Theoretical astrophysics and plasma physics at RPC

Abstract:

An extension of Schwarzschild's galaxy-building technique is presented that enables one to build Schwarzschild models with known distribution functions (DFs). The new extension makes it possible to combine a DF that depends only on classical integrals with orbits that respect non-classical integrals. With such a combination, Schwarzschild's orbits are used only to represent the difference between the true galaxy DF and an approximating classical DF. The new method is used to construct a dynamical model of the inner Galaxy. The model is based on an orbit library that contains 22 168 regular orbits. The model aims to reproduce the three-dimensional mass density of Binney, Gerhard & Spergel, which was obtained through deprojection of the COBE surface photometry, and to reproduce the observed kinematics in three windows - namely Baade's Window with (l, b) = (10, -40) and two off-axis fields at (80, 70) and (120, 30). The viewing angle is assumed to be 200 the long axis of the bar and the pattern speed is taken to be 60km s^-1 kpc^-1. The model fits essentially all the available data within the innermost 3 kpc. The axial ratio and the morphology of the projected density contours of the COBE bar are recovered to excellent accuracy within corotation. The kinematic quantities - the line-of-sight streaming velocity and velocity dispersion, as well as the proper motions when available - are recovered, not merely for the fitted fields at (10, -40) and (80, 70), but also for three new fields at (80.4, -60), (10.21, -10.67) and (-10.14, 10.81). The dynamical model deviates most from the input density close to the Galactic plane just outside corotation, where the deprojection of the surface photometry is suspect. The dynamical model does not reproduce the kinematics at the most distant window, (120, 30), where disc contamination of the data may be severe. Maps of microlensing optical depth are presented both for randomly chosen stars and for stars that belong to individual components within the model. While the optical depth to a randomly chosen star in Baade's Window is half what measurements imply, the optical depth to stars in a particular component can be as high as the measured values. The contributions to the optical depth towards randomly chosen stars from lenses in different components are also given.

Abstract:

Possible orbital histories of the Sgr dwarf galaxy are explored. A special-purpose N-body code is used to construct the first models of the Milky Way-Sgr dwarf system in which both the Milky Way and the Sgr dwarf are represented by full N-body systems and followed for a Hubble time. These models are used to calibrate a semi-analytic model of the Sgr dwarf's orbit that enables us to explore a wider parameter space than is accessible to the N-body models. We conclude that the extant data on the Sgr dwarf are compatible with a wide range of orbital histories. At one extreme the Sgr dwarf initially possesses ∼1011 M⊙ and starts from a Galactocentric distance RD(0) ≳ 200 kpc. At the other extreme the Sgr dwarf starts with ∼109 M⊙ and RD(0) ∼60 kpc, similar to its present apocentric distance. In all cases the Sgr dwarf is initially dark matter dominated and the current velocity dispersion of the Sgr dwarf's dark matter is tightly constrained to be 21 ± 2 km s-1. This number is probably compatible with the smaller measured dispersion of the Sgr dwarf's stars because of (i) the dynamical difference between dark and luminous matter, and (ii) velocity anisotropy.

Abstract:

The stratification of entropy and the stratification of angular momentum are closely analogous. The analogy has been developed for a number of different problems in the fluid literature, but its consequences for the behavior of turbulent accretion disks are less appreciated. Of particular interest is the behavior of disks in which angular momentum transport is controlled by convection, and heat transport by dynamical turbulence. In both instances we argue that the transport must proceed "backward" relative to the sense one would expect from a simple enhanced diffusion approach. Reversed angular momentum transport has already been seen in numerical simulations; contragradient thermal diffusion should be amenable to numerical verification as well. These arguments also bear on the observed nonlinear local stability of isolated Keplerian disks: locally generated turbulence in such a disk would require simultaneous inward and outward angular momentum transport, which is, of course, impossible. We also describe a diffusive instability that is the entropy analogue to the magnetorotational instability. It affects thermally stratified layers when Coulomb conduction and a weak magnetic field are present. The plasma must be sufficiently dilute that heat is channeled only along field lines. The criterion for convective instability goes from one of upwardly decreasing entropy to one of upwardly decreasing temperature. The instability remains formally viable if radiative heat transport is also present, but the equilibrium is much more unstable if Coulomb transport is dominant. In that case, the maximum growth rate is of the order of the inverse sound crossing time, independent of the thermal conductivity. The indifference of the growth rate to the conduction coefficient, its simple dynamical scaling, and the replacement in the stability criterion of a conserved quantity (entropy) gradient by a free energy (temperature) gradient are properties similar to those exhibited by the magnetorotational instability.

Abstract:

Because we know little about the Galactic force-field away from the plane, the Galactic mass distribution is very ill-determined. I show that a microlensing survey of galaxies closer than 50 Mpc would enable us to map in three dimensions the Galactic density of stellar mass, which should be strictly less than the total mass density. A lower limit can be placed on the stellar mass needed at R

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