2,448 research outputs found
Strange quark matter fragmentation in astrophysical events
The conjecture of Bodmer-Witten-Terazawa suggesting a form of quark matter
(Strange Quark Matter) as the ground state of hadronic interactions has been
studied in laboratory and astrophysical contexts by a large number of authors.
If strange stars exist, some violent events involving these compact objects,
such as mergers and even their formation process, might eject some strange
matter into the interstellar medium that could be detected as a trace signal in
the cosmic ray flux. To evaluate this possibility, it is necessary to
understand how this matter in bulk would fragment in the form of strangelets
(small lumps of strange quark matter in which finite effects become important).
We calculate the mass distribution outcome using the statistical
multifragmentation model and point out several caveats affecting it. In
particular, the possibility that strangelets fragmentation will render a tiny
fraction of contamination in the cosmic ray flux is discussed.Comment: 13 pages, 4 figure
Interaction of strangelets with ordinary nuclei
Strangelets (hypothetical stable lumps of strange quarkmatter) of
astrophysical origin may be ultimately detected in specific cosmic ray
experiments. The initial mass distribution resulting from the possible
astrophysical production sites would be subject to reprocessing in the
interstellar medium and in the earth's atmosphere. In order to get a better
understanding of the claims for the detection of this still hypothetic state of
hadronic matter, we present a study of strangelet-nucleus interactions
including several physical processes of interest (abrasion, fusion, fission,
excitation and de-excitation of the strangelets), to address the fate of the
baryon number along the strangelet path. It is shown that, although fusion may
be important for low-energy strangelets in the interstellar medium (thus
increasing the initial baryon number A), in the earth's atmosphere the loss of
the baryon number should be the dominant process. The consequences of these
findings are briefly addressed
Dark Matter, Dark Energy and Modern Cosmology: The Case For a Kuhnian Paradigm Shift
Several works in the last few years devoted to measure fundamental probes of contemporary cosmology have suggested the existence of a delocalized dominant component (the "dark energyâ), in addition to the several-decade-old evidence for "dark matterâ other than ordinary baryons, both assuming the description of gravity to be correct. Either we are faced to accept the ignorance of at least 95 % of the content of the universe or consider a deep change of the conceptual framework to understand the data. Thus, the situation seems to be completely favorable for a Kuhnian paradigm shiftin either particle physics or cosmology. We attempt to offer here a brief discussion of these issues from this particular perspective, arguing that the situation qualifies as a textbook Kuhnian anomaly, and offer a tentative identification of some of the actual elements typically associated with the paradigm shift process "in the worksâ in contemporary science
Ten Years Of Latin-American Journal Of Astronomy Education RELEA: Achievements And Challenges For International Astronomy Education Development
This study reviews 10 years of Latin-American Journal of Astronomy Education (RELEA), showing that the journal has become a valuable resource for publishing and highlights its pathway as scholarly journal. Furthermore, it is also a call to astronomy education specialists to consolidate their efforts considering similar journals worldwide. Publishing policies of the RELEA and their context are presented in relation to submission, refereeing and publication. The 75 articles published in 18 editions are analyzed and classified. The results showed an acceptance rate of 60.2%; an average of 7.5 articles per year/4.2 per issue. RELEA´s authors are mostly based in Brazil (81.3%); articles target chiefly a school level (university education: 28.0%, high school: 28.0%); their main study focus is learning and teaching (34.7%) and general astronomy content (33.3%). Our results show that compared to other Brazilian journals of education, RELEA had twice as much astronomy articles. In the international scene there has been about a half of astronomy education research, but twice of astronomy in science education research. The challenges related to improve the article submission are discussed: how to increase their number, the submission of Latin American countries, and how to bring in the issues and subjects not addressed until now. It is also encouraged graduate studies, new lines of research in astronomy education, and advertising the journal in universities and schools for professors and teachers. Finally, future possibilities are mentioned given the International Astronomical Unionâs development programs
Deflection of ultra high energy cosmic rays by the galactic magnetic field: from the sources to the detector
We report the results of 3D simulations of the trajectories of ultra-high
energy protons and Fe nuclei (with energies and ) propagating through the galactic magnetic field from the
sources to the detector. A uniform distribution of anti-particles is
backtracked from the detector, at the Earth, to the halo of the Galaxy. We
assume an axisymmetric, large scale spiral magnetic field permeating both the
disc and the halo. A normal field component to the galactic plane () is
also included in part of the simulations. We find that the presence of a large
scale galactic magnetic field does not generally affect the arrival directions
of the protons, although the inclusion of a component may cause
significant deflection of the lower energy protons (
eV). Error boxes larger than or equal to are most expected in
this case. On the other hand, in the case of heavy nuclei, the arrival
direction of the particles is strongly dependent on the coordinates of the
particle source. The deflection may be high enough () as to make
extremely difficult any identification of the sources unless the real magnetic
field configuration is accurately determined. Moreover, not every incoming
particle direction is allowed between a given source and the detector. This
generates sky patches which are virtually unobservable from the Earth. In the
particular case of the UHE events of Yakutsk, Fly's Eye, and Akeno, they come
from locations for which the deflection caused by the assumed magnetic field is
not significant.Comment: LaTeX + 2 postscript figures - Color versions of both figures (highly
recommended) available via anonymous ftp at
ftp://capc07.ast.cam.ac.uk/pub/uhecr_gmf as fig*.g
Mu and Tau Neutrino Thermalization and Production in Supernovae: Processes and Timescales
We investigate the rates of production and thermalization of and
neutrinos at temperatures and densities relevant to core-collapse
supernovae and protoneutron stars. Included are contributions from electron
scattering, electron-positron annihilation, nucleon-nucleon bremsstrahlung, and
nucleon scattering. For the scattering processes, in order to incorporate the
full scattering kinematics at arbitrary degeneracy, the structure function
formalism developed by Reddy et al. (1998) and Burrows and Sawyer (1998) is
employed. Furthermore, we derive formulae for the total and differential rates
of nucleon-nucleon bremsstrahlung for arbitrary nucleon degeneracy in
asymmetric matter. We find that electron scattering dominates nucleon
scattering as a thermalization process at low neutrino energies
( MeV), but that nucleon scattering is always faster
than or comparable to electron scattering above MeV. In
addition, for g cm, MeV, and
neutrino energies MeV, nucleon-nucleon bremsstrahlung always
dominates electron-positron annihilation as a production mechanism for
and neutrinos.Comment: 29 pages, LaTeX (RevTeX), 13 figures, submitted to Phys. Rev. C. Also
to be found at anonymous ftp site http://www.astrophysics.arizona.edu; cd to
pub/thompso
- âŚ