12,167 research outputs found
Probing r-Process Production of Nuclei Beyond Bi209 with Gamma Rays
We estimate gamma-ray fluxes due to the decay of nuclei beyond Bi209 from a
supernova or a supernova remnant assuming that the r-process occurs in
supernovae. We find that a detector with a sensitivity of about 10**(-7)
photons/cm**2/s at energies of 40 keV to 3 MeV may detect fluxes due to the
decay of Ra226, Th229, Am241, Am243, Cf249, and Cf251 in the newly discovered
supernova remnant near Vela. In addition, such a detector may detect fluxes due
to the decay of Ac227 and Ra228 produced in a future supernova at a distance of
about 1 kpc. As nuclei with mass numbers A > 209 are produced solely by the
r-process, such detections are the best proof for a supernova r-process site.
Further, they provide the most direct information on yields of progenitor
nuclei with A > 209 at r-process freeze-out. Finally, detection of fluxes due
to the decay of r-process nuclei over a range of masses from a supernova or a
supernova remnant provides the opportunity to compare yields in a single
supernova event with the solar r-process abundance pattern.Comment: 24 pages, 3 figures, to appear in the October 10, 1999 issue of Ap
Supernovae as the Site of the r-Process: Implications for Gamma-Ray Astronomy
We discuss how detection of gamma-ray emission from the decay of r-process
nuclei can improve our understanding of r-process nucleosynthesis. We find that
a gamma-ray detector with a sensitivity of 10**(-7)/cm**2/s at 100-700 keV may
detect the emission from the decay of Sb125, Cs137, Ce144, Eu155, and Os194
produced in a future Galactic supernova. In addition, such a detector may
detect the emission from the decay of Sn126 in the Vela supernova remnant and
the diffuse emission from the decay of Sn126 produced by past supernovae in our
Galaxy. The required detector sensitivity is similar to what is projected for
the proposed Advanced Telescope for High Energy Nuclear Astrophysics (ATHENA).
Both the detection of gamma-ray emission from the decay of several r-process
nuclei (e.g., Sb125 and Os194) produced in future Galactic supernovae and the
detection of emission from the decay of Sn126 in the Vela supernova remnant
would prove that supernovae are a site of the r-process. Furthermore, the
former detection would allow us to determine whether or not the r-process
nuclei are produced in relative proportions specified by the solar r-process
abundance pattern in supernova r-process events. Finally, detection of diffuse
emission from the decay of Sn126 in our Galaxy would eliminate neutron
star/neutron star mergers as the main source for the r-process nuclei near mass
number A=126.Comment: 14 pages, AASTeX, submitted to the Astrophysical Journa
Neutrino-induced nucleosynthesis and the site of the r process
If the r process occurs deep within a type II supernova, probably the most popular of the proposed sites, abundances of r-process elements may be altered by the intense neutrino flux. We point out that the effects would be especially pronounced for eight isotopes that can be efficiently synthesized by the neutrino reactions following r-process freeze-out. We show that the observed abundances of these isotopes are entirely consistent with neutrino-induced nucleosynthesis, strongly arguing for a supernova r-process site. The deduced neutrino fluences place stringent constraints on the freeze-out radius and dynamic time scale of the r process
The last glacial-interglacial cycle in Lake Ohrid (Macedonia/Albania): testing diatom response to climate
Lake Ohrid is a site of global importance for palaeoclimate research. This study presents results of diatom analysis of a ca. 136 ka sequence, Co1202, from the northeast of the lake basin. It offers the opportunity to test diatom response across two glacial-interglacial transitions and within the Last Glacial, while setting up taxonomic protocols for future research. The results are outstanding in demonstrating the sensitivity of diatoms to climate change, providing proxy evidence for temperature change marked by glacial-interglacial shifts between the dominant planktonic taxa, Cyclotella fottii and C. ocellata, and exact correlation with geochemical proxies to mark the start of the Last Interglacial at ca. 130 ka. Importantly, diatoms show much stronger evidence in this site for warming during MIS3 than recorded in other productivity-related proxies, peaking at ca. 39 ka, prior to the extreme conditions of the Last Glacial maximum. In the light of the observed patterns, and from the results of analysis of early Holocene sediments from a second core, Lz1120, the lack of a response to Late Glacial and early Holocene warming from ca. 15-7.4 ka suggests the Co1202 sequence may be compromised during this phase. After ca. 7.4 ka, there is evidence for enhanced nutrient enrichment compared to the Last Interglacial, following by a post-Medieval cooling trend. Taxonomically, morphological variability in C. fottii shows no clear trends linked to climate, but an intriguing change in central area morphology occurs after ca. 48.7 ka, coincident with a tephra layer. In contrast, C. ocellata shows morphological variation in the number of ocelli between interglacials, suggesting climatically-forced variation or evolutionary selection pressure. The application of a simple dissolution index does not track preservation quality very effectively, underlining the importance of diatom concentration data in future studies
Detecting a set of entanglement measures in an unknown tripartite quantum state by local operations and classical communication
We propose a more general method for detecting a set of entanglement
measures, i.e. negativities, in an \emph{arbitrary} tripartite quantum state by
local operations and classical communication. To accomplish the detection task
using this method, three observers, Alice, Bob and Charlie, do not need to
perform the partial transposition maps by the structural physical
approximation; instead, they are only required to collectively measure some
functions via three local networks supplemented by a classical communication.
With these functions, they are able to determine the set of negativities
related to the tripartite quantum state.Comment: 16 pages, 2 figures, revte
Environmental Federalism in the European Union and the United States
The United States (US) and the European Union (EU) are federal systems in which the responsibility for environmental policy-making is divided or shared between the central government and the (member) states. The attribution of decision-making power has important policy implications. This chapter compares the role of central and local authorities in the US and the EU in formulating environmental regulations in three areas: automotive emissions for health related (criteria) pollutants, packaging waste, and global climate change. Automotive emissions are relatively centralised in both political systems. In the cases of packaging waste and global climate change, regulatory policy-making is shared in the EU, but is primarily the responsibility of local governments in the US. Thus, in some important areas, regulatory policy-making is more centralised in the EU. The most important role local governments play in the regulatory process is to help diffuse stringent local standards through more centralised regulations, a dynamic which has become recently become more important in the EU than in the US.
Detection of Dense Molecular Gas in Inter-Arm Spurs in M51
Spiral arm spurs are prominent features that have been observed in extinction
and 8m emission in nearby galaxies. In order to understand their molecular
gas properties, we used the Owens Valley Radio Observatory to map the
CO(J=1--0) emission in three spurs emanating from the inner northwestern spiral
arm of M51. We report CO detections from all three spurs. The molecular gas
mass and surface density are M M_{\sun} and
50 M_{\sun} pc. Thus, relative to the spiral arms,
the spurs are extremely weak features. However, since the spurs are extended
perpendicular to the spiral arms for 500 pc and contain adequate fuel for
star formation, they may be the birthplace for observed inter-arm HII regions.
This reduces the requirement for the significant time delay that would be
otherwise needed if the inter-arm star formation was initiated in the spiral
arms. Larger maps of galaxies at similar depth are required to further
understand the formation and evolution of these spurs and their role in star
formation - such data should be forthcoming with the new CARMA and future ALMA
telescopes and can be compared to several recent numerical simulations that
have been examining the evolution of spiral arm spurs.Comment: 6 pages, 3 figures, emulate-apj format, accepted in Ap
Diverse Supernova Sources for the r-Process
(Abridged) It is shown that a semi-quantitative agreement with the gross
solar r-process abundance pattern near and above mass number A=130 can be
obtained by a superposition of two distinctive kinds of supernova r-process
events. These correspond to a low frequency case L and a high frequency case H,
which takes into account the low abundance of I129 and the high abundance of
Hf182 in the early solar nebula. The lifetime of Hf182 associates the events in
case H with the most common Type II supernovae. These events would be mainly
responsible for the r-process nuclei near and above A=195. They would also make
a significant amount of the nuclei between A=130 and 195, including Hf182, but
very little I129. In order to match the solar r-process abundance pattern and
to satisfy the I129 and Hf182 constraints, the events in case L, which would
make the r-process nuclei near A=130 and the bulk of those between A=130 and
195, must occur 10 times less frequently but eject 10--20 times more r-process
material in each event. We speculate that the usual neutron star remnants, and
hence prolonged ejection of r-process material, are associated with the events
in case L, whereas the more frequently occurring events in case H have ejection
of other r-process material terminated by black hole formation during the
neutrino cooling phase of the protoneutron star.Comment: 23 pages, AAS LATEX, 8 Postscript figure
Myosin motors fragment and compact membrane-bound actin filaments
Cell cortex remodeling during cell division is a result of myofilament-driven contractility of the cortical membrane-bound actin meshwork. Little is known about the interaction between individual myofilaments and membrane-bound actin filaments. Here we reconstituted a minimal actin cortex to directly visualize the action of individual myofilaments on membrane-bound actin filaments using TIRF microscopy. We show that synthetic myofilaments fragment and compact membrane-bound actin while processively moving along actin filaments. We propose a mechanism by which tension builds up between the ends of myofilaments, resulting in compressive stress exerted to single actin filaments, causing their buckling and breakage. Modeling of this mechanism revealed that sufficient force (âŒ20 pN) can be generated by single myofilaments to buckle and break actin filaments. This mechanism of filament fragmentation and compaction may contribute to actin turnover and cortex reorganization during cytokinesis
Quantum tomography as normalization of incompatible observations
Quantum states are successfully reconstructed using the maximum likelihood
estimation on the subspace where the measured projectors reproduce the identity
operator. Reconstruction corresponds to normalization of incompatible
observations. The proposed approach handles the noisy data corresponding to
realistic incomplete observation with finite resolution.Comment: RevTeX, 4 pages, 3 figure
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