Formation of a Propeller Structure by a Moonlet in a Dense Planetary Ring

The Cassini spacecraft discovered a propeller-shaped structure in Saturn's A ring. This propeller structure is thought to be formed by gravitational scattering of ring particles by an unseen embedded moonlet. Self-gravity wakes are prevalent in dense rings due to gravitational instability. Strong gravitational wakes affect the propeller structure. Here, we derive the condition for formation of a propeller structure by a moonlet embedded in a dense ring with gravitational wakes. We find that a propeller structure is formed when the wavelength of the gravitational wakes is smaller than the Hill radius of the moonlet. We confirm this formation condition by performing numerical simulations. This condition is consistent with observations of propeller structures in Saturn's A ring.Comment: 12 pages, 4 figures. Accepted for publication in ApJ Letter

Toward first-principle simulations of galaxy formation: I. How should we choose star formation criteria in high-resolution simulations of disk galaxies?

We performed 3-dimensional N-body/SPH simulations to study how mass resolution and other model parameters such as the star formation efficiency parameter, C* and the threshold density, nth affect structures of the galactic gaseous/stellar disk in a static galactic potential. We employ 10^6 - 10^7 particles to resolve a cold and dense (T 100 cm^{-3}) phase. We found that structures of the ISM and the distribution of young stars are sensitive to the assumed nth. High-nth models with nth = 100 cm^{-3} yield clumpy multi-phase features in the ISM. Young stars are distributed in a thin disk of which half-mass scale height is 10 - 30 pc. In low-nth models with nth = 0.1 cm^{-3}, the stellar disk is found to be several times thicker, and the gas disk appears smoother than the high-nth models. A high-resolution simulation with high-nth is necessary to reproduce the complex structure of the gas disk. The global properties of the model galaxies in low-nth models, such as star formation histories, are similar to those in the high-nth models when we tune the value of C* so that they reproduce the observed relation between surface gas density and surface star formation rate density. We however emphasize that high-nth models automatically reproduce the relation, regardless of the values of C*. The ISM structure, phase distribution, and distributions of young star forming region are quite similar between two runs with values of C* which differ by a factor of 15. We also found that the timescale of the flow from n_H ~1 cm^{-3} to n_H > 100 cm^{-3} is about 5 times as long as the local dynamical time and is independent of the value of C*. The use of a high-nth criterion for star formation in high-resolution simulations makes numerical models fairy insensitive to the modelling of star formation. (Abridged)Comment: 15 pages, 14 figures, accepted for publication in PASJ. Abridged abstract. For high resolution figures, see http://www.cfca.nao.ac.jp/~saitoh/Papers/2008/Saitoh+2008a.pd

Shock-induced star cluster formation in colliding galaxies

We studied the formation process of star clusters using high-resolution N-body/smoothed particle hydrodynamcs simulations of colliding galaxies. The total number of particles is 1.2x10^8 for our high resolution run. The gravitational softening is 5 pc and we allow gas to cool down to \sim 10 K. During the first encounter of the collision, a giant filament consists of cold and dense gas found between the progenitors by shock compression. A vigorous starburst took place in the filament, resulting in the formation of star clusters. The mass of these star clusters ranges from 10^{5-8} Msun. These star clusters formed hierarchically: at first small star clusters formed, and then they merged via gravity, resulting in larger star clusters.Comment: 4 pages, 3 figures, Proceedings of IAU Symposium 270, Computational Star Formatio

Toward First-Principle Simulations of Galaxy Formation: II. Shock-Induced Starburst at a Collision Interface During the First Encounter of Interacting Galaxies

We investigated the evolution of interacting disk galaxies using high-resolution $N$-body/SPH simulations, taking into account the multiphase nature of the interstellar medium (ISM). In our high-resolution simulations, a large-scale starburst occurred naturally at the collision interface between two gas disks at the first encounter, resulting in the formation of star clusters. This is consistent with observations of interacting galaxies. The probability distribution function (PDF) of gas density showed clear change during the galaxy-galaxy encounter. The compression of gas at the collision interface between the gas disks first appears as an excess at $n_{\rm H} \sim 10{\rm cm^{-3}}$ in the PDF, and then the excess moves to higher densities ($n_{\rm H} \gtrsim 100{\rm cm^{-3}}$) in a few times $10^7$ years where starburst takes place. After the starburst, the PDF goes back to the quasi-steady state. These results give a simple picture of starburst phenomena in galaxy-galaxy encounters.Comment: 6 pages, 6 figures, accepted to PASJ. For high resolution figures, see http://www.cfca.nao.ac.jp/~saitoh/Papers/2009/Saitoh+2009a.pd

REBOUND: An open-source multi-purpose N-body code for collisional dynamics

REBOUND is a new multi-purpose N-body code which is freely available under an open-source license. It was designed for collisional dynamics such as planetary rings but can also solve the classical N-body problem. It is highly modular and can be customized easily to work on a wide variety of different problems in astrophysics and beyond. REBOUND comes with three symplectic integrators: leap-frog, the symplectic epicycle integrator (SEI) and a Wisdom-Holman mapping (WH). It supports open, periodic and shearing-sheet boundary conditions. REBOUND can use a Barnes-Hut tree to calculate both self-gravity and collisions. These modules are fully parallelized with MPI as well as OpenMP. The former makes use of a static domain decomposition and a distributed essential tree. Two new collision detection modules based on a plane-sweep algorithm are also implemented. The performance of the plane-sweep algorithm is superior to a tree code for simulations in which one dimension is much longer than the other two and in simulations which are quasi-two dimensional with less than one million particles. In this work, we discuss the different algorithms implemented in REBOUND, the philosophy behind the code's structure as well as implementation specific details of the different modules. We present results of accuracy and scaling tests which show that the code can run efficiently on both desktop machines and large computing clusters.Comment: 10 pages, 9 figures, accepted by A&A, source code available at https://github.com/hannorein/reboun

Stochastic orbital migration of small bodies in Saturn's rings

Many small moonlets, creating propeller structures, have been found in Saturn's rings by the Cassini spacecraft. We study the dynamical evolution of such 20-50m sized bodies which are embedded in Saturn's rings. We estimate the importance of various interaction processes with the ring particles on the moonlet's eccentricity and semi-major axis analytically. For low ring surface densities, the main effects on the evolution of the eccentricity and the semi-major axis are found to be due to collisions and the gravitational interaction with particles in the vicinity of the moonlet. For large surface densities, the gravitational interaction with self-gravitating wakes becomes important. We also perform realistic three dimensional, collisional N-body simulations with up to a quarter of a million particles. A new set of pseudo shear periodic boundary conditions is used which reduces the computational costs by an order of magnitude compared to previous studies. Our analytic estimates are confirmed to within a factor of two. On short timescales the evolution is always dominated by stochastic effects caused by collisions and gravitational interaction with self-gravitating ring particles. These result in a random walk of the moonlet's semi-major axis. The eccentricity of the moonlet quickly reaches an equilibrium value due to collisional damping. The average change in semi-major axis of the moonlet after 100 orbital periods is 10-100m. This translates to an offset in the azimuthal direction of several hundred kilometres. We expect that such a shift is easily observable.Comment: 13 pages, 6 figures, submitted to A&A, comments welcom