2,308 research outputs found
Echo traces associated with thermoclines
Echoes received from the depth of the thermocline in Windermere, the North Sea, the Barents Sea and off Spitsbergen have been associated with aggregations of plankton organisms: zooplankton at a density of 25/litre in Windermere and phytoplankton at a density of 100,000 cells/litre (Thalassiosira gravida and Chaetoceros sp.) in the Barents Sea
FeH Absorption in the Near-Infrared Spectra of Late M and L Dwarfs
We present medium-resolution z-, J-, and H-band spectra of four late-type
dwarfs with spectral types ranging from M8 to L7.5. In an attempt to determine
the origin of numerous weak absorption features throughout their near-infrared
spectra, and motivated by the recent tentative identification of the E 4\Pi- A
^4\Pi system of FeH near 1.6 microns in umbral and cool star spectra, we have
compared the dwarf spectra to a laboratory FeH emission spectrum. We have
identified nearly 100 FeH absorption features in the z-, J-, and H-band spectra
of the dwarfs. In particular, we have identified 34 features which dominate the
appearance of the H-band spectra of the dwarfs and which appear in the
laboratory FeH spectrum. Finally, all of the features are either weaker or
absent in the spectrum of the L7.5 dwarf which is consistent with the weakening
of the known FeH bandheads in the spectra of the latest L dwarfs.Comment: accepted by Ap
The Unusual Object IC 2144/MWC 778
IC 2144 is a small reflection nebula located in the zone of avoidance near
the Galactic anticenter. It has been investigated here largely on the basis of
Keck/HIRES optical spectroscopy (R ~ 48,000) and a SpeX spectrogram in the
near-IR (R = 2000) obtained at the NASA IRTF. The only star in the nebula that
is obvious in the optical or near-IR is the peculiar emission-line object MWC
778 (V = 12.8), which resembles a T Tauri star in some respects. What appear to
be F- or G-type absorption features are detectable in its optical region under
the very complex emission line spectrum; their radial velocity agrees with the
CO velocity of the larger cloud in which IC 2144 is embedded. There are
significant differences between the spectrum of the brightest area of the
nebula and of MWC 778, the presumed illuminator, an issue discussed in some
detail. The distance of IC 2144 is inferred to be about 1.0 kpc by reference to
other star-forming regions in the vicinity. The extinction is large, as
demonstrated by [Fe II] emission line ratios in the near-IR and by the strength
of the diffuse interstellar band spectrum; a provisional value of A_V of 3.0
mag was assumed. The SED of MWC 778 rises steeply beyond about 1 m, with a
slope characteristic of a Class I source. Integration of the flux distribution
leads to an IR luminosity of about 510 L_solar. If MWC 778 is indeed a F- or
G-type pre--main-sequence star several magnitudes above the ZAMS, a population
of faint emission Halpha stars would be expected in the vicinity. Such a
search, like other investigations that are recommended in this paper, has yet
to be carried out.Comment: 36 pages, 13 figures, accepted by A
Solving the measurement problem: de Broglie-Bohm loses out to Everett
The quantum theory of de Broglie and Bohm solves the measurement problem, but
the hypothetical corpuscles play no role in the argument. The solution finds a
more natural home in the Everett interpretation.Comment: 20 pages; submitted to special issue of Foundations of Physics, in
honour of James T. Cushin
Quantum mechanics is about quantum information
I argue that quantum mechanics is fundamentally a theory about the
representation and manipulation of information, not a theory about the
mechanics of nonclassical waves or particles. The notion of quantum information
is to be understood as a new physical primitive -- just as, following
Einstein's special theory of relativity, a field is no longer regarded as the
physical manifestation of vibrations in a mechanical medium, but recognized as
a new physical primitive in its own right.Comment: 17 pages, forthcoming in Foundations of Physics Festschrift issue for
James Cushing. Revised version: some paragraphs have been added to the final
section clarifying the argument, and various minor clarifying remarks have
been added throughout the tex
Mismatch in microbial food webs: predators but not prey perform better in their local biotic and abiotic conditions
Understanding how trophic levels respond to changes in abiotic and biotic conditions is key for predicting how food webs will react to environmental perturbations. Different trophic levels may respond disproportionately to change, with lower levels more likely to react faster, as they typically consist of smaller-bodied species with higher reproductive rates. This response could cause a mismatch between trophic levels, in which predators and prey will respond differently to changing abiotic or biotic conditions. This mismatch between trophic levels could result in altered top-down and bottom-up control and changes in interaction strength. To determine the possibility of a mismatch, we conducted a reciprocal-transplant experiment involving Sarracenia purpurea food webs consisting of bacterial communities as prey and a subset of six morphologically similar protozoans as predators. We used a factorial design with four temperatures, four bacteria and protozoan biogeographic origins, replicated four times. This design allowed us to determine how predator and prey dynamics were altered by abiotic (temperature) conditions and biotic (predators paired with prey from either their local or non-local biogeographic origin) conditions. We found that prey reached higher densities in warmer temperature regardless of their temperature of origin. Conversely, predators achieved higher densities in the temperature condition and with the prey from their origin. These results confirm that predators perform better in abiotic and biotic conditions of their origin while their prey do not. This mismatch between trophic levels may be especially significant under climate change, potentially disrupting ecosystem functioning by disproportionately affecting top-down and bottom-up control
Productivity responses of a widespread marine piscivore, Gadus morhua, to oceanic thermal extremes and trends
Climate change will have major consequences for population dynamics and life histories of marine biota as it progresses in the twenty-first century. These impacts will differ in magnitude and direction for populations within individual marine species whose geographical ranges span large gradients in latitude and temperature. Here we use meta-analytical methods to investigate how recruitment (i.e. the number of new fish produced by spawners in a given year which subsequently grow and survive to become vulnerable to fishing gear) has reacted to temperature fluctuations, and in particular to extremes of temperature, in cod populations throughout the north Atlantic. Temperature has geographically explicit effects on cod recruitment. Impacts differ depending on whether populations are located in the upper (negative effects) or in the lower (positive effects) thermal range. The probabilities of successful year-classes in populations living in warm areas is on average 34 per cent higher in cold compared with warm seasons, whereas opposite patterns exist for populations living in cold areas. These results have implications for cod dynamics, distributions and phenologies under the influence of ocean warming, particularly related to not only changes in the mean temperature, but also its variability (e.g. frequency of exceptionally cold or warm seasons)
Magnetic and Metal-Insulator Transitions in beta-Na0.5CoO2 and gamma-K0.5CoO2 -NMR and Neutron Diffraction Studies-
Co-oxides beta-Na0.5CoO2 and gamma-K0.5CoO2 have been prepared by the Na
de-intercalation from alpha-NaCoO2 and by the floating-zone method,
respectively. It has been found that successive phase transitions take place at
temperatures Tc1 and Tc2 in both systems. The appearance of the internal
magnetic field at Tc1 with decreasing temperature T indicates that the
antiferromagnetic order exists at T < Tc1, as in gamma-Na0.5CoO2. For
beta-Na0.5CoO2, the transition temperatures and the NMR parameters determined
from the data taken for magnetically ordered state are similar to those of
gamma-Na0.5CoO2, indicating that the difference of the stacking ways of the
CoO2 layers between these systems do not significantly affect their physical
properties. For gamma-K0.5CoO2, the quantitative difference of the physical
quantities are found from those of beta- and gamma-Na0.5CoO2. The difference
between the values of Tci (i = 1 and 2) of these systems might be explained by
considering the distance between CoO2 layers.Comment: 8 pages, 14 figures, 1 Tabl
Grounding Bohmian Mechanics in Weak Values and Bayesianism
Bohmian mechanics (BM) is a popular interpretation of quantum mechanics in
which particles have real positions. The velocity of a point x in configuration
space is defined as the standard probability current j(x) divided by the
probability density P(x). However, this ``standard'' j is in fact only one of
infinitely many that transform correctly and satisfy \dot P + \del . j=0. In
this article I show that there is a unique j that can be determined
experimentally as a weak value using techniques that would make sense to a
classical physicist. Moreover, this operationally defined j equals the standard
j, so, assuming \dot x = j/P, the possible Bohmian paths can also be determined
experimentally from a large enough ensemble. Furthermore, this approach to
deriving BM singles out x as the hidden variable, because (for example) the
operationally defined momentum current is in general incompatible with the
evolution of the momentum distribution. Finally I discuss how, in this setting,
the usual quantum probabilities can be derived from a Bayesian standpoint, via
the principle of indifference.Comment: 11 page
Crystal Structure of the Sodium Cobaltate Deuterate Superconductor NaxCoO2o4xD2O (x=1/3)
Neutron and x-ray powder diffraction have been used to investigate the
crystal structures of a sample of the newly-discovered superconducting sodium
cobaltate deuterate compound with composition Na0.31(3)CoO2o1.25(2)D2O and its
anhydrous parent compound Na0.61(1)CoO2. The deuterate superconducting compound
is formed by coordinating four D2O molecules (two above and two below) to each
Na ion in a way that gives Na-O distances nearly equal to those in the parent
compound. One deuteron of the D2O molecule is hydrogen bonded to an oxygen atom
in the CoO2 plane and the oxygen atom and the second deuteron of each D2O
molecule lie approximately in a plane between the Na layer and the CoO2 layers.
This coordination of Na by four D2O molecules leads to ordering of the Na ions
and D2O molecules. The sample studied here, which has Tc=4.5 K, has a refined
composition of Na0.31(3)CoO2o1.25(2)D2O, in agreement with the expected 1:4
ratio of Na to D2O. These results show that the optimal superconducting
composition should be viewed as a specific hydrated compound, not a solid
solution of Na and D2O (H2O) in NaxCoO2oyD2O. Studies of physical properties
vs. Na or D2O composition should be viewed with caution until it is verified
that the compound remains in the same phase over the composition range of the
study.Comment: 22 pages, 8 figure
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