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1.
Stars form within molecular clouds but our understanding of this fundamental process remains hampered by the complexity of the physics that drives their evolution. We review our observational and theoretical knowledge of molecular clouds trying to confront the two approaches wherever possible. After a broad presentation of the cold interstellar medium and molecular clouds, we emphasize the dynamical processes with special focus to turbulence and its impact on cloud evolution. We then review our knowledge of the velocity, density and magnetic fields. We end by openings towards new chemistry models and the links between molecular cloud structure and star-formation rates. 相似文献
2.
In order to investigate the differences between the molecular clouds which are associated with the massive star forming regions and those which are not, we have performed the single-dish simultaneous observations of 12CO J=2-1 and J=3-2 lines toward a sample of 59 Spitzer Extended Green Objects (EGOs) as the massive star forming regions in the northern sky. Combining our results with the data of the 12CO J=1-0 observations toward the same sample EGOs in the literature, we have made the statistical comparisons on the intensities and linewidths of multiple 12CO lines between the molecular clouds associated with EGOs (EGO molecular clouds, in brief) and other non-EGO molecular clouds. On this basis, we have discussed the effects of the gas temperature, density, and velocity field distributions on the statistical characteristics of the two kinds of molecular clouds. It is found that both the EGO molecular clouds and non-EGO molecular clouds have similar mass ranges, hence we conclude that for the formation of massive stars, the key-important factor is probably not the total mass of a giant molecular cloud (GMC), but the volume filling factor of the molecular clumps in the GMC (or the compression extent of the molecular gas in the cloud). 相似文献
3.
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
4.
J.E. Pringle Ronald J. Allen S.H. Lubow 《Monthly notices of the Royal Astronomical Society》2001,327(2):663-668
In a recent paper, Elmegreen has made a cogent case, from an observational point of view, that the lifetimes of molecular clouds are comparable to their dynamical time-scales. If so, this has important implications for the mechanisms by which molecular clouds form. In particular, we consider the hypothesis that molecular clouds may form not by in situ cooling of atomic gas, but rather by the agglomeration of the dense phase of the interstellar medium, much, if not most, of which is already in molecular form. 相似文献
5.
Javier Ballesteros-Paredes 《Astrophysics and Space Science》2004,289(3-4):243-254
Molecular clouds (MC) are the densest and coldestcomponent of the interstellar gas, and the sites of starformation. They are also turbulent and fractaland theirmasses and sizes span several orders of magnitude. It is also generally believed that they are close to Virial equilibrium (VE).Since this statement has beenquestioned by a number of authors, with important implicationson molecular clouds’ lifetimes, we will review this subjectwithin the context of a turbulent ISM. In this framework, there issignificant numerical evidence that MCs are not in VE, that there is a strong exchange of mass, momentum and energy between clouds and their surrounding medium, andthat it is difficult (if not impossible) to form quasistatic coresinside MCs, suggesting that they must be transient, short-livedphenomena. Thus, their formation and disruption must be primarily dynamical, and probably not due tojust a single mechanism, but rather to the combination of severalprocesses. This picture seems consistent withrecent estimates of ages of stars in the solar neighborhood. 相似文献
6.
Shocks propagating in the interstellar medium (ISM) play an important role in the life of molecular clouds. Through a theoretical study of interaction between clouds and shocks we can understand, for example, the density distribution of observed molecular clouds and the first steps of star formation. The only way to study of interaction in detail is via a numerical hydrodynamical simulation. In this paper we present the first results of a hydrocode which is able to follow the processes after the collision between the cloud and shock front.Our main theoretical result is that the chemical processes (e.g. H2 dissociation) can affect the dynamical processes significantly. Global parameters of the cloud are calculated for the comparision of the simulation and the observations. 相似文献
7.
Daniele Galli 《Astrophysics and Space Science》2005,295(1-2):43-51
We review the basic theoretical elements leading to our current understanding of the role of magnetic fields in the process of star formation. In particular, we concentrate on: (i) the relevance of the mass-to-flux ratio for the stability of molecular clouds; (ii) the consequences of magnetic flux leakage for the evolution of cloud cores; (iii) the phase of anisotropic dynamical collapse following the formation of strongly peaked density distributions; (iv) the mechanism of magnetic braking as a possible solution to the angular momentum problem in star formation. 相似文献
8.
Two models of molecular cloud in disk galaxies are proposed to investigate the formation of giant molecular clouds (GMCs)
under the gravitational instability and random collision using PP(Particle–Particle) simulation. Having analysed simulation
outputs of the two models and comparing them with observation, we are able to draw some general conclusions, the most significant
ones of which are: 1) Similar to results obtained previously, the gravitational instability can make small clouds form large
clouds faster than random collision. 2) The differential rotation in gravitational instability model plays a positive role
in agglomeration of molecular clouds.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
9.
The distribution of proper motions of stars in the Pup-CMa association is presented. The stars' velocities are approximately parallel to each other, which indicates that the stars are close together in space. The mutual distribution of stars and molecular clouds in the association is interpreted as proof that the stars emerged from a single gigantic primordial molecular cloud (or several large clouds), destroyed by radiation and/or stellar wind coming from those stars. It is assumed that part of that cloud is being dissipated, while part is being broken into several small clouds, which we are observing at present. 相似文献
10.
Sandra R. Oliveira Oswaldo D. Miranda & José C. N. de Araujo Reuven Opher 《Monthly notices of the Royal Astronomical Society》1998,301(1):115-130
In this article we extend the study performed in our previous article of the collapse of primordial objects. We here analyse the behaviour of the physical parameters for clouds ranging from 107 to 1015 M⊙ . We study the dynamical evolution of these clouds in two ways: as purely baryonic clouds and as clouds with non-baryonic dark matter included. We start the calculations at the beginning of the recombination era, following the evolution of the structure until the collapse (which we defined as the time when the density contrast of the baryonic matter is greater than 104 ). We analyse the behaviour of several physical parameters of the clouds (e.g. the density contrast and the velocities of the baryonic matter and the dark matter) as a function of time and radial position in the cloud. In this study all physical processes that are relevant to the dynamical evolution of the primordial clouds, such as for example photon drag (due to the cosmic background radiation) and hydrogen molecular production, besides the expansion of the Universe, are included in the calculations. In particular we find that the clouds with dark matter collapse at higher redshift when we compare the results with the purely baryonic models. As a general result we find that the distribution of the non-baryonic dark matter is more concentrated than the baryonic one. It is important to stress that we do not take into account the putative virialization of the non-baryonic dark matter; we just follow the time and spatial evolution of the cloud, solving its hydrodynamical equations. We also studied the role of cooling–heating processes in the purely baryonic clouds. 相似文献
11.
Supernova remnants accelerate particles up to energies of at least 100 TeV as established by observations in very-high-energy γ-ray astronomy. Molecular clouds in their vicinity provide an increased amount of target material for proton-proton interaction and subsequent neutral pion decay into γ-rays of accelerated hadrons escaping the remnant. Therefore, these molecular clouds are potential γ-ray sources. The γ-ray emission from these clouds provides a unique environment to derive information on the propagation of very-high-energy particles through the interstellar medium as well as on the acceleration of hadrons in supernova remnants. Current Imaging Atmospheric Cherenkov Telescope systems are suitable to explore a large parameter space of the propagation properties depending on the age of the supernova remnant and the distance between the remnant and the nearby molecular cloud.In this paper we present our strategy and results of a systematic search for γ-ray emitting molecular clouds near supernova remnants which are potentially detectable with current experiments in the TeV energy range and explore the prospects of future experiments. 相似文献
12.
S. Viti 《Astrophysics and Space Science》2003,285(3-4):791-800
In this article, some aspects of the clumpy nature of molecular clouds are reviewed. In particular the observational evidence
for small-scale structures both in low and high mass star forming regions will be discussed. I will review some examples of
`clumpiness' such as: i) the molecular clumps ahead of HH objects and how the study of the physical and chemical nature of
these clumps is important for the understanding of the clumpiness of the Interstellar Medium; and ii)hot cores and their use
as a tool to study the early phases of massive star formation.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
13.
J. Bland-Hawthorn S.L. Lumsden G.M. Voit G.N. Cecil J.C. Weisheit 《Astrophysics and Space Science》1997,248(1-2):177-190
The radio jet axis of NGC 1068 is characterised by energetic activity from x-ray to radio wavelengths. Detailed kinematic
and polarization studies have shown that this activity is confined to bipolar cones centered on the AGN which intersect the
plane of the disk. Thus, molecular clouds at 1 kpc distance along this axis are an important probe of the nuclear ionizing
luminosity and spectrum. Extended 10.8μm emission coincident with the clouds is reasonably understood by dust heated to high
temperatures by the nuclear radiation field. This model predicts that the nuclear spectrum is quasar-like (power law + blue
excess) with a luminosity 2-5 times higher than inferred by Pier et al. (1994). Consequently, there is little or no polyaromatic
hydrocarbon (PAH) emission associated with the radio-axis molecular clouds. We review this model in the light of new observations.
A multi-waveband collage is included to illustrate the possible orientations of the double cones to our line of sight and
the galaxian plane.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
14.
Summary Cosmic dust grains play an important role for the thermal, dynamical, and chemical structure of the interstellar medium. This is especially true for the star formation process and the late stages of stellar evolution. Dust grains determine the spectral appearance of protostars, very young stellar objects with disk-like structures as well as of evolved stars with circumstellar envelopes.In this review, we will demonstrate that solid particles in interstellar space are both agent and subject of galactic evolution. We will especially discuss the different dust populations in circumstellar envelopes, the diffuse interstellar medium, and the molecular clouds with strong emphasis on the evolutionary aspects and the metamorphosis of these populations. 相似文献
15.
We describe the results of a study of the mean lifetime of molecular clouds based on actual observations. Using the model of growth from cloud-cloud collision and observations of 13CO along the galactic equator between longitudes 27°.85 and 40°, we derive a lower limit of 1 × 109 yr for the lifetime of the interstellar molecular clouds 相似文献
16.
We develop a three-dimensional numerical model for an ensemble of molecular clouds moving in the fixed gravitational potential of a galaxy. This scheme is a modification of the widely known model of Oort and includes different processes of coagulation and fragmentation of clouds under pairwise collisions, interaction of clouds with the diffuse interstellar medium, and also feedback: the breaking up of clouds into small fragments under the action of stars arising in them. This model makes it possible to study the influence of various parameters of both the galaxy itself and the ensemble of molecular clouds on the process of large-scale star formation connected with giant molecular clouds and on the temporal changes of the global structure of the interstellar medium. We give as an example a computation of the evolution of the energy characteristics of an ensemble of molecular clouds in a spiral galaxy.Translated fromAstrofizika, Vol. 37, No. 4, 1994. 相似文献
17.
18.
V. A. Dogiel A. V. Gurevich Ya. N. Istomin K. P. Zybin 《Astrophysics and Space Science》2005,297(1-4):201-211
Acceleration of charged particles by neutral gas turbulence in giant molecular clouds is considered. The gamma-ray emission
from these clouds is estimated. It is shown that molecular clouds can be the counterparts of some of unidentified sources. 相似文献
19.
Akiko Kawamura 《Astrophysics and Space Science》2008,313(1-3):145-151
Star formation is a fundamental process that dominates the life-cycle of various matters in galaxies: Stars are formed in
molecular clouds, and the formed stars often affect the surrounding materials strongly via their UV photons, stellar winds,
and supernova explosions. It is therefore revealing the distribution and properties of molecular gas in a galaxy is crucial
to investigate the star formation history and galaxy evolution. Recent progress in developing millimeter and sub-millimeter
wave receiver systems has enabled us to rapidly increase our knowledge on molecular clouds. In this proceedings, the recent
results from the surveys of the molecular clouds in the Milky Way and the Magellanic Clouds as well as the Galactic center
as the most active regions in the Milky Way are presented. The high sensitivity with unrivaled high resolution of ALMA will
play a key role in detecting denser gas that is tightly connected to star formation. 相似文献
20.
M. Gritschneder T. Naab A. Burkert S. Walch F. Heitsch M. Wetzstein 《Monthly notices of the Royal Astronomical Society》2009,393(1):21-31
We present a three-dimensional, fully parallelized, efficient implementation of ionizing ultraviolet (UV) radiation for smoothed particle hydrodynamics ( sph ) including self-gravity. Our method is based on the sph / tree code vine . We therefore call it iVINE (for Ionization + VINE). This approach allows detailed high-resolution studies of the effects of ionizing radiation from, for example, young massive stars on their turbulent parental molecular clouds. In this paper, we describe the concept and the numerical implementation of the radiative transfer for a plane-parallel geometry and we discuss several test cases demonstrating the efficiency and accuracy of the new method. As a first application, we study the radiatively driven implosion of marginally stable molecular clouds at various distances of a strong UV source and show that they are driven into gravitational collapse. The resulting cores are very compact and dense exactly as it is observed in clustered environments. Our simulations indicate that the time of triggered collapse depends on the distance of the core from the UV source. Clouds closer to the source collapse several 105 yr earlier than more distant clouds. This effect can explain the observed age spread in OB associations where stars closer to the source are found to be younger. We discuss possible uncertainties in the observational derivation of shock front velocities due to early stripping of protostellar envelopes by ionizing radiation. 相似文献