arXiv subjects

Massimo Ricotti

Publications and source records attributed to Massimo Ricotti.

110 records · Page 7Linked to original sources

arXiv1998

On Nonshearing Magnetic Configurations in Differentially Rotating Disks

A new class of disk MHD equilibrium solutions is described, which is valid within the standard local (``shearing sheet'') approximation scheme. These solutions have the following remarkable property: velocity streamlines and magnetic lines of force rotate rigidly, even in the presence of differential rotation. This situation comes about because the Lorentz forces acting upon modified epicycles compel fluid elements to follow magnetic lines of force. Field line (and streamline) configurations may be elliptical or hyperbolic, prograde or retrograde. These structures have previously known hydrodynamical analogs: the ``planet'' solutions described by Goodman, Narayan, & Goldreich. The primary focus of this investigation is configurations in the disk plane. A related family of solutions lying in a vertical plane is briefly discussed; other families of solutions may exist. Whether these MHD structures are stable is not yet known, but could readily be determined by three-dimensional simulations. If stable or quasi-stable, these simple structures may find important applications in both accretion and galactic disks.

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arXiv1997

Energy Dissipation in Interstellar Cloud Collisions

We present a study of the kinetic energy dissipation in interstellar cloud collisions. The main aim is to understand the dependence of the elasticity (defined as the ratio of the final to the initial kinetic energy of the clouds) on the velocity and mass ratio of the colliding clouds, magnetic field strength, and gas metallicity for head-on collisions. The problem has been studied both analytically and via numerical simulations. We have derived handy analytical relationships that well approximate the analogous numerical results. The main findings of this work are: (i) the kinetic energy dissipation in cloud collisions is minimum (i.e. the collision elasticity is maximum) for a cloud relative velocity $v_r \simeq 30 km s^{-1}$; (ii) the above minimum value is proportional $Z L_c^2$, where $Z$ is the metallicity and $L_c$ is the cloud size: the larger is $Z L_c^2$ the more dissipative (i.e. inelastic) the collision will be; (iii) in general, we find that the energy dissipation decreases when the magnetic field strength, and mass ratio of the clouds are increased and the metallicity is decreased, respectively. We briefly discuss the relevance of this study to the global structure of the interstellar medium and to galaxy formation and evolution.

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