Accretion by isolated neutron stars     

N. R. Ikhsanov 《Astrophysics and Space Science》2007,308(1-4):137-140

Accretion of interstellar material by an isolated neutron star is discussed. The point I address here is the interaction between the accretion flow and the stellar magnetosphere. I show that the interchange instabilities of the magnetospheric boundary under the conditions of interest are basically suppressed. The entry of the material into the magnetosphere is governed by diffusion. Due to this reason the persistent accretion luminosity of isolated neutron stars is limited to <4×10²⁶ erg s⁻¹. These objects can also appear as X-ray bursters with the burst durations of ∼30 min and repetition time of ∼10⁵ yr. This indicates that the number of the accreting isolated neutron stars which could be observed with recent and modern X-ray missions is a few orders of magnitude smaller than that previously estimated.   相似文献   

The Galactic Center: a laboratory for AGN?     

Peter G. Mezger  Wolfgang J. Duschl  Robert Zylka 《Astronomy and Astrophysics Review》1996,7(4):289-388

Summary. This paper reviews the physical state of stars and Interstellar Matter in the Galactic Bulge (radius kpc from the dynamical center of the Galaxy), in the Nuclear Bulge (kpc) and in the Sgr A Radio and GMC Complex, i.e. the central \,pc of our Galaxy. The Galactic Bulge is devoid of cold Interstellar Matter and consists mainly of old stars, while the Nuclear Bulge accounts for of the mass of all of the Interstellar Matter in the Galaxy. A similar ratio holds for the formation rate of medium and high mass stars in Bulge and Disk. The metal abundance of the Interstellar Matter in the Galactic Bulge is found to be . The H-to-CO conversion factors to be applied to molecular gas in the Central Region are by factors 3 (Arimoto et al. 1996) to 10 (Sodroski et al. 1995) lower than in the solar vicinity. Hence, most H masses derived for the Central Region appear to be considerably overestimated. The Nuclear Bulge is pervaded by a thermal plasma (K) which is responsible for the diffuse free-free emission. Lyman continuum photon and dust IR luminosity of the Nuclear Bulge again account for of the respective total luminosities of the Galaxy. Magnetic fields in the Nuclear Bulge are strong (up to mG) as compared with the Galactic Disk (a few tens of G). The field lines are oriented parallel to the galactic plane inside giant molecular clouds and perpendicular to the plane in the intercloud medium. The compact source Sgr A* is close to or at the dynamical center of the Galaxy. Its radio spectrum with a high frequency cut-off at GHz, a low frequency turnover at GHz and a flux density dependence in between can be explained by synchrotron emission from quasi-monoenergetic relativistic electrons. Due to an extinction between Sun and Galactic Center corresponding to , an intrinsic weakness of this source in the near infrared, and a strong background emission from warm dust there are only upper limits available for the flux density of Sgr A* in the far, mid and near infrared and X-ray regime. The size of Sgr A* in the radio regime is cm, its dereddened K-band flux density is mJy, its luminosity has upper limits of (if radiation comes from an Accretion Disk) and (if black-body radiation from an object with a single temperature of K is assumed). If anyone of the soft X-ray sources detected by ROSAT actually coincides with Sgr A*, its X-ray luminosity would be less than a few . With a dark mass of Sgr A* is the best candidate for a starving black hole, although there are no observational indications for the presence of a (Standard) Accretion Disk. While the radio/IR spectrum of Sgr A* is purely nonthermal, the spectrum integrated over the central parsec resembles that of a Seyfert galaxy. Sgr A* is embedded in the Hii region Sgr A West with part of the ionized gas forming a minispiral. Sgr A West is surrounded by the Circum Nuclear Disk, an irregular shaped assembly of molecular gas which extends from pc and rotates around the Galactic Center with an estimated dynamical time scale of \,yr. The total luminosity of of the central parsec is due to the radiation of early-type stars of which have now been directly identified as luminous blue supergiants. It is still debated, however, if these stars can also account for all of the ionization of Sgr A West. In addition, the central parsec contains red giants, AGB stars, and a few super giants of which the brightest are now identified by direct imaging. These stars – together with a few million low mass main sequence stars – account for the bulk of the 2.2\,m emission. The spatial distributions of the three stellar populations in the central pc are remarkably different. Sgr A* is – along the line-of-sight – presumably located close to the center of the Hii region Sgr A West, which in turn is located in front of the extended (pc) synchrotron source Sgr A East, which appears to be the remnant of a gigantic explosion (of the order of the energy of a single supernova explosion) which took place yr ago inside the GMC Sgr A East Core. X-ray observations show within pc a pervasive hot (keV) plasma of expansion age of yr. Both phenomena – as well as the formation of the Circum Nuclear Disk – may have the same origin. Influx of material is observed within the Nuclear Bulge on all distance scales. In the Nuclear Bulge (pc) as well as in the Circum Nuclear Disk (pc) inflow towards the Galactic Center occurs primarily in the galactic plane and amounts to a few . The accretion rate into the central Black Hole, deduced from the luminosity of Sgr A*, however, appears to be lower by at least five orders of magnitude (assuming standard disk accretion). But in an equilibrium state only part of the infalling mass which is not accreted by the Black Hole can be consumed by star formation. A mass inflow rate varying with time is a more natural explanation. Comparing the physical state of the Center of our Galaxy with that of Active Galactic Nuclei derived from observations and modelling, we find that most of the basic characteristics of an AGN are also present in the Galactic Center. Lacking are, however, both the evidence for a standard Accretion Disk and a hard UV spectrum with accompanying high excitation emission lines in the Galactic Center which are characteristic for AGN. The luminosity of the central parsec, , amounts to only of the total luminosity of the Galaxy of . Seen from a distance of M31 (kpc) with an angular resolution of (corresponding to a linear size of pc) the Center of our Galaxy would appear as a mildly active nucleus with some starburst activity and would probably be classified as a weak Seyfert galaxy. The synchrotron spectrum of Sgr A*, however, would be completely masked by reprocessed stellar light (i.e. free-free and dust emission). Received: October 21, 1996  相似文献   

Generation of Magnetic Fields in Partially Ionized Dusty Plasmas Caused by Sheared Flows     

Birk  G.T.  Kopp  A.  Lesch  H. 《Studia Geophysica et Geodaetica》1998,42(3):404-409

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Ultrasoft X-ray components in the high-temperature extreme     

E. M. Puchnarewicz 《Monthly notices of the Royal Astronomical Society》1998,299(1):299-304

A search for high-redshift ( z  > 0.5) ultrasoft X-ray AGN in pointed ROSAT PSPC observations is made, using the wgacat catalogue. Evidence for a further three such objects is found, adding to three identified previously (E1346+266, EXO 1346.4+2637 and RX J0947.0+4721). The flux of one new object [1WGA J1342.4+2720; L (0.1–2 keV)∼ 2 × 10⁴⁵ erg s⁻¹)] was found to be variable (a factor of 1.7 over 2 d), and this object has relatively narrow permitted line emission. These properties, also seen in E1346+266 and EXO 1346.4+2637, are typical of ultrasoft AGN at all redshifts, in particular the narrow-line Seyfert 1s (NLS1s). The rarity of high- z ultrasoft AGN places limits on the 'temperature' of the soft X-ray component in the rest frame of AGN in general. It also provides an opportunity to investigate models for the soft component in the high-temperature extreme, e.g., for accretion disc models, testing the physics at the inner edge of the disc, and the effects of Comptonization in a hot, optically thin corona.  相似文献   

Effects of T-shape groin parameters on beach accretion   总被引：1，自引：0，他引：1  

&#x;smail Hakk&#x; zler  Murat &#x;hsan Kmürcü  Ali Remzi Birben   mer Yüksek  Servet Karasu 《Ocean Engineering》2006,33(3-4):382-403

The effects of various groin parameters (length, head length and opening) and wave parameters (wave height, wave period and wave angle) on the accretion of the area protected by T-shape groins are studied in a physical model. The model studies are performed using regular waves in a basin. A numerical model which depends primarily on a CERC model is employed to examine the effects of some of the above parameters. Good agreement is found in the results of physical and numerical models. The results of a numerical model are compared with field data obtained by deep sounding measurements at Of coasts, Trabzon Province, Turkey. The finding from this study may be employed in designing T-shape groins.  相似文献   

Differential Late Paleozoic active margin evolution in South-Central Chile (37°S–40°S) – the Lanalhue Fault Zone     

Johannes Glodny  Helmut Echtler  Santiago Collao  Mary Ardiles  Pablo Burn  Oscar Figueroa 《Journal of South American Earth Sciences》2008,26(4):397-411

The N–S oriented Coastal Cordillera of South Central Chile shows marked lithological contrasts along strike at 38°S. Here, the sinistral NW–SE-striking Lanalhue Fault Zone (nomen novum) juxtaposes Permo-Carboniferous magmatic arc granitoids and associated, frontally accreted metasediments (Eastern Series) in the northeast with a Late Carboniferous to Triassic basal-accretionary forearc wedge complex (Western Series) in the southwest. The fault is interpreted as an initially ductile deformation zone with divergent character, located in the eastern flank of the basally growing, upwarping, and exhuming Western Series. It was later transformed and reactivated as a semiductile to brittle sinistral transform fault. Rb–Sr data and fluid inclusion studies of late-stage fault-related mineralizations revealed Early Permian ages between 280 and 270 Ma for fault activity, with subsequent minor erosion. Regionally, crystallization of arc intrusives and related metamorphism occurred between 306 and 286 Ma, preceded by early increments of convergence-related deformation. Basal Western Series accretion started at >290 Ma and lasted to 250 Ma. North of the Lanalhue fault, Late Paleozoic magmatic arc granitoids are nearly 100 km closer to the present day Andean trench than further south. We hypothesize that this marked difference in paleo-forearc width is due to an Early Permian period of subduction erosion north of 38°S, contrasting with ongoing accretion further south, which kinematically triggered the evolution of the Lanalhue Fault Zone. Permo-Triassic margin segmentation was due to differential forearc accretion and denudation characteristics, and is now expressed in contrasting lithologies and metamorphic signatures in todays Andean forearc region north and south of the Lanalhue Fault Zone.  相似文献   

Geophysical consequences of planetary-scale impacts into a Mars-like planet     

Margarita M. Marinova  Oded Aharonson 《Icarus》2011,211(2):960-8

All planetary bodies with old surfaces exhibit planetary-scale impact craters: vast scars caused by the large impacts at the end of Solar System accretion or the late heavy bombardment. Here we investigate the geophysical consequences of planetary-scale impacts into a Mars-like planet, by simulating the events using a smoothed particle hydrodynamics (SPH) model. Our simulations probe impact energies over two orders of magnitude (2 × 10²⁷-6 × 10²⁹ J), impact velocities from the planet’s escape velocity to twice Mars’ orbital velocity (6-50 km/s), and impact angles from head-on to highly oblique (0-75°). The simulation results confirm that for planetary-scale impacts, surface curvature, radial gravity, the large relative size of the impactor to the planet, and the greater penetration of the impactor, contribute to significant differences in the geophysical expression compared to small craters, which can effectively be treated as acting in a half-space. The results show that the excavated crustal cavity size and the total melt production scale similarly for both small and planetary-scale impacts as a function of impact energy. However, in planetary-scale impacts a significant fraction of the melt is sequestered at depth and thus does not contribute to resetting the planetary surface; complete surface resetting is likely only in the most energetic (6 × 10²⁹ J), slow, and head-on impacts simulated. A crater rim is not present for planetary-scale impacts with energies >10²⁹ J and angles ?45°, but rather the ejecta is more uniformly distributed over the planetary surface. Antipodal crustal removal and melting is present for energetic (>10²⁹ J), fast (>6 km/s), and low angle (?45°) impacts. The most massive impactors (with both high impact energy and low velocity) contribute sufficient angular momentum to increase the rotation period of the Mars-sized target to about a day. Impact velocities of >20 km/s result in net mass erosion from the target, for all simulated energies and angles. The hypothesized impact origin of planetary structures may be tested by the presence and distribution of the geochemically-distinct impactor material.  相似文献   

Appearance of layered structures in numerical simulations of polydisperse bodies accretion: Application to cometary nuclei     

J. Lasue  R. Botet  E. Hadamcik 《Icarus》2011,213(1):369-381

A model for the aggregation of size distribution of cometesimals (Gaussian or power law) into cometary nuclei is developed. Upon disruption induced by collisions, sticking and evolution of the tensile strength and density of the cometesimals by sintering processes are taken into account. The resulting cometary nuclei present specific internal structures that have been quantified to allow the comparison with observational constraints and future in situ observations and cometary nucleus sounding with the CONSERT radar on-board the Rosetta mission. A parameter called the homogeneity exponent, μ, determines different aggregation regimes. Fractal aggregates are formed for μ < 0.4. Radial variations in tensile strength appear for 0.4 < μ < 0.6 and vanish for larger values of μ. The initial size distribution (following a Gaussian or power law) of aggregating cometesimals does not influence strongly these values but can change the extent of corresponding layers. If the layering observed on the surface of some cometary nuclei occurs often and originates from primordial structures, this constrains the velocity distribution of aggregating bodies to follow v∝m^-0.25, while a differential size distribution following a power law with exponent between −2 and −3 should result for large bodies, in agreement with current estimations of the size distributions. Such a layered structure would lead to more cohesive, dense and less porous material located near the center of mass of the nucleus predicting an increase of bulk density of comet nuclei with their erosion state.  相似文献   

The convective stability of a thin viscous disc     

M. Abramowicz  J. Papaloizou  E. Szuszkiewicz 《地球物理与天体物理流体动力学》2013,107(1-4):215-233

Abstract

We prove that the presence of viscosity does not affect stability to axisymmetric convective modes of a thin differentially rotating disc with no thermal conduction but in which viscosity is taken fully into account. In such a case the Schwarzschild criterion is necessary and sufficient for convective stability to local perturbations. In the proof we use a general formulation of local stability analysis, which allows a rigorous demonstration. Restricted particular forms of the viscous stress tensor introduced in the modelling of thin accretion discs may lead to viscous overstabilities. The additional instability found by Elstner et al. (1989) and described by the authors as a correction to the Schwarzschild criterion is a manifestation of these. However, when viscosity is taken fully into account, such instabilities cannot be discussed within the framework of a local analysis, a fully global treatment being required.  相似文献   

How rapidly did Mars accrete? Uncertainties in the Hf-W timing of core formation     

F. Nimmo  T. Kleine 《Icarus》2007,191(2):497-504

Estimates for the martian core formation timescale based on the hafnium-tungsten (Hf-W) isotopic system have varied by almost an order of magnitude, because of uncertainties in the martian mantle Hf/W ratio. Here we argue that the Hf/W ratio is ∼4 but is uncertain by ∼25%, resulting in (instantaneous) martian core formation timescales ranging from 0 to 10 Myr; accordingly, Hf-W isotope observations currently have limited utility in distinguishing between scenarios in which Mars formed as a stranded embryo and scenarios in which Mars suffered a prolonged accretion history.  相似文献