Supersonic Collisions between Two Gas Streams

A star around a massive black hole can be disrupted tidally by the gravity of the black hole. Then, its debris may form a precessing stream which may even collide with itself. In order to understand the dynamical effects of the stream-stream collision on the eventual accretion of the stellar debris onto the black hole, we have studied how gas flow behaves when the outgoing stream collides supersonically with the incoming stream. We have investigated the problem analytically with one-dimensional plane-parallel streams and numerically with more realistic three-dimensional streams. A shock formed around the contact surface converts the bulk of the orbital streaming kinetic energy into thermal energy. In three-dimensional simulations, the accumulated hot post-shock gas then expands adiabatically and drives another shock into the low density ambient region. Through this expansion, thermal energy is converted back to the kinetic energy associated with the expanding motion. Thus, in the end, only a small fraction of the orbital kinetic energy is actually converted to the thermal energy, while most of it is transferred to the kinetic energy of the expanding gas. Nevertheless the collision is effective in circularizing the debris orbit, because the shock efficiently transforms the ordered motion of the streams into the expanding motion in directions perpendicular to the streams. The circularization efficiency decreases, if two colliding streams have a large ratio of cross sections and a large density contrast. But even in such cases, the main shock extends beyond the overlapping contact surface and the high pressure region behind the shock keeps the stream of the larger cross section from passing freely. Thus the stream-stream collisions are still expected to circularize the stellar debris rather efficiently, unless the ratioComment: ApJ accepted, 14 pages with 9 figures, uuencoded, gzipped, tarred postscript files, or available upon request to [email protected]

Comments on the Moduli Dynamics of 1/4 BPS Dyons

We rederive the nonrelativistic Lagrangian for the low energy dynamics of 1/4 BPS dyons by considering the time dependent fluctuations around classical 1/4 BPS configurations. The relevant fluctuations are the zero modes of the underlying 1/2 BPS monopoles.Comment: 5 page

Moduli Space Dynamics of a First-Order Vortex System

The moduli space dynamics of vortices in the Jackiw-Pi model where a non-relativistic Schrodinger field couples minimally to Chern-Simons gauge field, is considered. It is shown that the difficulties in direct application of Manton's method to obtain a moduli-space metric in the first order system can be circumvented by turning the Lagrangian into a second order system. We obtain exact metrics for some simple cases and describe how the vortices respond to an external U(1) field. We then construct an effective Lagrangian describing dynamics of the vortices. In addition, we clarify strong-weak coupling duality between fundamental particles and vortices.Comment: 9 pages, Latex, Corrections include

Cosmic Rays in Clusters of Galaxies

We argue that clusters of galaxies have an intergalactic medium, which is permeated by strong magnetic fields and also has a contribution of pressure from cosmic rays. These two components of total pressure are probably highly time dependent, and range probably between 1/10 of the gas pressure up to equipartition between gas pressure and the sum of the two other components. Radio galaxies are likely to provide the main source for both magnetic fields and cosmic rays. In this concept it becomes easy to understand the occasional mismatch between the total mass inferred from the assumption of hydrostatic equilibrium derived purely from gas, and the total mass derived from lensing data. We also suggest that the structure and topology of the magnetic field may be highly inhomogeneous - at least over a certain range of scales, and may contain long twisted filaments of strong magnetic fields, as on the Sun. The analogy with the interstellar medium may be fruitful to explore further, where we do not know where magnetic fields come from, but suspect that the cosmic rays derive from supernova explosions. In such an analogy it becomes useful to refer to "radio galaxy explosions" in clusters of galaxies. A full scale exploration of all the implications, especially of the notion that occasionally complete equipartition may be reached, is a task for the future.Comment: 21 pages with 6 figures (reduced resolution). Invited talk presented at the "Matter and Energy in Clusters of Galaxies" workshop in Taipei, 23-27 April, 2002. To appear in the proceedings, published by PASP. eds: Chorng-Yuan Hwang and Stu Bowyer. Pdf version with full resolution figures can be downloaded from ftp://canopus.chungnam.ac.kr/ryu/ryu1.pd

AD4RL: Autonomous Driving Benchmarks for Offline Reinforcement Learning with Value-based Dataset

Offline reinforcement learning has emerged as a promising technology by enhancing its practicality through the use of pre-collected large datasets. Despite its practical benefits, most algorithm development research in offline reinforcement learning still relies on game tasks with synthetic datasets. To address such limitations, this paper provides autonomous driving datasets and benchmarks for offline reinforcement learning research. We provide 19 datasets, including real-world human driver's datasets, and seven popular offline reinforcement learning algorithms in three realistic driving scenarios. We also provide a unified decision-making process model that can operate effectively across different scenarios, serving as a reference framework in algorithm design. Our research lays the groundwork for further collaborations in the community to explore practical aspects of existing reinforcement learning methods. Dataset and codes can be found in https://sites.google.com/view/ad4rl.Comment: ICRA 2024 Website at: https://sites.google.com/view/ad4r

Supertubes connecting D4 branes

We find and explore a class of dyonic instanton solutions which can be identified as the supertubes connecting two D4 branes. They correspond to a single monopole string and a pair of monopole antimonopole strings from the worldvolume view point of D4 branes.Comment: 12 pages, no figures, a reference adde