5,102 research outputs found
Puncture discharges in surface dielectrics as contaminant sources in spacecraft environments
Spacecraft in geosynchronous orbits are known to become charged to large negative potentials during the local midnight region of the satellite orbit. Such discharges have been studied by the electron beam irradiation of dielectric samples in a vacuum environment. In addition to static measurements and photographic examination of the puncture discharges in Teflon samples, the transient characteristics of the electrical discharges are determined from oscillographs of voltage and current and by charged particle measurements employing a biased Faraday cup and a retarding potential analyzer. Using these latter techniques, studies of angular and energy distributions of charged particles have indicated an initial burst of high energy electrons (5 x 10 to the 13th power per discharge at energies greater than 300 eV) followed by a less intense burst of lower energy negative particles. Positive ions are emitted from the discharge site in an initial high velocity burst followed by a lower velocity burst tentatively identified as carbon
Optimal traps in graphene
We transform the two-dimensional Dirac-Weyl equation, which governs the
charge carriers in graphene, into a non-linear first-order differential
equation for scattering phase shift, using the so-called variable phase method.
This allows us to utilize the Levinson Theorem to find zero-energy bound states
created electrostatically in realistic structures. These confined states are
formed at critical potential strengths, which leads to us posit the use of
`optimal traps' to combat the chiral tunneling found in graphene, which could
be explored experimentally with an artificial network of point charges held
above the graphene layer. We also discuss scattering on these states and find
the zero angular momentum states create a dominant peak in scattering
cross-section as energy tends towards the Dirac point energy, suggesting a
dominant contribution to resistivity.Comment: 11 pages, 5 figure
Discovery of Multi-Phase Cold Accretion in a Massive Galaxy at z=0.7
We present detailed photo+collisional ionization models and kinematic models
of the multi-phase absorbing gas, detected within the HST/COS, HST/STIS, and
Keck/HIRES spectra of the background quasar TON 153, at 104 kpc along the
projected minor axis of a star-forming spiral galaxy (z=0.6610). Complementary
g'r'i'Ks photometry and stellar population models indicate that the host galaxy
is dominated by a 4 Gyr stellar population with slightly greater than solar
metallicity and has an estimated log(M*)=11 and a log(Mvir)=13. Photoionization
models of the low ionization absorption, (MgI, SiII, MgII and CIII) which trace
the bulk of the hydrogen, constrain the multi-component gas to be cold
(logT=3.8-5.2) and metal poor (-1.68<[X/H]<-1.64). A lagging halo model
reproduces the low ionization absorption kinematics, suggesting gas coupled to
the disk angular momentum, consistent with cold accretion mode material in
simulations. The CIV and OVI absorption is best modeled in a separate
collisionally ionized metal-poor (-2.50<[X/H]<-1.93) warm phase with logT=5.3.
Although their kinematics are consistent with a wind model, given the 2-2.5dex
difference between the galaxy stellar metallicity and the absorption
metallicity indicates the gas cannot arise from galactic winds. We discuss and
conclude that although the quasar sight-line passes along the galaxy minor axis
at projected distance of 0.3 virial radii, well inside its virial shock radius,
the combination of the relative kinematics, temperatures, and relative
metallicities indicated that the multi-phase absorbing gas arises from cold
accretion around this massive galaxy. Our results appear to contradict recent
interpretations that absorption probing the projected minor axis of a galaxy is
sampling winds.Comment: 16 pages, 11 figures, accepted for publication in MNRA
Galaxy Morphology - Halo Gas Connections
We studied a sample of 38 intermediate redshift MgII absorption-selected
galaxies using (1) Keck/HIRES and VLT/UVES quasar spectra to measure the halo
gas kinematics from MgII absorption profiles and (2) HST/WFPC-2 images to study
the absorbing galaxy morphologies. We have searched for correlations between
quantified gas absorption properties, and host galaxy impact parameters,
inclinations, position angles, and quantified morphological parameters. We
report a 3.2-sigma correlation between asymmetric perturbations in the host
galaxy morphology and the MgII absorption equivalent width. We suggest that
this correlation may indicate a connection between past merging and/or
interaction events in MgII absorption-selected galaxies and the velocity
dispersion and quantity of gas surrounding these galaxies.Comment: 6 pages; 3 figures; contributed talk for IAU 199: Probing Galaxies
through Quasar Absorption Line
QSO Absorption Line Constraints on Intragroup High-Velocity Clouds
We show that the number statistics of moderate redshift MgII and Lyman limit
absorbers may rule out the hypothesis that high velocity clouds are infalling
intragroup material.Comment: 4 pages, no figures; submitted to Astrophysical Journal Letters;
revised version, more general and includes more about Braun and Burton CHVC
A Catalog of Absorption Lines in Eight HST/STIS E230M 1.0 < z < 1.7 Quasar Spectra
We have produced a catalog of line identifications and equivalent width
measurements for all absorption features in eight ultraviolet echelle quasar
spectra. These spectra were selected as having the highest signal-to-noise
among the HST/STIS spectra obtained with the E230M grating. We identify 56
metal-line systems toward the eight quasars, and present plots of detected
transitions, aligned in velocity-space. We found that about 1/4 - 1/3 of the
features in the Lya forest region, redward of the incidence of the Lyb forest,
are metal lines. High ionization transitions are common. We see both O VI and C
IV in 88 - 90% of the metal-line systems for which the spectra cover the
expected wavelength. Si III is seen in 58%, while low ionization absorption in
C II, Si II, and/or Al II is detected in 50% of the systems for which they are
covered. This catalog will facilitate future studies of the Lya forest and of
metal-line systems of various types.Comment: 13 pages, 2 figures, submitted to Monthly Notices of the Royal
Astronomical Society, a complete version with the appendix and all figures is
available at http://www.astro.psu.edu/users/misawa/pub/Paper/qalcat.pdf.g
Quenched Cold Accretion of a Large Scale Metal-Poor Filament due to Virial Shocking in the Halo of a Massive z=0.7 Galaxy
Using HST/COS/STIS and HIRES/Keck high-resolution spectra, we have studied a
remarkable HI absorbing complex at z=0.672 toward the quasar Q1317+277. The HI
absorption has a velocity spread of 1600 km/s, comprises 21 Voigt profile
components, and resides at an impact parameter of D=58 kpc from a bright, high
mass [log(M_vir/M_sun) ~ 13.7] elliptical galaxy that is deduced to have a 6
Gyr old, solar metallicity stellar population. Ionization models suggest the
majority of the structure is cold gas surrounding a shock heated cloud that is
kinematically adjacent to a multi-phase group of clouds with detected CIII, CIV
and OVI absorption, suggestive of a conductive interface near the shock. The
deduced metallicities are consistent with the moderate in situ enrichment
relative to the levels observed in the z ~ 3 Ly-alpha forest. We interpret the
HI complex as a metal-poor filamentary structure being shock heated as it
accretes into the halo of the galaxy. The data support the scenario of an early
formation period (z > 4) in which the galaxy was presumably fed by cold-mode
gas accretion that was later quenched via virial shocking by the hot halo such
that, by intermediate redshift, the cold filamentary accreting gas is
continuing to be disrupted by shock heating. Thus, continued filamentary
accretion is being mixed into the hot halo, indicating that the star formation
of the galaxy will likely remain quenched. To date, the galaxy and the HI
absorption complex provide some of the most compelling observational data
supporting the theoretical picture in which accretion is virial shocked in the
hot coronal halos of high mass galaxies.Comment: 20 pages, 9 figures, submitted to Ap
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