Results 1 - 10 of 3194
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[en] We examine the recent star formation associated with four supergiant shells in the Large Magellanic Cloud (LMC): LMC 1, 4, 5, and 6, which have been shown to have simple expanding-shell structures. H II regions and OB associations are used to infer star formation in the last few Myr, while massive young stellar objects reveal the current ongoing star formation. Distributions of ionized H I and molecular components of the interstellar gas are compared with the sites of recent and current star formation to determine whether triggering has taken place. We find that a great majority of the current star formation has occurred in gravitationally unstable regions, and that evidence of triggered star formation is prevalent at both large and local scales.
[en] By a comparison of Trapezium-type multiple star systems in the Abastumani Catalogue and the famous Catalogue of Stellar Associations and Clusters, it is confirmed that the great number of Trapezium-type multiple stars, which belong mostly to spectral classes O-B2, are found in associations and clusters. In 13 T-associations, 120 Trapezium-type multiple systems and 182 common multiple and double stars could be detected. Based on present-day observational data and the author's photographic observations, the kinematic of Trapezium-type multiple stars of spectral class O-B2 is studied. (Auth.)
[en] Even though it is likely that the process of creation of an inverted electron distribution in the slowly converging magnetic field lines would not arise because of the fast destruction of anisotropy by collisions, it is possible that a rapid build-up of an inverted population, as might occur in a shock transition, could be preserved for a relatively long time by means of transfer of momentum, rather than energy, from an inverted population of protons
[en] After discussing the basic facts about geometrically thin accretion discs, the authors discuss the motivations which have led to the theory of geometrically thick ones and present their main properties. They review the observational evidence for the existence of thick accretion discs, concentrating on recent results in the extragalactic context
[en] We discuss the effect of chemical separation as matter freezes at the base of the ocean of an accreting neutron star, and argue that the retention of light elements in the liquid acts as a source of buoyancy that drives a slow but continual mixing of the ocean, enriching it substantially in light elements, and leading to a relatively uniform composition with depth. We first consider the timescales associated with different processes that can redistribute elements in the ocean, including convection, sedimentation, crystallization, and diffusion. We then calculate the steady-state structure of the ocean of a neutron star for an illustrative model in which the accreted hydrogen and helium burn to produce a mixture of O and Se. Even though the H/He burning produces only 2% oxygen by mass, the steady-state ocean has an oxygen abundance more than 10 times larger, almost 40% by mass. Furthermore, we show that the convective motions transport heat inward, with a flux of ∼0.2 MeV nucleon-1 for an O-Se ocean, heating the ocean and steepening the outward temperature gradient. The enrichment of light elements and heating of the ocean due to compositionally driven convection likely have important implications for carbon ignition models of superbursts.
[en] The purpose of this note is to present preliminary results of deceleration calculations for matter infalling onto strongly magnetized neutron star. We discuss the validity of both models of thermalization. It has been found that for accretion rates > or approx. 105 gr.cm-2 sec-1, stationary, collisionless shock solutions do not exist. (orig.)
[en] Results of analytical study of early stages of a single star evolution are used investigation of the rotational fragmentation of the collapsing gas-dust cloud which leads to formation of two types of binaries. Wide α-systems (Psub(orb) > or approximately 100 yrs) with usually unequal masses of components are formed before the formation of gas-dust core in hydrostatical equilibrium. Close #betta#-systems (Psub(orb) < or approximation 100 yrs) with usually nearly equal masses of components are formed in the course of the collapse of gas-dust core
[en] Some questions concerning formation and early evolution of protoplanetary disks around single solar type stars are discussed. A new class of models of circumstellar protoplanetary disks are proposed, the parameters of the model being the disk's mass and angular momentum and the central star's mass and luminosity. Since disks probably exist for a long time (up to 100 million years), it would be interesting to search for them about stars in disintegrating clusters
[en] We study the 205.9 Hz pulsations of the accreting millisecond X-ray pulsar NGC 6440 X-2 across all outbursts observed with the Rossi X-ray Timing Explorer over a period of 800 days. We find the pulsations are highly sinusoidal with a fundamental amplitude of 5%–15% rms and a second harmonic that is only occasionally detected with amplitudes of ≲2% rms. By connecting the orbital phase across multiple outbursts, we obtain an accurate orbital ephemeris for this source and constrain its 57 minute orbital period to sub-millisecond precision. We do not detect an orbital period derivative to an upper limit of We investigate the possibility of coherently connecting the pulse phase across all observed outbursts, but find that due to the poorly constrained systematic uncertainties introduced by a flux-dependent bias in the pulse phase, multiple statistically acceptable phase-connected timing solutions exist.
[en] Data for tangential velocities of the condensations groups which make up the Herbig-Haro (HH objects) HH 1, HH 2 and HH 39 indicate that these condensations are expanding in the center-of-mass systems. On the basis of an analysis of condensation distributions and the electron density of the objects, it is concluded that these groups were formed by the decay of a more massive body. A schematic diagram of the center-of-mass system for the objects is provided which gives the directions and velocities of the condensations groups. 10 references