Rachid Ouyed

Numerical Simulations of Astrophysical Jets from Keplerian Disks. II. Episodic Outflows

We present 2.5-dimensional time-dependent simulations of the nonlinear evolution of nonrelativistic outNows from Keplerian accretion disks orbiting low-mass protostars or black holes accreting at sub- Eddington rates. The gas is ejected from the surface of the disk (which is a -xed platform in these simulations) into a cold corona in stable equilibrium. The initial magnetic -eld lines are taken to be uniform and parallel to the disk axis (z-axis). Because of the gradient force in the nonlinear torsional Alfvec n waves generated by the rotor at the footpoints of the -eld lines, the initial magnetic con-guration opens up in a narrow region on the diskIs surface located at with being the innermost r i \ r \ 2r i r i radius of the disk. Within this narrow region, a wind is ejected from the -eld lines that have opened to less than the critical angle (^60i), as expected from the centrifugally driven wind theory. Our simula- tions show that the strong toroidal magnetic -eld generated recollimates the Now toward the diskIs axis and, through magnetohydrodynamic (MHD) shocks, produces knots. The knot generation mechanism occurs at a distance of about from the surface of the disk. Knots propagate down the length of z ^ 8r i the jet at speeds less than the di†use component of the outNow. The knot generator is episodic and is inherent to the jet. Subject headings: accretion, accretion disks E galaxies: jets E ISM: jets and outNows E MHD

Three-dimensional simulations of the reorganization of a quark star's magnetic field as induced by the meissner effect

In a previous paper (Ouyed et al. 2004) we presented a new model for soft gamma-ray repeaters (SGR), based on the onset of colour superconductivity in quark stars. In this model, the bursts result from the reorganization of the exterior magnetic field following the formation of vortices that confine the internal magnetic field (the Meissner effect). Here we extend the model by presenting full 3-dimensional simulations of the evolution of the inclined exterior magnetic field immediately following vortex formation. The simulations capture the violent reconnection events in the entangled surface magnetic field as it evolves into a smooth, more stable, configuration which consists of a dipole field aligned with the star’s rotation axis. The total magnetic energy dissipated in this process is found to be of the order of 10 44 erg and, if it is emitted as synchrotron radiation, peaks typically at 280keV. The intensity decays temporally in a way resembling SGRs and AXPs (anomalous X-ray pulsars), with a tail lasting from a few to a few hundred times the rotation period of the star, depending on the initial inclination between the rotation and dipole axis. One of the obvious consequences of our model’s final state (aligned rotator) is the suppression of radio-emission in SGRs and AXPs following their bursting era. We suggest that magnetar-like magnetic field strength alone cannot be responsible for the properties of SGRs and AXPs, while a quark star entering the “Meissner phase” is compatible with the observational facts. We compare our model to observations and highlight our predictions.

Pricing European Options with a Log Student's t-Distribution: a Gosset Formula

The distributions of returns for stocks are not well described by a normal probability density function (pdf). Student’s t-distributions, which have fat tails, are known to fit the distributions of the returns. We present pricing of European call or put options using a log Student’s t-distribution, which we call a Gosset approach in honour of W.S. Gosset, the author behind the nom de plume Student. The approach that we present can be used to price European options using other distributions and yields the Black–Scholes formula for returns described by a normal pdf.

D-D FUSION IN THE INTERIOR OF JUPITER?

One of the still uncertain and debated questions about Jupiter is the origin of its excess heat. Understanding the source of such heat will certainly shed some light on the physics of the interior of the planet and on scenarios of its formation. Recent measurements of sound velocities in Jupiter show substantial disagreement with the existing models for the Jovian interior. Analysis of these measurements suggests that helium (He) sedimentation (through H-He phase separation) is plausible in the planets interior, contrary to what is believed from numerical calculations of H-He mixture at conditions prevailing in Jupiters deep interior. This signals the need for a revision of the existing models of Jupiter and allows for new models to be explored. While He sedimentation might help shift the calculated sound velocities toward the observed ones, we find that it cannot explain the excess heat. Here, we analyze the consequences of deuterium (D) sedimentation on Jupiters excess heat and discuss its effects on the sound profiles. Such a sedimentation is assumed to have occurred in the early stages of planet formation (here the core-instability model) through planetesimal vaporization in the deeper parts of the envelope. Our interest in investigating D sedimentation resulted from the recent extrapolations of D-D, D-T, D-3He, and p-D fusion to electron volt temperatures, which indicate that D-D fusion largely dominates the other reactions under conditions thought to prevail in the interior of early Jupiter. We find that with a modest degree of interior stratification of D (5%-15% of the total D of the planet), D-D burning naturally explains the excess heat given off by the planet. For our model to operate, we find that D sedimentation must occur during the early stages of planet formation (core-instability scenario) when interior temperatures of 16-18 eV where available. Our model is applied to the family of the Jovian planets as a whole.

QUARK-NOVAE, COSMIC REIONIZATION, AND EARLY r-PROCESS ELEMENT PRODUCTION

We examine the case for quark-novae (QNe) as possible sources for the reionization and early metal enrichment of the universe. QNe are predicted to arise from the explosive collapse (and conversion) of sufficiently massive neutron stars into quark stars (QSs). A QN can occur over a range of timescales following the supernova (SN) event. For QNe that arise days to weeks after the SNe, we show that dual shock that arises as the QN ejecta encounter the SN ejecta can produce enough photons to reionize hydrogen in most of the intergalactic medium (IGM) by z ~ 6. Such events can explain the large optical depth τ e ~ 0.1 as measured by WMAP, if the clumping factor, C, of the material being ionized is smaller than 10. We suggest a way in which a normal initial mass function for the oldest stars can be reconciled with a large optical depth as well as the mean metallicity of the early IGM post reionization. We find that QN also make a contribution to r-process element abundances for atomic numbers A ≥ 130. We predict that the main cosmological signatures of QNe are the gamma-ray bursts that announce their birth. These will be clustered at redshifts in the range z ~ 7-8 in our model.

Cyclical Variability in MHD Disk Winds

Jets in star forming regions are observed to be intrinsically time-dependent phenomena whose episodic eruptions lead to moving knots in the flow. We demonstrate using time-dependent magnetohydrodynamical simulations that magnetic fields anchored in a gaseous accretion disks orbiting a star can accelerate and collimate a bipolar jet which originates from the surface of the disk. Moreover, for certain magnetic field geometries, we find that episodic eruptions originate within the jet close to the central object. The link between the accretion disk and the wind variability is analysed.

FORBIDDEN LINE DIAGNOSTICS OF DISK WINDS IN YSOS

The last decade of work on bipolar molecular outflows and optical jets from the vicinity of young stellar objects (≡ YSOs) has established that most, if not all stars go through an outflow phase during their formation. CO outflows are the earliest signposts of star formation and may continue to be active for up to 2 × 105 yrs, (see Parker, Padman, and Scott 1991). This time scale is also approximately the free-fall time for the gaseous cores in which stars form