Analytical Modelling of Galaxy Formation     

Alberto Manrique  Eduard Salvador-Solé 《Astrophysics and Space Science》1998,263(1-4):39-42

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Variable synchrotron emission from BL Lacertae objects. II. Optical and X-ray flares in HBL source 1ES 1959+650     

Bidzina Z. Kapanadze  Eduard Janiashvili 《Astrophysics and Space Science》2012,339(2):339-353

This paper presents the results of the optical R band and 1.5–12 keV band X-ray monitoring of the high-energy peaked BL Lacertae source 1ES 1959+650 performed during 2002–2007 with the 70 cm Meniscus Telescope of Abastumani Astrophysical Observatory (Georgia) and the All-Sky Monitor on board the Rossi X-ray Time Explorer, respectively. The observed long- and short-term outbursts are fitted with the lightcurves obtained by means of the modeling of synchrotron flares that are assumed to be the result of a propagation of the relativistic shock waves through the jet of 1ES 1959+650, pointed to the observer. Different values of the input parameters (shock velocity, particles’ spectral index, sizes of emission region, minimum and maximum Lorentz factors of the particles etc.) are used in order to fit the simulated lightcurves whose constructed by means of observational data. This investigation shows that both shock velocity and physical conditions in the jet of 1ES 1959+650 should be variable from flare to flare. The shocks are found to be mildly relativistic with the apparent speeds β=0.46–0.85, expressed in the units of c. Spectral index of the particle energy distribution varied from 2.10 to 2.17 for the long-term flares while it is higher in the case of short-term outbursts: s=2.32–2.45 that is suggested to be a result of the deceleration of shock front during its passage through the shell situated downstream the Mach disc. The average strength of a turbulent magnetic field ranged from 0.025 gauss to 0.04 gauss for different long-term flares while the values of 0.07–0.14 gauss were adopted for the different short-term outbursts. The lengths of variable jet area found to be of 0.13–0.47 pc with the transverse extents of (0.5–1.0)×10¹⁷ cm in the case of long-term flares. The same characteristics for short-term outbursts were (2.74–5.5)×10¹⁶ cm and (0.2–04)×10¹⁷ cm, respectively. We conclude that both shock velocity and properties of pre-shocked plasma were not the same in 1ES 1959+650 for the different flaring epochs.  相似文献   

Building the cosmic distance scale: from Hipparcos to Gaia     

Catherine Turon  Xavier Luri  Eduard Masana 《Astrophysics and Space Science》2012,341(1):15-29

Hipparcos, the first ever experiment of global astrometry, was launched by ESA (European Space Agency) in 1989 and its results published in 1997 (Perryman et al. in Astron. Astrophys. 323:L49, 1997; Perryman & ESA (eds.) in The Hipparcos and Tycho catalogues, ESA SP-1200, 1997). A new reduction was later performed using an improved satellite attitude reconstruction leading to an improved accuracy for stars brighter than 9th magnitude (van Leeuwen & Fantino in Astron. Astrophys. 439:791, 2005; van Leeuwen in Astron. Astrophys. 474:653, 2007a). The Hipparcos Catalogue provided an extended dataset of very accurate astrometric data (positions, trigonometric parallaxes and proper motions), enlarging by two orders of magnitude the quantity and quality of distance determinations and luminosity calibrations. The availability of more than 20 000 stars (22 000 for the original catalogue, 30 000 for the re-reduction) with a trigonometric parallax known to better than 10% opened the way to a drastic revision of our 3-D knowledge of the solar neighbourhood and to a renewal of the calibration of many distance indicators and age estimations. The prospects opened by Gaia, the next ESA cornerstone, planned for launch in 2013 (Perryman et al., in Astron. Astrophys. 369:339, 2001), are still much more dramatic: a billion objects with systematic and quasi simultaneous astrometric, spectrophotometric and spectroscopic observations, about 150 million stars with expected distances to better than 10%, all over the Galaxy. All stellar distance indicators, in very large numbers, will be directly measured, providing a direct calibration of their luminosity and making possible detailed studies of the impacts of various effects linked to chemical element abundances, age or cluster membership. With the help of simulations of the data expected from Gaia, obtained from the mission simulator developed by DPAC (Gaia Data Processing and Analysis Consortium), we will illustrate what Gaia can provide with some selected examples.  相似文献   

The migrating embryo model for disk evolution     

Shantanu BASU  Eduard I. VOROBYOV 《Meteoritics & planetary science》2012,47(12):1907-1914

Abstract– A new view of disk evolution is emerging from self‐consistent numerical simulation modeling of the formation of circumstellar disks from the direct collapse of prestellar cloud cores. This has implications for many aspects of star and planet formation, including the growth of dust and high‐temperature processing of materials. A defining result is that the early evolution of a disk is crucially affected by the continuing mass loading from the core envelope, and is driven into recurrent phases of gravitational instability. Nonlinear spiral arms formed during these episodes fragment to form gaseous clumps in the disk. These clumps generally migrate inward due to gravitational torques arising from their interaction with a trailing spiral arm. Occasionally, a clump can open up a gap in the disk and settle into a stable orbit, revealing a direct pathway to the formation of companion stars, brown dwarfs, or giant planets. At other times, when multiple clumps are present, a low mass clump may even be ejected from the system, providing a pathway to the formation of free‐floating brown dwarfs and giant planets in addition to low mass stars. Finally, it has been suggested that the inward migration of gaseous clumps can provide the proper conditions for the transport of high‐temperature processed solids from the outer disk to the inner disk, and even possibly accelerate the formation of terrestrial planets in the inner disk. All of these features arising from clump formation and migration can be tied together conceptually in a migrating embryo model for disk evolution that can complement the well‐known core accretion model for planet formation.  相似文献   

Effect of the initial density and angular-velocity profiles of pre-stellar cores on the properties of young stellar objects     

Eduard?I.?VorobyovEmail author 《Astronomy Reports》2012,56(3):179-192

The physical properties of young stellar objects are studied as functions of the initial spatial distributions of the gas surface density Σ and angular velocity Ω in pre-stellar cores using numerical hydrodynamic simulations. Two limiting cases are considered: spatially homogeneous cores with Σ = const and Ω = const and centrally concentrated cores with radius-dependent densities Σ ∝ r ⁻¹ and Ω ∝ r ⁻¹. The degree of gravitational instability and protostellar disk fragmentation is mostly determined by the initial core mass and the ratio of the rotational to the gravitational energy, and depends only weakly on the initial spatial configuration of pre-stellar cores, except for the earliest stages of evolution, when models with spatially homogeneous cores can be more gravitationally unstable. The accretion of disk matter onto a protostar also depends weakly on the initial distributions of Σ and Ω, with matter from the collapsing core falling onto the disk at a rate that is slightly higher in models with spatially homogeneous cores. An appreciable dependence of the disk mass, disk radius, and the disk-to-protostar mass ratio on the initial density and angular velocity profiles of the parent core is found only for class 0 young objects; this relationship is not systematic in the later I and II stages of stellar evolution. The mass of the central protostar depends weakly on the initial core configuration in all three evolutionary stages.  相似文献   

Geomorphic and stratigraphic evidence for an unusual tsunami or storm a few centuries ago at Anegada, British Virgin Islands   总被引：1，自引：1，他引：0  

Brian F. Atwater  Uri S. ten Brink  Mark Buckley  Robert S. Halley  Bruce E. Jaffe  Alberto M. López-Venegas  Eduard G. Reinhardt  Maritia P. Tuttle  Steve Watt  Yong Wei 《Natural Hazards》2012,63(1):51-84

Waters from the Atlantic Ocean washed southward across parts of Anegada, east-northeast of Puerto Rico, during a singular event a few centuries ago. The overwash, after crossing a fringing coral reef and 1.5?km of shallow subtidal flats, cut dozens of breaches through sandy beach ridges, deposited a sheet of sand and shell capped with lime mud, and created inland fields of cobbles and boulders. Most of the breaches extend tens to hundreds of meters perpendicular to a 2-km stretch of Anegada??s windward shore. Remnants of the breached ridges stand 3?m above modern sea level, and ridges seaward of the breaches rise 2.2?C3.0?m high. The overwash probably exceeded those heights when cutting the breaches by overtopping and incision of the beach ridges. Much of the sand-and-shell sheet contains pink bioclastic sand that resembles, in grain size and composition, the sand of the breached ridges. This sand extends as much as 1.5?km to the south of the breached ridges. It tapers southward from a maximum thickness of 40?cm, decreases in estimated mean grain size from medium sand to very fine sand, and contains mud laminae in the south. The sand-and-shell sheet also contains mollusks??cerithid gastropods and the bivalve Anomalocardia??and angular limestone granules and pebbles. The mollusk shells and the lime-mud cap were probably derived from a marine pond that occupied much of Anegada??s interior at the time of overwash. The boulders and cobbles, nearly all composed of limestone, form fields that extend many tens of meters generally southward from limestone outcrops as much as 0.8?km from the nearest shore. Soon after the inferred overwash, the marine pond was replaced by hypersaline ponds that produce microbial mats and evaporite crusts. This environmental change, which has yet to be reversed, required restriction of a former inlet or inlets, the location of which was probably on the island??s south (lee) side. The inferred overwash may have caused restriction directly by washing sand into former inlets, or indirectly by reducing the tidal prism or supplying sand to post-overwash currents and waves. The overwash happened after A.D. 1650 if coeval with radiocarbon-dated leaves in the mud cap, and it probably happened before human settlement in the last decades of the 1700s. A prior overwash event is implied by an inland set of breaches. Hypothetically, the overwash in 1650?C1800 resulted from the Antilles tsunami of 1690, the transatlantic Lisbon tsunami of 1755, a local tsunami not previously documented, or a storm whose effects exceeded those of Hurricane Donna, which was probably at category 3 as its eye passed 15?km to Anegada??s south in 1960.  相似文献   

Homotrema rubrum (Lamarck) taphonomy as an overwash indicator in Marine Ponds on Anegada, British Virgin Islands     

Jessica E. Pilarczyk  Eduard G. Reinhardt 《Natural Hazards》2012,63(1):85-100

Marine hypersaline ponds on Anegada, British Virgin Islands contain stratigraphic evidence (Shell and Sand Sheet) of a A.D. 1650?C1800 overwash event that could have formed through a hurricane or tsunami. Candidates for the deposit include far-field (e.g. 1755 Lisbon tsunami) and local Puerto Rico Trench events (e.g. 1690), but hurricanes cannot be ignored. The goal of this study is to provide additional information to assess the origin of the deposit by examining the taphonomic characters of Homotrema rubrum, a common encrusting foraminifer in Caribbean reef settings. Surface samples (n?=?12) from major sub-environments (reef-flat, beach, storm wrack, and dune) and pond sections (n?=?6; 20?C80?cm thick) are analyzed for their Homotrema concentration (specimens/cm³) and taphonomic character. Particle-size analysis was conducted on the same sections and samples. Highly Preserved (red colored, angular, intact chambers) Homotrema dominate the beach, storm wrack, and reef-flat deposits relative to the dune sand, but the beach and storm wrack contain the largest specimens. The Shell and Sand Sheet in the pond has Highly Preserved and abundant Homotrema (specimens/cm³) versus other sedimentary units in the ponds (e.g. Mud Cap and Shelly Mud). Its taphonomic character is most similar (test size and condition) to the storm wrack deposit on the beach indicating an outside provenance for the sand. Concentration of Homotrema in the Shell and Sand Sheet declined southward indicating a northerly reef-flat provenance for the overwash, although it does not preclude a southern inundation as well. It is unclear whether Homotrema individuals originated from the reef itself or were eroded from older beach ridge deposits during the overwash event. Conclusions from Homotrema taphonomic analysis were limited by the lack of comparative data from known hurricane and tsunami deposits in other Caribbean regions.  相似文献   

The Investigation of the Dynamical Evolution of Extrasolar Three-planetary System GJ 3138     

Alexander Perminov  Eduard Kuznetsov 《天文和天体物理学研究(英文版)》2022,(1):68-84

This article is devoted to studying the dynamical evolution and orbital stability of compact extrasolar three-planetary system GJ 3138.In this system,all semima...  相似文献   

Magnetic Stratigraphy from Deep Clastic Turbidites: An Example from the Eocene Hecho Group (Southern Pyrenees)     

Oms  Oriol  Dinarès-Turell  Jaume  Remacha  Eduard 《Studia Geophysica et Geodaetica》2003,47(2):275-288

A paleomagnetic study of a 2200-meter thick section of clastic turbidites from the Eocene Hecho group (southcentral Pyrenees, Spain) allows defining its magnetostratigraphic record. Natural remanent magnetization is carried by up to three components: a viscous low-temperature component; a second component unblocked between 300°C and 345°C, likely carried by iron-sulphides; and a third component which demagnetizes at temperatures higher than 345°C and is likely carried by magnetite. The second and third components may display opposite polarities at the same site. The magnetite component delineates different polarity zones and has a more consistent behavior along section if compared with the iron-sulphide component, which displays either a normal or a reverse direction without any stratigraphic consistency along section. The bulk of iron sulphides are interpreted to be secondary in origin and to carry diagenetic overprints acquired at different times after deposition and the magnetite component is taken as the characteristic primary magnetization. As supported by biostratigraphy, the section is correlated from chrons C20r to C18n.2n (Lutetian-Bartonian transition), which indicates a mean sediment accumulation rate of about 52 cm/ky for the studied section. The new chronostratigraphy allows constraining the age of the upper Hecho Group (Banastón and Jaca allogroups) to an unprecedented level and is consistent with previous magnetostratigraphic work in younger sediments from the Jaca Basin. Deep clastic sedimentary systems should not be neglected as a target for magnetostratigraphic studies despite diagenetic growth of secondary minerals may mask the primary signal.  相似文献   

Periglacial processes and landforms in Britain and Ireland. Edited by J. BOARDMAN Publisher Cambridge University Press 1987 £40.00 or US $79.00 (hardback) (296 pp) ISBN 0 521 33250 8     

Eduard A. Koster 《第四纪科学杂志》1989,4(2):184-185

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