4,319 research outputs found
Planetary Core Formation with Collisional Fragmentation and Atmosphere to Form Gas Giant Planets
Massive planetary cores ( Earth masses) trigger rapid gas accretion
to form gas giant planets \rev{such as} Jupiter and Saturn. We investigate the
core growth and the possibilities for cores to reach such a critical core mass.
At the late stage, planetary cores grow through collisions with small
planetesimals. Collisional fragmentation of planetesimals, which is induced by
gravitational interaction with planetary cores, reduces the amount of
planetesimals surrounding them, and thus the final core masses. Starting from
small planetesimals that the fragmentation rapidly removes, less massive cores
are formed. However, planetary cores acquire atmospheres that enlarge their
collisional cross section before rapid gas accretion. Once planetary cores
exceed about Mars mass, atmospheres significantly accelerate the growth of
cores. We show that, taking into account the effects of fragmentation and
atmosphere, initially large planetesimals enable formation of sufficiently
massive cores. On the other hand, because the growth of cores is slow for large
planetesimals, a massive disk is necessary for cores to grow enough within a
disk lifetime. If the disk with 100\,km-sized initial planetesimals is 10 times
as massive as the minimum mass solar nebula, planetary cores can exceed 10
Earth masses in the Jovian planet region (AU).Comment: accepted for publication in Ap
Habituation to novel visual vestibular environments with special reference to space flight
The etiology of space motion sickness and the underlying physiological mechanisms associated with spatial orientation in a space environment were investigated. Human psychophysical experiments were used as the basis for the research concerning the interaction of visual and vestibular cues in the development of motion sickness. Particular emphasis is placed on the conflict theory in terms of explaining these interactions. Research on the plasticity of the vestibulo-ocular reflex is discussed
Random skew plane partitions with a piecewise periodic back wall
Random skew plane partitions of large size distributed according to an
appropriately scaled Schur process develop limit shapes. In the present work we
consider the limit of large random skew plane partitions where the inner
boundary approaches a piecewise linear curve with non-lattice slopes,
describing the limit shape and the local fluctuations in various regions. This
analysis is fairly similar to that in [OR2], but we do find some new behavior.
For instance, the boundary of the limit shape is now a single smooth (not
algebraic) curve, whereas the boundary in [OR2] is singular. We also observe
the bead process introduced in [B] appearing in the asymptotics at the top of
the limit shape.Comment: 24 pages. This version to appear in Annales Henri Poincar
The CAVE (TM) automatic virtual environment: Characteristics and applications
Virtual reality may best be defined as the wide-field presentation of computer-generated, multi-sensory information that tracks a user in real time. In addition to the more well-known modes of virtual reality -- head-mounted displays and boom-mounted displays -- the Electronic Visualization Laboratory at the University of Illinois at Chicago recently introduced a third mode: a room constructed from large screens on which the graphics are projected on to three walls and the floor. The CAVE is a multi-person, room sized, high resolution, 3D video and audio environment. Graphics are rear projected in stereo onto three walls and the floor, and viewed with stereo glasses. As a viewer wearing a location sensor moves within its display boundaries, the correct perspective and stereo projections of the environment are updated, and the image moves with and surrounds the viewer. The other viewers in the CAVE are like passengers in a bus, along for the ride. 'CAVE,' the name selected for the virtual reality theater, is both a recursive acronym (Cave Automatic Virtual Environment) and a reference to 'The Simile of the Cave' found in Plato's 'Republic,' in which the philosopher explores the ideas of perception, reality, and illusion. Plato used the analogy of a person facing the back of a cave alive with shadows that are his/her only basis for ideas of what real objects are. Rather than having evolved from video games or flight simulation, the CAVE has its motivation rooted in scientific visualization and the SIGGRAPH 92 Showcase effort. The CAVE was designed to be a useful tool for scientific visualization. The Showcase event was an experiment; the Showcase chair and committee advocated an environment for computational scientists to interactively present their research at a major professional conference in a one-to-many format on high-end workstations attached to large projection screens. The CAVE was developed as a 'virtual reality theater' with scientific content and projection that met the criteria of Showcase
Considerations for the future development of virtual technology as a rehabilitation tool
BACKGROUND: Virtual environments (VE) are a powerful tool for various forms of rehabilitation. Coupling VE with high-speed networking [Tele-Immersion] that approaches speeds of 100 Gb/sec can greatly expand its influence in rehabilitation. Accordingly, these new networks will permit various peripherals attached to computers on this network to be connected and to act as fast as if connected to a local PC. This innovation may soon allow the development of previously unheard of networked rehabilitation systems. Rapid advances in this technology need to be coupled with an understanding of how human behavior is affected when immersed in the VE. METHODS: This paper will discuss various forms of VE that are currently available for rehabilitation. The characteristic of these new networks and examine how such networks might be used for extending the rehabilitation clinic to remote areas will be explained. In addition, we will present data from an immersive dynamic virtual environment united with motion of a posture platform to record biomechanical and physiological responses to combined visual, vestibular, and proprioceptive inputs. A 6 degree-of-freedom force plate provides measurements of moments exerted on the base of support. Kinematic data from the head, trunk, and lower limb was collected using 3-D video motion analysis. RESULTS: Our data suggest that when there is a confluence of meaningful inputs, neither vision, vestibular, or proprioceptive inputs are suppressed in healthy adults; the postural response is modulated by all existing sensory signals in a non-additive fashion. Individual perception of the sensory structure appears to be a significant component of the response to these protocols and underlies much of the observed response variability. CONCLUSION: The ability to provide new technology for rehabilitation services is emerging as an important option for clinicians and patients. The use of data mining software would help analyze the incoming data to provide both the patient and the therapist with evaluation of the current treatment and modifications needed for future therapies. Quantification of individual perceptual styles in the VE will support development of individualized treatment programs. The virtual environment can be a valuable tool for therapeutic interventions that require adaptation to complex, multimodal environments
“I don’t use a condom (with my regular partner) because I know that I’m faithful, but with everyone else I do”: The cultural and socioeconomic determinants of sexual partner concurrency in young South Africans
Finding ways to reduce the high rates of sexual partner concurrency is increasingly believed to be vital to controlling HIV spread in southern Africa. We describe the frequency and correlates of sexual partner concurrency in a representative sample of 3 324 young South Africans aged 14-25. Of the 2 468 individuals who were sexually active 21% had engaged in concurrent sexual partnerships. Multivariate analysis revealed that concurrency was more common with males, Africans, those who knew their partner had anotherpartner, early age of sexual debut, four or more lifetime sexual partners, alcohol consumption, and self-perception of being at high risk for acquisition of HIV. If the respondent’s partner knew his or her friends (termed high structural embeddedness) this was associated with a 52% reduction in concurrency rates. There are significant differences in both the rates of concurrency and the risk factors underpinning these in the different racial/ethnic groups. Analysis of these underlying determinants suggests that cultural rather than socioeconomic factors predominate, which has important implications for the design and targeting of prevention efforts
Planetary Growth with Collisional Fragmentation and Gas Drag
As planetary embryos grow, gravitational stirring of planetesimals by embryos
strongly enhances random velocities of planetesimals and makes collisions
between planetesimals destructive. The resulting fragments are ground down by
successive collisions. Eventually the smallest fragments are removed by the
inward drift due to gas drag. Therefore, the collisional disruption depletes
the planetesimal disk and inhibits embryo growth. We provide analytical
formulae for the final masses of planetary embryos, taking into account
planetesimal depletion due to collisional disruption. Furthermore, we perform
the statistical simulations for embryo growth (which excellently reproduce
results of direct -body simulations if disruption is neglected). These
analytical formulae are consistent with the outcome of our statistical
simulations. Our results indicate that the final embryo mass at several AU in
the minimum-mass solar nebula can reach about Earth mass within
years. This brings another difficulty in formation of gas giant planets,
which requires cores with Earth masses for gas accretion. However, if
the nebular disk is 10 times more massive than the minimum-mass solar nebula
and the initial planetesimal size is larger than 100 km, as suggested by some
models of planetesimal formation, the final embryo mass reaches about 10 Earth
masses at 3-4 AU. The enhancement of embryos' collisional cross sections by
their atmosphere could further increase their final mass to form gas giant
planets at 5-10 AU in the solar system.Comment: Accepted for publication in Icaru
The statistics of the photometric accuracy based on MASS data and the evaluation of high-altitude wind
The effect of stellar scintillation on the accuracy of photometric
measurements is analyzed. We obtain a convenient form of estimaton of this
effect in the long exposure regime, when the turbulence shift produced by the
wind is much larger than the aperture of the telescope. A simple method is
proposed to determine index introduced by perture of the Kenyon et al.
(2006), directly from the measurements with the Multi Aperture Scintillation
Sensor (MASS) without information on vertical profile of the wind. The
statistics resulting from our campaign of 2005 -- 2007 at Maidanak
observatory is presented. It is shown that these data can be used to estimate
high-altitude winds at pressure level 70 -- 100 mbar. Comparison with the wind
speed retrieved from the NCEP/NCAR global models shows a good agreement. Some
prospects for retrieval of the wind speed profile from the MASS measurements
are outlined.Comment: 11 pages, 9 figures, accepted for publication in Astronomy Letter
Debris Disks: Seeing Dust, Thinking of Planetesimals and Planets
Debris disks are optically thin, almost gas-free dusty disks observed around
a significant fraction of main-sequence stars older than about 10 Myr. Since
the circumstellar dust is short-lived, the very existence of these disks is
considered as evidence that dust-producing planetesimals are still present in
mature systems, in which planets have formed - or failed to form - a long time
ago. It is inferred that these planetesimals orbit their host stars at asteroid
to Kuiper-belt distances and continually supply fresh dust through mutual
collisions. This review outlines observational techniques and results on debris
disks, summarizes their essential physics and theoretical models, and then
places them into the general context of planetary systems, uncovering
interrelations between the disks, dust parent bodies, and planets. It is shown
that debris disks can serve as tracers of planetesimals and planets and shed
light on the planetesimal and planet formation processes that operated in these
systems in the past.Comment: Review paper, accepted for publication in "Research in Astronomy and
Astrophysics
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