arXiv subjects

R. P. Nelson

Publications and source records attributed to R. P. Nelson.

28 records · Page 2Linked to original sources

arXiv2008

An Investigation on the Morphological Evolution of Bright-Rimmed Clouds

A new Radiative Driven Implosion (RDI) model based on Smoothed Particle Hydrodynamics (SPH) technique is developed and applied to investigate the morphological evolutions of molecular clouds under the effect of ionising radiation. This model self-consistently includes the self-gravity of the cloud in the hydrodynamical evolution, the UV radiation component in the radiation transfer equations, the relevant heating and cooling mechanisms in the energy evolution and a comprehensive chemical network. The simulation results reveal that under the effect of ionising radiation, a molecular cloud may evolve through different evolutionary sequences. Dependent on its initial gravitational state, the evolution of a molecular cloud does not necessarily follow a complete morphological evolution sequence from type A to B to C, as described by previous RDI models. When confronted with observations, the simulation results provide satisfactory physical explanations for a series of puzzles derived from Bright-Rimmed Clouds (BRCs) observations. The consistency of the modelling results with observations shows that the self-gravity of a molecular cloud should not be neglected in any investigation on the dynamical evolution of molecular clouds when they are exposed to ionising radiation.

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arXiv2006

A comparative study of disc-planet interaction

We perform numerical simulations of a disc-planet system using various grid-based and smoothed particle hydrodynamics (SPH) codes. The tests are run for a simple setup where Jupiter and Neptune mass planets on a circular orbit open a gap in a protoplanetary disc during a few hundred orbital periods. We compare the surface density contours, potential vorticity and smoothed radial profiles at several times. The disc mass and gravitational torque time evolution are analyzed with high temporal resolution. There is overall consistency between the codes. The density profiles agree within about 5% for the Eulerian simulations while the SPH results predict the correct shape of the gap although have less resolution in the low density regions and weaker planetary wakes. The disc masses after 200 orbital periods agree within 10%. The spread is larger in the tidal torques acting on the planet which agree within a factor 2 at the end of the simulation. In the Neptune case the dispersion in the torques is greater than for Jupiter, possibly owing to the contribution from the not completely cleared region close to the planet.

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arXiv2006

Triggered star formation in Bright-Rimmed Clouds: The Eagle Nebula revisited

A three dimensional Smoothed Particle Hydrodynamical (SPH) model has been extended to study the radiative driven implosion effect of massive stars on the dynamical evolutions of surrounding molecular clouds. The new elements in the upgraded code are the inclusion of Lyman continuum in the incident radiation flux and the treatment of hydrogen ionisation process; introducing ionisation heating & recombination cooling effects; and adding a proper description of the magnetic and turbulent pressures to the internal pressure of the molecular cloud. The application of this newly developed model to the structure of the middle Eagle Nebula finger suggests that the shock induced by the ionising radiation at the front side of the head precedes an ionisation front moving towards the center of the core, and that the core at the fingertip is at transition stage evolving toward a state of induced star formation. The dynamical evolution of the velocity field of the simulated cloud structure is discussed to illustrate the role of the self-gravity and the different cloud morphologies which appear at different stages in the evolutionary process of the cloud. The modelled gas evaporation rate is consistent with that of current other models and the density, temperature and chemical profiles are agreement with the observed values. The relative lifetimes of different simulated cloud morphologies suggests a possible answer to the question of why more bright-rimmed clouds are observed to possess a flat-core than an elongated-core morphology. [Abridged]

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arXiv2006

Disk-Planet Interactions During Planet Formation

The discovery of close orbiting extrasolar giant planets led to extensive studies of disk planet interactions and the forms of migration that can result as a means of accounting for their location. Early work established the type I and type II migration regimes for low mass embedded planets and high mass gap forming planets respectively. While providing an attractive means of accounting for close orbiting planets intially formed at several AU, inward migration times for objects in the earth mass range were found to be disturbingly short, making the survival of giant planet cores an issue. Recent progress in this area has come from the application of modern numerical techniques which make use of up to date supercomputer resources. These have enabled higher resolution studies of the regions close to the planet and the initiation of studies of planets interacting with disks undergoing MHD turbulence. This work has led to indications of how the inward migration of low to intermediate mass planets could be slowed down or reversed. In addition, the possibility of a new very fast type III migration regime, that can be directed inwards or outwards, that is relevant to partial gap forming planets in massive disks has been investigated.

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arXiv2001

On disc driven inward migration of resonantly coupled planets with application to the system around GJ876

We consider two protoplanets gravitationally interacting with each other and a protoplanetary disc. The two planets orbit interior to a tidally maintained disc cavity while the disc interaction indices inward migration. When the migration is slow enough, the more rapidly migrating outer protoplanet approaches and becomes locked in a 2:1 commensurability with the inner one. This is maintained in subsequent evolution. We study this evolution using a simple anaytic model, full hydrodynamic 2D simulations of the disc planet system and longer time N body integrations incorporating simple prescriptions for the effect of the disc on the planet orbits. The eccentricity of the protoplanets are found to be determined by the migration rate induced in the outer planet orbit by the external disc. We apply our results to the recently discovered resonant planets around GJ876. Simulation shows that a disc with parameters expected for protoplanetary discs causes trapping in the 2:1 commensurability when the planets orbit in an inner cavity and that eccentricities in the observed range may be obtained.

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arXiv2001

Dynamical modeling of large scale asymmetries in the beta Pictoris dust disk

We report a new and complete model of the beta Pictoris disk, which succeeds in accounting for both the surface brightness distribution, warp characteristics, the outer ``butterfly'' asymmetry as observed by HST/STIS in scattered light, as well as the infrared emission. Our model includes the presence of a disk of planetesimals extending out to 120-150 AU, perturbed gravitationally by a giant planet on an inclined orbit, following the approach of Mouillet et al. (1997b). At any time, the planetesimal disk is assumed to be the source of a distribution of grains produced through collisional evolution, with the same initial orbital parameter distribution. The steady state spatial grain distribution is found incorporating the effects of radiation pressure which can cause the distribution of the smallest particles to become very distended. With realistic assumptions about the grains' chemical properties, the modeling confirms the previously evident need for an additional population of hot grains close to the star, to account for the 12 microns fluxes at short distances from the star. It also indicates that this population cannot explain the outer 12 microns flux distribution when the effects of gravity and radiation pressure determine the distribution. Very small grains, produced by collisions among aggregates, are tentatively proposed to account for this 12 microns outer emission.

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arXiv1998

On the tidal interaction of a solar-type star with an orbiting companion: Excitation of g mode oscillation and orbital evolution

We calculate the dynamical tides raised on a non-rotating solar-type star by a close stellar or planetary companion. Dissipation arising from a turbulent viscosity operating in the convection zone and radiative damping in the radiative core are considered. We compute the torque exerted on the star by a companion in circular orbit, and determine the potentially observable magnitude of the tidally induced velocity at the stellar photosphere. These calculations are compared with the results obtained by assuming that a very small frequency limit can be taken in order to calculate the tidal response (equilibrium tide). For a standard solar model, the latter is found to give a relatively poor approximation at the periods of interest of several days, even when the system is far from resonance with a normal mode. It is shown that although the companion may go through a succession of resonances as it spirals in under the action of the tides, for a fixed spectrum of normal modes its migration is controlled essentially by the non-resonant interaction. We find that the turbulent viscosity that is required to provide the observed circularization rates of main sequence solar-type binaries is about fifty times larger than that simply estimated from mixing length theory for non-rotating stars. We discuss the means by which this enhanced viscosity might be realized. These calculations are applied to 51 Pegasi. We show that the perturbed velocity induced by the tides at the stellar surface is too small to be observed.

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arXiv1997

Oscillations in solar-type stars tidally induced by orbiting planets

We examine the effect of dynamical tides raised by a companion on a solar-type star. In these binaries, gravity or g mode oscillations are excited by the companion in the radiative region beneath the convective envelope of the star. They become evanescent in the convection zone. This is of particular interest in connection with the newly discovered planets, some of which are found to orbit around solar-type stars with a period comparable to that of the high order g modes of the star. One such example is 51 Pegasi. Here, we determine the magnitude of the perturbed velocity induced by the tides at the stellar surface. We show that, in the case of 51 Pegasi, this velocity is too small to be observed. This result is insensitive to the magnitude of the stellar turbulent viscosity assumed and is not affected by the possibility of resonance, which occurs when the frequency of the tidal disturbance is close to that of some normal mode of the star. We also discuss the orbital evolution and synchronization timescales associated with the tidal interaction.

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arXiv1997

Precessing warped discs in close binary systems

We describe some recent nonlinear three dimensional hydrodynamic simulations of accretion discs in binary systems where the orbit is circular and not necessarily coplanar with the disc midplane. The calculations are relevant to a number of observed astrophysical phenomena, including the precession of jets associated with young stars, the high spectral index of some T Tauri stars, and the light curves of X-ray binaries such as Hercules X-1 which suggest the presence of precessing accretion discs.

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arXiv1996

The tidally induced warping, precession and truncation of accretion discs in binary systems: three-dimensional simulations

We present the results of non linear, hydrodynamic simulations, in three dimensions, of the tidal perturbation of accretion discs in binary systems where the orbit is circular and not necessarily coplanar with the disc mid-plane. The accretion discs are assumed to be geometrically thin, and of low mass relative to the stellar mass so that they are governed by thermal pressure and viscosity, but not self-gravity. The parameters that we consider in our models are the ratio of the orbital distance to the disc radius, D/R, the binary mass ratio, the initial inclination angle between the orbit and disc planes and the Mach number in the outer parts of the unperturbed disc. For binary mass ratios of around unity and D/R in the range 3 to 4, we find that the global evolution of the discs is governed primarily by the value of the Mach number. For relatively low Mach numbers (i.e. 10 to 20) we find that the discs develop a mildly warped structure, are tidally truncated, and undergo a near rigid body precession at a rate which is in close agreement with analytical arguments. For higher Mach numbers (approx. 30), the evolution is towards a considerably more warped structure, but the disc none the less maintains itself as a long-lived, coherent entity. Additionally, it is found that the inclination angle between the disc and orbital angular momentum vectors evolves on a longer timescale which is probably the viscous evolution timescale of the disc. The calculations are relevant to a number of observed astrophysical phenomena, including the precession of jets associated with young stars, the high spectral index of some T-Tauri stars, and the light curves of X-ray binaries such as Hercules X-1 which suggest the presence of precessing accretion discs.

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