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排序方式: 共有106条查询结果,搜索用时 15 毫秒
31.
Active galactic nuclei can produce extremely powerful jets. While tightly collimated, the scale of these jets and the stellar density at galactic centres implies that there will be many jet/star interactions, which can mass load the jet through stellar winds. Previous work employed modest wind mass outflow rates, but this does not apply when mass loading is provided by a small number of high mass-loss stars. We construct a framework for jet mass loading by stellar winds for a broader spectrum of wind mass-loss rates than has previously been considered. Given the observed stellar mass distributions in galactic centres, we find that even highly efficient (0.1 Eddington luminosity) jets from supermassive black holes of masses M BH ≲ 104 M⊙ are rapidly mass loaded and quenched by stellar winds. For 104 M⊙ < M BH < 108 M⊙ , the quenching length of highly efficient jets is independent of the jet's mechanical luminosity. Stellar wind mass loading is unable to quench efficient jets from more massive engines, but can account for the observed truncation of the inefficient M87 jet, and implies a baryon-dominated composition on scales ≳2 kpc therein even if the jet is initially pair plasma dominated. 相似文献
32.
Ilian T. Iliev Benedetta Ciardi Marcelo A. Alvarez Antonella Maselli rea Ferrara Nickolay Y. Gnedin Garrelt Mellema Taishi Nakamoto Michael L. Norman Alexei O. Razoumov Erik-Jan Rijkhorst Jelle Ritzerveld Paul R. Shapiro Hajime Susa Masayuki Umemura Daniel J. Whalen 《Monthly notices of the Royal Astronomical Society》2006,371(3):1057-1086
Radiative transfer (RT) simulations are now at the forefront of numerical astrophysics. They are becoming crucial for an increasing number of astrophysical and cosmological problems; at the same time their computational cost has come within reach of currently available computational power. Further progress is retarded by the considerable number of different algorithms (including various flavours of ray tracing and moment schemes) developed, which makes the selection of the most suitable technique for a given problem a non-trivial task. Assessing the validity ranges, accuracy and performances of these schemes is the main aim of this paper, for which we have compared 11 independent RT codes on five test problems: (0) basic physics; (1) isothermal H ii region expansion; (2) H ii region expansion with evolving temperature; (3) I-front trapping and shadowing by a dense clump and (4) multiple sources in a cosmological density field. The outputs of these tests have been compared and differences analysed. The agreement between the various codes is satisfactory although not perfect. The main source of discrepancy appears to reside in the multifrequency treatment approach, resulting in different thicknesses of the ionized-neutral transition regions and the temperature structure. The present results and tests represent the most complete benchmark available for the development of new codes and improvement of existing ones. To further this aim all test inputs and outputs are made publicly available in digital form. 相似文献
33.
As a result of feedback from massive stars, via their intense winds and/or supernova explosions, massive star‐forming regions are entirely filled with hot, X‐ray emitting plasmas, which escape into the ambient ISM. As shown recently by Townsley et al. for several “extreme” cases (Carina, M17, NGC 3576, NGC 3603, 30 Dor), by way of large Chandra ACIS mosaics, extra, non‐thermal emission lines are present on top of the standard lines emitted by hot plasmas. Some of them are very close to lines characteristic of charge‐exchange reactions between the hot plasma and the cold surrounding material, suggesting that this mechanism operates on large spatial scales (several 10 pc) in star‐forming regions in general. The connection with starburst galaxies is briefly mentioned, and it is pointed out that supernovae interacting with molecular clouds may also provide a good environment to look for charge exchange processes (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
34.
35.
A. Gil de Paz J. Zamorano J. Gallego 《Monthly notices of the Royal Astronomical Society》1999,306(4):975-987
We have studied the velocity field of the blue compact dwarf galaxy Mrk 86 (NGC 2537) using data provided by 14 long-slit optical spectra obtained in 10 different orientations and positions. This kinematical information is complemented with narrow-band ([O iii ]5007 Å and H α ) and broad-band ( B , V , Gunn r and K ) imaging. The analysis of the galaxy global velocity field suggests that the ionized gas could be distributed in a rotating inclined disc, with projected central angular velocity of Ω=34 km s−1 kpc−1 . The comparison between the stellar, H i and modelled dark matter density profile indicates that the total mass within its optical radius is dominated by the stellar component. Peculiarities observed in its velocity field can be explained by irregularities in the ionized gas distribution or local motions induced by star formation.
Kinematical evidences for two expanding bubbles, Mrk 86–B and Mrk 86–C, are given. They show expanding velocities of 34 and 17 km s−1 , H α luminosities of 3×1038 and 1.7×1039 erg s−1 , and physical radii of 374 and 120 pc, respectively. The change in the [S ii ]/H α , [N ii ]/H α , [O ii ]/[O iii ] and [O iii ]/H β line ratios with the distance to the bubble precursor suggests a diminution in the ionization parameter and, in the case of Mrk 86–B, an enhancement of the shock-excited gas emission. The optical–near-infrared colours of the bubble precursors are characteristic of low‐metallicity star‐forming regions (∼0.2 Z⊙ ) with burst strengths of about 1 per cent in mass. 相似文献
Kinematical evidences for two expanding bubbles, Mrk 86–B and Mrk 86–C, are given. They show expanding velocities of 34 and 17 km s
36.
37.
The gravitational instability of expanding shells triggering the formation of clouds and stars is analyzed. Disks with different
scale-heights, ambient and shell velocity dispersions, mid-plane densities, rotation rates and shear rates are explored with
three dimensional numerical simulations in the thin shell approximation. Three conditions for the shell collapse are specified:
the first is that it happens before a significant blow-out, the second requires that the shell collapses before it is distorted
by Coriolis forces and shear, and the third requires that the internal pressure in the accumulated gas is small and the fragmentation
is achieved within the expansion time. The gas-rich and slowly rotating galaxies are the best sites of the triggered star
formation, concluding that its importance has been much larger at the times of galaxy formation compared to the present epoch.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
38.
39.
We study the self-similar magnetohydrodynamics (MHD) of a quasi-spherical expanding void (viz. cavity or bubble) surrounding the centre of a self-gravitating gas sphere with a general polytropic equation of state. We show various analytic asymptotic solutions near the void boundary in different parameter regimes and obtain the corresponding void solutions by extensive numerical explorations. We find novel void solutions of zero density on the void boundary. These new void solutions exist only in a general polytropic gas and feature shell-type density profiles. These void solutions, if not encountering the magnetosonic critical curve (MCC), generally approach the asymptotic expansion solution far from the central void with a velocity proportional to radial distance. We identify and examine free-expansion solutions, Einstein–de Sitter expansion solutions, and thermal-expansion solutions in three different parameter regimes. Under certain conditions, void solutions may cross the MCC either smoothly or by MHD shocks, and then merge into asymptotic solutions with finite velocity and density far from the centre. Our general polytropic MHD void solutions provide physical insight for void evolution, and may have astrophysical applications such as massive star collapses and explosions, shell-type supernova remnants and hot bubbles in the interstellar and intergalactic media, and planetary nebulae. 相似文献