1,720,967 research outputs found
Early spectrum and bolometric corrections for SN 1987A
No full textIn order to relate the visual magnitudes observed at early times for SN 1987A with the theoretical luminosities obtained from supernova models, it is necessary to know the bolometric corrections for the just-exploded star. In this paper we develop an analytic model to derive the continuum spectrum and thence the absolute bolometric magnitudes and the bolometric corrections of SN 1987A in the first 2 days after the explosion, i.e., up until the time that the full spectrum was measured.
We integrate the equation of radiative transfer over realistic envelope profiles given by computer outputs by deriving an analytical relation between optical depth, density, and temperature and by extending the Milne-Schwarzschild type of solution to the case of spherical geometry; in order to account for the strong dominance of scattering over absorption in the early atmosphere due to the high degree of ionization of hydrogen and helium, we calculate self-consistently the nonthermal source function with an interative procedure. We can in this way describe both the extended nature of the envelope and the non-LTE state of the radiation field, which both cause a dilution of the emerging flux and thence yield an effective temperature lower than the color temperature.
The explicit results we obtain for Woosley's model 10H at t = 0.5 days show indeed a significant deviation from the simple "blackbody approximation" used so far: the calculated spectrum has a net excess of higher frequency emission when compared with blackbody, and the total luminosity is much lower than in the thermal case. The precise values for the magnitudes derived from this depend on the absorptive processes, here represented by the Doppler-enhanced line opacity due to the metals in the rapidly expanding atmosphere. With a physically reasonable estimate of these effects, we get for the visual brightness a value which is about half a magnitude fainter than the one calculated so far using a sharp photosphere, blackbody sources, and "static" line opacities and which, unlike the previous one, fits the observations quite well. Yet we point out that this alone is not enough to single out Woosley's model 10H as the right model for SN 1987A
Vortex-lattice interaction in Pulsar Glitches
No full textIn this proceeding we present a quantitative model to study the pinning interaction between a neutron vortex and the nuclear lattice in the inner crust of a neutron star, which is formed by nuclear clusters immersed in a superfluid neutron gas. This is one of the most important quantities in the vortex model for pulsar glitches, where the sudden variations of the rotation velocity observed in some neutron stars are explained in terms of pinning-depinning processes between vortices and the nuclear lattice. The study has been done through an actual counting of the number of pinning sites between the vortex and the nuclear lattice. Moreover, it has been recognized that the vortices can have a generic orientation with respect to the lattice and thus the simple pinned-unpinned scenario (that is characteristic of the aligned case and has been assumed in previous qualitative models) is not acceptable for a realistic calculation. The forces that we have found are more than two orders of magnitude smaller than those calculated so far in the literature, and of the same order as what suggested to explain real Vela-like glitches. They can thus be applied to quantitative models for the vortex dynamics, in order to better simulate and understand these remarkable astrophysical phenomena
Mass-Energy Relation for SN 1987A from Observations
No full textWe derive a relation between mass and kinetic energy for the hydrogen envelope of SN 1987A, using direct observations of the luminosities and photospheric velocities of the supernova. With a schematic but realistic treatment of the radiative transfer problem, which allows us to follow the position in mass of the photosphere as a function of time, we find that the observations determine uniquely the kinetic energy of the envelope once its mass is known. We do not use any input from the existing computer calculations of supernova explosions: therefore, our results are independent from any model assumed for the explosion mechanism and can actually constrain such models. Although the mass of the envelope has not so far been determined from observations, from different studies of the progenitor star there seems to be evidence for a quite large value, in the range Menv > 7 M ⊙. Corresponding to this, our approach gives for SN 1987 A a kinetic energy of the ejecta > 1 foe (=1051 ergs), the energy scaling roughly linearly with Menv
Is there nuclear pinning of vortices in superfluid pulsars?
Full text linkWe develop a fully consistent semianalytical model in order to study the vortex-nucleus interaction in the inner crust of neutron stars. In the framework of the local-density approximation and assuming a constant pairing gap and a square-well nuclear potential, the model takes into account all energy contributions and determines unambiguously the structure of the vortex core. The results show that, irrespective of the value of the pairing gap, only interstitial pinning takes place all along the inner crust. This is in contrast with all existing calculations, which predict nuclear pinning in the deeper layers of the crust. Should further studies confirm this surprising result, the explanation of pulsar glitches in terms of depinning of vortices will have to be carefully revisited
Critical dimensions for strungs on group-manifolds from path-integral methods
No full textWe derive the critical dimensions of bosonic string on SO(N) group-manifolds for arbitrary level k, using path-integral methods and non-abelian bosonization techniques. We obtain a recursive equation for the critical dimensions, whose solution is in agreement with the operator approach results. We also discuss the application of this method to the linear sigma models
Neutrino capture cross sections for 40Ar and β-decay of 40Ti
Full text linkShell-model calculations of solar neutrino absorption cross sections for Ar-40, the proposed component of the ICARUS detector, are presented. It is found that low-lying Gamow-Teller transitions lead to a significant enhancement of the absorption rate over that expected from the Fermi transition between the isobaric analog states, leading to an overall absorption cross section for B-8 neutrinos of (11.5 +/- 0.7) x 10(-43) Cm-2 or a total expected rate of 6.7 + 2.5 SNU. We also note that the pertinent Gamow-Teller transitions in Ar-40, experimentally accessible from the P-decay of the mirror nucleus Ti-40. Predictions for the branching ratios to states in (SC)-S-40 are presented, and the theoretical halflife of 55 +/- 5 ms is found to be in good agreement with the experimental value of 56(-12)(+18) ms
The Physics and Astrophysics of Neutron Stars
No full textThis book summarizes the recent progress in the physics and astrophysics of neutron stars and, most importantly, it identifies and develops effective strategies to explore, both theoretically and observationally, the many remaining open questions in the field.
Because of its significance in the solution of many fundamental questions in nuclear physics, astrophysics and gravitational physics, the study of neutron stars has seen enormous progress over the last years and has been very successful in improving our understanding in these fascinating compact objects.
The book addresses a wide spectrum of readers, from students to senior researchers. Thirteen chapters written by internationally renowned experts offer a thorough overview of the various facets of this interdisciplinary science, from neutron star formation in supernovae, pulsars, equations of state super dense matter, gravitational wave emission, to alternative theories of gravity.
The book was initiated by the European Cooperation in Science and Technology (COST) Action MP1304 “Exploring fundamental physics with compact stars” (NewCompStar)
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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