排序方式: 共有17条查询结果,搜索用时 31 毫秒
11.
A. Frank 《Astrophysics and Space Science》2007,307(1-3):35-39
The expected lifetimes for molecular clouds has become a topic of considerable debate as numerical simulations have shown
that MHD turbulence, the nominal means of support for clouds against self-gravity, will decay on short timescales. Thus it
appears that either molecular clouds are transient features or they are resupplied with turbulent energy through some means.
Jets and molecular outflows are recognized as a ubiquitous phenomena associated with star formation. Stars however form not
isolation but in clusters of different density and composion. The ubiquity and high density of outflows from young stars in
clusters make them an intriguing candidate for the source of turbulence energy in molecular clouds. In this contribution we
present new studies, both observational and theoretical, which address the issue of jet/outflow interactions and their abilityto
drive turbulent flows in molecular clouds. Our studies focus on scales associated with young star forming clusters. In particular
we first show that direct collisions between active outflows are not effective at stirring the ambient medium. We then show
that fossil cavities from “extinct” outflows may provide the missing link in terms of transferring momentum and energy to
the cloud. 相似文献
12.
13.
Surface meteorological observations, associated with gust fronts produced by thunderstorm outflows over Tehran, an area surrounded by mountains, have been analyzed. Distinctive features are sudden drop in air temperature, up to 10℃, sharp increase in wind speed, up to 30 m s-1, with wind shift, to northwesterly, ressure jump, up to 4 hPa, humidity increase, up to 40%, and rain after some 20 min. Gust fronts which often occur in spring time, have a typical thickness of about 1.5 km and produce vertical wind shear of the order of 10-2s-1. Although these features seem to be common for most of the events, their intensities differ from one event to another, indicating that the gust fronts may occur in different sizes and shapes. Apart from a dominant effect on the formation of the original thunderstorms, topography appears to break up the frontal structure of the gust fronts. The internal Rossby radius of deformation for these flows is small enough (~ 100 km) for rotational effects to be minor.A laboratory model of the gust front (gravity current) also shows that it initially has a distinctive head with a turbulent wake, and can be broken up by topography. It is shown that when the environment is stratified, turbulence due to lobes and clefts instabilities near the nose of the current is suppressed. When the ground is rough, these instabilities are highly amplified and the internal Froude number of the flow is reduced. The bottom slope in the presence of rough topography leads to the break up of the current head and produces a broad and highly non-uniform head, recognized in the density signals. 相似文献
14.
S. Cabrit 《Astrophysics and Space Science》1995,233(1-2):81-96
In this contribution, I present a broad historical review of the various hydrodynamical models that have been considered for explaining molecular outflows, and of their merits and failures when compared with observations. Wind-driven bubbles, viscous jet mixing-layers, and jet bowshocks, are discussed in turn. Most general properties of outflows can be understood in terms of a simple bowshock model. However, the detailed structure of outflows is more complex and not yet fully understood, given the presence of time variability in the jet velocity and/or direction. Finally, I discuss constraints on wind properties (momentum, mass-loss rate, radius) that can be derived from molecular outflows driven by jet bowshocks. 相似文献
15.
E. Y. Vilkoviskij R. V. E. Lovelace L. A. Pavlova M. M. Romanova S. N. Yefimov 《Astrophysics and Space Science》2006,306(3):129-137
We discuss a self-consistent unified model of the matter outflows from AGNs based on a theoretical approach and involving
data on AGN evolution and structure. The model includes a unified geometry, two-phase gas dynamics, radiation transfer, and
absorption spectrum calculations in the UV and X-ray bands. We briefly discuss several questions about the mass sources of
the flows, the covering factors, and the stability of the narrow absorption details. 相似文献
16.
Markus Böttcher 《Astrophysics and Space Science》2007,307(1-3):69-75
Some recent developments in the study of relativistic jets in active galactic nuclei and microquasars are reviewed. While
it has been well established for some time that extragalactic jets found in radio galaxies, quasars, and BL Lac objects are
the site of ultrarelativistic particle acceleration, the recent identification of the Galactic jet source and microquasar
LS~5039 as a source of very-high-energy gamma-ray emission has underlined the striking similarity between the two types of
astrophysical jet sources. In this paper, I will present an overview of the dominant radiation and particle acceleration processes
and observational tests to distinguish between such processes. The wide-ranging analogies between Galactic and extragalactic
jets, but also their distinct differences, in particular those caused by the presence of the companion star in Galactic microquasar
systems, will be exposed. 相似文献
17.
High velocity jets from young stars interact with the surrounding molecular environment and molecular outflows quite possibly are the result. This interaction can take place through the formation of a turbulent mixing layer. Models have been constructed (following Cant/'o and Raga) of a plane mixing layer in the boundary between a high velocity, atomic wind (i.e., the stellar jet) and a stationary, molecular environment, computed considering a detailed chemical network.The chemical composition of the mixing layer initially corresponds to the direct mixture of the (atomic) jet and (molecular) environmental material. However, we find that the mixing layer is hot (with temperatures exceeding 104 K), and the surprising only partial dissociation of H2 means that a number of molecules are either created or survive in the high velocity gas. This contrasts with the slower, cooler flows that have tended to be termed a molecular outflow.The emission from such atomic jet/molecular environment mixing layers is dominated by emission in the rotational and vibrational lines of H2. As a result of the high temperatures and velocities (ranging from zero to the jet velocity) of these mixing layers, the predicted H2 emission line spectrum has interesting characteristics. 相似文献