160 research outputs found
The surprisingly low carbon mass in the debris disk around HD 32297
Gas has been detected in a number of debris disks. It is likely secondary,
i.e. produced by colliding solids. Here, we report ALMA Band 8 observations of
neutral carbon in the CO-rich debris disk around the 15--30 Myr old A-type star
HD 32297. We find that C is located in a ring at 110 au with a FWHM
of 80 au, and has a mass of M.
Naively, such a surprisingly small mass can be accumulated from CO
photo-dissociation in a time as short as 10 yr. We develop a simple
model for gas production and destruction in this system, properly accounting
for CO self-shielding and shielding by neutral carbon, and introducing a
removal mechanism for carbon gas. We find that the most likely scenario to
explain both C and CO observations, is one where the carbon gas is rapidly
removed on a timescale of order a thousand years and the system maintains a
very high CO production rate of 15 M Myr, much higher
than the rate of dust grind-down. We propose a possible scenario to meet these
peculiar conditions: the capture of carbon onto dust grains, followed by rapid
CO re-formation and re-release. In steady state, CO would continuously be
recycled, producing a CO-rich gas ring that shows no appreciable spreading over
time. This picture might be extended to explain other gas-rich debris disks.Comment: accepted for publication in the Ap
Community structure of Pleistocene coral reefs of Curaçao, Netherlands Antilles
The Quaternary fossil record of living coral reefs is fundamental for understanding modern ecological patterns. Living reefs generally accumulate in place, so fossil reefs record a history of their former biological inhabitants and physical environments. Reef corals record their ecological history especially well because they form large, resistant skeletons, which can be identified to species. Thus, presence-absence and relative abundance data can be obtained with a high degree of confidence. Moreover, potential effects of humans on reef ecology were absent or insignificant on most reefs until the last few hundred years, so that it is possible to analyze "natural" distribution patterns before intense human disturbance began. We characterized Pleistocene reef coral assemblages from Curacao, Netherlands Antilles, Caribbean Sea, focusing on predictability in species abundance patterns from different reef environments over broad spatial scales. Our data set is composed of >2 km of surveyed Quaternary reef. Taxonomic composition showed consistent differences between environments and along secondary environmental gradients within environments. Within environments, taxonomic composition of communities was markedly similar indicating nonrandom species associations and communities composed of species occurring in characteristic abundances. This community similarity was maintained with little change over a 40-km distance. The nonrandom patterns in species abundances were similar to those found in the Caribbean before the effects of extensive anthropogenic degradation of reefs in the late 1970s and early 1980s. The high degree of order observed in species abundance patterns of fossil reef coral communities on a scale of tens of kilometers contrasts markedly with patterns observed in previous small-scale studies of modern reefs. Dominance of Acropora palmata in the reef crest zone and patterns of overlap and nonoverlap of species in the Montastraea ''annularis'' sibling species complex highlight the tendency for distribution and abundance patterns of Pleistocene corals to reflect environmental preferences at multiple spatial scales. Wave energy is probably the most important physical environmental variable structuring these coral communities. The strong similarity between ancient and pre-1980s Caribbean reefs and the nonrandom distribution of coral species in space and time indicate that recent variability noted at much smaller time scales may be due to either unprecedented anthropogenic influences on reefs or fundamentally different patterns at varying spatio-temporal scales
Analytical case study of the Korean Educational Development Institute
노트 : Korean Educational Development Project(1969-1980)Prokect Documents(Volume VI 15)Learning Systems InstituteComplied December, 1980Florida State Universit
Delivery of Dark Material to Vesta via Carbonaceous Chondritic Impacts
NASA's Dawn spacecraft observations of asteroid (4) Vesta reveal a surface
with the highest albedo and color variation of any asteroid we have observed so
far. Terrains rich in low albedo dark material (DM) have been identified using
Dawn Framing Camera (FC) 0.75 {\mu}m filter images in several geologic
settings: associated with impact craters (in the ejecta blanket material and/or
on the crater walls and rims); as flow-like deposits or rays commonly
associated with topographic highs; and as dark spots (likely secondary impacts)
nearby impact craters. This DM could be a relic of ancient volcanic activity or
exogenic in origin. We report that the majority of the spectra of DM are
similar to carbonaceous chondrite meteorites mixed with materials indigenous to
Vesta. Using high-resolution seven color images we compared DM color properties
(albedo, band depth) with laboratory measurements of possible analog materials.
Band depth and albedo of DM are identical to those of carbonaceous chondrite
xenolith-rich howardite Mt. Pratt (PRA) 04401. Laboratory mixtures of Murchison
CM2 carbonaceous chondrite and basaltic eucrite Millbillillie also show band
depth and albedo affinity to DM. Modeling of carbonaceous chondrite abundance
in DM (1-6 vol%) is consistent with howardite meteorites. We find no evidence
for large-scale volcanism (exposed dikes/pyroclastic falls) as the source of
DM. Our modeling efforts using impact crater scaling laws and numerical models
of ejecta reaccretion suggest the delivery and emplacement of this DM on Vesta
during the formation of the ~400 km Veneneia basin by a low-velocity (<2
km/sec) carbonaceous impactor. This discovery is important because it
strengthens the long-held idea that primitive bodies are the source of carbon
and probably volatiles in the early Solar System.Comment: Icarus (Accepted) Pages: 58 Figures: 15 Tables:
Variability of Disk Emission in Pre-Main Sequence and Related Stars. I. HD 31648 and HD 163296 - Isolated Herbig Ae Stars Driving Herbig-Haro Flows
Infrared photometry and spectroscopy covering a time span of a quarter
century are presented for HD 31648 (MWC 480) and HD 163296 (MWC 275). Both are
isolated Herbig Ae stars that exhibit signs of active accretion, including
driving bipolar flows with embedded Herbig-Haro (HH) objects. HD 163296 was
found to be relatively quiescent photometrically in its inner disk region, with
the exception of a major increase in emitted flux in a broad wavelength region
centered near 3 microns in 2002. In contrast, HD 31648 has exhibited sporadic
changes in the entire 3-13 micron region throughout this span of time. In both
stars the changes in the 1-5 micron flux indicate structural changes in the
region of the disk near the dust sublimation zone, possibly causing its
distance from the star to vary with time. Repeated thermal cycling through this
region will result in the preferential survival of large grains, and an
increase in the degree of crystallinity. The variability observed in these
objects has important consequences for the interpretation of other types of
observations. For example, source variability will compromise models based on
interferometry measurements unless the interferometry observations are
accompanied by nearly-simultaneous photometric data.Comment: 55 pages, 18 figures, 2 tables, Accepted by Ap
Study of Gluon versus Quark Fragmentation in and Events at \sqrt{s}=10 GeV
Using data collected with the CLEO II detector at the Cornell Electron
Storage Ring, we determine the ratio R(chrg) for the mean charged multiplicity
observed in Upsilon(1S)->gggamma events, to the mean charged multiplicity
observed in e+e- -> qqbar gamma events. We find R(chrg)=1.04+/-0.02+/-0.05 for
jet-jet masses less than 7 GeV.Comment: 15 pages, postscript file also available through
http://w4.lns.cornell.edu/public/CLN
Clinical Trials in Head Injury
Traumatic brain injury (TBI) remains a major public health problem globally. In the United States the incidence of closed head injuries admitted to hospitals is conservatively estimated to be 200 per 100,000 population, and the incidence of penetrating head injury is estimated to be 12 per 100,000, the highest of any developed country in the world. This yields an approximate number of 500,000 new cases each year, a sizeable proportion of which demonstrate signficant long-term disabilities. Unfortunately, there is a paucity of proven therapies for this disease. For a variety of reasons, clinical trials for this condition have been difficult to design and perform. Despite promising pre-clinical data, most of the trials that have been performed in recent years have failed to demonstrate any significant improvement in outcomes. The reasons for these failures have not always been apparent and any insights gained were not always shared. It was therefore feared that we were running the risk of repeating our mistakes. Recognizing the importance of TBI, the National Institute of Neurological Disorders and Stroke (NINDS) sponsored a workshop that brought together experts from clinical, research, and pharmaceutical backgrounds. This workshop proved to be very informative and yielded many insights into previous and future TBI trials. This paper is an attempt to summarize the key points made at the workshop. It is hoped that these lessons will enhance the planning and design of future efforts in this important field of research.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/63185/1/089771502753754037.pd
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