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1.
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
2.
S. I.Bastrukov J.Yang D. V.Podgainy 《Monthly notices of the Royal Astronomical Society》2002,330(4):901-906
We investigate the evolution of the magnetic flux density in a magnetically supported molecular cloud driven by Hall and Ohmic components of the electric field generated by the flows of thermal electrons. Particular attention is given to the wave transport of the magnetic field in a cloud whose gas dynamics is dominated by electron flows; the mobility of neutrals and ions is regarded as heavily suppressed. It is shown that electromagnetic waves penetrating such a cloud can be converted into helicons – weakly damped, circularly polarized waves in which the densities of the magnetic flux and the electron current undergo coherent oscillations. These waves are interesting in their own right, because for electron magnetohydrodynamics the low-frequency helicoidal waves have the same physical significance as the transverse Alfvén waves do for a single-component magnetohydrodynamics. The latter, as is known, are considered to be responsible for the widths of molecular lines detected in dark, magnetically supported clouds. From our numerical estimates for the group velocity and the rate of dissipation of helicons it follows that a possible contribution of these waves to the broadening of molecular lines is consistent with the conditions typical of dark molecular clouds. 相似文献
3.
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
4.
The heating of the ion-neutral (or ambipolar) diffusion may affect the thermal phases of the molecular clouds. We present an investigation on the effect of this heating mechanism in the thermal instability of the molecular clouds. A weakly ionized one-dimensional slab geometry, which is allowed for self-gravity and ambipolar diffusion, is chosen to study its thermal phases. We use the thermodynamic evolution of the slab to obtain the regions where slab cloud becomes thermally unstable. We investigate this evolution using the model of ambipolar diffusion with two-fluid smoothed particle hydrodynamics, as outlined by Hosking and Whitworth. Firstly, some parts of the technique are improved to test the pioneer works on behavior of the ambipolar diffusion in an isothermal self-gravitating slab. Afterwards, the improved two-fluid technique is used for thermal evolution of the slab. The results show that the thermal instability may persist inhomogeneities with a large density contrast at the intermediate parts of the cloud. We suggest that this feature may be responsible for the planet formation in the intermediate regions of a collapsing molecular cloud and/or may also be relevant to the formation of star forming dense cores in the clumps. 相似文献
5.
We estimated the gravitational influence of giant molecular clouds passing near the Solar system on the orbital evolution of Oort cloud comets. We performed a comparative analysis of the accuracies of the following two methods of allowance for the perturbations from giant molecular clouds: the impulse approximation and numerical integration. The impulse approximation yields fairly accurate estimates of the change in the energy of Oort cloud comets and the probability of their ejection under the influence of a molecular cloud if the path of the Solar system does not cross its boundary and if the molecular cloud may be treated as a point perturbing mass. The comet survival probability in the Oort cloud depends significantly on the internal structure of the perturbing molecular cloud and the impact parameter of the encounter. The most massive injection of comets into the planetary region and their ejection from the Oort cloud take place if the Solar system passes through a giant molecular cloud composed of several high-mass condensations. In this case, most of the comets injected into the planetary region were initially comets of the inner Oort cloud (a 10–4 AU) with high orbital eccentricities. 相似文献
6.
K. Tassis C. D. Dowell R. H. Hildebrand L. Kirby J. E. Vaillancourt 《Monthly notices of the Royal Astronomical Society》2009,399(4):1681-1693
We present a novel statistical analysis aimed at deriving the intrinsic shapes and magnetic field orientations of molecular clouds using dust emission and polarization observations by the Hertz polarimeter. Our observables are the aspect ratio of the projected plane-of-the-sky cloud image and the angle between the mean direction of the plane-of-the-sky component of the magnetic field and the short axis of the cloud image. To overcome projection effects due to the unknown orientation of the line-of-sight, we combine observations from 24 clouds, assuming that line-of-sight orientations are random and all are equally probable. Through a weighted least-squares analysis, we find that the best-fitting intrinsic cloud shape describing our sample is an oblate disc with only small degrees of triaxiality. The best-fitting intrinsic magnetic field orientation is close to the direction of the shortest cloud axis, with small (∼24°) deviations towards the long/middle cloud axes. However, due to the small number of observed clouds, the power of our analysis to reject alternative configurations is limited. 相似文献
7.
S. Rodriguez S. Le Mouélic P. Rannou C. Sotin R.H. Brown J.W. Barnes C.A. Griffith J. Burgalat K.H. Baines B.J. Buratti R.N. Clark P.D. Nicholson 《Icarus》2011,216(1):89-110
Since Saturn orbital insertion in July 2004, the Cassini orbiter has been observing Titan throughout most of the northern winter season (October 2002–August 2009) and the beginning of spring, allowing a detailed monitoring of Titan’s cloud coverage at high spatial resolution with close flybys on a monthly basis. This study reports on the analysis of all the near-infrared images of Titan’s clouds acquired by the Visual and Infrared Mapping Spectrometer (VIMS) during 67 targeted flybys of Titan between July 2004 and April 2010.The VIMS observations show numerous sporadic clouds at southern high and mid-latitudes, rare clouds in the equatorial region, and reveal a long-lived cloud cap above the north pole, ubiquitous poleward of 60°N. These observations allow us to follow the evolution of the cloud coverage during almost a 6-year period including the equinox, and greatly help to further constrain global circulation models (GCMs). After 4 years of regular outbursts observed by Cassini between 2004 and 2008, southern polar cloud activity started declining, and completely ceased 1 year before spring equinox. The extensive cloud system over the north pole, stable between 2004 and 2008, progressively fractionated and vanished as Titan entered into northern spring. At southern mid-latitudes, clouds were continuously observed throughout the VIMS observing period, even after equinox, in a latitude band between 30°S and 60°S. During the whole period of observation, only a dozen clouds were observed closer to the equator, though they were slightly more frequent as equinox approached.We also investigated the distribution of clouds with longitude. We found that southern polar clouds, before disappearing in mid-2008, were systematically concentrated in the leading hemisphere of Titan, in particular above and to the east of Ontario Lacus, the largest reservoir of hydrocarbons in the area. Clouds are also non-homogeneously distributed with longitude at southern mid-latitudes. The n = 2-mode wave pattern of the distribution, observed since 2003 by Earth-based telescopes and confirmed by our Cassini observations, may be attributed to Saturn’s tides.Although the latitudinal distribution of clouds is now relatively well reproduced and understood by the GCMs, the non-homogeneous longitudinal distributions and the evolution of the cloud coverage with seasons still need investigation. If the observation of a few single clouds at the tropics and at northern mid-latitudes late in winter and at the start of spring cannot be further interpreted for the moment, the obvious shutdown of the cloud activity at Titan’s poles provides clear signs of the onset of the general circulation turnover that is expected to accompany the beginning of Titan’s northern spring. According to our GCM, the persistence of clouds at certain latitudes rather suggests a ‘sudden’ shift in near future of the meteorology into the more illuminated hemisphere. Finally, the observed seasonal change in cloud activity occurred with a significant time lag that is not predicted by our model. This may be due to an overall methane humidity at Titan’s surface higher than previously expected. 相似文献
8.
An active hydrological cycle has been added to the EPIC general circulation model (GCM) for planetary applications, with a special emphasis on Jupiter. Scientists have suspected for decades that clouds, and in particular latent heating, strongly influence Jupiter's atmospheric dynamics and this research provides a tool to investigate this phenomenon. Components of the model have been adapted for the planetary setting from recently published Earth microphysics schemes. The behavior of the cloud model is investigated in two steps. First, we explore in detail the runtime properties of a nominal model, and second, through sensitivity tests we determine how the full microphysics and selected components of the scheme affect the formation and evolution of clouds and precipitation. Results from our one-dimensional (vertical) simulations match expectations based on thermochemical models about the vertical positioning of ammonia and water clouds, and the nature of precipitation. Using (two-dimensional) meridional plane simulations, we investigate the latitudinal variation of clouds. We conclude that the zonal-wind structure under the visible cloud deck strongly affects the position of the cloud bases, also that the atmospheric dynamics modifies the resulting cloud structure that we can determine in 1D models. We describe in detail an equatorial storm system observed in our 2D simulations. We also show that simplification of our microphysics scheme would improperly simulate large-scale weather phenomena on Jupiter. We support future laboratory tests and in situ measurements that would improve the cloud parameterization scheme and would also add more constraints on the global distribution of condensibles and on the zonal wind-structure. The complete computer program resulting from this research can be downloaded as open-source software from NASA's Planetary Data System (PDS) Atmospheres node. 相似文献
9.
Antonella Natta 《Earth, Moon, and Planets》1980,22(3):331-346
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
10.
Summary This review consists of five sections. In the introduction, we briefly review the development of the study of molecular clouds. In the second section, we review the theories of molecular cloud structure and compare these predictions with the statistical properties of the clouds. In Sect. 3 we give an overview of current approaches to determinations of mass and local density in clouds. In the fourth part, we discuss the observations of a selected sample of individual sources. The emphasis here is on high resolution studies of regions of star formation. The final section contains a discussion of instrumental limitations and mentions some future developments. 相似文献
11.
12.
Anthony J. Horton Matthew R. Bate Ian A. Bonnell 《Monthly notices of the Royal Astronomical Society》2001,321(3):585-592
We consider how the tidal potential of a stellar cluster or a dense molecular cloud affects the fragmentation of gravitationally unstable molecular cloud cores. We find that molecular cloud cores which would collapse to form a single star in the absence of tidal shear, can be forced to fragment if they are subjected to tides. This may enhance the frequency of binaries in star-forming regions such as Ophiuchus and the frequency of binaries with separations ≲100 au in the Orion Trapezium Cluster. We also find that clouds which collapse to form binary systems in the absence of a tidal potential will form bound binary systems if exposed to weak tidal shear. However, if the tidal shear is sufficiently strong, even though the cloud still collapses to form two fragments, the fragments are pulled apart while they are forming by the tidal shear and two single stars are formed. This sets an upper limit for the separation of binaries that form near dense molecular clouds or in stellar clusters. 相似文献
13.
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
14.
Richard M. Crutcher 《Astrophysics and Space Science》2004,292(1-4):225-237
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
15.
A time-dependent microphysical model is used to study the evolution of ethane ice clouds in Titan’s atmosphere. The model simulates nucleation, condensational growth, evaporation, coagulation, and transport of particles. For a critical saturation of 1.15 (a lower limit, determined by laboratory experiments), we find that ethane clouds can be sustained between altitudes of 8 and 50 km. Growth due to coalescence is inefficient, limiting the peak in the size distribution (by number) to 10 μm. These clouds vary with a period of about 20 days. This periodicity disappears for higher critical saturation values where clouds remain subvisible. Rainout of ethane due to methane cloud formation raises the altitude of the ethane cloud bottom to near the tropopause and may eliminate ethane clouds entirely if methane cloud formation occurs up to 30 km. However, clouds formed above the troposphere from other gases in Titan’s atmosphere could be sustained even with rainout up to 30 km. Although the optical depth of ethane clouds above 20 km is typically low, short-lived clouds with optical depths of order 0.1-1 can be created sporadically by dynamically driven atmospheric cooling. Ethane cloud particles larger than 25 μm can fall to the surface before total evaporation. However, ethane clouds remain only a small sink for tholin particles. At the peak of their cycle, the optical depth of ethane clouds could be comparable to that of tholin in the near-infrared, resulting in a 5% increase in Titan’s albedo for wavelengths between 1 and 2 μm. A number of factors limit our ablility to predict the ethane cloud properties. These factors include the mixing time in the troposphere, the critical saturation ratio for ethane ice, the existence of a surface reservoir of ethane, the magnitude and timing of dynamically driven temperature perturbations, and the abundance and life cycle of methane clouds. 相似文献
16.
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
17.
This work deals with a CCD imaging study at optical and near‐infrared wavelength oftwo giant molecular clouds (plus a control field) in the southern region of the Large Magellanic Cloud, one ofwhich shows multiple signs of star formation, whereas the other does not. The observational data from VLT FORS2 (R band) and NTT SOFI (Ks band) have been analyzed to derive luminosity functions and color‐magnitude diagrams. The young stellar content of these two giant molecular clouds is compared and confirmed to be different, in the sense that the apparently “starless” cloud has so far formed only low‐luminosity, low‐mass stars (fainter than mKs ∽ 16.5 mag, not seen by 2MASS), while the other cloud has formed both faint low‐mass and luminous high‐mass stars. The surface density excess oflow‐luminosity stars (∽2 per square arcmin) in the “starless” cloud with respect to the control field is about 20% whereas the excess is about a factor of 3 in the known star‐forming cloud. The difference may be explained theoretically by the gravo‐turbulent evolution of giant molecular clouds, one being younger and less centrally concentrated than the other (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
18.
Philippe André 《Astrophysics and Space Science》2002,281(1-2):51-66
Detailed studies of nearby cluster-forming molecular clouds can help us understand the physical processes by which most stars
form in galaxies. I review recent advances made on this subject. Submillimeter observations of nearby protoclusters suggest
that stars are generally built from finite, detached reservoirs of mass inside molecular cloud cores, and point to a cloud
fragmentation origin for the IMF. Much progress in this field will come from future large submillimeter instruments such as
Herschel and ALMA.
This revised version was published online in September 2006 with corrections to the Cover Date. 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.
The Sun may well have formed in the type of interstellar cloud currently referred to as a cold, dark cloud. We present current tabulations of the totality of known interstellar molecules and of the subset which have been identified in cold clouds. Molecular abundances are given for two such clouds which show interesting chemical differences in spite of strong physical similarities, Taurus Molecular Cloud 1 (TMC-1) and Lynd's 134N (L134N, also referred to as L183). These regions may be at different evolutionary stages. 相似文献