A. Santillan

The Collisions of HVCs with a Magnetized Gaseous Galactic Disk

Resumen en: We discuss 2-D MHD numerical simulations for the interaction of high-velocit y clouds with a magnetized Galactic disk. The initial magnetic field is orie...

The Parker Instability in a Thick Galactic Gaseous Disk. I. Linear Stability Analysis and Nonlinear Final Equilibria

A linear stability analysis of a multicomponent and magnetized Galactic disk model is presented. The disk model uses the observed stratifications for the gas density and gravitational acceleration at the solar neighborhood, and in this sense it can be called a realistic model. The distribution of the total gas pressure is defined by these observed stratifications, and the gaseous disk is assumed isothermal. The initial magnetic field is taken parallel to the disk, with a midplane value of 5 μG, and its stratification along the z-axis is derived from the condition of magnetohydrostatic equilibrium in an isothermal atmosphere. The resulting isothermal sound speed is ~8.4 km s-1, similar to the velocity dispersion of the main gas components within 1.5 kpc from the midplane. The thermal-to-magnetic pressure ratio decreases with [z], and the warm model is Parker unstable. The dispersion relations show that the fastest growing mode has a wavelength of about 3 kpc, for both symmetric and antisymmetric perturbations, and the corresponding growth timescales are of about 3 × 107 yr. The structure of the final equilibrium stage is also derived, and we find that the midplane antisymmetric (MA) mode gathers more gas in the magnetic valleys. The resulting MA gas condensations have larger densities, and the column density enhancement is a factor of about 3 larger than the value of the initial stage. The unstable wavelengths and growth times for the multicomponent disk model are substantially larger than those of a thin-disk model, and some of the implications of these results are discussed.

THE PARKER INSTABILITY IN A THICK GASEOUS DISK. II. NUMERICAL SIMULATIONS IN TWO DIMENSIONS

We present 2D, ideal–MHD numerical simulations of the Parker instability in a multi–component warm disk model. The calculations were done using two numerical codes with different algorithms, TVD and ZEUS-3D. The outcome of the numerical experiments performed with both codes is very similar, and confirms the results of the linear analysis for the undular mode derived by Kim et al. (2000): the most unstable wavelength is about 3 kpc and its growth timescale is between 30–50 Myr (the growth rate is sensitive to the position of the upper boundary of the numerical grid). Thus, the time and length scales of this multicomponent disk model are substantially larger than those derived for thin disk models. We use three different types of perturbations, random, symmetric, and antisymmetric, to trigger the instability. The antisymmetric mode is dominant, and determines the minimum time for the onset of the nonlinear regime. The instability generates dense condensations and the final peak column density value in the antisymmetric case, as also derived by Kim et al. (2000), is about a factor of 3 larger than its initial value. These wavelengths and density enhancement factors indicate that the instability alone cannot be the main formation mechanism of giant molecular clouds in the general interstellar medium. The role of the instability in the formation of large-scale corrugations along spiral arms is briefly discussed.

Exploring cloudy gas accretion as a source of interstellar turbulence in the outskirts of disks

High-resolution two-dimensional MHD numerical simulations have been carried out to investigate the effects of the continuing infall of clumpy gas in extended H I galactic disks. Given a certain accretion rate, the response of the disk depends on its surface gas density and temperature. For Galactic conditions at a galactocentric distance of ~20 kpc, and for mass accretion rates consistent with current empirical and theoretical determinations in the Milky Way, the rain of compact high-velocity clouds onto the disk can maintain transonic turbulent motions in the warm phase (~2500 K) of H I. Hence, the H I line width is expected to be ~6.5 km s-1 for a gas layer at 2500 K, if infall is the only mechanism driving the turbulence. Some statistical properties of the resulting force flow are shown in this Letter. The radial dependence of the gas velocity dispersion is also discussed.

A Numerical Study on the Evolution of CMEs and Shocks in the Interplanetary Medium

We studied the evolution in the solar wind of four CMEs detected by SOHO‐LASCO which were associated with ICMEs and interplanetary (IP) shocks detected afterward by Wind at 1 AU. The study is based on a 1‐D hydrodynamic single fluid model using the ZEUS code. These simple numerical simulations of CME like pulses illuminate several aspects of the heliocentric evolution of the ICME front and its associated IP shock and we were able to reproduce some characteristics of the IP shocks and ICMEs inferred from the two‐point measurements from spacecraft. The simulation shows that ICMEs and IP shocks follow different evolutions in the interplanetary medium both having phases of about constant speed propagation followed by an exponential deceleration with heliocentric distance. IP shocks always propagate faster than their associated ICME drivers and the former began to decelerate well before the IP shock. The results indicate that, in general, although an IP shock is driven by its ICME in the inner heliosphere in m...

INTERACTION OF HIGH VELOCITY CLOUDS WITH MAGNETIZED DISKS: THREE-DIMENSIONAL NUMERICAL SIMULATIONS

High-velocity clouds are flows of neutral hydrogen, located at high galactic latitudes, with large velocities () that do not match a simple model of circular rotation for our Galaxy. Numerical simulations have been performed for the last 20 years to study the details of their evolution, and their possible interaction with the Galactic disk. Here we present a brief review of the models that have been already published, and describe newly performed three-dimensional magnetohydrodynamic simulations.

Magnetic fields: impact on the rotation curve of the Galaxy

We quantify the effects of magnetic fields, cosmic rays and gas pressure on the rotational velocity of HI gas in the Milky Way, at galactic distances between Rsun and 2Rsun. The magnetic field is modelled by two components; a mainly azimuthal magnetic component and a small-scale tangled field. We construct a range of plausible axisymmetric models consistent with the strength of the total magnetic field as inferred from radio synchrotron data. In a realistic Galactic disk, the pressure by turbulent motions, cosmic rays and the tangled turbulent field provide radial support to the disk. Large-scale (ordered) magnetic fields may or may not provide support to the disk, depending on the local radial gradient of the azimuthal field. We show that for observationally constrained models, magnetic forces cannot appreciably alter the tangential velocity of HI gas within a galactic distance of 2Rsun.

Torques on Low-mass Bodies in Retrograde Orbit in Gaseous Disks

We evaluate the torque acting on a gravitational perturber on a retrograde circular orbit in the midplane of a gaseous disk. We assume that the mass of this satellite is so low it weakly disturbs the disk (type I migration). The perturber may represent the companion of a binary system with a small mass ratio. We compare the results of hydrodynamical simulations with analytic predictions. Our two-dimensional (2D) simulations indicate that the torque acting on a perturber with softening radius

Co-existence and switching between fast and Ω-slow wind solutions in rapidly rotating massive stars

R_{\rm soft}

TURBULENCE IN THE OUTSKIRTS OF THE MILKY WAY

can be accounted for by a scattering approach if