8,684 research outputs found
Investigation of atomic oxygen-surface interactions related to measurements with dual air density explorer satellites
For a number of candidate materials of construction for the dual air density explorer satellites the rate of oxygen atom loss by adsorption, surface reaction, and recombination was determined as a function of surface and temperature. Plain aluminum and anodized aluminum surfaces exhibit a collisional atom loss probability alpha .01 in the temperature range 140 - 360 K, and an initial sticking probability. For SiO coated aluminum in the same temperature range, alpha .001 and So .001. Atom-loss on gold is relatively rapid alpha .01. The So for gold varies between 0.25 and unity in the temperature range 360 - 140 K
Research related to measurements of atomic species in the earth's upper atmosphere Final report
Interaction kinetics of atomic oxygen and hydrogen on metal surfaces of satellite-borne mass spectrometer
Comment on Decay
We calculate the rate for decay using Chiral
Perturbation Theory. This isospin violating process results from -
mixing, and its amplitude is proportional to . Experimental information on the branching
ratio for can provide insight into the pattern of
violation in radiative decays.Comment: 7 pages with 2 figures not included but available upon request,
CALT-68-191
Dwarf Galaxies with Ionizing Radiation Feedback. I: Escape of Ionizing Photons
We describe a new method for simulating ionizing radiation and supernova
feedback in the analogues of low-redshift galactic disks. In this method, which
we call star-forming molecular cloud (SFMC) particles, we use a ray-tracing
technique to solve the radiative transfer equation for ultraviolet photons
emitted by thousands of distinct particles on the fly. Joined with high
numerical resolution of 3.8 pc, the realistic description of stellar feedback
helps to self-regulate star formation. This new feedback scheme also enables us
to study the escape of ionizing photons from star-forming clumps and from a
galaxy, and to examine the evolving environment of star-forming gas clumps. By
simulating a galactic disk in a halo of 2.3e11 Msun, we find that the average
escape fraction from all radiating sources on the spiral arms (excluding the
central 2.5 kpc) fluctuates between 0.08% and 5.9% during a ~20 Myr period with
a mean value of 1.1%. The flux of escaped photons from these sources is not
strongly beamed, but manifests a large opening angle of more than 60 degree
from the galactic pole. Further, we investigate the escape fraction per SFMC
particle, f_esc(i), and how it evolves as the particle ages. We discover that
the average escape fraction f_esc is dominated by a small number of SFMC
particles with high f_esc(i). On average, the escape fraction from a SFMC
particle rises from 0.27% at its birth to 2.1% at the end of a particle
lifetime, 6 Myrs. This is because SFMC particles drift away from the dense gas
clumps in which they were born, and because the gas around the star-forming
clumps is dispersed by ionizing radiation and supernova feedback. The framework
established in this study brings deeper insight into the physics of photon
escape fraction from an individual star-forming clump, and from a galactic
disk.Comment: 15 pages, 12 figures, Accepted for publication in the Astrophysical
Journal, Image resolution reduced, High-resolution version of this article is
available at http://www.jihoonkim.org/index/research.html#sfm
Dwarf Galaxies with Ionizing Radiation Feedback. II: Spatially-resolved Star Formation Relation
We investigate the spatially-resolved star formation relation using a
galactic disk formed in a comprehensive high-resolution (3.8 pc) simulation.
Our new implementation of stellar feedback includes ionizing radiation as well
as supernova explosions, and we handle ionizing radiation by solving the
radiative transfer equation rather than by a subgrid model. Photoheating by
stellar radiation stabilizes gas against Jeans fragmentation, reducing the star
formation rate. Because we have self-consistently calculated the location of
ionized gas, we are able to make spatially-resolved mock observations of star
formation tracers, such as H-alpha emission. We can also observe how stellar
feedback manifests itself in the correlation between ionized and molecular gas.
Applying our techniques to the disk in a galactic halo of 2.3e11 Msun, we find
that the correlation between star formation rate density (estimated from mock
H-alpha emission) and molecular hydrogen density shows large scatter,
especially at high resolutions of <~ 75 pc that are comparable to the size of
giant molecular clouds (GMCs). This is because an aperture of GMC size captures
only particular stages of GMC evolution, and because H-alpha traces hot gas
around star-forming regions and is displaced from the molecular hydrogen peaks
themselves. By examining the evolving environment around star clusters, we
speculate that the breakdown of the traditional star formation laws of the
Kennicutt-Schmidt type at small scales is further aided by a combination of
stars drifting from their birthplaces, and molecular clouds being dispersed via
stellar feedback.Comment: 16 pages, 15 figures, Accepted for publication in the Astrophysical
Journal, Image resolution greatly reduced, High-resolution version of this
article is available at http://www.jihoonkim.org/index/research.html#sfm
A precise determination of the charm quark's mass in quenched QCD
We present a lattice determination of the charm quark's mass, using the mass
of the D_s meson as experimental input. All errors are under control with the
exception of the quenched approximation. Setting the scale with F_K=160 MeV,
our final result for the renormalization group invariant (RGI) quark mass is
M_c = 1.654(45) GeV, which translates to m_c(m_c) =1.301(34) GeV for the
running mass in the MSbar scheme. A 6 percent increase of the RGI quark mass is
observed when the scale is set by the nucleon mass. This is a typical quenched
scale ambiguity, which is reduced to about 3 percent for m_c(m_c), and to 4
percent for the mass ratio M_c/M_s. In contrast, the mass splitting
m(Dstar_s)-m(D_s) changes from 117(11) MeV to 94(11) MeV, which is
significantly smaller than the experimental value of 144 MeV.Comment: 27 pages, 5 figure
Chiral Estimates of Strong CP Violation Revisited
The effects of the CP violating term in the QCD Lagrangian upon low
energy hadronic phenomenology are reconsidered. Strong CP violating
interactions among Goldstone bosons and octet baryons are incorporated into an
effective chiral Lagrangian framework. The term's impact upon the
decays and is then investigated but
found to be extremely small. A refined model independent estimate of
nonanalytic contributions to the neutron electric dipole moment is also
determined using velocity dependent Baryon Chiral Perturbation Theory. We
obtain the approximate upper bound .Comment: 11 pages with 3 figures not included but available upon request,
CALT-68-184
Resolving the Formation of Protogalaxies. III. Feedback from the First Stars
The first stars form in dark matter halos of masses ~10^6 M_sun as suggested
by an increasing number of numerical simulations. Radiation feedback from these
stars expels most of the gas from their shallow potential well of their
surrounding dark matter halos. We use cosmological adaptive mesh refinement
simulations that include self-consistent Population III star formation and
feedback to examine the properties of assembling early dwarf galaxies. Accurate
radiative transport is modeled with adaptive ray tracing. We include supernova
explosions and follow the metal enrichment of the intergalactic medium. The
calculations focus on the formation of several dwarf galaxies and their
progenitors. In these halos, baryon fractions in 10^8 solar mass halos decrease
by a factor of 2 with stellar feedback and by a factor of 3 with supernova
explosions. We find that radiation feedback and supernova explosions increase
gaseous spin parameters up to a factor of 4 and vary with time. Stellar
feedback, supernova explosions, and H_2 cooling create a complex, multi-phase
interstellar medium whose densities and temperatures can span up to 6 orders of
magnitude at a given radius. The pair-instability supernovae of Population III
stars alone enrich the halos with virial temperatures of 10^4 K to
approximately 10^{-3} of solar metallicity. We find that 40% of the heavy
elements resides in the intergalactic medium (IGM) at the end of our
calculations. The highest metallicity gas exists in supernova remnants and very
dilute regions of the IGM.Comment: 15 pages, 16 figures, accepted to ApJ. Many changes, including
estimates of metal line cooling. High resolution images and movies available
at http://www.slac.stanford.edu/~jwise/research/PGalaxies3
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