years.

DYNAMICAL HEATING INDUCED BY DWARF PLANETS ON COLD KUIPER BELT–LIKE DEBRIS DISKS

With the use of long-term numerical simulations, we study the evolution and orbital behavior of cometary nuclei in cold Kuiper belt-like debris disks under the gravitational influence of dwarf planets (DPs); we carry out these simulations with and without the presence of a Neptune-like giant planet. This exploratory study shows that in the absence of a giant planet, 10 DPs are enough to induce strong radial and vertical heating on the orbits of belt particles. On the other hand, the presence of a giant planet close to the debris disk, acts as a stability agent reducing the radial and vertical heating. With enough DPs, even in the presence of a Neptune-like giant planet some radial heating remains; this heating grows steadily, re-filling resonances otherwise empty of cometary nuclei. Specifically for the solar system, this secular process seems to be able to provide material that, through resonant chaotic diffusion, increase the rate of new comets spiraling into the inner planetary system, but only if more than the

Solar wind‐driven plasma fluxes from the Venus ionosphere

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