Conditions for Gravitational Instability in Protoplanetary Disks

Gravitational instability is one of considerable mechanisms to explain the formation of giant planets. We study the gravitational stability for the protoplanetary disks around a protostar. The temperature and Toomre's Q-value are calculated by assuming local equilibrium between viscous heating and radiative cooling (local thermal equilibrium). We assume constant $\alpha$ viscosity and use a cooling function with realistic opacity. Then, we derive the critical surface density $\Sigma_{\rm{c}}$ that is necessary for a disk to become gravitationally unstable as a function of $r$. This critical surface density $\Sigma_{\rm c}$ is strongly affected by the temperature dependence of the opacity. At the radius $r_{\rm c}\sim 20$AU, where ices form, the value of $\Sigma_{\rm c}$ changes discontinuously by one order of magnitude. This $\Sigma_{\rm c}$ is determined only by local thermal process and criterion of gravitational instability. By comparing a given surface density profile to $\Sigma_{\rm c}$, one can discuss the gravitational instability of protoplanetary disks. As an example, we discuss the gravitational instability of two semi-analytic models for protoplanetary disks. One is the steady state accretion disk, which is realized after the viscous evolution. The other is the disk that has the same angular momentum distribution with its parent cloud core, which corresponds to the disk that has just formed. As a result, it is found that the disks tend to become gravitationally unstable for $r\ge r_{\rm c}$ because ices enable the disks to become low temperature. In the region closer to the protostar than $r_{\rm c}$, it is difficult for a typical protoplanetary disk to fragment because of the high temperature and the large Coriolis force. From this result, we conclude that the fragmentation near the central star is possible but difficult.Comment: accepted for publication in PASJ. Draft version with 26 pages, 8 figures, 1 tabl

Second-Harmonic Generation in Thermally Poled Twin-Hole Silica-Glass Fiber: Quasi-Phase Matching with Mercury-Lamp Exposure and its Optimization by Fiber Stretch

The 9th International Conference on the Science and Technology for Advanced Ceramics/The 9th International Symposium on Transparent Oxide and Related Materials for Electronics and Optics (STAC-9&TOEO-9), October 19-21, 2015, Epocal Tsukuba, Ibaraki, Japa

マグネシウムの水素吸蔵／放出反応における遷移金属酸化物の触媒機構

広島大学(Hiroshima University)博士(学術)Philosophydoctora

Parallel Viterbi Decoding Implementation by Multi-microprocessors

The Viterbi algorithm is a well-established technique for channel and source decoding in high performance digital communication systems. However, excessive time consumption makes it difficult to design an efficient highspeed decoder for practical application. The central unit of a Viterbi decoder is a data-dependent feedback loop which performs an add-compare-select (ACS) operation. This nonlinear recurrence is the bottleneck for a high-speed parallel implementation. This paper describes the implementation of parallel Viterbi algorithm by multi-microprocessors. Internal computations are performed in a parallel fashion. The use of microprocessors allows low-cost implementation with moderate complexity. An organization network, separate memory blocks and programs provide proper operation. For a fixed processing speed of given hardware parallel Viterbi decoding allows a linear speed up in the throughput rate by a linear increase in hardware complexity

bcl-x Prevents Apoptotic Cell Death of Both Primitive and Definitive Erythrocytes at the End of Maturation

bcl-x is a member of the bcl-2 gene family, which regulates apoptotic cell death in various cell lineages. There is circumstantial evidence suggesting that bcl-x might play a role in the apoptosis of erythroid lineage cells, although there is no direct evidence. In this study, we used Bcl-X null mouse embryonic stem (ES) cells, and showed that Bcl-X is indispensable for the production of both embryonic primitive erythrocytes (EryP) and adult definitive erythrocytes (EryD) at the end of their maturation. In vivo, bcl-x−/− ES cells did not contribute to circulating EryD in adult chimeric mice that were produced by blastocyst microinjection of the bcl-x−/− ES cells. bcl-x−/− EryP and EryD were produced by in vitro differentiation induction of ES cells on macrophage colony-stimulating factor–deficient stromal cell line OP9, and further analysis was carried out. The emergence of immature EryP and EryD from bcl-x−/− ES cells was similar to that from bcl-x+/+ ES cells. However, prominent cell death of bcl-x−/− EryP and EryD occurred when the cells matured. The data show that the antiapoptotic function of bcl-x acts at the very end of erythroid maturation