1,964 research outputs found
Evolution of iron core white dwarfs
Recent measurements made by Hipparcos (Provencal et al. 1998) present
observational evidence supporting the existence of some white dwarf (WD) stars
with iron - rich, core composition. In this connection, the present paper is
aimed at exploring the structure and evolution of iron - core WDs by means of a
detailed and updated evolutionary code. In particular, we examine the evolution
of the central conditions, neutrino luminosity, surface gravity,
crystallization, internal luminosity profiles and ages. We find that the
evolution of iron - rich WDs is markedly different from that of their carbon -
oxygen counterparts. In particular, cooling is strongly accelerated as compared
with the standard case. Thus, if iron WDs were very numerous, some of them
would have had time enough to evolve at lower luminosities than that
corresponding to the fall - off in the observed WD luminosity function.Comment: 8 pages, 21 figures. Accepted for publication in MNRA
Charged Condensate and Helium Dwarf Stars
White dwarf stars composed of carbon, oxygen or heavier elements are expected
to crystallize as they cool down below certain temperatures. Yet, simple
arguments suggest that the helium white dwarf cores may not solidify, mostly
because of zero-point oscillations of the helium ions that would dissolve the
crystalline structure. We argue that the interior of the helium dwarfs may
instead form a macroscopic quantum state in which the charged helium-4 nuclei
are in a Bose-Einstein condensate, while the relativistic electrons form a
neutralizing degenerate Fermi liquid. We discuss the electric charge screening,
and the spectrum of this substance, showing that the bosonic long-wavelength
fluctuations exhibit a mass gap. Hence, there is a suppression at low
temperatures of the boson contribution to the specific heat -- the latter being
dominated by the specific heat of the electrons near the Fermi surface. This
state of matter may have observational signatures.Comment: 10 pages; v2: to appear in JCAP, brief comments and section titles
added, typos correcte
Analysis of defect structure in silicon. Characterization of SEMIX material. Silicon sheet growth development for the large area silicon sheet task of the low-cost solar array project
Statistically significant quantitative structural imperfection measurements were made on samples from ubiquitous crystalline process (UCP) Ingot 5848 - 13C. Important correlation was obtained between defect densities, cell efficiency, and diffusion length. Grain boundary substructure displayed a strong influence on the conversion efficiency of solar cells from Semix material. Quantitative microscopy measurements gave statistically significant information compared to other microanalytical techniques. A surface preparation technique to obtain proper contrast of structural defects suitable for quantimet quantitative image analyzer (QTM) analysis was perfected and is used routinely. The relationships between hole mobility and grain boundary density was determined. Mobility was measured using the van der Pauw technique, and grain boundary density was measured using quantitative microscopy technique. Mobility was found to decrease with increasing grain boundary density
Effects of low surfactant Sb coverage on Zn and C incorporation in GaP
Journal ArticleThe use of surfactants during the vapor phase growth of III-V materials to control fundamental characteristics of epitaxial layers is becoming increasingly important. We have investigated the remarkable effects of Sb, from triethylantimony (TESb) Pyrolysis, on the Zn doping during the organometallic vapor phase epitaxial growth (OMVPE) of GaP. Antimony is isoelectric with the P host; therefore it is not a dopant in this material. It is also much larger than P so little incorporation occurs
Urban grasslands support threatened water voles
Urbanisation is often linked with habitat loss and a reduction in species richness but some species may be able to adapt to urban environments. Water voles Arvicola amphibius, a rapidly declining species in the UK, have recently been recorded in isolated grassland habitats in Glasgow, Scotland’s largest city (human population 1.2 million). The aim of this study was to determine the distribution and habitat characteristics of water vole populations occupying these dry grasslands. Field work was undertaken from March to October 2014 in a 34 km2 study area located 3 km east of the city centre. Field sign transects recorded water vole presence in 21/65 (32%) and 19/62 (31%) surveyed sites in spring and autumn, respectively. Vole occupancy increased with distance from water and was greatest in parkland, followed by sites with rank vegetation and roadside habitats. Occupancy was lower where signs of predators were recorded but surprisingly occupancy was found to be greater in the most disturbed sites, perhaps linked to the fact that many of these sites were public parks containing suitable grassland. Sites occupied by water voles were classed as neutral grasslands with species composition dominated by two main species. The number of grassland sites occupied by water voles, especially within public areas suggests that careful management of these urban grassland habitats will benefit the conservation of this highly threatened species in the UK
Development and operation of research-scale III-V nanowire growth reactors
III-V nanowires are useful platforms for studying the electronic and
mechanical properties of materials at the nanometer scale. However, the costs
associated with commercial nanowire growth reactors are prohibitive for most
research groups. We developed hot-wall and cold-wall metal organic vapor phase
epitaxy (MOVPE) reactors for the growth of InAs nanowires, which both use the
same gas handling system. The hot-wall reactor is based on an inexpensive
quartz tube furnace and yields InAs nanowires for a narrow range of operating
conditions. Improvement of crystal quality and an increase in growth run to
growth run reproducibility are obtained using a homebuilt UHV cold-wall reactor
with a base pressure of 2 X 10 Torr. A load-lock on the UHV reactor
prevents the growth chamber from being exposed to atmospheric conditions during
sample transfers. Nanowires grown in the cold-wall system have a low defect
density, as determined using transmission electron microscopy, and exhibit
field effect gating with mobilities approaching 16,000 cm(V.s).Comment: Related papers at http://pettagroup.princeton.ed
Correcting for Distortions due to Ionization in the STAR TPC
Physics goals of the STAR Experiment at RHIC in recent (and future) years
drive the need to operate the STAR TPC at ever higher luminosities, leading to
increased ionization levels in the TPC gas. The resulting ionic space charge
introduces field distortions in the detector which impact tracking performance.
Further complications arise from ionic charge leakage into the main TPC volume
from the high gain anode region. STAR has implemented corrections for these
distortions based on measures of luminosity, which we present here.
Additionally, we highlight a novel approach to applying the corrections on an
event-by-event basis applicable in conditions of rapidly varying ionization
sources.Comment: 6 pages, 7 figures, proceedings of the Workshop on Tracking in High
Multiplicity Environments (TIME 05) in Zurich, Switzerland, submitted to
Nucl. Instr. and Meth.
The infrared properties of the new outburst star IRAS 05436-0007 in quiescent phase
We compiled and investigated the infrared/sub-mm/mm SED of the new outburst
star IRAS 05436-0007 in quiescent phase. The star is a flat-spectrum source,
with an estimated total luminosity of L_bol ~ 5.6 L_sun, typical of low-mass T
Tauri stars. The derived circumstellar mass of 0.5 M_sun is rather high among
low-mass YSOs. The observed SED differs from the SEDs of typical T Tauri stars
and of 4 well-known EXors, and resembles more the SEDs of FU Orionis objects
indicating the presence of a circumstellar envelope. IRAS 05436-0007 seems to
be a Class II source with an age of approximately 4x10^5 yr. In this
evolutionary stage an accretion disk is already fully developed, though a
circumstellar envelope may also be present. Observations of the present
outburst will provide additional knowledge on the source.Comment: 4 pages, 4 figures, to be published in Astronomy & Astrophysics
Letter
Topological Homogeneity for Electron Microscopy Images
In this paper, the concept of homogeneity is defined, from a
topological perspective, in order to analyze how uniform is the material
composition in 2D electron microscopy images. Topological multiresolution
parameters are taken into account to obtain better results than
classical techniques.Ministerio de Economía y Competitividad MTM2016-81030-PMinisterio de Economía y Competitividad TEC2012-37868-C04-0
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