2,875 research outputs found
First principles numerical model of avalanche-induced arc discharges in electron-irradiated dielectrics
The model consists of four phases: single electron dynamics, single electron avalanche, negative streamer development, and tree formation. Numerical algorithms and computer code implementations are presented for the first three phases. An approach to developing a code description of fourth phase is discussed. Numerical results are presented for a crude material model of Teflon
15 Epidemiology of nontuberculous mycobacteria in the USA: Pilot study utilizing closed healthcare systems
Feedback in the local LBG Analog Haro 11 as probed by far-UV and X-ray observations
We have re-analyzed FUSE data and obtained new Chandra observations of Haro
11, a local (D_L=88 Mpc) UV luminous galaxy. Haro 11 has a similar far-UV
luminosity (10^10.3 L_\odot), UV surface brightness (10^9.4 L_\odot kpc^-2),
SFR, and metallicity to that observed in Lyman Break Galaxies (LBGs). We show
that Haro 11 has extended, soft thermal (kT~0.68 keV) X-ray emission with a
luminosity and size which scales with the physical properties (e.g. SFR,
stellar mass) of the host galaxy. An enhanced alpha/Fe, ratio of ~4 relative to
solar abundance suggests significant supernovae enrichment. These results are
consistent with the X-ray emission being produced in a shock between a
supernovae driven outflow and the ambient material. The FUV spectra show strong
absorption lines similar to those observed in LBG spectra. A blueshifted
absorption component is identified as a wind outflowing at ~200-280 km/s.
OVI\lambda\lambda1032,1038 emission, the dominant cooling mechanism for coronal
gas at T~10^5.5 K is also observed. If associated with the outflow, the
luminosity of the OVI emission suggests that <20% of the total mechanical
energy from the supernovae and solar winds is being radiated away. This implies
that radiative cooling through OVI is not significantly inhibiting the growth
of the outflowing gas. In contradiction to the findings of Bergvall et al 2006,
we find no convincing evidence of Lyman continuum leakage in Haro 11. We
conclude that the wind has not created a `tunnel' allowing the escape of a
significant fraction of Lyman continuum photons and place a limit on the escape
fraction of f_{esc}<2%. Overall, both Haro 11 and a previously observed LBG
analogue VV 114, provide an invaluable insight into the X-ray and FUV
properties of high redshift LBGs.Comment: Accepted for publication in ApJ, 40 pages, 17 figure
High-resolution X-ray spectroscopy and imaging of the nuclear outflow of the starburst galaxy NGC 253
Aims: Using XMM-Newton data, we have aimed to study the nuclear outflow of
the nearby starburst galaxy NGC 253 in X-rays with respect to its morphology
and to spectral variations along the outflow. Methods: We analysed XMM-Newton
RGS spectra, RGS brightness profiles in cross-dispersion direction, narrow band
RGS and EPIC images and EPIC PN brightness profiles of the nuclear region and
of the outflow of NGC 253. Results: We detect a diversity of emission lines
along the outflow of NGC 253. This includes the He-like ions of Si, Mg, Ne and
O and their corresponding ions in the next higher ionisation state.
Additionally transitions from Fe XVII and Fe XVIII are prominent. The derived
temperatures from line ratios along the outflow range from 0.21+/-0.01 to
0.79+/-0.06 keV and the ratio of Fe XVII lines indicates a predominantly
collisionally ionised plasma. Additionally we see indications of a recombining
or underionized plasma in the Fe XVII line ratio. Derived electron densities
are 0.106+/-0.018 cm^-3 for the nuclear region and 0.025+/-0.003 cm^-3 for the
outflow region closest to the centre. The RGS image in the O VIII line energy
clearly shows the morphology of an outflow extending out to ~750 pc along the
south-east minor axis, while the north-west part of the outflow is not seen in
O VIII due to the heavy absorption by the galactic disc. This is the first time
that the hot wind fluid has been detected directly. The limb brightening seen
in Chandra and XMM-Newton EPIC observations is only seen in the energy range
containing the Fe XVII lines (550-750 eV). In all other energy ranges between
400 and 2000 eV no clear evidence of limb brightening could be detected.Comment: 14 pages, 7 figures, 3 tables, accepted for publication on A&A, v2:
typos corrected, electron densities and table with emission line flux added,
discussion improve
Ferromagnetic redshift of the optical gap in GdN
We report measurements of the optical gap in a GdN film at temperatures from
300 to 6K, covering both the paramagnetic and ferromagnetic phases. The gap is
1.31eV in the paramagnetic phase and red-shifts to 0.9eV in the spin-split
bands below the Curie temperature. The paramagnetic gap is larger than was
suggested by very early experiments, and has permitted us to refine a
(LSDA+U)-computed band structure. The band structure was computed in the full
translation symmetry of the ferromagnetic ground state, assigning the
paramagnetic-state gap as the average of the majority- and minority-spin gaps
in the ferromagnetic state. That procedure has been further tested by a band
structure in a 32-atom supercell with randomly-oriented spins. After fitting
only the paramagnetic gap the refined band structure then reproduces our
measured gaps in both phases by direct transitions at the X point.Comment: 5 pages, 4 figure
Recommended from our members
Development of a massively parallel parachute performance prediction code
The Department of Energy has given Sandia full responsibility for the complete life cycle (cradle to grave) of all nuclear weapon parachutes. Sandia National Laboratories is initiating development of a complete numerical simulation of parachute performance, beginning with parachute deployment and continuing through inflation and steady state descent. The purpose of the parachute performance code is to predict the performance of stockpile weapon parachutes as these parachutes continue to age well beyond their intended service life. A new massively parallel computer will provide unprecedented speed and memory for solving this complex problem, and new software will be written to treat the coupled fluid, structure and trajectory calculations as part of a single code. Verification and validation experiments have been proposed to provide the necessary confidence in the computations
- …