13,285 research outputs found
On the Maximum Mass of Accreting Primordial Supermassive Stars
Supermassive primordial stars are suspected to be the progenitors of the most
massive quasars at z~6. Previous studies of such stars were either unable to
resolve hydrodynamical timescales or considered stars in isolation, not in the
extreme accretion flows in which they actually form. Therefore, they could not
self-consistently predict their final masses at collapse, or those of the
resulting supermassive black hole seeds, but rather invoked comparison to
simple polytropic models. Here, we systematically examine the birth, evolution
and collapse of accreting non-rotating supermassive stars under accretion rates
of 0.01-10 solar masses per year, using the stellar evolution code KEPLER. Our
approach includes post-Newtonian corrections to the stellar structure and an
adaptive nuclear network, and can transition to following the hydrodynamic
evolution of supermassive stars after they encounter the general relativistic
instability. We find that this instability triggers the collapse of the star at
masses of 150,000-330,000 solar masses for accretion rates of 0.1-10 solar
masses per year, and that the final mass of the star scales roughly
logarithmically with the rate. The structure of the star, and thus its
stability against collapse, is sensitive to the treatment of convection, and
the heat content of the outer accreted envelope. Comparison with other codes
suggests differences here may lead to small deviations in the evolutionary
state of the star as a function of time, that worsen with accretion rate. Since
the general relativistic instability leads to the immediate death of these
stars, our models place an upper limit on the masses of the first quasars at
birth.Comment: 5 pages, 4 figures. Accepted ApJ letter
Airborne measurements of cloud-forming nuclei and aerosol particles in stabilized ground clouds produced by solid rocket booster firings
Airborne measurements of cloud volumes, ice nuclei and cloud condensation nuclei, liquid particles, and aerosol particles were obtained from stabilized ground clouds (SGCs) produced by Titan 3 launches at Kennedy Space Center, 20 August and 5 September 1977. The SGCs were bright, white, cumulus clouds early in their life and contained up to 3.5 g/m3 of liquid in micron to millimeter size droplets. The measured cloud volumes were 40 to 60 cu km five hours after launch. The SGCs contained high concentrations of cloud condensation nuclei active at 0.2%, 0.5%, and 1.0% supersaturation for periods of three to five hours. The SGCs also contained high concentrations of submicron particles. Three modes existed in the particle population: a 0.05 to 0.1 micron mode composed of aluminum-containing particles, a 0.2 to 0.8 micron mode, and a 2.0 to 10 micron mode composed of particles that contained primarily aluminum
Dynamics Of Australian Dairy-Food Supply Chain: Strategic Options For Participants In A Deregulated Environment
Following deregulation, participants in Australian dairy-food supply chains are confronted with a more complex and rapidly changing environment. In a study conducted between March 2002 and April 2003, major supermarkets emerged as the dominant power in chain development, with a trend towards greater interdependence and coordination between the chain participants. Future supply chain development will depend on the capabilities of the chain participants in operational and strategic management within the firm, and also in successfully negotiating linkages within the chains. In addition the organizational structures of both the firms and the chains need to be responsive to changing end-user needs and the dynamic business environment
LIN 358: A symbiotic binary accreting above the steady hydrogen fusion limit
Symbiotic binaries are long period interacting binaries consisting of a white
dwarf (WD) accreting material from a cool evolved giant star via stellar winds.
In this paper we study the symbiotic binary LIN 358 located in the SMC. We have
observed LIN 358 with the integral field spectrograph WiFeS and obtained its
line emission spectrum. With the help of the plasma simulation and spectral
synthesis code Cloudy, we have constructed a 2D photo-ionisation model of LIN
358. From comparison with the observations, we have determined the colour
temperature of the WD in LIN 358 to be 19 eV, its bolometric luminosity erg s, and the mass-loss rate from the
donor star to be M yr. Assuming a solar
H to He ratio in the wind material, a lower limit to the accreted mass fraction
in LIN 358 is 0.31. The high mass-accretion efficiency of a wind Roche lobe
overflow implies that the WD is accreting above the upper boundary of stable
hydrogen fusion and thus growing in mass with the maximal rate of M yr. This causes the WD photosphere to
expand, which explains its low colour temperature. Our calculations show that
the circumstellar material in LIN 358 is nearly completely ionized except for a
narrow cone around the donor star, and that the WD emission is freely escaping
the system. However, due to its low colour temperature, this emission can be
easily attenuated by even moderate amounts of neutral ISM. We speculate that
other symbiotic systems may be operating in a similar regime, thus explaining
the paucity of observed systems.Comment: 14 pages, 13 figures. Accepted for publication in MNRA
Cosmic cookery : making a stereoscopic 3D animated movie.
This paper describes our experience making a short stereoscopic movie visualizing the development of structure in
the universe during the 13.7 billion years from the Big Bang to the present day. Aimed at a general audience for
the Royal Society's 2005 Summer Science Exhibition, the movie illustrates how the latest cosmological theories
based on dark matter and dark energy are capable of producing structures as complex as spiral galaxies and
allows the viewer to directly compare observations from the real universe with theoretical results. 3D is an
inherent feature of the cosmology data sets and stereoscopic visualization provides a natural way to present the
images to the viewer, in addition to allowing researchers to visualize these vast, complex data sets.
The presentation of the movie used passive, linearly polarized projection onto a 2m wide screen but it was
also required to playback on a Sharp RD3D display and in anaglyph projection at venues without dedicated
stereoscopic display equipment. Additionally lenticular prints were made from key images in the movie. We
discuss the following technical challenges during the stereoscopic production process; 1) Controlling the depth
presentation, 2) Editing the stereoscopic sequences, 3) Generating compressed movies in display speci¯c formats.
We conclude that the generation of high quality stereoscopic movie content using desktop tools and equipment
is feasible. This does require careful quality control and manual intervention but we believe these overheads
are worthwhile when presenting inherently 3D data as the result is signi¯cantly increased impact and better
understanding of complex 3D scenes
A stacking-fault based microscopic model for platelets in diamond
We propose a new microscopic model for the planar defects in
diamond commonly called platelets. This model is based on the formation of a
metastable stacking fault, which can occur because of the ability of carbon to
stabilize in different bonding configurations. In our model the core of the
planar defect is basically a double layer of three-fold coordinated
carbon atoms embedded in the common diamond structure. The properties of
the model were determined using {\it ab initio} total energy calculations. All
significant experimental signatures attributed to the platelets, namely, the
lattice displacement along the direction, the asymmetry between the
and the directions, the infrared absorption peak
, and broad luminescence lines that indicate the introduction of
levels in the band gap, are naturally accounted for in our model. The model is
also very appealing from the point of view of kinetics, since naturally
occurring shearing processes will lead to the formation of the metastable
fault.Comment: 5 pages, 4 figures. Submitted for publication on August 2nd, 200
Resonance line-profile calculations based on hydrodynamical models of cataclysmic variable winds
We present synthetic line profiles as predicted by the models of 2-D line-
driven disk winds due to Proga, Stone & Drew. We compare the model line
profiles with HST observations of the cataclysmic variable IX Vel. The model
wind consists of a slow outflow that is bounded on the polar side by a fast
stream. We find that these two components of the wind produce distinct spectral
features. The fast stream produces profiles which show features consistent with
observations. These include the appearance of the P-Cygni shape for a range of
inclinations, the location of the maximum depth of the absorption component at
velocities less than the terminal velocity, and the transition from absorption
to emission with increasing inclination. However the model profiles have too
little absorption or emission equivalent width. This quantitative difference
between our models and observations is not a surprise because the line-driven
wind models predict a mass loss rate that is lower than the rate required by
the observations. We note that the model profiles exhibit a double-humped
structure near the line center which is not echoed in observations. We identify
this structure with a non-negligible redshifted absorption which is formed in
the slow component of the wind where the rotational velocity dominates over
expansion velocity. We conclude that the next generation of disk wind models,
developed for application to CVs, needs to yield stronger wind driving out to
larger disk radii than do the present models.Comment: LaTeX, 19 pages, to appear in Ap
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