Gravitationally unstable protoplanetary discs

The possibility that protoplanetary gaseous discs are dynamically unstable to axisymmetric and non-axisymmetric gravity perturbations (e.g. those produced by spontaneous disturbances) with characteristic scales larger than the vertical scale height is discussed analytically, using a local Wentzel–Kramers–Brillouin (WKB) approach. It is shown that such discs might be clumpy, and these gravitationally bound clumps may later collapse to become giant planets ('hot Jupiters'). The chief aim in this paper is to underscore a fact of vital importance for application in the planetary formation process: gravitationally unstable non-axisymmetric (spiral) perturbations can effectively transport both the angular momentum and the mass in a spatially inhomogeneous disc.     

Gas-phase models for the evolved planetary nebulae NGC 6781, M4-9 and NGC 7293

We have studied the chemistry of the molecular gas in evolved planetary nebulae. Three pseudo-time-dependent gas-phase models have been constructed for dense (10⁴–10⁵ cm⁻³) and cool ( T ∼15 K) clumpy envelopes of the evolved nebulae NGC 6781, M4-9 and NGC 7293. The three nebulae are modelled as carbon-rich stars evolved from the asymptotic giant branch to the late planetary nebula phase. The clumpy neutral envelopes are subjected to ultraviolet radiation from the central star and X-rays that enhance the rate of ionization in the clumps. With the ionization rate enhanced by four orders of magnitude over that of the ISM, we find that resultant abundances of the species HCN, HNC, HC₃N and SiC₂ are in good agreement with observations, while those of CN, HCO⁺, CS and SiO are in rough agreement. The results indicate that molecular species such as CH, CH₂, CH₂⁺ , HCl, OH and H₂O are anticipated to be highly abundant in these objects.     

Truncated Collimated Flows in Abell 30, Abell 78 and the Honeycomb Nebula

Localised collimated flows of ionized gas are found in two hydrogen deficient planetary nebulae, Abell 30 and Abell 78 as well as in the Honeycomb complex of interlocking shells in halo of 30 Dor in the Large Magellanic Cloud. One common feature of these flows, in seemingly disparate objects, is that they all terminate at around the same difference in radial velocity (with respect to that of the systemic radial velocity). A possible explanation involves high speed flows being decelerated by mass-loading. In Abell 30 and 78, mass is injected by clumps embedded in the fast wind. In the Honeycomb nebula, a supernova blast wave has pierced an old dense shell wall which adds mass to the post-shock flow via a boundary layer. This revised version was published online in July 2006 with corrections to the Cover Date.     

Chemical Evolution from the AGB to the Planetary Nebula Phase

An overview is given on the wealth of data recently provided by large mm-wave radiotelescopes on AGB stars, planetary nebulae (PNe), and transition objects. The observations reveal that there is an observable chemical evolution in the neutral gas as a star evolves beyond the AGB, through the proto-PN and PN phases. Significant changes in the abundances of some key molecules (such as CS, CN, HCO+, HCN, and HC3N) take place during the fast evolution of the envelopes. Chemistry can thus be used as a rough clock to date the evolutionary stage of post-AGB envelopes and proto-PN objects. However, once the PN is formed, the observed abundances in the molecular clumps of the envelope remain relatively constant. The chemical evolution of the molecular envelopes likely occurs through the development of photon-dominated regions produced by the ultraviolet field of the central star. The main chemical processes which likely control the evolution are also reviewed. This revised version was published online in September 2006 with corrections to the Cover Date.     

Unlocking the Keyhole: H2 and PAH emission from molecular clumps in the Keyhole Nebula

To better understand the environment surrounding CO emission clumps in the Keyhole Nebula, we have made images of the region in H₂ 1–0 S(1) (2.122-μm) emission and polycyclic aromatic hydrocarbon (PAH) emission at 3.29 μm. Our results show that the H₂ and PAH emission regions are morphologically similar, existing as several clumps, all of which correspond to CO emission clumps and dark optical features. The emission confirms the existence of photodissociation regions (PDRs) on the surface of the clumps. By comparing the velocity range of the CO emission with the optical appearance of the H₂ and PAH emission, we present a model of the Keyhole Nebula whereby the most negative velocity clumps are in front of the ionization region, the clumps at intermediate velocities are in it and those which have the least negative velocities are at the far side. It may be that these clumps, which appear to have been swept up from molecular gas by the stellar winds from η  Car, are now being overrun by the ionization region and forming PDRs on their surfaces. These clumps comprise the last remnants of the ambient molecular cloud around η Car.     

Disentangling Saturn's F Ring. I. Clump orbits and lifetimes

A comprehensive analysis of the Voyager images reveals the kinematics and lifetimes of clumps in the F Ring. At any given time, the ring has 2-3 major clumps, each several times brighter than the typical ring, plus numerous smaller features. A total of 34 individual clumps have been tracked over periods of 1-7 weeks. The clumps orbited at measurably different rates, implying a 100-km range of semimajor axes centered on 140,220 km. Most are centered around the nominal mean motion of the ring's core, but a few outliers may be associated with a different strand, or with no strand at all. Most clumps change very little over the ∼30 days that they can be detected; however, no clump persisted for the nine-month interval between the two Voyager encounters. The brightest Voyager 2 clump is unusual in that it travels at a rapid mean motion and seems to be associated with the formation of several other clumps.     

Chemistry ahead of Herbig-Haro objects

There is now compelling evidence that dark molecular clouds are clumpy. Much of the clumpiness is unresolved by single-dish telescopes but is apparent in the data from array telescopes. Molecular clumps may also be observed close to Herbig-Haro (HH) objects. These clumps are easily observable because they are `illuminated' due to the UV radiation from the shock front of the HH jet. A detailed observational and theoretical study of one HH clump has been performed and it indicates that this clump must be transient and has a similar density and temperature to those clumps detected in the cloud interior. Thus, HH clumps may be used as an independent method of determining physical parameters of the clumpiness of molecular clouds.     

Dynamics of an ensemble of clumps embedded in a magnetized ADAF

We investigate the effects of a global magnetic field on the dynamics of an ensemble of clumps within a magnetized advection-dominated accretion flow by ignoring interactions between the clumps and then solving the collisionless Boltzman equation.In the strong-coupling limit,in which the averaged radial and rotational velocities of the clumps follow dynamics described by an Advection-Dominated Accretion Flow(ADAF),the root mean square radial velocity of the clumps is calculated analytically for different magnetic field configurations.The value of the root mean square radial velocity of the clumps increases by increasing the strength of the radial or vertical components of the magnetic field,but a purely toroidal magnetic field geometry leads to a reduction in the value of the root mean square radial velocity of the clumps in the inner parts by increasing the strength of this component.Moreover,dynamics of the clumps strongly depend on the amount of advected energy so that the value of the root mean square radial velocity of the clumps increases in the presence of a global magnetic field as the flow becomes more advective.     

Assessing the performance of molecular gas clump identification algorithms

The detection of clumps(cores) in molecular clouds is an important issue in sub-millimetre astronomy. However, the completeness of the identification and the accuracy of the returned parameters of the automated clump identification algorithms are still not clear. In this work, we test the performance and bias of the GaussClumps, ClumpFind, FellWalker, Reinhold, and Dendrograms algorithms in identifying simulated clumps. By designing the simulated clumps with various sizes, peak brightness, and crowdedness, we investigate the characteristics of the algorithms and their performance. In the aspect of detection completeness, FellWalker, Dendrograms, and GaussClumps are the first, second, and third best algorithms, respectively. The numbers of correct identifications of the six algorithms gradually increase as the size and signal-to-noise ratio(SNRs) of the simulated clumps increase and they decrease as the crowdedness increases. In the aspect of the accuracy of retrieved parameters, FellWalker and Dendrograms exhibit better performance than the other algorithms. The average deviations in clump parameters for all algorithms gradually increase as the size and SNR of clumps increase. Most of the algorithms except FellWalker exhibit significant deviation in extracting the total flux of clumps. Taken together, FellWalker, GaussClumps,and Dendrograms exhibit the best performance in detection completeness and extracting parameters. The deviation in virial parameter for the six algorithms is relatively low. When applying the six algorithms to the clump identification for the Rosette molecular cloud, ClumpFind1994, ClumpFind2006, GaussClumps,FellWalker, and Reinhold exhibit performance that is consistent with the results from the simulated test.     

A Monte Carlo study of the spectra from inhomogeneous accretion flow

A model of an inhomogeneous accretion flow,in which cold clumps are surrounded by hot gas or corona,has been proposed to explain the spectral features of black hole X-ray binaries.In this work,we try to find possible observational features in the continuum that can indicate the existence of clumps.The spectra of an inhomogeneous accretion flow are calculated via the Monte Carlo method.Since the corresponding accretion flow is unsteady and complex,the accretion flow is described by a set of free parameters,the ranges of which can include the real cases.The influences of the parameters are investigated.It is found that the thermal component of the spectra deviates from multi-color black body spectra in the middle power-law part.On one hand,a warp appears due to the gaps between the clumps and the outer cold disk,and on the other hand,the slope of the line connecting the thermal peaks deviates from 4/3.The warp feature,as well as the correlation between the thermal peak at higher frequency and the spectral index,possibly indicate the existence of clumps,and are worthy of further investigation with more self-consistent models.     

Planetary nebulae in the Galaxy

It is shown that the planetary nebulae can be divided into three types according to the values of the mass of shell and a central star. The criteria are given using which one can determine the mass type of the nebula. The distance scale of each mass-type planetary nebulae is given. The distribution of planetary nebulae in the Galaxy, their formation rate, scale-height and other physical and kinematic characteristics are investigated. A catalogue of planetary nebulae emitting in the radio range is given.     

The dynamical evolution of substructure

The evolution of substructure embedded in non-dissipative dark haloes is studied through N -body simulations of isolated systems, both in and out of initial equilibrium, complementing cosmological simulations of the growth of structure. We determine by both analytic calculations and direct analysis of the N -body simulations the relative importance of various dynamical processes acting on the clumps, such as the removal of material by global tides, clump–clump heating, clump–clump merging and dynamical friction. The ratio of the internal clump velocity dispersion to that of the dark halo is an important parameter; as this ratio approaches a value of unity, heating by close encounters between clumps becomes less important, while the other dynamical processes continue to increase in importance. Our comparison between merging and disruption processes implies that spiral galaxies cannot be formed in a protosystem that contains a few large clumps, but can be formed through the accretion of many small clumps; elliptical galaxies form in a more clumpy environment than do spiral galaxies. Our results support the idea that the central cusp in the density profiles of dark haloes is the consequence of self-limiting merging of small, dense haloes. This implies that the collapse of a system of clumps/substructure is not sufficient to form a cD galaxy, with an extended envelope; plausibly, subsequent accretion of large galaxies is required. The post-collapse system is in general triaxial, with rounder systems resulting from fewer, but more massive, clumps. Persistent streams of material from disrupted clumps can be found in the outer regions of the final system, and at an overdensity of around 0.75, can cover 10 to 30 per cent of the sky.     

Resonant periodic orbits in the exoplanetary systems

The planetary dynamics of 4/3, 3/2, 5/2, 3/1 and 4/1 mean motion resonances is studied by using the model of the general three body problem in a rotating frame and by determining families of periodic orbits for each resonance. Both planar and spatial cases are examined. In the spatial problem, families of periodic orbits are obtained after analytical continuation of vertical critical orbits. The linear stability of orbits is also examined. Concerning initial conditions nearby stable periodic orbits, we obtain long-term planetary stability, while unstable orbits are associated with chaotic evolution that destabilizes the planetary system. Stable periodic orbits are of particular importance in planetary dynamics, since they can host real planetary systems. We found stable orbits up to 60^° of mutual planetary inclination, but in most families, the stability does not exceed 20^°–30^°, depending on the planetary mass ratio. Most of these orbits are very eccentric. Stable inclined circular orbits or orbits of low eccentricity were found in the 4/3 and 5/2 resonance, respectively.     

In situ 3‐D mapping of pore structures and hollow grains of interplanetary dust particles with phase contrast X‐ray nanotomography

Unlocking the 3‐D structure and properties of intact chondritic porous interplanetary dust particles (IDPs) in nanoscale detail is challenging, which is also complicated by atmospheric entry heating, but is important for advancing our understanding of the formation and origins of IDPs and planetary bodies as well as dust and ice agglomeration in the outer protoplanetary disk. Here, we show that indigenous pores, pristine grains, and thermal alteration products throughout intact particles can be noninvasively visualized and distinguished morphologically and microstructurally in 3‐D detail down to ~10 nm by exploiting phase contrast X‐ray nanotomography. We have uncovered the surprisingly intricate, submicron, and nanoscale pore structures of a ~10‐μm‐long porous IDP, consisting of two types of voids that are interconnected in 3‐D space. One is morphologically primitive and mostly submicron‐sized intergranular voids that are ubiquitous; the other is morphologically advanced and well‐defined intragranular nanoholes that run through the approximate centers of ~0.3 μm or lower submicron hollow grains. The distinct hollow grains exhibit complex 3‐D morphologies but in 2‐D projections resemble typical organic hollow globules observed by transmission electron microscopy. The particle, with its outer region characterized by rough vesicular structures due to thermal alteration, has turned out to be an inherently fragile and intricately submicron‐ and nanoporous aggregate of the sub‐μm grains or grain clumps that are delicately bound together frequently with little grain‐to‐grain contact in 3‐D space.     

Multiple clump structures within photoionized regions

We present 3D simulations of a system of four neutral clumps embedded in a photoionized region. In this system, we have three small clumps which partially shield a single, larger clump from the stellar ionizing photons. This flow evolves to form a neutral structure with a main body and three neutral 'columns' pointing towards the central star. Qualitatively, similar structures are seen in the 'Finger' of the Carina Nebula.     

Planetary nebulae in the Magellanic Clouds (I)

A classification scheme for planetary nebulae in the Magellanic Clouds using four key criteria is suggested. All nebulae can then be divided into three classes. The features of physical parameters and evolution of the Magellanic Cloud planetary nebulae and the way in which they differ from galactic planetary nebulae are investigated separately for each class of nebulae.     

Planetary nebulae in the Magellanic Clouds (II)

Using the classification scheme for planetary nebulae in the Magellanic Clouds using four criteria proposed in Paper I, all nebulae are divided into three classes on the basis of the mass of their central stars. The features of individual chemical abundances in the Magellanic Cloud planetary nebulae and the way in which these differ from the galactic planetary nebulae are investigated separately for each class of nebulae. The role of CN and ON cycling in intermediate mass star evolution is discussed.     

Stability of outer planetary systems

The conditions for stability in the Liapunov-Hill sense of outer planetary systems are given in terms of radii of planetary orbits. The outer planets of the solar system are found stable and the possible existence of other than the presently known planets between Jupiter and Pluto are indicated. The existence of other planetary systems with arbitrary mass ratios of the primaries is suggested, and the stability conditions for such systems are derived.Proceedings of the Sixth Conference on Mathematical Methods in Celestial Mechanics held at Oberwolfach (West Germany) from 14 to 19 August, 1978.     

Extrasolar planetary systems

The article deals with the occurrence of planetary systems in the Universe. In Section I, the terms “planet” and “planet-like objects” are defined. Two definitions proposed for the term “planetary system” are examined from the point of view (1) of the relation between planetary systems and binary and multiple star systems and (2) of planetary systems as abodes of intelligent beings. In Section II, the observational search for extrasolar planetary systems is described, as performable by earthbound optical telescopes, by space probes, by long baseline radio interferometry, and finally by inference from the reception of signals sent by intelligent beings in other worlds.In Section III we show that any planetary system must be preceded by a rotating disk of gas and dust around a central mass. Both observational evidence and theoretical reasons indicate the ease of formation of such disk structures in the cosmos. The time scale of collapse of a gaseous medium into a disk and that of the latter's dissipation are examined. This provides us with a new empirical approach and leads us to consider the problem of the frequency of occurrence of planetary systems to be ripe for scientific study. In Section IV, a brief review of theories of the formation of the solar system is given along with a proposed scheme for classification of these theories. In Section V, the evidence for magnetic activity in the early stages of stellar evolution is presented, as developed from six independent clues: the nuclear abundance of light elements, the behavior of flare stars, the intensities of H and K emission in stars, the nonthermal radiation of premain sequence stars, the properties of meteorites, and finally the existence of contact binaries. The magnetic braking theories of solar and stellar rotation are discussed in Section VI, thereby introducing the idea of formation of a rotating disk of gas and dust around stars in Section VII. From this disk a planetary system emerges.Section VIII gives an estimate for the frequency of occurrence of planetary systems in the Universe. It is based on the rotational behavior of main-sequence stars, and concludes that planetary systems have a far greater chance to appear around single main-sequence stars of spectral types later than F5 than around any other kind of star. The combined probability distribution of sizes and masses could be obtained. From physical considerations, it appears that sizes of planetary systems around stars of any given spectral type may not vary greatly from one to another.     

On the propensity of the formation of massive clumps via fragmentation of driven shells

Early type massive stars drive thin, dense shells whose edges often show evidence of star-formation. The possibility of fragmentation of these shells, leading to the formation of putative star-forming clumps is examined with the aid of semi-analytic arguments. We also derive a mass-spectrum for clumps condensing out of these shells by performing Monte–Carlo simulations of the problem. By extending on results from our previous work on the stability of thin, dense shells, we argue that clump-mass estimated by other authors in the past, under a set of simplifying assumptions, are several orders of magnitude smaller than those calculated here. Using the expression for the fastest growing unstable mode in a shock-confined shell, we show that fragmentation of a typical shell can produce clumps with a typical mass ?10³  M_⊙. It is likely that such clumps could spawn a second generation of massive and/or intermediate-mass stars which could in turn, trigger the next cycle of star-formation. We suggest that the ratio of shell thickness-to-radius evolves only weakly with time. Calculations have been performed for stars of seven spectral types, ranging from B1 to O5. We separately consider the stability of supernova remnants.