It has been believed that spiral arms in pure stellar disks, especially the ones spontaneously formed, decay in several galactic rotations due to the increase of stellar velocity dispersions. Therefore, some cooling mechanism, for example dissipational effects of the interstellar medium, was assumed to be necessary to keep the spiral arms. Here we show that stellar disks can maintain spiral features for several tens of rotations without the help of cooling, using a series of high-resolution three-dimensional N -body simulations of pure stellar disks. We found that if the number of particles is sufficiently large, e.g., 3 × 10 6 , multi-arm spirals developed in an isolated disk can survive for more than 10 Gyrs. We confirmed that there is a self-regulating mechanism that maintains the amplitude of the spiral arms. Spiral arms increase Toomre's Q of the disk, and the heating rate correlates with the squared amplitude of the spirals. Since the amplitude itself is limited by Q, this makes the dynamical heating less effective in the later phase of evolution. A simple analytical argument suggests that the heating is caused by gravitational scattering of stars by spiral arms and that the self-regulating mechanism in pure-stellar disks can effectively maintain spiral arms on a cosmological timescale. In the case of a smaller number of particles, e.g., 3 × 10 5 , spiral arms grow faster in the beginning of the simulation (while Q is small) and they cause a rapid increase of Q. As a result, the spiral arms become faint in several Gyrs.

About 20% of all massive stars in the Milky Way have unusually high velocities, the origin of which has puzzled astronomers for half a century. We argue that these velocities originate from strong gravitational interactions between single stars and binaries in the centers of star clusters. The ejecting binary forms naturally during the collapse of a young ( < ∼ 1 Myr) star cluster. This model replicates the key characteristics of OB runaways in our galaxy and it explains the > ∼ 100 M ⊙ runaway stars around young star clusters, e.g. R136 and Westerlund 2. The high proportion and the distributions in mass and velocity of runaways in the Milky Way is reproduced if the majority of massive stars are born in dense and relatively low-mass (5000 − 10000 M ⊙ ) clusters.Most stars in our galaxy have a relatively low velocity. However, there is a population of fast moving stars, called OB runaways (1), which have considerably higher space motions of > 30 km/s (2). The origin of such velocities can be attained in two very distinct ways: A runaway can be launched when its binary companion explodes in a supernova (3), or by the ejection via a dynamical slingshot (4). The relative importance of both mechanisms has remained elusive, mainly because both are associated with young stellar populations and the high speed of a star is generally observed long after it has moved away from its birth place.A massive star can be accelerated effectively by a three-body dynamical interaction (4). Every hard interaction eventually results in a collision between two or all three participating stars, or in the escape of one star and one binary (5). The velocity acquired by the ejected star easily exceeds the escape speed of a star cluster: For a binary with total mass M b and semi-major axis a, the typical velocity with which the single star is ejected is vIt is typically the least massive star that is ejected (5), and both components of the ejecting (bully) binary are therefore likely to be more massive than the escaping star. For 1

The rapid neutron-capture process (r-process) is a major process to synthesize elements heavier than iron, but the astrophysical site(s) of r-process is not identified yet. Neutron star mergers (NSMs) are suggested to be a major r-process site from nucleosynthesis studies. Previous chemical evolution studies however require unlikely short merger time of NSMs to reproduce the observed large starto-star scatters in the abundance ratios of r-process elements relative to iron, [Eu/Fe], of extremely metal-poor stars in the Milky Way (MW) halo. This problem can be solved by considering chemical evolution in dwarf spheroidal galaxies (dSphs) which would be building blocks of the MW and have lower star formation efficiencies than the MW halo. We demonstrate that enrichment of r-process elements in dSphs by NSMs using an N -body/smoothed particle hydrodynamics code. Our highresolution model reproduces the observed [Eu/Fe] by NSMs with a merger time of 100 Myr when the effect of metal mixing is taken into account. This is because metallicity is not correlated with time up to ∼ 300 Myr from the start of the simulation due to low star formation efficiency in dSphs. We also confirm that this model is consistent with observed properties of dSphs such as radial profiles and metallicity distribution. The merger time and the Galactic rate of NSMs are suggested to be 300 Myr and ∼ 10 −4 yr −1 , which are consistent with the values suggested by population synthesis and nucleosynthesis studies. This study supports that NSMs are the major astrophysical site of r-process.