33,222 research outputs found
Time variations of the narrow FeII and HI spectral emission lines from the close vicinity of Eta Carinae during the spectral event of 2003
The spectrum of Eta Carinae and its ejecta shows slow variations over a
period of 5.5 years. However, the spectrum changes drastically on a time scale
of days once every period called the 'spectral event'. We report on variations
in the narrow emission line spectrum of gas condensations (the Weigelt blobs)
close to the central star during a spectral event. The rapid changes in the
stellar radiation field illuminating the blobs make the blobs a natural
astrophysical laboratory to study atomic photoprocesses. The different
responses of the HI Paschen lines, fluorescent lines and forbidden
[FeII] lines allow us to identify the processes and estimate physical
conditions in the blobs. This paper is based on observations from the Pico dos
Dias Observatory (LNA/Brazil) during the previous event in June 2003.Comment: Accepted for publication in A&
A Radiative Cycle with Stimulated Emission from Atoms (Ions) in an astrophysical Plasma
We propose that a radiative cycle operates in atoms (ions) located in a
rarefied gas in the vicinity of a hot star. Besides spontaneous transitions the
cycle includes a stimulated transition in one very weak intermediate channel.
This radiative "bottle neck" creates a population inversion, which for an
appropriate column density results in amplification and stimulated radiation in
the weak transition. The stimulated emission opens a fast decay channel leading
to a fast radiative cycle in the atom (or ion). We apply this model by
explaining two unusually bright Fe II lines at 250.7 and 250.9 nm in the UV
spectrum of gas blobs close to h Carinae, one of the most massive and luminous
stars in the Galaxy. The gas blobs are spatially resolved from the central star
by the Hubble Space Telescope (HST). We also suggest that in the frame of a
radiative cycle stimulated emission is a key phenomenon behind many spectral
lines showing anomalous intensities in spectra of gas blobs outside eruptive
stars.Comment: Accepted for publication in Phys. Rev. Letter
Precision laboratory UV and IR wavelengths for cosmological and astrophysical applications
The quality of astronomical spectra is now so high that the accuracy of the
laboratory data is getting more and more important for the analysis and
interpretation. Both in astrophysics and cosmology the needs for accurate
laboratory wavelengths have increased with the development of new ground-based
and air-borne telescopes and spectrographs. The high resolution UV Fourier
Transform spectrometer at Lund Observatory is being used for studying
laboratory spectra of astrophysically important elements. Measurements of
accurate laboratory UV and IR wavelengths have been made for cosmological and
astrophysical applications.Comment: To appear in the proceedings of "Precision Spectroscopy in
Astrophysics", Aveiro, Portugal, Sep. 2006, eds Pasquini et al., ESO
Astrophysics Symposia. 2 pages, 2 figure
Drivers and food web effects of Gonyostomum semen blooms
The flagellate Gonyostomum semen forms dense late-summer blooms in humic lakes and is a nuisance to swimmers because it forms a slimy coat on the skin, causing irritation in sensitive individuals. Increasing occurrence and bloom incidence of G. semen has been reported during recent decades, but it is not clear which factors affect the distribution and bloom formation of this alga. Large cell size, ejection of long, slimy threads (trichocysts), and nighttime migration to the hypolimnion may limit grazing on G. semen by herbivorous zooplankton, resulting in a decreased coupling between phytoplankton and higher trophic levels during blooms. The studies included in this thesis investigate which factors affect G. semen occurrence and bloom formation and how G. semen blooms affect the community composition and trophic interactions in boreal, humic lakes.
The occurrence of G. semen has increased between 1995 and 2010, especially in southern Sweden. Bloom incidence and total biomass did not increase continually, but fluctuated among years and peaked in the middle of the study period. Temperature and length of the growing season affected the occurrence and, to a lesser extent, bloom formation of G. semen, but local factors such as pH and water colour were more important for bloom formation. More lakes may become suitable habitats with the ongoing increase in water colour and increasing temperatures may result in a more frequent occurrence and bloom formation of G. semen.
Blooms resulted in a shift in zooplankton assemblages toward predominance by small cladocerans, which were not able to feed on G. semen but instead fed more on heterotrophic food resources, supporting the hypothesis of a reduced coupling between phytoplankton and zooplankton. Zooplankton assemblages predominated by small animals feeding on low-quality resources may reduce the food quality for planktivorous fish. Instead, the invertebrate predator C. flavicans appeared to benefit from G. semen blooms, as indicated by its high abundance in bloom-lakes. Calanoid copepods and a large cladoceran fed efficiently on G. semen in the laboratory, indicating that there is, however, some trophic coupling between G. semen and higher trophic levels. This supports the use of biomanipulation of fish communities for controlling G. semen blooms
Astrophysical Lasers Operating in optical Fe II Lines Lines in Stellar Ejecta of Eta Carinae
After the discovery of space masers based on OH radicals (Weaver et al, 1965)
and H2O (Cheung et al, 1969) such microwave lasers have been found to work in
more than 100 molecular species (Elitzur, 1992; Townes, 1997), as well as in
highly excited H atoms (Strelnitski et al,1996). In the IR region (10 microns),
the effect of stimulated emission of radiation in the CO2 molecule has been
discovered in the Martian and Venus' atmospheres (Betz et al, 1976; Mumma et
al, 1981). We report here on the discovery of laser action in the range 0.9-2
micr. in several spectral lines of Fe II, which are associated with transitions
from "pseudo-metastable" states populated by spontaneous transitions from
Ly-alpha pumped Fe II levels. The intense Ly-alpha radiation is formed in the
HII region of gas condensations close to the star Eta Carinae. The laser
transitions form together with spontaneous transitions closed radiative cycles,
one of which includes the extremely bright 2507/09 A lines. Closed radiative
cycles, together with an accidental mixing of energy levels, may provide an
explanation of the abnormal intensities of these UV non-lasing lines. Using the
complicated energy level diagram of Fe II we present those peculiar features,
which are essential for the inverted population and laser effect: the pumping,
the level mixing, and the "bottle neck" for spontaneous decay. The laser action
is a new indicator of non-equilibrium and spatially non-homogeneous physical
conditions as well as a high brightness temperature of Ly-alpha in ejecta from
eruptive stars. Such conditions are very difficult to probe by existing
methods, and we propose some future experiments. The fact, that the lasing
near-IR lines appear in the spectrum with about the same inten- sity as
non-lasing lines is discussed and compared with the situation in masers.Comment: 27 pages, 11 figures; to be published in A&A; also available at
http://130.235.102.158/blackhole/astrolaser.pd
Emergent Behaviors over Signed Random Networks in Dynamical Environments
We study asymptotic dynamical patterns that emerge among a set of nodes that
interact in a dynamically evolving signed random network. Node interactions
take place at random on a sequence of deterministic signed graphs. Each node
receives positive or negative recommendations from its neighbors depending on
the sign of the interaction arcs, and updates its state accordingly. Positive
recommendations follow the standard consensus update while two types of
negative recommendations, each modeling a different type of antagonistic or
malicious interaction, are considered. Nodes may weigh positive and negative
recommendations differently, and random processes are introduced to model the
time-varying attention that nodes pay to the positive and negative
recommendations. Various conditions for almost sure convergence, divergence,
and clustering of the node states are established. Some fundamental
similarities and differences are established for the two notions of negative
recommendations
The Evolution of Beliefs over Signed Social Networks
We study the evolution of opinions (or beliefs) over a social network modeled
as a signed graph. The sign attached to an edge in this graph characterizes
whether the corresponding individuals or end nodes are friends (positive links)
or enemies (negative links). Pairs of nodes are randomly selected to interact
over time, and when two nodes interact, each of them updates its opinion based
on the opinion of the other node and the sign of the corresponding link. This
model generalizes DeGroot model to account for negative links: when two enemies
interact, their opinions go in opposite directions. We provide conditions for
convergence and divergence in expectation, in mean-square, and in almost sure
sense, and exhibit phase transition phenomena for these notions of convergence
depending on the parameters of the opinion update model and on the structure of
the underlying graph. We establish a {\it no-survivor} theorem, stating that
the difference in opinions of any two nodes diverges whenever opinions in the
network diverge as a whole. We also prove a {\it live-or-die} lemma, indicating
that almost surely, the opinions either converge to an agreement or diverge.
Finally, we extend our analysis to cases where opinions have hard lower and
upper limits. In these cases, we study when and how opinions may become
asymptotically clustered to the belief boundaries, and highlight the crucial
influence of (strong or weak) structural balance of the underlying network on
this clustering phenomenon
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