The migrating embryo model for disk evolution

Abstract– A new view of disk evolution is emerging from self‐consistent numerical simulation modeling of the formation of circumstellar disks from the direct collapse of prestellar cloud cores. This has implications for many aspects of star and planet formation, including the growth of dust and high‐temperature processing of materials. A defining result is that the early evolution of a disk is crucially affected by the continuing mass loading from the core envelope, and is driven into recurrent phases of gravitational instability. Nonlinear spiral arms formed during these episodes fragment to form gaseous clumps in the disk. These clumps generally migrate inward due to gravitational torques arising from their interaction with a trailing spiral arm. Occasionally, a clump can open up a gap in the disk and settle into a stable orbit, revealing a direct pathway to the formation of companion stars, brown dwarfs, or giant planets. At other times, when multiple clumps are present, a low mass clump may even be ejected from the system, providing a pathway to the formation of free‐floating brown dwarfs and giant planets in addition to low mass stars. Finally, it has been suggested that the inward migration of gaseous clumps can provide the proper conditions for the transport of high‐temperature processed solids from the outer disk to the inner disk, and even possibly accelerate the formation of terrestrial planets in the inner disk. All of these features arising from clump formation and migration can be tied together conceptually in a migrating embryo model for disk evolution that can complement the well‐known core accretion model for planet formation.     

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.     

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.     

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.     

A possible EGRET source with a two-sided radio jet structure on a parsec scale – 3EG J0808+5114

Recent observations have revealed that damped Lyα clouds (DLAs) host star formation activity. In order to examine if such star formation activity can be triggered by ionization fronts, we perform high-resolution hydrodynamics and radiative transfer simulations of the effect of radiative feedback from propagating ionization fronts on high-density clumps. We examine two sources of ultraviolet (UV) radiation field to which high-redshift ( z ∼ 3) galaxies could be exposed: one corresponding to the UV radiation originating from stars within the DLA, itself, and the other corresponding to the UV background radiation. We find that, for larger clouds, the propagating I-fronts created by local stellar sources can trigger cooling instability and collapse of significant part, up to 85 per cent, of the cloud, creating conditions for star formation in a time-scale of a few Myr. The passage of the I-front also triggers collapse of smaller clumps (with radii below ∼4 pc), but in these cases the resulting cold and dense gas does not reach conditions conducive to star formation. Assuming that 85 per cent of the gas initially in the clump is converted into stars, we obtain a star formation rate of  ∼0.25 M_⊙ yr⁻¹ kpc⁻²  . This is somewhat higher than the value derived from recent observations. On the other hand, the background UV radiation which has harder spectrum fails to trigger cooling and collapse. Instead, the hard photons which have long mean free-path heat the dense clumps, which as a result expand and essentially dissolve in the ambient medium. Therefore, the star formation activity in DLAs is strongly regulated by the radiative feedback, both from the external UV background and internal stellar sources and we predict quiescent evolution of DLAs (not starburst-like evolution).     

High-resolution simulations of clump–clump collisions using SPH with particle splitting

We investigate, by means of numerical simulations, the phenomenology of star formation triggered by low-velocity collisions between low-mass molecular clumps. The simulations are performed using a smoothed particle hydrodynamics code which satisfies the Jeans condition by invoking on-the-fly particle splitting.
Clumps are modelled as stable truncated (non-singular) isothermal, i.e. Bonnor–Ebert, spheres. Collisions are characterized by M ₀ (clump mass), b (offset parameter, i.e. ratio of impact parameter to clump radius) and     (Mach number, i.e. ratio of collision velocity to effective post-shock sound speed). The gas subscribes to a barotropic equation of state, which is intended to capture (i) the scaling of pre-collision internal velocity dispersion with clump mass, (ii) post-shock radiative cooling and (iii) adiabatic heating in optically thick protostellar fragments.
The efficiency of star formation is found to vary between 10 and 30 per cent in the different collisions studied and it appears to increase with decreasing M ₀, and/or decreasing b , and/or increasing     . For   b < 0.5  collisions produce shock-compressed layers which fragment into filaments. Protostellar objects then condense out of the filaments and accrete from them. The resulting accretion rates are high,     , for the first     . The densities in the filaments,     , are sufficient that they could be mapped in NH₃ or CS line radiation, in nearby star formation regions.     

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.     

Planetesimal dissolution in the envelopes of the forming,giant planets

We have assessed the ability of planetesimals to penetrate through the envelopes of growing giant planets that form by a “core-instability” mechanism. According to this mechanism, a core grows by the accretion of solid bodies in the solar nebula and the growing core becomes progressively more effective in gravitationally concentrating gas from the surrounding solar nebula in an envelope until a “runaway” accretion of gas occurs. In performing this assessment, we have considered the ability of gas drag to slow down a planetesimal; the effectiveness of gas dynamical pressure in fracturing and ultimately finely fragmenting it; the ability of its strength and self-gravity to resist such fracturing; and the degree to which it is evaporated due to heating by the surrounding envelope, including shock heating that develops during the supersonic portion of its trajectory. We also consider what happens if the planetesimal is able to reach the core at free-fall velocity and the ability of the envelope to convectively mix dissolved materials to different radial distances. These calculations were performed for various epochs in the growth of a giant planet with the model envelopes derived by Bodenheimer and Pollack (1986,67, 391–408). As might have been anticipated, our results vary significantly with the size of the planetesimal, its composition, and the stage of growth of the giant planet and hence the mass of its envelope. Over much of the growth phase of the core, prior to its reaching its critical mass for runaway gas accretion, icy planetesimals less than about 1 m in size dissolve in the outer region of the envelope, ones larger than about 1 m and smaller than about 1 km dissolve in the middle region of the envelope, ones larger than 1 km either reach the core interface or dissolve in the deeper regions of the envelope. Similarly rocky planetesimals smaller than about a kilometer dissolve in the middle portion of the envelope, while larger ones can penetrate more deeply. Furthermore, the convection zones of the envelopes during this stage are confined to localized regions and hence dissolved materials experience little radial mixing then. Thus, if much of the accreted mass is contained in planetesimals larger than about a kilometer, the critical core mass for runaway accretion is not expected to change significantly when planetesimal dissolution is taken into account. After accretion is terminated and the planet contracts toward its present size, the convection zone grows until it encompasses the entire envelope. Therefore, dissolved material should eventually become well mixed through the envelope. We proposed that the envelopes of the giant planets should contain significant enhancements above solar proportions in the abundances of virtually all elements relative to that of hydrogen, with the magnitude of the enhancement increasing approximately linearly with the ratio of the high Z mass to the (H, He) mass for the bulk of the planet. This prediction is in accord both qualitatively and quantitatively with the systematic increase in the atmospheric C/H ratio from Jupiter to Saturn to Uranus and Neptune and semiquantitatively with the results of recent interior models of the giant planets. It is not clear whether it is consistent with the abundances of H₂O and NH₃ in the atmospheres of some of the outer planets. Finally, the complete reduction of some dissolved materials, especially C containing compounds, is expected to consume some of the H₂ in the envelopes. Consequently, the He/H₂ ratios in the atmospheres of Uranus and Neptune may be slightly enhanced over the solar ratio. We estimate that the He/H₂ ratios for Uranus' and Neptune's atmospheres should be about 6 and 15% larger, respectively, than the solar ratio.     

Role of Gas Dynamical Friction in the Evolution of Embedded Stellar Clusters

Two puzzles associated with open clusters have attracted a lot of attention – their formation, with densities and velocity dispersions that are not too different from those of the star forming regions in the galaxy, given that the observed Star Formation Efficiencies (SFE) are low and, the mass segregation observed/inferred in some of them, at ages significantly less than the dynamical relaxation times in them. Gas dynamical friction has been considered before as a mechanism for contracting embedded stellar clusters, by dissipating their energy. This would locally raise the SFE which might then allow bound clusters to form. Noticing that dynamical friction is inherently capable of producing mass segregation, since here, the dissipation rate is proportional to the mass of the body experiencing the force, we explore further, some of the details and implications of such a scenario, vis-à-vis observations. Making analytical approximations, we obtain a boundary value for the density of a star forming clump of a given mass, such that, stellar clusters born in clumps which have densities higher than this, could emerge bound after gas loss. For a clump of given mass and density, we find a critical mass such that, sub-condensations with larger masses than this could suffer significant segregation within the clump.     

Étude du système AX Mon

We have divided this investigation into four main parts. In the first part, we study the ways in which the envelope affects the composition and the spectral variations, and the orbital motion acts on the envelope absorption intensities and how the extent of the envelope constantly decreases during the 11 yr of our observations. This phenomenon regarding AX Mon has not been previously reported in the literature. However, it explains the appearance of the α Cygni spectrum which occurs according to an arbitrary integral multiple of the orbital periods. Two absorbing envelopes seem to exist: an exterior shell of hydrogen and an interior metallic shell, which appears only when the last hydrogen line of the Balmer series of the envelope is H 27. In the second part, the study of the line Feii λ 4233 shows the influence of the orbital motion on the profile and, in particular, its effect on the absorption in the envelope. The decrease of the extent of the envelope is shown by means of a series of ‘isophase curves’, which indicates that there is maximum emission whenever, there is maximum envelope extension. The existence of satellite components to the red or violet gives evidence for the existence of heterogeneous velocity layers, contracting more rapidly than the bulk of the envelope during the cycle 100 and expanding more rapidly during cycle 116. The study of theV/R ratio shows that this ratio is independent of the orbital motion and always remains bigger than one. Some layers fall down to the photosphere as the envelope decreases (cycle 100 to 108). These layers participate in the general motion of the envelope (cycles 109 to 112) and then are strongly accelerated towards the border when the general contraction in the envelope increases again (cycles 112 to 116). The nature of the radial velocities indicates a pulsation of the emission layers which is connected with the orbital motion. The direction of the acceleration in the internal layers is reversed from apastron to periastron, while at the same time the acceleration of the internal layers increases. These motions cause changes in the density which could explain the variations of intensity in the spectrum of the envelope. In the third part, we study the photometric variations referring to three time-scales:

1. very short time-scale variations (≤4 hr). These variations can be important, reaching 0.1 mag, and can be described by a model matter ejection in very hot gaseous streams.
2. short time-scale variations (>1 day). These variations are connected with the intensity of the satellite absorptions of the line Feii λ 4233 and could be interpreted as absorption variations in the low layers of the envelope.
3. long time-scale variations (≤3 yr). We observe a perceptible decrease of the amplitude of (b) type variations, but no change in their mean value. We can then determine the AX Mon indices (V=6.77;B?V=+0.33;U?B=?0.66). The amplitude of the variations seems to increase as the envelope increases.

In the fourth part, we show that the spectral type of the hotter star can be estimated to be B0.5 V and for the cold star to be K2 II. The mass-ratio is estimated by choosing the velocity curve of the α Cygni spectrum to represent the B star, this choice leading to results which agree very well with the observations. In the resulting model, the secondary star fills the Roche's lobe and mass exchange can occur between the K star and the B star. The large value of the inclination (i=79°) leads to a quasiequatorial observation and explains why, in spite of the small eccentricity, we can observe tidal effects and the resulting spectral changes. The study of the evolution of the system by means of the theories by Crawford and Plavec shows that the mass exchange began with the commencement of nuclear reactions at the border of the B star. During this evolution, the role of the two components has changed, the original primary becoming the secondary. In this assumption, the present system has exchanged about half of the permitted mass. The emissive zone radius is estimated to be 70R _⊙ by means of Sobolev's theory. This zone is entirely contained within the Roche's lobe of the star and is very sensitive to the gravitational action of the K star.     

Forming dust precursors in the inner envelopes of carbon-rich AGB stars

The formation of polycyclic aromatic hydrocarbon (PAH) molecules is studied in the inner envelope of a typical carbon-rich AGB star. The deep envelope is formed of layers of gas that experience the passage of strong periodic shocks forming close to the stellar photosphere. The parcels of gas then follow quasi-ballistic trajectories which are characterized by high gas densities. A chemical scheme based on combustion chemistry is applied to shocked layers of gas, and a PAH formation yield is calculated. PAHs up to coronene (C₂₄H₁₂) survive shocks with strengths of 10 km s^–1, and they accumulate in the gas parcel over several stellar pulsations. This result illustrates that any C-rich AGB star can nucleate dust precursors in its envelope.     

A simple model of phase-changing variation between be stars and shell stars

Phase changing variations between Be and shell stars are considered from the viewpoint of the formation of shell absorption lines in the envelopes of these stars. Typical shell stars are characterized by large optical depths of the envelopes in the H line (H) in a range of 2000 to 5000, whereas the envelopes of typical Be stars are optically thinner with the values of (H)100. This infers that the envelopes of Be stars should be fully expanded as compared to those of shell stars, so as to reduce the optical thickness. Spectral formation in shell stars shows that their envelopes are well condensed near the equatorial planes forming disks or rings. In this paper a simple model of transformation from a disk envelope of shell star to a spherical envelope of Be star is considered to show the relative volume and volume emission measures of the envelopes in both phases. The phase change variations observed in Pleione and in other Be and shell stars are discussed based on this simple consideration. Some implications of the present model in the linear polarization, IR-excess, UV spectra and the radiation field of the envelope are also discussed briefly.     

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.     

SO and SiO emission around the young cluster in the CB 34 globule

The globule CB 34, which harbours a cluster of class 0 young stellar object (YSO) protostars, has been investigated through a multiline SO and SiO survey at millimetre wavelengths. The SO data reveal that the globule consists of three quiescent high-density (∼10⁵ cm⁻³) clumps, labelled A, B and C, with sizes of ∼  0.2–0.3 pc  . The SiO data provide evidence for high-velocity gas across the globule. Most likely, the high-velocity gas is distributed in three distinct high-velocity outflows associated with the YSOs in each of the three clumps. High-velocity SO features have been detected only towards the two brightest SiO outflows. These broad SO components exhibit spatial and spectral distributions which are consistent with those of the SiO emission, so they can also be used as tracers of the outflows.
The comparison between the spatial and spectral properties of the SO and SiO emissions in the three clumps suggests different evolutionary stages for the embedded YSOs. In particular, the YSO associated with clump C exhibits some peculiarities, namely smaller SiO linewidths, lower SiO column densities, a lack of extended SiO structure and of SO wings, and the presence of a SO spatial distribution which is displaced with respect to the location of the YSO. This behaviour is well explained if the SiO and SO molecules which were produced at high velocities in the shocked region have been destroyed or slowed down because of the interaction with the ambient medium, and the chemistry is dominated again by low-temperature reactions. Thus our observations strongly suggest that the YSO in clump C is in a more evolved phase than the other members of the cluster.     

Interaction of planetary nebulae with prenebulae debris

In the present poster we suggest that some of the structures observed in the envelopes of planetary nebulae are caused by the interaction of central star wind and radiation with preplanetary nebula debris: planets, moons, minor objects and ring and ring arcs.Recently considerable amount of planetary material has been reported to exist around solar type stars, this debris could be evaporated during the envelope ejection and alter the chemical abundance and produce some of the envelope inhomogeneities.If there are massive enough rings of material surrounding the progenitor and planets in their vicinity, arc rings could be formed. If the rings are viewed pole on when the envelope is detached from the central star, it will interact with the arc ring material and produce ansae and pedal and garden-hose-shape structures observed in some planetaries.Paper presented at the Conference onPlanetary Systems: Formation, Evolution, and Detection held 7–10 December, 1992 at CalTech, Pasadena, California, U.S.A.     

Self-consistent photoionization models of planetary nebulae luminescence: stellar ionizing continuum,nebular abundances,and evolution of the envelopes

The modern self-consistent photoionization model of planetary nebula luminescence is described. All of the processes which play an important role in the ionization and thermal equilibrium of the nebular gas are taken into consideration. The diffuse ionizing radiation is taken into account completely. The construction of the model is carried out for the radial distribution of gas density in the nebular envelope which is consistent with isophotal map of the nebula. The application of the model is illustrated on the example of the planetary nebulae BD+30°3639 and NGC 7293. It is shown that the continuum of the central star at 912 Å does not correspond to the blackbody spectrum but agrees with the spectrum of corresponding non-LTE model atmosphere. The radial distributions of electron density, electron temperature, and other parameters in the nebular envelopes are found.The evolution of the radial distribution of gas density in the planetary nebulae envelopes is investigated. Approximative analytical expression which describe both such distribution and its change with time is adjusted. It is shown that the nebular envelope is formed as a result of quiet evolution of the slow stellar wind of star-precursor, and the formation of the envelope begins from the decrease of star-precursor's mass loss rate. Obtained radial distributions of gas density in the envelopes of young nebulae rule out the idea that the planetary nebula is formed as a result of a rapid ejection of clear-cut envelope. So, there is no necessity for the superwind which is used for this purpose in theoretical calculations.A new method of the determination of planetary nebulae abundances is proposed. Unobserved ionization stages are taken into account with aid of the correlations between relative abundances of various ions which had been obtained from the grid of the photoionization models of planetary nebulae luminescence. Simple approximative expressions for the determination of He/H, C/H, N/H, O/H, Ne/H, Mg/H, Si/H, S/H, and Ar/H are found. The chemical composition of 130 Galactic planetary nebulae is revised. A comparative analysis of the abundances in the Galactic disk, bulge, and halo nebulae is carried out.     

Axisymmetry in protoplanetary nebulae: using imaging polarimetry to investigate envelope structure

We use ground-based imaging polarimetry to detect and image the dusty circumstellar envelopes of a sample of protoplanetary nebulae (PPNe) at near-infrared wavelengths. This technique allows the scattered light from the faint envelope to be separated from the glare of the bright central star, and is particularly well suited to this class of object. We detect extended (up to 9-arcsec diameter) circumstellar envelopes around 15 out of 16 sources with a range of morphologies including bipolars and shells. The distribution of scattered light in combination with its polarization (up to 40 per cent) provides unambiguous evidence for axisymmetry in 14 objects, showing this to be a common trait of PPNe. We suggest that the range of observed envelope morphologies results from the development of an axisymmetric dust distribution during the superwind phase at the end of the AGB. We identify shells seen in polarized light with scattering from these superwind dust distributions, which allows us to provide constraints on the duration of the superwind phase. In one object (IRAS 19475+3119) the circumstellar envelope has a two-armed spiral structure, which we suggest results from the interaction of the mass-losing star with a binary companion.     

Planetesimal formation by turbulent concentration

The formation of 1-1000 km diameter planetesimals from dust grains in a protoplanetary disk is a key step in planet formation. Conventional models for planetesimal formation involve pairwise sticking of dust grains, or the sedimentation of dust grains to a thin layer at the disk midplane followed by gravitational instability. Each of these mechanisms is likely to be frustrated if the disk is turbulent. Particles with stopping times comparable to the turnover time of the smallest eddies in a turbulent disk can become concentrated into dense clumps that may be the precursors of planetesimals. Such particles are roughly millimeter-sized for a typical protoplanetary disk. To survive to become planetesimals, clumps need to form in regions of low vorticity to avoid rotational breakup. In addition, clumps must have sufficient self gravity to avoid break up due to the ram pressure of the surrounding gas. Given these constraints, the rate of planetesimal formation can be estimated using a cascade model for the distribution of particle concentration and vorticity within eddies of various sizes in a turbulent disk. We estimate planetesimal formation rates and planetesimal diameters as a function of distance from a star for a range of protoplanetary disk parameters. For material with a solar composition, the dust-to-gas ratio is too low to allow efficient planetesimal formation, and most solid material will remain in small particles. Enhancement of the dust-to-gas ratio by 1-2 orders of magnitude, either vertically or radially, allows most solid material to be converted into planetesimals within the typical lifetime of a disk. Such dust-to-gas ratios may occur near the disk midplane as a result of vertical settling of short-lived clumps prior to clump breakup. Planetesimal formation rates are sensitive to the assumed size and rotational speed of the largest eddies in the disk, and formation rates increase substantially if the largest eddies rotate more slowly than the disk itself. Planetesimal formation becomes more efficient with increasing distance from the star unless the disk surface density profile has a slope of −1.5 or steeper as a function of distance. Planetesimal formation rates typically increase by an order-of-magnitude or more moving outward across the snow line for a solid surface density increase of a factor of 2. In all cases considered, the modal planetesimal size increases with roughly the square root of distance from the star. Typical modal diameters are 100 km and 400 km in the regions corresponding to the asteroid belt and Kuiper belt in the Solar System, respectively.     

Self-similar pressure oscillations in neutron star envelopes as probes of neutron star structure

We study eigenmodes of acoustic oscillations of high multipolarity l ∼ 100–1000 and high frequency (∼100 kHz), localized in neutron star envelopes. We show that the oscillation problem is self-similar. Once the oscillation spectrum is calculated for a given equation of state (EOS) in the envelope and given stellar mass M and radius R , it can be rescaled to a star with any M and R (but the same EOS in the envelope). For l ≳ 300, the modes can be subdivided into the outer and inner ones. The outer modes are mainly localized in the outer envelope. The inner modes are mostly localized near the neutron drip point, being associated with the softening of the EOS after the neutron drip. We calculate oscillation spectra for the EOSs of cold-catalyzed and accreted matter and show that the spectra of the inner modes are essentially different. A detection and identification of high-frequency pressure modes would allow one to infer M and R and determine also the EOS in the envelope (accreted or ground state) providing a new, potentially powerful method to explore the main parameters and internal structure of neutron stars.     

Cooling flows in clusters of galaxies

Summary X-ray images and spectra of clusters of galaxies show strong evidence for cooling flows. In many clusters, the hot gas in the core is cooling at rates of 100Myr^–1 and greater. Few traces of the cooled gas have been observed, but it probably forms into low-mass stars (perhaps brown dwarf or even Jupiter-mass objects). X-ray surface-brightness profiles show that the cooling gas is highly inhomogeneous. Overdense gas cools rapidly to form cooled clumps distributed throughout the flow, with little of the gas ever reaching the cluster centre. Cooled and cooling clumps are disrupted because of their motion relative to the remainder of the gas, tending to produce small cooled fragments and, ultimately, low-mass stars. Large molecular clouds, which are the sites of massive star formation in our galaxy, do not occur in the outer parts of cooling flows. There is evidence of larger gas clumps and the formation of more massive stars in the central few kpc of some cooling flows. It is argued that cooling flows efficiently form dark matter. This has wider implications for the formation of dark matter in massive galaxies.