1,068 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
Physiological Functions of Mitochondrial Reactive Oxygen Species
Mitochondria are the major energy producers within a cell in the form of adenosine triphosphate by oxidative phosphorylation. Normal mitochondrial metabolism inevitably generates reactive oxygen species (ROS), which have been considered to solely cause cellular damage. Increase of oxidative stress has been linked to various pathologies. Thus, mitochondrial ROS (mROS) were basically proposed as byproducts of oxidative metabolism, which undergo normalized by antioxidant enzymes. However, the mROS have extensively been esteemed to function as signalling molecules to regulate a wide variety of physiology. These phenomena are indeed dependent on mitochondrial redox status, which is dynamically altered under different physiological and pathological conditions. The oxidative stress is incurred by which the redox status is inclined to exceeded oxidation or reduction. Here, we attempt to integrate the recent advances in our understanding of the physiological functions of mROS
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
Applicaton of USB Serial Communication to Radon Measuring System
The USB serial communication such as USB-Serial-for PC and USB-Serial-for-Android is studied in order to monitor the measure radon data using a PC screen or a smart phone screen. Through some experimental studies, we believe that the USB serial communication module is useful for checking the data transmitted to a PC from a microcontroller
Effect of Si and SiO 2
Carbon coils could be synthesized using C2H2/H2 as source gases and SF6 as an incorporated additive gas under thermal chemical vapor deposition system. Si substrate, SiO2 thin film deposited Si substrate (SiO2 substrate), and quartz substrate were employed to elucidate the effect of substrate on the formation of carbon coils. The characteristics (formation densities, morphologies, and geometries) of the deposited carbon coils on the substrate were investigated. In case of Si substrate, the microsized carbon coils were dominant on the substrate surface. While, in case of SiO2 substrate, the nanosized carbon coils were prevailing on the substrate surface. The surface morphologies of samples were investigated step by step during the reaction process. The cause for the different geometry formation of carbon coils according to the different substrates was discussed in association with the different thermal expansion coefficient values of Si and SiO2 substrates and the different etched characteristics of Si and SiO2 substrates by SF6 + H2 flow
Modeling UV Radiation Feedback from Massive Stars: III. Escape of Radiation from Star-forming Giant Molecular Clouds
Using a suite of radiation hydrodynamic simulations of star cluster formation
in turbulent clouds, we study the escape fraction of ionizing (Lyman continuum)
and non-ionizing (FUV) radiation for a wide range of cloud masses and sizes.
The escape fraction increases as H II regions evolve and reaches unity within a
few dynamical times. The cumulative escape fraction before the onset of the
first supernova explosion is in the range 0.05-0.58; this is lower for higher
initial cloud surface density, and higher for less massive and more compact
clouds due to rapid destruction. Once H II regions break out of their local
environment, both ionizing and non-ionizing photons escape from clouds through
fully ionized, low-density sightlines. Consequently, dust becomes the dominant
absorber of ionizing radiation at late times and the escape fraction of
non-ionizing radiation is only slightly larger than that of ionizing radiation.
The escape fraction is determined primarily by the mean
and width of the optical-depth distribution in the large-scale cloud,
increasing for smaller and/or larger . The escape
fraction exceeds (sometimes by three orders of magnitude) the naive estimate
due to non-zero induced by turbulence. We
present two simple methods to estimate, within , the escape fraction
of non-ionizing radiation using the observed dust optical depth in clouds
projected on the plane of sky. We discuss implications of our results for
observations, including inference of star formation rates in individual
molecular clouds, and accounting for diffuse ionized gas on galactic scales.Comment: 27 pages, 16 figures, accepted for publication in Ap
- …