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1.
Frictional heating by the ion-neutral drift is calculated and its effect on the isobaric thermal instability is studied. Ambipolar drift heating of a one-dimensional self-gravitating magnetized molecular slab is used under the assumptions of quasi-magnetohydrostatic and local ionization equilibrium. We see that ambipolar drift heating is inversely proportional to density and its value in some regions of the slab can be significantly larger than the average heating rates of cosmic rays and turbulent motions. The results show that isobaric thermal instability can occur in some regions of the slab, and thus it may produce slab fragmentation and formation of astronomical unit scale condensations.  相似文献   

2.
A systematic study of the linear thermal instability of a self-gravitating magnetic molecular cloud is carried out for the case when the unperturbed background is subject to local expansion or contraction. We consider the ambipolar diffusion, or ion-neutral friction on the perturbed states. In this way, we obtain a non-dimensional characteristic equation that reduces to the prior characteristic equation in the non-gravitating stationary background. By parametric manipulation of this characteristic equation, we conclude that there are, not only oblate condensation forming solutions, but also prolate solutions according to local expansion or contraction of the background. We obtain the conditions for existence of the Field lengths that thermal instability in the molecular clouds can occur. If these conditions establish, small-scale condensations in the form of spherical, oblate, or prolate shape may be produced via thermal instability.  相似文献   

3.
Using the results of optical and molecular line observations of the dark cloud L134, some basic cloud parameters are obtained and the stability and energy of the cloud are discussed.It is found that thermal pressure and rotation are unimportant, while internal magnetic field may be effective for supporting the cloud against gravitational collapse. And the cloud could not collapse on the free-fall time scale but on the longer time scale of ambipolar diffusion.The cooling and heating rates in L134 are also calculated. The results show that the work done by gravitation against thermal pressure is not an effective heating source; cosmic rays, however, may provide as much as 20% heating energy required. Calculation shows that internal magnetic energy released through the processes of ambipolar diffusion can supply the most part of the energy required, therefore, it may be the most important source.  相似文献   

4.
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   

5.
《New Astronomy》2003,8(4):295-311
Observations of molecular clouds point to the existence of supersonic, turbulent flows. Therefore, any theory which attempts to describe molecular cloud evolution and star formation must include a consideration of the dynamics of colliding flows. Previous studies have considered the collision of supersonic streams or clouds. The resultant instabilities provide a mechanism which may give rise to observable cloud morphologies and enhance the star formation rate. One such instability is the nonlinear thin shell instability (NTSI) of a shock-bounded slab. This process is driven by ram pressure and efficient cooling. In this study, I use numerical simulations to examine the head-on collision of supersonic gas streams in a cold, molecular gas. A dense slab forms in the collision midplane and is prone to a number of instabilities, including the NTSI. The thermodynamic processes involved are found to have a controlling influence upon the instability and fragmentation of the slab. Although some minimal amount of cooling is needed to drive the instability, too rapid a cooling rate gives rise to smaller wavelength instabilities which wipe out the NTSI. The growth rate of the NTSI in a gas undergoing molecular cooling corresponds to a timescale of order 1012 s, in general agreement with the theoretical value for an isothermal gas. The NTSI may provide a viable mechanism for the instigation of rapid star formation.  相似文献   

6.
The breakdown of flux-freezing in molecular clouds and protostellar discs is usually approximated by ambipolar diffusion at low densities or by resistive diffusion at high densities. Here an intermediate regime is discussed in which the Hall term in the conductivity tensor is significant, and the vector evolution of the magnetic field, and therefore the evolution of the system under consideration is dramatically altered. Calculations of charged particle abundances in dense gas in molecular clouds and protostellar discs demonstrate that Hall diffusion is important over a surprisingly broad range of conditions.  相似文献   

7.
利用暗星云L134的光学观测及分子谱线观测的结果,估算了它的某些基本物理参数,讨论了该星云的稳定性及能量问题。 发现在L134的演化过程中,热压力及转动在制约其自引力坍缩中起一定作用,但不是重要因素,而磁场的支撑作用可能是比较重要的,L134不是在自由下落时间尺度t_(ff)上引力坍缩,而可能是在一个比较长的双极扩散时间尺度t_D上收缩。 本文还计算了L134的冷却率及加热率,表明引力克服热压力作功不是暗星云L134的有效加热源;宇宙线是L134的一个加热源,它能提供所需能量的～20％;磁场通过双极扩散释放能量可能为L134提供了一个重要的加热源。  相似文献   

8.
Astrophysical fluids under the influence of magnetic fields are often subjected to single- or two-fluid approximations. In the case of weakly ionized plasmas, however, this can be inappropriate due to distinct responses from the multiple constituent species to both collisional and non-collisional forces. As a result, in dense molecular clouds and protostellar accretion discs, for instance, the conductivity of the plasma may be highly anisotropic leading to phenomena such as Hall and ambipolar diffusion strongly influencing the dynamics.
Diffusive processes are known to restrict the stability of conventional numerical schemes which are not implicit in nature. Furthermore, recent work establishes that a large Hall term can impose an additional severe stability limit on standard explicit schemes. Following a previous paper, which presented the one-dimensional case, we describe a fully three-dimensional method which relaxes the normal restrictions on explicit schemes for multifluid processes. This is achieved by applying the little-known Super TimeStepping technique to the symmetric (ambipolar) component of the evolution operator for the magnetic field in the local plasma rest frame, and the new Hall Diffusion Scheme to the skew-symmetric (Hall) component.  相似文献   

9.
Current theoretical models for what drives star formation (especially low-mass star formation) are: (1) magnetic support of self-gravitating clouds with ambipolar diffusion removing support in cores and triggering collapse and (2) compressible turbulence forming self-gravitating clumps that collapse as soon as the turbulent cascade produces insufficient turbulent support. Observations of magnetic fields can distinguish between these two models because of different predictions in three areas: (1) magnetic field morphology, (2) the scaling of field strength with density and non-thermal velocities, and (3) the mass to magnetic flux ratio, M/Φ. We first discuss the techniques and limitations of methods for observing magnetic fields in star formation regions, then describe results for the L1544 prestellar core as an exemplar of the observational results. Application of the three tests leads to the following conclusions. The observational data show that both magnetic fields and turbulence are important in molecular cloud physics. Field lines are generally regular rather than chaotic, implying strong field strengths. But fields are not aligned with the minor axes of oblate spheroidal clouds, suggesting that turbulence is important. Field strengths appear to scale with non-thermal velocity widths, suggesting a significant turbulent support of clouds. Giant Molecular Clouds (GMCs) require mass accumulation over sufficiently large volumes that they would likely have an approximately critical M/Φ. Yet H I clouds are observed to be highly subcritical. If self-gravitating (molecular) clouds form with the subcritical M/Φ of H I clouds, the molecular clouds will be subcritical. However, the observations of molecular cloud cores suggest that they are approximately critical, with no direct evidence for subcritical molecular clouds or cloud envelopes. Hence, the observations remain inconclusive in deciding between the two extreme-case models of what drives star formation. What is needed to further advance our understanding of the role of magnetic fields in the star formation process are additional high sensitivity surveys of magnetic field strengths and other cloud properties in order to further refine the assessment of the importance of magnetic fields in molecular cores and envelopes.  相似文献   

10.
We consider the thermal properties of cold, dense clouds of molecular hydrogen and atomic helium. For cloud masses below 10-1.7 M middle dot in circle, the internal pressure is sufficient to permit the existence of particles of solid or liquid hydrogen at temperatures above the cosmic microwave background temperature. Optically thin thermal continuum emission by these particles can balance cosmic-ray heating of the cloud, leading to equilibria that are thermally stable even though the heating rate is independent of cloud temperature. For the Galaxy, the known heating rate in the disk sets a minimum mass of order 10-6 M middle dot in circle necessary for survival. Clouds of this type may in principle comprise most of the dark matter in the Galactic halo. However, we caution that the equilibria do not exist at redshifts z greater, similar1 when the temperature of the microwave background was substantially larger than its current value; therefore, the formation and the survival of such clouds to the present epoch remain open questions.  相似文献   

11.
The formation of quasar broad-line region (BLR) clouds via thermal instability in the presence of Alfvén heating has been discussed by Gonçalves, Jatenco-Pereira & Opher. In particular, these studies showed the relevance of Alfvén heating in establishing the stability of BLR clouds in the intercloud medium. The present paper shows the results of time-dependent calculations (we use a time-dependent hydrodynamic code) following the evolution of BLR clouds, since their formation from the 107-K intercloud medium. We also calculate the UV and optical line emission associated with the clouds in order to compare with observations. Our results are compared with those of UV and optical monitoring of well-studied AGN, which suggest that the BLR is most probably composed of at least two different regions, each one giving rise to a kind of line variability, since low- and high-ionization lines present different patterns of variability. We discuss the alternative scenario in which lines of different ionization could be formed at the same place but heated/excited by distinct mechanisms, considering the Alfvén heating as the non-radiative mechanism.  相似文献   

12.
We investigate with 3D hydrodynamical simulations the role played by thermal processes in the dynamical evolution of the interstellarmedium (ISM). A parametric approach of the coolingprocess shows that the observed mass fraction of the cold (< 300 K)and unstable gas (300K < T < 6000K) can not be produced by turbulentcompression or background heating of the medium alone. An analysis of theproperties of the clouds that are formed by the combined effect of the thermal and gravitational instability shows that the cloud’s scaling relations imprinted by the thermal instability (TI) are in good agreementwith observational values.  相似文献   

13.
In view of the Turbulent Cooling Flows scenario we carry out several 3D axisymmetric calculations to follow the evolution of magnetically subcritical weakly ionized and rotating turbulent cloud cores. Turbulent Cooling Flows appear to pronounce the effects of ambipolar diffusion considerably, inducing thereby a runaway collapse of the core already on a diluted free-fall time scale. Ambipolar diffusion significantly weakens the efficiency of magnetic braking. This implies that most of the rotational energy is trapped into the dynamically collapsing core and that initiation of outflows is prevented at least in the early isothermal phases. The trapped rotational energy is found to enhance the formation of rings that may afterwards fragment. It is shown that the central region of a strongly ionized magnetically subcritical core is principally overdense, with central density up to one order of magnitude larger than the surroundings. These results confirm that large scale magnetic fields threading a cloud core relax the supersonic random motions on an Alfvén wave crossing time. Moreover, ambipolar diffusion enhances dissipation of supersonic turbulence even more.  相似文献   

14.
To investigate the occurrence of low temperatures and the formation of noctilucent clouds in the summer mésosphere a one-dimensional time-dependent photochemical-thermal numerical model of the atmosphere between 50 and 120 km has been constructed. The model includes the important chemistry of the hydrogen and oxygen species and transport by eddy and molecular processes. The thermal balance incorporates: heating by solar ultraviolet radiation; transport of chemical potential energy; eddy diffusion and dissipation; molecular conduction; airglow emissions; and infrared cooling by carbon dioxide. A non- LTE parameterization is used to calculate 15 μm band cooling by carbon dioxide. The model self-consistently solves the coupled photochemical and thermal equations as perturbation equations from a reference state assumed to be in equilibrium and is used to consider the effect of variability in water vapor in the lower mesosphere on the temperature in the region of noctilucent cloud formation. It is found that change in water vapor from an equilibrium value of 5 ppm at 50 km to a value of 10 ppm, a variation consistent with observations, can produce a ~ 15 K drop in temperature at 82 km. It is suggested that this process may produce long periods (weeks) of cold temperatures and influence noctilucent cloud formation.  相似文献   

15.
We study the magneto-thermal instability in ionized plasmas including the effects of Ohmic, ambipolar and Hall diffusion. The magnetic field in the single-fluid approximation does not allow for transverse thermal condensations; however, non-ideal effects highly diminish the stabilizing role of the magnetic field in thermally unstable plasmas. Therefore, the enhanced growth rate of thermal condensation modes in the presence of the diffusion mechanisms speed up the rate of structure formation.  相似文献   

16.
Star formation     
Various topics on star formation, centered on the observed properties of young stars and their environment, are reviewed. (a) In our Galaxy, young stellar objects are generally associated with giant molecular clouds. (b) Giant molecular clouds cannot be in free-fall collapse. They are probably stabilized by magnetic fields, which are then likely to dominate the dynamical evolution of the clouds themselves. (c) Star formation occurs mostly in spiral arms. The role of spiral density waves is however not yet clearly understood. (d) The formation of massive stars can perturb the evolution of the progenitor cloud, and possibly trigger the sequential formation of OB subgroups. (e) There is a large number of clouds in the Galaxy associated only with low and intermediate mass young stars. These clouds are not perturbed by the presence of massive stars, and are probably the best source of information on the primary triggering mechanism, active on a galactic scale, and on the initial conditions for star formation.Paper presented at the European Workshop on Planetary Sciences, organised by the Laboratorio di Astrofisica Spaziale di Frascati, and held between April 23–27, 1979, at the Accademia Nazionale del Lincei in Rome, Italy.  相似文献   

17.
Improving our understanding of the earliest stages of star formation is crucial to gain insight into the origin of stellar masses, multiple systems, and protoplanetary disks. We discuss recent advances made in this area, thanks to submillimeter mapping observations with large single-dish telescopes and interferometers. Although ambipolar diffusion appears to be too slow cores, there is nevertheless good evidence that the gravitational collapse of isolated protostellar cores is strongly magnetically controlled. We also argue that the beginning of protostellar collapse is much more violent in cluster-forming clouds than in regions of distributed star formation.  相似文献   

18.
We employ the first fully three-dimensional simulation to study the role of magnetic fields and ion–neutral friction in regulating gravitationally driven fragmentation of molecular clouds. The cores in an initially subcritical cloud develop gradually over an ambipolar diffusion time while the cores in an initially supercritical cloud develop in a dynamical time. The infalling speeds on to cores are subsonic in the case of an initially subcritical cloud, while an extended (≳0.1 pc) region of supersonic infall exists in the case of an initially supercritical cloud. These results are consistent with previous two-dimensional simulations. We also found that a snapshot of the relation between density (ρ) and the strength of the magnetic field ( B ) at different spatial points of the cloud coincides with the evolutionary track of an individual core. When the density becomes large, both the relations tend to   B ∝ρ0.5  .  相似文献   

19.
It is generally accepted that the lifetime of molecular clouds does not exceed 3×107 yr due to disruption by stellar feedback. We put together some arguments giving evidence that a substantial fraction of molecular clouds (primarily in the outer regions of a disc) may avoid destruction process for at least 108 yr or even longer. A molecular cloud can live long if massive stars are rare or absent. Massive stars capable to destroy a cloud may not form for a long time if a cloud is low massive, or stellar initial mass function is top-light, or if there is a delay of the beginning of active star formation. A long duration of the inactive phase of clouds may be reconciled with the low amount of the observed starless giant molecular clouds if to propose that they were preceded by slowly contraction phase of the magnetized dark gas, non-detected in CO-lines.  相似文献   

20.
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