共查询到20条相似文献，搜索用时 31 毫秒
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Ryuichi Kurosawa M. M. Romanova Tim J. Harries 《Monthly notices of the Royal Astronomical Society》2008,385(4):1931-1945
We present 3D simulations of rotationally induced line variability arising from complex circumstellar environment of classical T Tauri stars (CTTS) using the results of the 3D magnetohydrodynamics (MHD) simulations of Romanova et al., who considered accretion on to a CTTS with a misaligned dipole magnetic axis with respect to the rotational axis. The density, velocity and temperature structures of the MHD simulations are mapped on to the radiative transfer grid, and corresponding line source function and the observed profiles of neutral hydrogen lines (Hβ, Paβ and Brγ) are computed using the Sobolev escape probability method. We study the dependency of line variability on inclination angles ( i ) and magnetic axis misalignment angles (Θ). We find the line profiles are relatively insensitive to the details of the temperature structure of accretion funnels, but are influenced more by the mean temperature of the flow and its geometry. By comparing our models with the Paβ profiles of 42 CTTS observed by Folha & Emerson, we find that models with a smaller misaligngment angle (Θ < ∼15°) are more consistent with the observations which show that majority of Paβ are rather symmetric around the line centre. For a high inclination system with a small dipole misalignment angle (Θ≈ 15°) , only one accretion funnel (on the upper hemisphere) is visible to an observer at any given rotational phase. This can cause an anticorrelation of the line equivalent to the width in the blue wing ( v < 0) and that in the red wing ( v > 0) over half of a rotational period, and a positive correlation over the other half. We find a good overall agreement of the line variability behaviour predicted by our model and those from observations. 相似文献
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J. S. Greaves W. S. Holland Marc W. Pound 《Monthly notices of the Royal Astronomical Society》2003,346(2):441-446
The discovery of isolated bodies of planetary mass has challenged the paradigm that planets form only as companions to stars. To determine whether 'isolated planets', brown dwarfs and stars can have a common origin, we have made deep submillimetre observations of part of the ρ Oph B star formation region. Spectroscopy of the 9-Jupiter-mass core Oph B-11 has revealed carbon monoxide line wings such as those of a protostar. Moreover, the estimated mass of outflowing gas lies on the force versus core-mass relation for protostars and protobrown dwarfs. This is evidence for a common process that can form any object between planetary and stellar masses in a molecular cloud. In a submillimetre continuum map, six compact cores in ρ Oph B were found to have masses presently below the deuterium-burning limit, extending the core mass function down to 0.01 M⊙ with the approximate form d N /d M ∝ M −3/2 . If these lowest-mass cores are not transient and can collapse under gravity, then isolated planets should be very common in ρ Oph in the future, as is the case in the Orion star formation region. In fact, the isolated planetary objects that may form from these cores would outnumber the massive planets that have been found as companions to stars. 相似文献
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High-velocity dust collisions: forming planetesimals in a fragmentation cascade with final accretion
In laboratory experiments we determine the mass gain and loss in central collisions between centimetre- to decimetre-size SiO2 dust targets and submillimetre- to centimetre-size SiO2 dust projectiles of varying mass, size, shape and at different collision velocities up to ∼56.5 m s−1 . Dust projectiles much larger than 1 mm lead to a small amount of erosion of the target but decimetre targets do not break up. Collisions produce ejecta, which are smaller than the incoming projectile. Projectiles smaller than 1 mm are accreted by a target even at the highest collision velocities. This implies that net accretion of decimetre and larger bodies is possible. Independent of the original size of a considered projectile, after several collisions, all fragments will be of submillimetre size which might then be (re)accreted in the next collision with a larger body. The experimental data suggest that collisional growth through fragmentation and reaccretion is a viable mechanism to form planetesimals. 相似文献
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We present an edge-on Keplerian disk model to explain the main component of the 12.2 and 6.7 GHz methanol maser emission detected toward NGC7538-IRS1N. The brightness distribution and spectrum of the line of bright masers are successfully modeled with high amplification of background radio continuum emission along velocity coherent paths through a maser disk. The bend seen in the position–velocity diagram is a characteristic signature of differentially rotating disks. For a central mass of 30M, suggested by other observations, our model fixes the masing disk to have inner and outer radii of 270 and 750 AU. 相似文献
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Barbara Lo Faro Pierluigi Monaco Eros Vanzella Fabio Fontanot Laura Silva Stefano Cristiani 《Monthly notices of the Royal Astronomical Society》2009,399(2):827-841
It has recently been shown that galaxy formation models within the Λ cold dark matter cosmology predict that, compared to the observed population, small galaxies (with stellar masses <1011 M⊙ ) form too early, are too passive since z ∼ 3 and host too old stellar populations at z = 0 . We then expect an overproduction of small galaxies at z ≳ 4 that should be visible as an excess of faint Lyman-break galaxies. To check whether this excess is present, we use the morgana galaxy formation model and grasil spectrophotometric + radiative transfer code to generate mock catalogues of deep fields observed with Hubble Space Telescope Advanced Camera for Surveys. We add observational noise and the effect of Lyman α emission, and perform colour–colour selections to identify Lyman-break galaxies. The resulting mock candidates have plausible properties that closely resemble those of observed galaxies. We are able to reproduce the evolution of the bright tail of the luminosity function of Lyman-break galaxies (with a possible underestimate of the number of the brightest i -dropouts), but uncertainties and degeneracies in dust absorption parameters do not allow to give strong constraints to the model. Besides, our model shows a clear excess with respect to observations of faint Lyman-break galaxies, especially of z 850 ∼ 27 V -dropouts at z ∼ 5 . We quantify the properties of these 'excess' galaxies and discuss the implications: these galaxies are hosted in dark matter haloes with circular velocities in excess of 100 km s−1 , and their suppression may require a deep rethinking of stellar feedback processes taking place in galaxy formation. 相似文献
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D. Lynden-Bell 《Monthly notices of the Royal Astronomical Society》2002,334(4):787-796
A strong emission line at 2.8935 μm discovered by Rubin et al. in an ISO SWS02 spectrum of the Orion Nebula is identified as the multiplet of O i . Line formation is due to de-excitation cascades following UV-pumping of high 3 So and 3 Do terms and occurs in the O i zone immediately behind the hydrogen ionization front. This cascade mechanism also accounts for permitted O i triplet lines in the optical spectrum of the Nebula, as shown by Grandi. An escape probability treatment of the O i cascades accounts for the strength of the λ 2.89-μm line and suggests interesting diagnostic possibilities for the optical lines. 相似文献
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H. Stiele H. Lesch F. Heitsch 《Monthly notices of the Royal Astronomical Society》2006,372(2):862-868
We revisit the problem of clump formation due to thermal instabilities in a weakly ionized plasma with the help of a linear perturbation analysis, as discussed by Nejad-Asghar & Ghanbari. In the absence of a magnetic field and ambipolar diffusion the characteristic equation reduces to the thermal instability described by Field. We derive the critical wavelengths, which separate the spatial ranges of stability and instability. Contrary to the original analysis of Nejad-Asghar & Ghanbari, perturbations with a wavelength larger than the critical wavelength destabilize the cloud. Moreover, the instability regime of isentropic perturbations is drastically reduced. Isobaric modes with real values of the critical wavelength appear only if the density dependence of the cooling rate is more pronounced than the temperature dependence. Isentropic modes arise only if the power of the density in the cooling rate is smaller than 1/2, which is not fulfilled for CO cooling. We find that ambipolar diffusion is not a dominating heating process in molecular gas. 相似文献
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S. C. O. Glover P. W. J. L. Brand 《Monthly notices of the Royal Astronomical Society》2001,321(3):385-397
The first star formation in the Universe is expected to take place within small protogalaxies, in which the gas is cooled by molecular hydrogen. However, if massive stars form within these protogalaxies, they may suppress further star formation by photodissociating the H2 . We examine the importance of this effect by estimating the time-scale on which significant H2 is destroyed. We show that photodissociation is significant in the least massive protogalaxies, but becomes less so as the protogalactic mass increases. We also examine the effects of photodissociation on dense clumps of gas within the protogalaxy. We find that while collapse will be inhibited in low-density clumps, denser ones may survive to form stars. 相似文献
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E. Podlewska E. Szuszkiewicz 《Monthly notices of the Royal Astronomical Society》2008,386(3):1347-1354
In this paper we investigate the evolution of a pair of interacting planets – a Jupiter-mass planet and a Super-Earth with a mass of 5.5 M ⊕ – orbiting a Solar-type star and embedded in a gaseous protoplanetary disc. We focus on the effects of type I and II orbital migrations, caused by the planet–disc interaction, leading to the capture of the Super-Earth in first-order mean-motion resonances by the Jupiter. The stability of the resulting resonant system in which the Super-Earth is on the internal orbit relative to the Jupiter is studied numerically by means of full 2D hydrodynamical simulations. Our main aim is to determine the Super-Earth behaviour in the presence of the gas giant in the system. It is found that the Jupiter captures the Super-Earth into the interior 3:2 or 4:3 mean-motion resonance, and that the stability of such configurations depends on the initial positions of the planets and on the evolution of the eccentricity. If the initial separation of the orbits of the planets is larger than or close to that required for the exact resonance, the final outcome is the migration of the pair of planets at a rate similar to that of the gas giant, at least for the time of our simulations. Otherwise, we observe a scattering of the Super-Earth from the disc. The evolution of planets immersed in a gaseous disc is compared with their behaviour in the case of the classical three-body problem when the disc is absent. 相似文献
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K. A. Pounds A. R. King K. L. Page P. T. O''Brien 《Monthly notices of the Royal Astronomical Society》2003,346(4):1025-1030
Following the discovery of X-ray absorption in a high-velocity outflow from the bright quasar PG 1211 + 143 we have searched for similar features in XMM–Newton archival data of a second (high accretion rate) quasar PG 0844+349. Evidence is found for several faint absorption lines in both the EPIC and RGS spectra, whose most likely identification with resonance transitions in H-like Fe, S and Ne implies an origin in highly ionized matter with an outflow velocity of order ∼0.2c. The line equivalent widths require a line-of-sight column density of N H ∼ 4 × 1023 cm−2 , at an ionization parameter of log ξ∼ 3.7 . Assuming a radial outflow being driven by radiation pressure from the inner accretion disc, as suggested previously for PG 1211 + 143, the flow in PG 0844+349 is also likely to be optically thick, in this case within ∼25 Schwarzschild radii. Our analysis suggests that a high-velocity, highly ionized outflow is likely to be a significant component in the mass and energy budgets of active galactic nuclei accreting at or above the Eddington rate. 相似文献