263 research outputs found
Self-Sustained Ionization and Vanishing Dead Zones in Protoplanetary Disks
We analyze the ionization state of the magnetohydrodynamically turbulent
protoplanetary disks and propose a new mechanism of sustaining ionization.
First, we show that in the quasi-steady state of turbulence driven by
magnetorotational instability in a typical protoplanetary disk with dust
grains, the amount of energy dissipation should be sufficient for providing the
ionization energy that is required for activating magnetorotational
instability. Second, we show that in the disk with dust grains the energetic
electrons that compose electric currents in weakly ionized gas can provide
collisional ionization, depending on the actual saturation state of
magnetorotational turbulence. On the other hand, we show that in the
protoplanetary disks with the reduced effect of dust grains, the turbulent
motion can homogenize the ionization degree, leading to the activation of
magnetorotational instability even in the absence of other ionization
processes. The results in this Letter indicate that most of the regions in
protoplanetary disks remain magnetically active, and we thus require a change
in the theoretical modeling of planet formation.Comment: 11 pages, 2 figures. Accepted for publication in The Astrophysical
Journal Letter
Saturation and Thermalization of the Magnetorotational Instability: Recurrent Channel Flows and Reconnections
The nonlinear evolution and the saturation mechanism of the magnetorotational
instability (MRI) are investigated using three-dimensional resistive MHD
simulations. A local shearing box is used for our numerical analysis and the
simulations begin with a purely vertical magnetic field. We find that the
magnetic stress in the nonlinear stage of the MRI is strongly fluctuating. The
time evolution shows the quasi-periodic recurrence of spike-shape variations
typically for a few orbits which correspond to the rapid amplification of the
magnetic field by the nonlinear growth of a two-channel solution followed by
the decay through magnetic reconnections. The increase rate of the total energy
in the shearing box system is analytically related to the volume-averaged
torque in the system. We find that at the saturated state this energy gain of
the system is balanced with the increase of the thermal energy mostly due to
the joule heating. The spike-shape time evolution is a general feature of the
nonlinear evolution of the MRI in the disks threaded by vertical fields and can
be seen if the effective magnetic Reynolds number is larger than about unity.Comment: 11 pages, 4 figures, accepted for publication in ApJ
Optical Circuit for Waveform-synthesis with Utilizing both GaAs- and Si-Optoelectronic Switches
This paper presents an optical-waveform synthesizer as one
of the applications of optical bistable devices. This device is advantageous in terms of faster operation in which width of each pulse obtained is down to the order of subnanosecond, because this device employs both GaAs- and Si-optoelectronic switches as photodetector. Optical pulse shaping as a preliminary experiment is also described
Angular Momentum Transport by MHD Turbulence in Accretion Disks: Gas Pressure Dependence of the Saturation Level of the Magnetorotational Instability
The saturation level of the magnetorotational instability (MRI) is
investigated using three-dimensional MHD simulations. The shearing box
approximation is adopted and the vertical component of gravity is ignored, so
that the evolution of the MRI is followed in a small local part of the disk. We
focus on the dependence of the saturation level of the stress on the gas
pressure, which is a key assumption in the standard alpha disk model. From our
numerical experiments it is found that there is a weak power-law relation
between the saturation level of the Maxwell stress and the gas pressure in the
nonlinear regime; the higher the gas pressure, the larger the stress. Although
the power-law index depends slightly on the initial field geometry, the
relationship between stress and gas pressure is independent of the initial
field strength, and is unaffected by Ohmic dissipation if the magnetic Reynolds
number is at least 10. The relationship is the same in adiabatic calculations,
where pressure increases over time, and nearly-isothermal calculations, where
pressure varies little with time. Our numerical results are qualitatively
consistent with an idea that the saturation level of the MRI is determined by a
balance between the growth of the MRI and the dissipation of the field through
reconnection. The quantitative interpretation of the pressure-stress relation,
however, may require advances in the theoretical understanding of non-steady
magnetic reconnection.Comment: 45 pages, 5 tables, 17 figures, accepted for publication in Ap
Identity Avoidance and Rendaku
One important observation that is made in the past phonological research is that constraints on output structures can both block and trigger phonological processes (i.e., conspiracy: Kisseberth 1970). This paper reports an experiment which shows that an Identity Avoidance constraint (a.k.a. the OCP) both blocks and triggers one phonological process in the same language, namely rendaku in Japanese. Our wug- experiment shows that rendaku is more likely to apply when the two CV moras across a morpheme boundary are identical; i.e. an Identity Avoidance constraint triggers rendaku. The experiment also shows that rendaku is less likely to apply when it would result in two adjacent identical CV moras across a morpheme boundary; i.e. the Identity Avoidance constraint blocks rendaku. These blocking and triggering effects of the general Identity Avoidance constraint on rendaku are a new discovery in Japanese phonology, despite the fact that rendaku has been studied extensively in the previous literature, suggesting the importance of experimentation in phonological research. Moreover, our case study offers experimental confirmation of the OCP conspiracy in natural languages
Effect of Pitch Difference on Anti-loosening Performance and Fatigue Strength for High Strength Bolts and Nuts
A slight pitch difference is considered between the bolt and nut in this study. Here, the pitch ofnut is α μm larger than the pitch of bolt. In the first place, the distance δ, of nut screwed onto bolt withoutany prevailing torque, is experimentally and analytically obtained for each level of α. Then, the relationshipbetween axial force and prevailing torque during tightening process is tested. According to the obtainedtorque-axial force relationship, for four different levels of α, the loosening experiment based on NAS3350(National Aerospace Standard) is performed, and the loosening and dropping status of the nuts areinvestigated. Considering both the anti-loosening ability and the clamping ability of the nut, the desirablerange of α is discussed. After that, according to the loosening experiment results, three levels of α areselected to make a further study about the effect of the pitch difference on the fatigue life of bolt. The fatigueexperiment is performed and the S-N curves are obtained. Finally, the finite element method is used to makea simulation of the fatigue experiment and the mean stress and stress amplitude at each thread bottom of boltare analyzed.13th International Conference on Fracture, June 16–21, 2013, Beijing, Chin
Reconstruction of putative DNA virus from endogenous rice tungro bacilliform virus-like sequences in the rice genome: implications for integration and evolution
BACKGROUND: Plant genomes contain various kinds of repetitive sequences such as transposable elements, microsatellites, tandem repeats and virus-like sequences. Most of them, with the exception of virus-like sequences, do not allow us to trace their origins nor to follow the process of their integration into the host genome. Recent discoveries of virus-like sequences in plant genomes led us to set the objective of elucidating the origin of the repetitive sequences. Endogenous rice tungro bacilliform virus (RTBV)-like sequences (ERTBVs) have been found throughout the rice genome. Here, we reconstructed putative virus structures from RTBV-like sequences in the rice genome and characterized to understand evolutionary implication, integration manner and involvements of endogenous virus segments in the corresponding disease response. RESULTS: We have collected ERTBVs from the rice genomes. They contain rearranged structures and no intact ORFs. The identified ERTBV segments were shown to be phylogenetically divided into three clusters. For each phylogenetic cluster, we were able to make a consensus alignment for a circular virus-like structure carrying two complete ORFs. Comparisons of DNA and amino acid sequences suggested the closely relationship between ERTBV and RTBV. The Oryza AA-genome species vary in the ERTBV copy number. The species carrying low-copy-number of ERTBV segments have been reported to be extremely susceptible to RTBV. The DNA methylation state of the ERTBV sequences was correlated with their copy number in the genome. CONCLUSIONS: These ERTBV segments are unlikely to have functional potential as a virus. However, these sequences facilitate to establish putative virus that provided information underlying virus integration and evolutionary relationship with existing virus. Comparison of ERTBV among the Oryza AA-genome species allowed us to speculate a possible role of endogenous virus segments against its related disease
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