5,359 research outputs found
Wind-shearing in gaseous protoplanetary disks
One of the first stages of planet formation is the growth of small
planetesimals and their accumulation into large planetesimals and planetary
embryos. This early stage occurs much before the dispersal of most of the gas
from the protoplanetary disk. Due to their different aerodynamic properties,
planetesimals of different sizes/shapes experience different drag forces from
the gas at these stage. Such differential forces produce a wind-shearing effect
between close by, different size planetesimals. For any two planetesimals, a
wind-shearing radius can be considered, at which the differential acceleration
due to the wind becomes greater than the mutual gravitational pull between the
planetesimals. We find that the wind-shearing radius could be much smaller than
the gravitational shearing radius by the Sun (the Hill radius), i.e. during the
gas-phase of the disk wind-shearing could play a more important role than tidal
perturbations by the Sun. Here we study the wind-shearing radii for
planetesimal pairs of different sizes and compare it with gravitational
shearing (drag force vs. gravitational tidal forces). We then discuss the role
of wind-shearing for the stability and survival of binary planetesimals, and
provide stability criteria for binary planetesimals embedded in a gaseous disk.Comment: To be published in the proceedings of IAU 276: The Astrophysics of
planetary systems - formation, structure, and dynamical evolutio
Power analysis for detecting trends in juvenile spotted seatrout abundance in Florida Bay
The spotted seatrout (Cynoscion nebulosus) is considered a key species relative to the implementation of the Comprehensive Everglades Restoration Plan (CERP). One of the goals of the CERP is to increase freshwater flows to Florida Bay. Increased freshwater flows can have potential
positive and negative impacts on spotted seatrout populations. At low salinities, the planktonic eggs of spotted seatrout sink to the bottom and are not viable (Alshuth and Gilmore, 1994; Holt and Holt, 2002). On the other hand, increased freshwater flows can alleviate hypersaline conditions that could result in an expansion of the distribution of the early life stages of spotted seatrout (Thayer et al., 1999; Florida Department of Environmental Protection1). Thus it would be useful to develop a monitoring program that can detect changes in seatrout abundance on time scales short enough to be useful to resource managers.
The NOAA Center for Coastal Fisheries and Habitat Research (NOAA) has made sporadic collections of juvenile seatrout using otter trawls since 1984 (see Powell et al, 2004). The results suggest that it might be useful to sample for seatrout in as many as eight different areas or basins
(Figure 1): Bradley Key, Sandy Key, Johnson Key, Palm Key, Snake Bight, Central, Whipray and Crocodile Dragover. Unfortunately, logistical constraints are likely to limit the number of tows to about 40 per month over a period of six months each year. Inasmuch as few seatrout are caught in
any given tow and the proportion of tows with zero seatrout is often high, it is important to determine how best to allocate this limited sampling effort among the various basins so that any trends in abundance may be detected with sufficient statistical confidence. (PDF contains 16 pages
Bond patterns and charge order amplitude in 1/4-filled charge-transfer solids
Metal-insulator transition accompanied by charge-ordering has been widely
investigated in quasi-one-dimensional conductors, including in particular
organic charge-transfer solids. Among such materials the 1/4-filled band
charge-transfer solids are of strong interest, because of the commensurate
nature of the charge-ordering in these systems. The period-four charge-order
pattern ...1100... here is accompanied by two distinct bond distortion
patterns, giving rise to bond-charge-density waves (BCDW) of types 1 and 2.
Using quantum Monte Carlo methods, we determine the phase diagram within the
extended Hubbard Hamiltonian that gives both types 1 and 2 BCDW in the
thermodynamic limit. We further investigate the effect of electron-electron and
electron-phonon interactions on the amount of charge disproportionation. Our
results show that between these two bond patterns, one (BCDW2) in general
coexists with a large magnitude charge order, which is highly sensitive to
electron-phonon interactions, while the other (BCDW1) is characterized by weak
charge order. We discuss the relevance of our work to experiments on several
1/4-filled conductors, focusing in particular on the materials (EDO-TTF)_2X and
(DMEDO-TTF)_2X with large amplitude charge-order.Comment: 7 pages, 8 figure
Can Food-for-Work Programmes Reduce Vulnerability?
Famine, Food aid, Poverty, Public employment programmes, Transfers
Ion mass spectrometer
An ion mass spectrometer is described which detects and indicates the characteristics of ions received over a wide angle, and which indicates the mass to charge ratio, the energy, and the direction of each detected ion. The spectrometer includes a magnetic analyzer having a sector magnet that passes ions received over a wide angle, and an electrostatic analyzer positioned to receive ions passing through the magnetic analyzer. The electrostatic analyzer includes a two dimensional ion sensor at one wall of the analyzer chamber, that senses not only the lengthwise position of the detected ion to indicate its mass to charge ratio, but also detects the ion position along the width of the chamber to indicate the direction in which the ion was traveling
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