6,526 research outputs found
VLTI observations of the dust geometry around R Coronae Borealis stars
We are investigating the formation and evolution of dust around the
hydrogen-deficient supergiants known as R Coronae Borealis (RCB) stars. We aim
to determine the connection between the probable merger past of these stars and
their current dust-production activities. We carried out high-angular
resolution interferometric observations of three RCB stars, namely RY Sgr, V
CrA, and V854 Cen with the mid-IR interferometer, MIDI on the VLTI, using two
telescope pairs. The baselines ranged from 30 to 60 m, allowing us to probe the
dusty environment at very small spatial scales (~ 50 mas or 400 stellar radii).
The observations of the RCB star dust environments were interpreted using both
geometrical models and one-dimensional radiative transfer codes. From our
analysis we find that asymmetric circumstellar material is apparent in RY Sgr,
may also exist in V CrA, and is possible for V854 Cen. Overall, we find that
our observations are consistent with dust forming in clumps ejected randomly
around the RCB star so that over time they create a spherically symmetric
distribution of dust. However, we conclude that the determination of whether
there is a preferred plane of dust ejection must wait until a time series of
observations are obtained.Comment: Accepted for publication in MNRAS; 14 pages, 10 figures, 6 table
Spin-1 Particles with Light-Front Approach
For the vector sector, i.e, mesons with spin-1, the electromagnetic form
factors and anothers observables are calculated with the light-front approach.
However, the light-front quantum field theory have some problems, for example,
the rotational symmetry breaking. We solve that problem added the zero modes
contribuition to the matrix elements of the electromagnetic current, besides
the valence contribuition. We found that among the four independent matrix
elements of the plus component in the light-front helicity basis only the one carries zero mode contributions.Comment: 5 pages. 3 Figures, use latex and EPJ styl
Cuttlefish retrieve whether they smelt or saw a previously encountered item.
According to the Source Monitoring Framework, the origin of a memory is remembered through the retrieval of specific features (e.g. perceptive, sensitive, affective signals). In two source discrimination tasks, we studied the ability of cuttlefish to remember the modality in which an item had been presented several hours ago. In Experiment 1, cuttlefish were able to retrieve the modality of presentation of a crab (visual vs olfactory) sensed before 1 h and 3 hrs delays. In Experiment 2, cuttlefish were trained to retrieve the modality of the presentation of fish, shrimp, and crabs. After training, cuttlefish performed the task with another item never encountered before (e.g. mussel). The cuttlefish successfully passed transfer tests with and without a delay of 3 hrs. This study is the first to show the ability to discriminate between two sensory modalities (i.e. see vs smell) in an animal. Taken together, these results suggest that cuttlefish can retrieve perceptual features of a previous event, namely whether they had seen or smelled an item
Growth of Carbon Grains in Supernova Ejecta
We present a chemical reaction network that describes the condensation chemistry of carbon dust grains in an expanding supernova shell. We assume that the region of interest consists solely of gaseous free carbon and oxygen atoms and that the buildup of CO is counteracted by the radioactive decay of 56Co, which breaks up the CO mol-ecule and allows C to condense into solids. Our chemical model takes C to first form linear chains, which, at some critical length, transition into ringed isomers. These isomers are more resistant to oxidation than linear chains. These ringed isomers form the nuclei for the growth of larger carbon solids. The effect of the disruption of CO on grain growth is displayed, leading to a rethinking of previous assumptions on the importance of CO disruption. How the abundance and size distribution of grains are affected by various parameters of the ejecta is also studied, providing insight into the possible sites of grain condensation
Condensation of Carbon in Radioactive Supernova Gas
The chemistry of carbon molecules leading to the formation of large carbon-bearing molecules and dust in the interior of an expanding supernova is explored and the equations governing their abundances are solved. A steady state between production and destruction is set up early and evolves adiabatically as the supernova evolves. Simple solutions for that steady state limit yield the abundance of each linear carbon molecule and its dependence on the C/O atomic ratio in the gas. Carbon dust condenses from initially gaseous C and O atoms because Compton electrons produced by the radioactivity cause dissociation of the CO molecules, which would otherwise form and limit the supply of C atoms. The resulting free C atoms enable carbon dust to grow faster by C association than its destruction by oxidation for various C/O ratios. Nucleation for graphite growth occurs when linear Cn molecules transition to ringed Cn molecules. We survey the dependence of the abundances of these molecules on the C/O ratio and on several other kinetic rate parameters. The concept of population control is significant for the maximum sizes of carbon particles grown during supernova expansion. Interpretation of presolar micrometer-sized carbon solids found in meteorites and of infrared emission from supernova is relaxed to allow O to be more abundant than C, but the maximum grain size depends upon that ratio
Animal minds: from computation to evolution.
notes: PMCID: PMC3427558types: Introductory Journal Article; Research Support, Non-U.S. Gov'tCopyright © 2012 The Royal Society. Post print version deposited in accordance with SHERPA RoMEO guidelines. The definitive version is available at: http://rstb.royalsocietypublishing.org/content/367/1603/2670.longIn the great Darwinian struggle for existence, all animals must tackle the problems posed by variable environments, be it finding and processing food, recognizing and attracting potential mates, avoiding predators, outcompeting rivals or navigating back to nesting sites. Although the mental processes by which different species deal with such challenges are varied, all animals share the fundamental problem of having to cope with the sheer abundance of information in the environment, much of which is likely to be irrelevant to the task at hand.David Phillips Fellowship from the BBSRC (A.T.)The Human Frontiers Science Programme Organization (U.G.
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Morgan's canon is not evidence
Mahr & Csibra's (M&C's) account of the communicative function of episodic memory relies more heavily on the case against episodic memory in nonhumans than their description suggests. Although the communicative function of episodic memory may be accurate as it pertains to human behaviour, we question whether Morgan's canon is a suitable foundation on which to build theories of supposedly human-specific traits
Electron Positron Annihilation Radiation from SgrA East at the Galactic Center
Maps of the Galactic electron-positron annihilation radiation show evidence
for three distinct and significant features: (1) a central bulge source, (2)
emission in the Galactic plane, and (3) an enhancement of emission at positive
latitudes above the Galactic Center. In this paper, we explore the possibility
that Sgr A East, a very prominent radio structure surrounding the Galactic
nucleus, may be a significant contributer to the central bulge feature. The
motivation for doing so stems from a recently proposed link between this radio
object and the EGRET gamma-ray source 2EG J1746-2852. If this association is
correct, then Sgr A East is also expected to be a source of copious positron
production. The results presented here show that indeed Sgr A East must have
produced a numerically significant population of positrons, but also that most
of them have not yet had sufficient time to thermalize and annihilate. As such,
Sgr A East by itself does not appear to be the dominant current source of
annihilation radiation, but it will be when the positrons have cooled
sufficiently and they have become thermalized. This raises the interesting
possibility that the bulge component may be due to the relics of earlier
explosive events like the one that produced Sgr A East.Comment: This manuscript was prepared with the AAS Latex macros v4.0 It is 37
pages long and has 16 figure
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