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Three different models have been proposed for LiBeB production bycosmic rays: the CRI model in which the cosmic rays areaccelerated
out of an ISM of solar composition scaled withmetallicity; the CRS model in which cosmic rays with compositionsimilar to that
of the current epoch cosmic rays are acceleratedout of fresh supernova ejecta; and the LECR model in which adistinct low energy
component coexists with the postulated cosmicrays of the CRI model. These models are usually distinguished bytheir predictions
concerning the evolution of the Be and Babundances. Here we emphasize the energetics which favor the CRSmodel. This model
is also favored by observations showing that thebulk (80 to 90%) of all supernovae occur in hot, low densitysuperbubbles,
where supernova shocks can accelerate the cosmicrays from supernova ejecta enriched matter.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
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We study the evolution of an ensemble of energetic (1 GeV) protons in a twisted force-free magnetic loop. The protons are followed with a bounce-average method and they are subjected to collisions with ambient gas and pitch-angle scattering by plasrma turbulence in the loop. The proton loss is initially by drift and later by scattering into the loss cone. Gamma rays are produced by pion decays resulting from nuclear reactions of these lost protons. It is found that in order to have long-lasting protons, one of the following scenarios should hold: (1) For small loops (of length 2 × 109 cm), the twist angle should be about 2 and the turbulence level below 10–8 erg cm–3. (2) For large loops ( 1010 cm), the turbulence level should be below 10–6 erg cm–3. These set the conditions for testing the trapping picture as a viable explanation for the observed eight-hour gamma-ray emission. 相似文献
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Sandra R. Oliveira Oswaldo D. Miranda & José C. N. de Araujo Reuven Opher 《Monthly notices of the Royal Astronomical Society》1998,301(1):115-130
In this article we extend the study performed in our previous article of the collapse of primordial objects. We here analyse the behaviour of the physical parameters for clouds ranging from 107 to 1015 M⊙ . We study the dynamical evolution of these clouds in two ways: as purely baryonic clouds and as clouds with non-baryonic dark matter included. We start the calculations at the beginning of the recombination era, following the evolution of the structure until the collapse (which we defined as the time when the density contrast of the baryonic matter is greater than 104 ). We analyse the behaviour of several physical parameters of the clouds (e.g. the density contrast and the velocities of the baryonic matter and the dark matter) as a function of time and radial position in the cloud. In this study all physical processes that are relevant to the dynamical evolution of the primordial clouds, such as for example photon drag (due to the cosmic background radiation) and hydrogen molecular production, besides the expansion of the Universe, are included in the calculations. In particular we find that the clouds with dark matter collapse at higher redshift when we compare the results with the purely baryonic models. As a general result we find that the distribution of the non-baryonic dark matter is more concentrated than the baryonic one. It is important to stress that we do not take into account the putative virialization of the non-baryonic dark matter; we just follow the time and spatial evolution of the cloud, solving its hydrodynamical equations. We also studied the role of cooling–heating processes in the purely baryonic clouds. 相似文献
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The intensity and frequency spectrum of gyro-synchrotron emission from energetic solar electrons radiating in coronal magnetic fields are calculated. These calculations, based on a recent study of the generation of gyro-synchrotron emission in a magnetoactive plasma, are applied to a Type IV radio burst originating at a high altitude in the solar corona. It is shown that the observed frequency spectrum of the burst, which exhibits very sharp low and high frequency cutoffs, can be best understood in terms of gyro-synchrotron emission in an ionized medium and that from the observed frequency spectrum and the ambient coronal density it is possible to deduce both the magnetic field at the site of the emission and the range of electron energies responsible for the burst.NAS-NASA Post-Doctoral Resident Research Associate.Research supported by the National Research Foundation under grant GP-849. 相似文献
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Solar Physics - We show that protons can be accelerated to several GeV in ≲10 s by Alfven turbulence whose energy density is greater than a few erg/cm3. We also show that electrons can be... 相似文献
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Oswaldo D. Miranda Merav Opher & Reuven Opher 《Monthly notices of the Royal Astronomical Society》1998,301(2):547-550
The origin of the magnetic field in galaxies is an open question in astrophysics. Several mechanisms have been proposed related, in general, to the generation of small seed fields amplified by a dynamo mechanism. In general, these mechanisms have difficulty in satisfying both the requirements of a sufficiently high strength for the magnetic field and the necessary large coherent scales. We show that the formation of dense and turbulent shells of matter, in the multiple explosion scenario of Miranda &38; Opher for the formation of the large-scale structures of the Universe, can naturally act as a seed for the generation of a magnetic field. During the collapse and explosion of Population III objects, a temperature gradient not parallel to a density gradient can naturally be established, producing a seed magnetic field through the Biermann battery mechanism. We show that seed magnetic fields ∼ 10−12 –10−14 G can be produced in this multiple explosion scenario on scales of the order of clusters of galaxies (with coherence length L ∼ 1.8 Mpc) and up to ∼ 4.5 × 10−10 G on scales of galaxies ( L ∼ 100 kpc). 相似文献
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Reuven Opher 《Astrophysics and Space Science》1997,256(1-2):37-50
We discuss some of the major areas in astronomy and cosmology where plasma physics is important: (1) origin of stars; (2) distortions of the microwave background radiation; (3) expansion rate of the Early Universe; (4) the magnetic fields and relativistic electrons in jets; (5) the collimation of jets; (6) the origin of stellar winds; (7) the origin of filaments and clouds not gravitationally bound; and (8) the origin of cosmic rays. 相似文献
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It is generally assumed that the magnetic fields of millisecond pulsars (MSPs) are ～108 G. We argue that this may not be true and the fields may be appreciably greater. We present six evidences for this: (1) The ～108G field estimate is based on magnetic dipole emission losses which is shown to be questionable; (2) The MSPs in low mass X-ray binaries (LMXBs) are claimed to have <1011 G on the basis of a Rayleygh-Taylor instability accretion argument. We show that the accretion argument is questionable and the upper limit 1011 G may be much higher; (3) Low magnetic field neutron stars have difficulty being produced in LMXBs; (4) MSPs may still be accreting indicating a much higher magnetic field; (5) The data that predict ～108 G for MSPs also predict ages on the order of, and greater than, ten billion years, which is much greater than normal pulsars. If the predicted ages are wrong, most likely the predicted ～108 G fields of MSPs are wrong; (6) When magnetic fields are measured directly with cyclotron lines in X-ray binaries, fields ?108 G are indicated. Other scenarios should be investigated. One such scenario is the following. Over 85% of MSPs are confirmed members of a binary. It is possible that all MSPs are in large separation binaries having magnetic fields >108 G with their magnetic dipole emission being balanced by low level accretion from their companions. 相似文献
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Ofer Marder Reuven Yeshurun Ronit Lupu Guy Bar‐Oz Miriam Belmaker Naomi Porat Hagai RON Amos Frumkin 《第四纪科学杂志》2011,26(8):769-780
Rantis Cave is a recently discovered filled cave in central Israel, displaying a rich faunal accumulation of micromammals, ungulates and carnivores. U–Th dating assigns the beginning of accumulation to ca. 140 ka. The accumulation is culturally assigned to the late half of the Middle Paleolithic (MP) period. Single‐grain optically stimulated luminescence measurements attest to a complex sedimentological history. We present the cross‐disciplinary results of taphonomic and geomorphological analyses, which point to the cave serving as a natural pitfall trap for the large fauna, with little human or carnivore activity. The fauna is dominated by Dama among the ungulates and by Microtus among the micromammals. These data in conjunction with ungulate tooth mesowear analysis suggest a xeric Mediterranean environment on the eastern margin of the southern Levantine foothills. The relative taxonomic abundance of ungulate taxa shows some differences from anthropogenic MP sites, possibly reflecting the prey choice patterns of MP hunters. Overall, the natural accumulation scenario for Rantis Cave provides a rare paleoenvironmental and paleoeconomic reference to the rich anthropogenic MP faunas of the Southern Levant, enabling the reconstruction of a rich and diverse environmental setting for this important human dispersal route. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献