The dynamics of spiral arms in pure stellar disks

It has been believed that spirals 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., $3\times 10^6$, 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 the value of $Q$, 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., $3\times 10^5$, 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.Comment: 18 pages, 19 figures, accepted for Ap

The formation of Kuiper-belt Binaries through Exchange Reactions

Recent observations have revealed an unexpectedly high binary fraction among the Trans-Neptunian Objects (TNOs) that populate the Kuiper-belt. The discovered binaries have four characteristics they comprise a few percent of the TNOs, the mass ratio of their components is close to unity, their internal orbits are highly eccentric, and the orbits are more than 100 times wider than the primary's radius. In contrast, theories of binary asteroid formation tend to produce close, circular binaries. Therefore, a new approach is required to explain the unique characteristics of the TNO binaries. Two models have been proposed. Both, however, require extreme assumptions on the size distribution of TNOs. Here we show a mechanism which is guaranteed to produces binaries of the required type during the early TNO growth phase, based on only one plausible assumption, namely that initially TNOs were formed through gravitational instabilities of the protoplanetary dust layer.Comment: 12pages, 4 figure

Dust accretion onto high-mass planets

We study the accretion of dust particles of various sizes onto embedded massive gas giant planets, where we take into account the structure of the gas disk due to the presence of the planet. The accretion rate of solids is important for the structure of giant planets: it determines the growth rate of the solid core that may be present as well as their final enrichment in solids. We use the RODEO hydrodynamics solver to solve the flow equations for the gas, together with a particle approach for the dust. The solver for the particles' equations of motion is implicit with respect to the drag force, which allows us to treat the whole dust size spectrum. We find that dust accretion is limited to the smallest particle sizes. The largest particles get trapped in outer mean-motion resonances with the planet, while particles of intermediate size are pushed away from the orbit of the planet by the density structure in the gas disk. Only particles smaller than approximately s_max =10 micron may accrete on a planet with the mass of Jupiter. For a ten times less massive planet s_max=100 micron. The strongly reduced accretion of dust makes it very hard to enrich a newly formed giant planet in solids.Comment: 15 pages, 18 figures, accepted for publication in A&

Fluoride-containing bioactive glasses: Effect of glass design and structure on degradation, pH and apatite formation in simulated body fluid

NOTICE: this is the author’s version of a work that was accepted for publication in Acta Biomaterialia. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Acta Biomaterialia, [VOL 6, ISSUE 8, (2010)] DOI: 10.1016/j.actbio.2010.01.04

Migration then assembly: Formation of Neptune mass planets inside 1 AU

We demonstrate that the observed distribution of `Hot Neptune'/`Super-Earth' systems is well reproduced by a model in which planet assembly occurs in situ, with no significant migration post-assembly. This is achieved only if the amount of mass in rocky material is $\sim 50$--$100 M_{\oplus}$ interior to 1 AU. Such a reservoir of material implies that significant radial migration of solid material takes place, and that it occur before the stage of final planet assembly. The model not only reproduces the general distribution of mass versus period, but also the detailed statistics of multiple planet systems in the sample. We furthermore demonstrate that cores of this size are also likely to meet the criterion to gravitationally capture gas from the nebula, although accretion is rapidly limited by the opening of gaps in the gas disk. If the mass growth is limited by this tidal truncation, then the scenario sketched here naturally produces Neptune-mass objects with substantial components of both rock and gas, as is observed. The quantitative expectations of this scenario are that most planets in the `Hot Neptune/Super-Earth' class inhabit multiple-planet systems, with characteristic orbital spacings. The model also provides a natural division into gas-rich (Hot Neptune) and gas-poor (Super-Earth) classes at fixed period. The dividing mass ranges from $\sim 3 M_{\oplus}$ at 10 day orbital periods to $\sim 10 M_{\oplus}$ at 100 day orbital periods. For orbital periods $< 10$ days, the division is less clear because a gas atmosphere may be significantly eroded by stellar radiation.Comment: 41 pages in preprint style, 15 figures, final version accepted to Ap

Functionalization of different polymers with sulfonic groups as a way to coat them with a biomimetic apatite layer

Covalent coupling of sulfonic group (–SO3H) was attempted on different polymers to evaluate efficacy of this functional group in inducing nucleation of apatite in body environment, and thereupon to design a simple biomimetic process for preparing bonelike apatite-polymer composites. Substrates of polyethylene terephthalate (PET), polycaprolactam (Nylon 6), high molecular weight polyethylene (HMWPE) and ethylene-vinyl alcohol copolymer (EVOH) were subjected to sulfonation by being soaked in sulfuric acid (H2SO4) or chlorosulfonic acid (ClSO3H) with different concentrations. In order to incorporate calcium ions, the sulfonated substrates were soaked in saturated solution of calcium hydroxide (Ca(OH)2). The treated substrates were soaked in a simulated body fluid (SBF). Fourier transformed infrared spectroscopy, thin-film X-ray diffraction, and scanning electron microscopy showed that the sulfonation and subsequent Ca(OH)2 treatments allowed formation of –SO3H groups binding Ca2+ ions on the surface of HMWPE and EVOH, but not on PET and Nylon 6. The HMWPE and EVOH could thus form bonelike apatite layer on their surfaces in SBF within 7 d. These results indicate that the –SO3H groups are effective for inducing apatite nucleation, and thereby that surface sulfonation of polymers are effective pre-treatment method for preparing biomimetic apatite on their surfaces