2,975 research outputs found
Ignition criteria for x-ray fast ignition inertial confinement fusion
The derivation of the ignition energy for fast ignition inertial confinement fusion is reviewed and one-dimensional simulations are used to produce a revised formula for the ignition energy of an isochoric central hot-spot, which accounts for variation in the radius of the hot-spot r_h as well as the density rho. The required energy may be as low as 1 kJ when rho*r_h ~ 0:36 g cm^-2; T ~ 20 keV, and rho greater or equal to 700 g cm^-2. Although there are many physical challenges to creating these conditions, a possible route to producing such a hot-spot is via a bright source of nonthermal soft x-rays. Further one-dimensional simulations are used to study the non-thermal soft x-ray heating of dense DT and it is found to offer the potential to significantly reduce hydrodynamic losses as compared to particle driven fast ignition due to the hotspot being heated supersonically in a layer-by-layer fashion. A sufficiently powerful soft x-ray source would be difficult to produce, but line emission from laser-produced-plasma is the most promising option
High Mass Triple Systems: The Classical Cepheid Y Car
We have obtained an HST STIS ultraviolet high dispersion Echelle mode
spectrum the binary companion of the double mode classical Cepheid Y Car. The
velocity measured for the hot companion from this spectrum is very different
from reasonable predictions for binary motion, implying that the companion is
itself a short period binary. The measured velocity changed by 7 km/ s during
the 4 days between two segments of the observation confirming this
interpretation. We summarize "binary" Cepheids which are in fact members of
triple system and find at least 44% are triples. The summary of information on
Cepheids with orbits makes it likely that the fraction is under-estimated.Comment: accepted by A
High Resolution Chandra Spectroscopy of Gamma Cassiopeia (B0.5IVe)
gamma Cas has long been famous for its unique hard X-ray characteristics. We
report herein on a 53 ks Chandra HETGS observation of this target. An
inspection of our spectrum shows that it is quite atypical for a massive star,
with abnormally weak Fe XXV, XXVI lines, Ly-alpha lines of H-like species from
Fe XVII, XXIII, XXIV, S XVI, Si XIV, Mg XII, Ne X, O VII, VIII, and N VII.
Also, line ratios of the rif-triplet of for a few He-like ions XVII are
consistent with the dominance of collisional atomic processes. Yet, the
presence of Fe and Si fluorescence K features indicates that photoionization
also occurs in nearby cold gas. The line profiles indicate a mean velocity at
rest and a broadening of 500 km/s. A global fitting analysis of the line and
continuum spectrum finds that there are 3-4 plasma emission components. The
dominant hot (12 keV) component and has a Fe abundance of 0.22 solar. Some
fraction of this component (10-30%) is heavily absorbed. The other 2-3
components, with temperatures 0.1, 0.4, 3 keV, are "warm," have a nearly solar
composition, a lower column absorption, and are responsible for most other
emission lines. The strength of the fluorescence features and the dual-column
absorption model for the hot plasma component suggest the presence near the hot
sites of a cold gas structure with a column density of 10^23 cm^-2. Since this
value is consistent with theoretical estimates of the vertical disk column of
this star, these attributes suggest that the X-rays originate near the star or
disk. It is possible that the Fe anomaly in the hot component is related to the
First Ionization Potential effect found in coronal structures around active
cool stars. This would be yet another indication that the X-rays -rays are
produced in the immediate vicinity of the Be star.Comment: 32 pages, 4 figures (Fig. 3 colorized.) To be published in 01/10/04
Astrophysical Journal, Main Journal; included figures and updated formattin
Improving Underrepresented Minority Student Persistence in STEM.
Members of the Joint Working Group on Improving Underrepresented Minorities (URMs) Persistence in Science, Technology, Engineering, and Mathematics (STEM)-convened by the National Institute of General Medical Sciences and the Howard Hughes Medical Institute-review current data and propose deliberation about why the academic "pathways" leak more for URM than white or Asian STEM students. They suggest expanding to include a stronger focus on the institutional barriers that need to be removed and the types of interventions that "lift" students' interests, commitment, and ability to persist in STEM fields. Using Kurt Lewin's planned approach to change, the committee describes five recommendations to increase URM persistence in STEM at the undergraduate level. These recommendations capitalize on known successes, recognize the need for accountability, and are framed to facilitate greater progress in the future. The impact of these recommendations rests upon enacting the first recommendation: to track successes and failures at the institutional level and collect data that help explain the existing trends
A spectroscopic study of IRAS F10214+4724
The z=2.286 IRAS galaxy F10214+4724 remains one of the most luminous galaxies
in the Universe, despite its gravitational lens magnification. We present
optical and near-infrared spectra of F10214+4724, with clear evidence for three
distinct components: lines of width ~1000 km/s from a Seyfert-II nucleus; <~200
km/s lines which are likely to be associated with star formation; and a broad
~4000 km/s CIII] 1909ang emission line which is blue-shifted by ~1000 km/s with
respect to the Seyfert-II lines. Our study of the Seyfert-II component leads to
several new results, including: (i) From the double-peaked structure in the Ly
alpha line, and the lack of Ly beta, we argue that the Ly alpha photons have
emerged through a neutral column of N_H ~ 2.5 x 10^{25}/m^2, possibly located
within the AGN narrow-line region as argued in several high redshift
radiogalaxies. (ii) The resonant O VI 1032,1036ang doublet (previously
identified as Ly beta) is in an optically thick (1:1) ratio. At face value this
implies an an extreme density (n_e ~ 10^{17}/m^3) more typical of broad line
region clouds. However, we attribute this instead to the damping wings of Ly
beta from the resonant absorption. (iii) A tentative detection of HeII 1086
suggests little extinction in the rest-frame ultraviolet.Comment: Accepted for publication in MNRAS. Uses BoxedEPS (included
350 Micron Dust Emission from High Redshift Objects
We report observations of a sample of high redshift sources (1.8<z<4.7),
mainly radio-quiet quasars, at 350 microns using the SHARC bolometer camera at
the Caltech Submillimeter Observatory. Nine sources were detected (>4-sigma)
and upper limits were obtained for 11 with 350 micron flux density limits
(3-sigma) in the range 30-125mJy. Combining published results at other
far-infrared and millimeter wavelengths with the present data, we are able to
estimate the temperature of the dust, finding relatively low values, averaging
50K. From the spectral energy distribution, we derive dust masses of a few 10^8
M_sun and luminosities of 4-33x10^{12} L_sun (uncorrected for any
magnification) implying substantial star formation activity. Thus both the
temperature and dust masses are not very different from those of local
ultraluminous infrared galaxies. For this redshift range, the 350 micron
observations trace the 60-100 micron rest frame emission and are thus directly
comparable with IRAS studies of low redshift galaxies.Comment: 5 pages, 2 PS figures. Accepted for publication in Astrophysical
Journal Letter
Long-term outcome after early infrainguinal graft failure
AbstractPurpose: To determine the long-term outcome and prognostic factors after early infrainguinal graft failure (<30 days).Methods: Retrospective analysis of limb salvage data, patency data, and prognostic risk factors in 112 new infrainguinal bypass grafts from 1985 to 1995 that occluded within 30 days of operation.Result: Thirty-six femoropopliteal and 76 femorotibial/femoropedal arterial bypass (âindexâ) procedures were performed for rest pain (50%), tissue loss (31%), or disabling claudication (19%). In 103 patients, an immediate additional revascularization (âtakebackâ) procedure was performed at the time of early graft failure. Life table analysis of the takeback procedures for threatened limbs (n = 84) revealed limb salvage rates of 74%, 54%, 40%, and 31% at 1 month, 1 year, 3 years, and 5 years, respectively. The 1-month limb salvage rate (threatened limbs) was 12% (1 of 8) in patients who were not taken back for revascularization and 33% (4 of 12) in patients who had undergone more than one takeback procedure within 30 days. The secondary graft patency rates for the takeback procedures (n = 103) were 70%, 37%, 27%, and 23% at 1 month, 1 year, 3 years, and 5 years, respectively. Univariate and life table analysis revealed that patients who were given anticoagulation medication after the index procedure (before graft thrombosis) or patients who had undergone previous ipsilateral leg revascularization had significantly lower rates of limb salvage and graft patency (p < 0.05). The limb salvage rate was also significantly worse in patients who had single-vessel runoff compared with those who had multiple-vessel runoff (p < 0.01). Thrombectomy and revision or complete graft replacement had a better secondary patency rate than thrombectomy alone (p < 0.05). Autogenous vein grafts had better outcome than polytetrafluoroethylene-containing grafts, but statistical significance was not achieved. No significant differences in limb salvage or graft patency rates were found between femoropopliteal versus femorotibial/femoropedal bypass grafting, age, gender, previous inflow surgery, diabetes, hypertension, smoking, or cardiac, renal, or pulmonary disease.Conclusion: The long-term limb salvage and graft patency rates after takeback revascularization procedures for early graft failure are poor. Despite poor outcome, a single takeback procedure appears warranted in all patients. Multiple takeback procedures, however, do not appear to be justified, especially in patients who are given anticoagulation medication after the index bypass procedure, repeat leg bypass procedures, or if there is no potential for graft revision
Dealing with mobility: Understanding access anytime, anywhere
The rapid and accelerating move towards the adoption and use of mobile technologies has increasingly provided people and organisations with the ability to work away from the office and on the move. The new ways of working afforded by these technologies are often characterised in terms of access to information and people âanytime, anywhereâ. This paper presents a study of mobile workers that highlights different facets of access to remote people and information, and different facets of anytime, anywhere. Four key factors in mobile work are identified from the study: the role of planning, working in âdead timeâ, accessing remote technological and informational resources, and monitoring the activities of remote colleagues. By reflecting on these issues, we can better understand the role of technology and artefact use in mobile work and identify the opportunities for the development of appropriate technological solutions to support mobile workers
Subnanosecond spectral diffusion measurement using photon correlation
Spectral diffusion is a result of random spectral jumps of a narrow line as a
result of a fluctuating environment. It is an important issue in spectroscopy,
because the observed spectral broadening prevents access to the intrinsic line
properties. However, its characteristic parameters provide local information on
the environment of a light emitter embedded in a solid matrix, or moving within
a fluid, leading to numerous applications in physics and biology. We present a
new experimental technique for measuring spectral diffusion based on photon
correlations within a spectral line. Autocorrelation on half of the line and
cross-correlation between the two halves give a quantitative value of the
spectral diffusion time, with a resolution only limited by the correlation
set-up. We have measured spectral diffusion of the photoluminescence of a
single light emitter with a time resolution of 90 ps, exceeding by four orders
of magnitude the best resolution reported to date
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