1,604 research outputs found
Disruption of Molecular Clouds by Expansion of Dusty H II Regions
Dynamical expansion of H II regions around star clusters plays a key role in
dispersing the surrounding dense gas and therefore in limiting the efficiency
of star formation in molecular clouds. We use a semi-analytic method and
numerical simulations to explore expansion of spherical dusty H II regions and
surrounding neutral shells and the resulting cloud disruption. Our model for
shell expansion adopts the static solutions of Draine (2011) for dusty H II
regions and considers the contact outward forces on the shell due to radiation
and thermal pressures as well as the inward gravity from the central star and
the shell itself. We show that the internal structure we adopt and the shell
evolution from the semi-analytic approach are in good agreement with the
results of numerical simulations. Strong radiation pressure in the interior
controls the shell expansion indirectly by enhancing the density and pressure
at the ionization front. We calculate the minimum star formation efficiency
required for cloud disruption as a function of the cloud's
total mass and mean surface density. Within the adopted spherical geometry, we
find that typical giant molecular clouds in normal disk galaxies have
%, with comparable gas and radiation pressure
effects on shell expansion. Massive cluster-forming clumps require a
significantly higher efficiency of % for disruption,
produced mainly by radiation-driven expansion. The disruption time is typically
of the order of a free-fall timescale, suggesting that the cloud disruption
occurs rapidly once a sufficiently luminous H II region is formed. We also
discuss limitations of the spherical idealization.Comment: 23 pages, 14 figures; Accepted for publication in Ap
Modeling UV Radiation Feedback from Massive Stars: I. Implementation of Adaptive Ray Tracing Method and Tests
We present an implementation of an adaptive ray tracing (ART) module in the
Athena hydrodynamics code that accurately and efficiently handles the radiative
transfer involving multiple point sources on a three-dimensional Cartesian
grid. We adopt a recently proposed parallel algorithm that uses non-blocking,
asynchronous MPI communications to accelerate transport of rays across the
computational domain. We validate our implementation through several standard
test problems including the propagation of radiation in vacuum and the
expansions of various types of HII regions. Additionally, scaling tests show
that the cost of a full ray trace per source remains comparable to that of the
hydrodynamics update on up to processors. To demonstrate
application of our ART implementation, we perform a simulation of star cluster
formation in a marginally bound, turbulent cloud, finding that its star
formation efficiency is when both radiation pressure forces and
photoionization by UV radiation are treated. We directly compare the radiation
forces computed from the ART scheme with that from the M1 closure relation.
Although the ART and M1 schemes yield similar results on large scales, the
latter is unable to resolve the radiation field accurately near individual
point sources.Comment: 20 pages, 14 figures; accepted for publication in Ap
A new dynamic property of human consciousness
As pointed out by William James, "the consciousness is a dynamic process, not a thing" , during which short term integration is succeeded by another differentiated neural state through the continual interplay between the environment, the body, and the brain itself. Thus, the dynamic structure underlying successive states of the brain is important for understanding human consciousness as a process. In order to investigate the dynamic property of human consciousness, we developed a new method to reconstruct a state space from electroencephalogram(EEG), in which a trajectory, reflecting states of consciousness, is constructed based on the global information integration of the brain. EEGs were obtained from 14 subjects received an intravenous bolus of propopol. Here we show that the degree of human consciousness is directly associated with the information integration capacity of gamma wave, which is significantly higher in the conscious state than in the unconscious state. And we found a new time evolutional property of human consciousness. The conscious state showed a lower dimensional dynamic process which changed to a random-like process after loss of consciousness. This characteristic dynamic property, appeared only in the gamma band, might be used as an indicator to distinguish the conscious and unconscious states and also considered as an important fact for the human consciousness model
An Active and Soft Hydrogel Actuator to Stimulate Live Cell Clusters by Self-folding
The hydrogels are widely used in various applications, and their successful uses depend on controlling the mechanical properties. In this study, we present an advanced strategy to develop hydrogel actuator designed to stimulate live cell clusters by self-folding. The hydrogel actuator consisting of two layers with different expansion ratios were fabricated to have various curvatures in self-folding. The expansion ratio of the hydrogel tuned with the molecular weight and concentration of gel-forming polymers, and temperature-sensitive molecules in a controlled manner. As a result, the hydrogel actuator could stimulate live cell clusters by compression and tension repeatedly, in response to temperature. The cell clusters were compressed in the 0.7-fold decreases of the radius of curvature with 1.0 mm in room temperature, as compared to that of 1.4 mm in 37 degrees C. Interestingly, the vascular endothelial growth factor (VEGF) and insulin-like growth factor-binding protein-2 (IGFBP-2) in MCF-7 tumor cells exposed by mechanical stimulation was expressed more than in those without stimulation. Overall, this new strategy to prepare the active and soft hydrogel actuator would be actively used in tissue engineering, drug delivery, and micro-scale actuators
Modeling UV Radiation Feedback from Massive Stars: II. Dispersal of Star-Forming Giant Molecular Clouds by Photoionization and Radiation Pressure
UV radiation feedback from young massive stars plays a key role in the
evolution of giant molecular clouds (GMCs) by photoevaporating and ejecting the
surrounding gas. We conduct a suite of radiation hydrodynamic simulations of
star cluster formation in marginally-bound, turbulent GMCs, focusing on the
effects of photoionization and radiation pressure on regulating the net star
formation efficiency (SFE) and cloud lifetime. We find that the net SFE depends
primarily on the initial gas surface density, , such that the SFE
increases from 4% to 51% as increases from to . Cloud destruction occurs within
- after the onset of radiation feedback, or within
- freefall times (increasing with ). Photoevaporation
dominates the mass loss in massive, low surface-density clouds, but because
most photons are absorbed in an ionization-bounded Str\"{o}mgren volume the
photoevaporated gas fraction is proportional to the square root of the SFE. The
measured momentum injection due to thermal and radiation pressure forces is
proportional to , and the ejection of neutrals substantially
contributes to the disruption of low-mass and/or high-surface density clouds.
We present semi-analytic models for cloud dispersal mediated by
photoevaporation and by dynamical mass ejection, and show that the predicted
net SFE and mass loss efficiencies are consistent with the results of our
numerical simulations.Comment: Accepted to ApJ. 26 pages, 18 figures, 2 tables. For a simulation
movie, see http://www.youtube.com/watch?v=_YC-ueHvEW
Propofol Induction Reduces the Capacity for Neural Information Integration: Implications for the Mechanism of Consciousness and General Anesthesia
The cognitive unbinding paradigm suggests that the synthesis of cognitive information is attenuated by general anesthesia. Here, we investigated the functional organization of brain activities in the conscious and anesthetized states, based on characteristic functional segregation and integration of electroencephalography (EEG). EEG recordings were obtained from 14 subjects undergoing induction of general anesthesia with propofol. We quantified changes in mean information integration capacity in each band of the EEG. After induction with propofol, mean information integration capacity was reduced most prominently in the gamma band of the EEG (p=0.0001). Furthermore, we demonstrate that loss of consciousness is reflected by the breakdown of the spatiotemporal organization of gamma waves. Induction of general anesthesia with propofol reduces the capacity for information integration in the brain. These data directly support the information integration theory of consciousness and the cognitive unbinding paradigm of general anesthesia
- …