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[en] The issue of giant planet formation by core accretion (CA) far from the central star is rather controversial because the growth of a massive solid core necessary for triggering the gas runaway can take longer than the lifetime of the protoplanetary disk. In this work, we assess the range of separations at which CA may operate by (1) allowing for an arbitrary (physically meaningful) rate of planetesimal accretion by the core and (2) properly taking into account the dependence of the critical mass for the gas runaway on the planetesimal accretion luminosity. This self-consistent approach distinguishes our work from similar studies in which only a specific planetesimal accretion regime was explored and/or the critical core mass was fixed at some arbitrary level. We demonstrate that the largest separation at which the gas runaway can occur within 3 Myr corresponds to the surface density of solids in the disk ∼>0.1 g cm-2 and is 40-50 AU in the minimum mass solar nebula. This limiting separation is achieved when the planetesimal accretion proceeds at the fastest possible rate, even though the high associated accretion luminosity increases the critical core mass, delaying the onset of gas runaway. Our constraints are independent of the mass of the central star and vary only weakly with the core density and its atmospheric opacity. We also discuss various factors that can strengthen or weaken our limits on the operation of CA.
[en] Conclusion: 10 years of discovery? • Large effort of exploration; • Large amount of compliant resources discovered or confirmed; • New process development for low cost and for low grade; • New production from this effort still limited < 10%; • Feasibilty studies must confirm viability of economic exploitation and therefore resources quality; • Consolidation to set up critical mass deposits. ► To be ready for the coming decades 2020 +A
[en] Current subcritical limits for a number of uranium and plutonium materials (metals and compounds) as given in the ANSI/ANS standards for criticality safety are based on evaluations performed in the late 1970s and early 1980s. This paper presents the results of an analytical study of the minimum critical mass values for a set of materials using current codes and standard cross section sets. This work is meant to produce a consistent set of minimum critical mass values that can form the basis for adding new materials to the single-parameter tables in ANSI/ANS-8.1. Minimum critical mass results are presented for bare and water reflected full-density spheres and for full density moist (1.5 wt-% water) as calculated with KENO-Va, MCNP4A and ONEDANT. Calculations were also performed for both dry and moist materials at one-half density. Some KENO calculations were repeated using several cross section sets to examine potential bias differences. The results of the calculations were compared to the currently accepted subcritical limits. The calculated minimum critical mass values are reasonably consistent for the three codes, and differences most likely reflect differences in the cross section sets. The results are also consistent with values given in ANSI/ANS-8.1. 3 refs., 2 tabs
[en] The critical mass of the reactor RP-0 of the Peruvian Institute of Nuclear Energy is determined by using the Fermi model of diffusion and age in connection with an experimental subcritical neutron population. A desk computer HP9825 is used with a program in HP4. The observed discrepancy between the calculated and the measured mass is 5%
[en] The results of various conventional tests with this 1000kW experimental reactor (KSTR) are shown in diagrams. The comparatively high negative temperature coefficient of the reactivity was found to be a great advantage. Some inexplicable temperature differences and temperature changes in the reactor vessel were found. (J.S.)
[en] We present high spatial (<300 AU) and spectral (0.07 km s–1) resolution Submillimeter Array observations of the dense starless cluster core Oph A-N6 in the 1 mm dust continuum and the 3-2 line of N2H+ and N2D+. The dust continuum observations reveal a compact source not seen in single-dish observations, of size ∼1000 AU and mass 0.005-0.01 M☉. The combined line and single-dish observations reveal a core of size 3000 × 1400 AU elongated in a NW-SE direction, with almost no variation in either line width nor line center velocity across the map, and very small non-thermal motions. The deuterium fraction has a peak value of ∼0.15 and is >0.05 over much of the core. The N2H+ column density profile across the major axis of Oph A-N6 is well represented by an isothermal cylinder, with temperature 20 K, peak density 7.1 × 106 cm–3, and N2H+ abundance 2.7 × 10–10. The mass of Oph A-N6 is estimated to be 0.29 M☉, compared to a value of 0.18 M☉ from the isothermal cylinder analysis, and 0.63 M☉ for the critical mass for fragmentation of an isothermal cylinder. Compared to isolated low-mass cores, Oph A-N6 shows similar narrow line widths and small velocity variation, with a deuterium fraction similar to 'evolved' dense cores. It is significantly smaller than isolated cores, with larger peak column and volume density. The available evidence suggests that Oph A-N6 has formed through the fragmentation of the Oph A filament and is the precursor to a low-mass star. The dust continuum emission suggests that it may already have begun to form a star.
[en] The hoop conjecture, introduced by Thorne almost five decades ago, asserts that black holes are characterized by the mass-to-circumference relation 4πM/C≥1, whereas horizonless compact objects are characterized by the opposite inequality 4πM/C<1 (here C is the circumference of the smallest ring that can engulf the self-gravitating compact object in all azimuthal directions). It has recently been proved that a necessary condition for the validity of this conjecture in horizonless spacetimes of spatially regular charged compact objects is that the mass M be interpreted as the mass contained within the engulfing sphere (and not as the asymptotically measured total ADM mass). In the present paper we raise the following physically intriguing question: is it possible to formulate a unified version of the hoop conjecture which is valid for both black holes and horizonless compact objects? In order to address this important question, we analyze the behavior of the mass-to-circumference ratio of Kerr–Newman black holes. We explicitly prove that if the mass M in the hoop relation is interpreted as the quasilocal Einstein–Landau–Lifshitz–Papapetrou and Weinberg mass contained within the black-hole horizon, then these charged and spinning black holes are characterized by the sub-critical mass-to-circumference ratio 4πM/C<1. Our results provide evidence for the non-existence of a unified version of the hoop conjecture which is valid for both black-hole spacetimes and spatially regular horizonless compact objects.
[en] We explore the idea of a network of domain walls appearing on the surface of a soliton star. We show that for a suitable fine-tuning among the parameters of the model we can find localized fermion zero modes only on the network of domain walls. In this scenario, the soliton star becomes unstable and decays into free particles before the cold-matter upper mass limit is achieved. However, if fermions do not bind to the network of domain walls, the network becomes neutral, imposing a new lower bound on the charge of the soliton star, slightly raising its critical mass
[en] We present a study of dense structures in the L 1495 filament in the Taurus Molecular Cloud and examine its star-forming properties. In particular, we construct a dust extinction map of the filament using deep near-infrared observations, exposing its small-scale structure in unprecedented detail. The filament shows highly fragmented substructures and a high mass-per-length value of Mline = 17 Msun pc-1, reflecting star-forming potential in all parts of it. However, a part of the filament, namely B 211, is remarkably devoid of young stellar objects. We argue that in this region the initial filament collapse and fragmentation is still taking place and star formation is yet to occur. In the star-forming part of the filament, we identify 39 cores with masses from 0.4 to 10 Msun and preferred separations in agreement with the local Jeans length. Most of these cores exceed the Bonnor-Ebert critical mass, and are therefore likely to collapse and form stars. The dense core mass function follows a power law with exponent Γ = 1.2 ± 0.2, a form commonly observed in star-forming regions.