Imaging of the shell galaxies NGC 474 and 7600, and implications for their formation

We present photometric observations of two shell galaxies, NGC 474 and 7600. We examine the photometric colours and surface brightnesses of the shells and their host galaxies, and the isophotal parameters of each galaxy. In the case of NGC 474, we find that the shell formation is consistent with a merger origin, although it is possible that the close companion NGC 470 is contributing to the shell system via mass transfer. NGC 7600 exhibits shell geometry and colours which also favour a merger origin.     

Triggered star formation in expanding shells

We discuss fragmentation processes which induce star formation in the dense walls of expanding shells. We test the influence of the energy input, the interstellar medium scaleheight and the speed of sound in the ambient medium, and formulate the condition for the gravitational fragmentation of expanding shells: if the total surface density of the disc is higher than a certain critical value, the shells are unstable. The value of the critical density depends on the energy of the shell and the sound speed in the interstellar medium.     

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.     

Non-linear hydrodynamical evolution of rotating relativistic stars: numerical methods and code tests

We present numerical hydrodynamical evolutions of rapidly rotating relativistic stars, using an axisymmetric, non-linear relativistic hydrodynamics code. We use four different high-resolution shock-capturing (HRSC) finite-difference schemes (based on approximate Riemann solvers) and compare their accuracy in preserving uniformly rotating stationary initial configurations in long-term evolutions. Among these four schemes, we find that the third-order piecewise parabolic method scheme is superior in maintaining the initial rotation law in long-term evolutions, especially near the surface of the star. It is further shown that HRSC schemes are suitable for the evolution of perturbed neutron stars and for the accurate identification (via Fourier transforms) of normal modes of oscillation. This is demonstrated for radial and quadrupolar pulsations in the non-rotating limit, where we find good agreement with frequencies obtained with a linear perturbation code. The code can be used for studying small-amplitude or non-linear pulsations of differentially rotating neutron stars, while our present results serve as testbed computations for three-dimensional general-relativistic evolution codes.     

Thin-Shell Magnetohydrodynamic Equations for the Solar Tachocline

We present a new system of equations designed to study global-scale dynamics in the stably-stratified portion of the solar tachocline. This system is derived from the 3D equations of magnetohydrodynamics in a rotating spherical shell under the assumption that the shell is thin and stably-stratified (subadiabatic). The resulting thin-shell model can be regarded as a magnetic generalization of the hydrostatic primitive equations often used in meteorology. It is simpler in form than the more general anelastic or Boussinesq equations, making it more amenable to analysis and interpretation and more computationally efficient. However, the thin-shell system is still three-dimensional and as such represents an important extension to previous 2D and shallow-water approaches. In this paper we derive the governing equations for our thin-shell model and discuss its underlying assumptions, its context relative to other models, and its application to the solar tachocline. We also demonstrate that the dissipationless thin-shell system conserves energy, angular momentum and magnetic helicity.     

Numerical model for the SNR DEM L316: simulated X-ray emission

We present a two-component supernova remnant model for the case of Magellanic remnant DEM L316 obtained from two-dimensional, axisymmetric hydrodynamic simulations. We study different scenarios which consider a possible collision between the shells and also the effects of thermal conduction. Synthetic X-ray maps were obtained from numerical results in order to directly compare with the observed morphology of this object. We find a good agreement is achieved when thermal conduction is included, reproducing both the observed morphology and the total X-ray luminosity very well. Finally, our results suggest that the two components of DEM L316 are not in physical interaction.     

The Triggered Star Formation in Rotating Disks

The gravitational instability of expanding shells triggering the formation of clouds and stars is analyzed. Disks with different scale-heights, ambient and shell velocity dispersions, mid-plane densities, rotation rates and shear rates are explored with three dimensional numerical simulations in the thin shell approximation. Three conditions for the shell collapse are specified: the first is that it happens before a significant blow-out, the second requires that the shell collapses before it is distorted by Coriolis forces and shear, and the third requires that the internal pressure in the accumulated gas is small and the fragmentation is achieved within the expansion time. The gas-rich and slowly rotating galaxies are the best sites of the triggered star formation, concluding that its importance has been much larger at the times of galaxy formation compared to the present epoch. This revised version was published online in September 2006 with corrections to the Cover Date.     

Scattered emission from a relativistic outflow and its application to gamma-ray bursts

We investigate a scenario of photon scattering by electrons within a relativistic outflow. The outflow is composed of discrete shells with different speeds. One shell emits radiation for a short duration. Some of this radiation is scattered by the shell(s) behind. We calculate in a simple two-shell model the observed scattered flux density as a function of the observed primary flux density, the normalized arrival time delay between the two emission components, the Lorentz factor ratio of the two shells and the scattering shell's optical depth. Thomson scattering in a cold shell and inverse Compton scattering in a hot shell are both considered. The results of our calculations are applied to the gamma-ray bursts and the afterglows. We find that the scattered flux from a cold slower shell is small and likely to be detected only for those bursts with very weak afterglows. A hot scattering shell could give rise to a scattered emission as bright as the X-ray shallow decay component detected in many bursts, on a condition that the isotropically equivalent total energy carried by the hot electrons is large, ∼10⁵²–10⁵⁶ erg. The scattered emission from a faster shell could appear as a late short γ-ray/MeV flash or become part of the prompt emission depending on the delay of the ejection of the shell.     

BV photometry of five shell galaxies

Current views consider shell structures as bona fide signatures of a recent minor/major merging event though also weak interaction models (WIM) could produce long lasting shells on host galaxies possessing a stellar thick disc.We present a B V band photometric study of a sample of 5 shell galaxies belonging to the Malin & Carter (1983) compilation. The structural properties and colors of the galaxies, as well as the colors of their shells are examined in detail. We did not find signatures of the presence of double nuclei. NGC 7585 is the only E galaxy in the sample and has a moderately boxy structure. The other galaxies have either a discy structure or are mixed E/S0 type galaxies. NGC 474 is a true lenticular. NGC 6776 shows a diffuse asymmetric outer structure and a system of tails of the the same color of the galaxy body; but not clear shells. In general, the color of the shells in our sample is similar or slightly redder than that of the host galaxy, whose color, in turn, is typical of the early‐type morphological class. One of the outer shells of NGC 474 is significantly bluer than the body of the galaxy. Since NGC 474 appears to be interacting with NGC 470, the color of this one shell could be explained as result of a recent acquisition of material through tidal interaction. The WIM hypothesis could explain both the red and the blue shells of NGC 474, this latter acquired from the fly‐by of the nearby companion NGC 470, but the lack of the constancy of shell surface brightness as a ratio of the underlying galaxy brightness argues against WIM. We speculate about evidence, which also comes from different observations, that suggests a merging/accretion origin of the shells. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

An analytic solution to the hypersonic, radiative blunt body problem

We present an analytic model for the thin-shell, radiative interaction between a hypersonic, plane-parallel wind and a rigid, spherical obstacle. This problem has clear applications, e.g., to the interaction of winds from young stars and dense cloudlets, and to the interaction of the wind from a binary partner with the photosphere of the second star. We also present a comparison of the analytic model with a full, axisymmetric numerical simulation. We find only a partial agreement between the numerical simulation and the analytic model, apparently as a result of the very strong 'thin-shell' instabilities of the post-bow shock flow. Our analytic model predicts the surface density, flow velocity and the energy radiated per unit area, as well as the total luminosity of the bow shock. The model can therefore be used directly for carrying out comparisons with observations of different astrophysical objects.     

Scattered light models of the dust shell around HD 179821

The dust shell around the evolved star HD 179821 has been detected in scattered light in near-IR imaging polarimetry observations. Here, we subtract the contribution of the unpolarized stellar light to obtain an intrinsic linear polarization of between 30 and 40 per cent in the shell that seems to increase with radial offset from the star. The J - and K -band data are modelled using a scattering code to determine the shell parameters and dust properties. We find that the observations are well described by a spherically-symmetric distribution of dust with an r ⁻² density law, indicating that when mass-loss was occurring, the mass-loss rate was constant. The models predict that the detached nature of a spherically-symmetric, optically-thin dust shell, with a distinct inner boundary, will only be apparent in polarized flux. This is in accordance with the observations of this and other optically-thin circumstellar shells, such as IRAS 17436+5003. By fitting the shell brightness we derive an optical depth to the star that is consistent with V -band observations and that, assuming a distance of 6 kpc, gives an inner-shell radius of     , a dust number density of     at r _in and a dust mass of     . We have explored axisymmetric shell models but conclude that any deviations from spherical symmetry in the shell must be slight, with an equator-to-pole density contrast of less than 2:1. We have not been able to fit simultaneously the high linear polarizations and the small     colour excess of the shell and we attribute this to the unusual scattering properties of the dust. We suggest that the dust grains around HD 179821 either are highly elongated or consist of aggregates of smaller particles.     

Numerical study of the adiabatic index effect for the Vishniac instability in supernova remnants

The Vishniac instability is supposed to explain the fragmentation of the thin shell of shocked matter in the radiative phase of supernova remnants. However its implication and its consequence on the morphological evolution of stellar systems is not fully demonstrated. The present paper tackles this subject by numerical simulations and focus on the role of the adiabatic index in the instability growth. The HYDRO-MUSCL 2D hydrodynamics code has been used to simulate the evolution of a supernova remnant thin shell and the triggering of the Vishniac instability in this thin shell. We have studied the temporal behavior of the perturbation. The first result of the numerical study is the existence of the Vishniac instability in the simulations. This result is proved by the overstability process observed in the simulations as predicted by the theoretical analysis. The second important result is the damping of the perturbation at late evolution and for all the set of parameters. Indeed the accretion of matter onto the shock damps the instability when theoretical analysis predicts its occurrence.     

Spectroscopy of six highly evolved Abell planetary nebulae

We have undertaken visual spectroscopy of the highly evolved planetary nebulae (PNe) A8, A13, A62, A72, A78 and A83 over a wavelength range  4330 < λ < 6830 Å  . This permits us to specify relative line intensities in various sectors of the nebular shells, and to investigate the variation of emission as a function of radius. We determine that the spectrum of the central star of A78 has varied appreciably over a period of 25 yr. There is now evidence for strong P Cygni absorption in the λ4589 and λ5412 transitions of He  ii , implying terminal velocities of the order of   V _∞≅ 3.83 × 10³ km s⁻¹  . We also note that the emission-line profiles of the sources can be used to investigate their intrinsic emission structures. We find that most PNe show appreciable levels of emission throughout their volumes; only one source (A13) possesses a thin-shell structure. Such results are in conformity with evolutionary theory, and probably reflect the consequences of adiabatic cooling in highly evolved outflows.     

On a Formation Scenario of Star Clusters

Most formation scenarios of globular clusters assume a molecular cloud as the progenitor of the stellar system. However, it is still unclear, how this cloud is transformed into a star cluster, i.e. how the destructive processes related to gas removal or low star formation effiency can be avoided. Here a scheme of supernova (SN) induced cluster formation is studied. According to this scenario an expanding SN shell accumulates the mass of the cloud. This is accompanied by fragmentation resulting in star formation in the shell. Provided the stellar shell expands sufficiently slow, its self-gravity stops the expansion and the shell recollapses, by this forming a stellar system. I present N-body simulations of collapsing shells which move in a galactic potential on circular and elliptic orbits. It is shown that typical shells (10⁵ M_⊙, 30 pc) evolve to twin clusters over a large range of galactocentric distances. Outside this range single stellar systems are formed, whereas at small galactocentric distances the shells are tidally disrupted. In that case many small fragments formed during the collapse survive as single bound entities. About 1/3 of the twin cluster systems formed on circular orbits merge within 400 Myr. On elliptic orbits the merger rate reduces to less than 4%. Thus, there could be a significant number of twin clusters even in our Galaxy, which, however, might be undetected as twins due to a large phase shift on their common orbit. This revised version was published online in September 2006 with corrections to the Cover Date.     

Formation of Asteroid Families by Catastrophic Disruption: Simulations with Fragmentation and Gravitational Reaccumulation

This paper builds on preliminary work in which numerical simulations of the collisional disruption of large asteroids (represented by the Eunomia and Koronis family parent bodies) were performed and which accounted not only for the fragmentation of the solid body through crack propagation, but also for the mutual gravitational interaction of the resulting fragments. It was found that the parent body is first completely shattered at the end of the fragmentation phase, and then subsequent gravitational reaccumulations lead to the formation of an entire family of large and small objects with dynamical properties similar to those of the parent body. In this work, we present new and improved numerical simulations in detail. As before, we use the same numerical procedure, i.e., a 3D SPH hydrocode to compute the fragmentation phase and the parallel N-body code pkdgrav to compute the subsequent gravitational reaccumulation phase. However, this reaccumulation phase is now treated more realistically by using a merging criterion based on energy and angular momentum and by allowing dissipation to occur during fragment collisions. We also extend our previous studies to the as yet unexplored intermediate impact energy regime (represented by the Flora family formation) for which the largest fragment's mass is about half that of the parent body. Finally, we examine the robustness of the results by changing various assumptions, the numerical resolution, and different numerical parameters. We find that in the lowest impact energy regime the more realistic physical approach of reaccumulation leads to results that are statistically identical to those obtained with our previous simplistic approach. Some quantitative changes arise only as the impact energy increases such that higher relative velocities are reached during fragment collisions, but they do not modify the global outcome qualitatively. As a consequence, these new simulations confirm previous main results and still lead to the conclusion that: (1) all large family members must be made of gravitationally reaccumulated fragments; (2) the original fragment size distribution and their orbital dispersion are respectively steeper and smaller than currently observed for the real families, supporting recent studies on subsequent evolution and diffusion of family members; and (3) the formation of satellites around family members is a frequent and natural outcome of collisional processes.     

The circumstellar envelopes of Be stars: viscous disc dynamics

We have constructed models of axisymmetric, circumstellar envelopes for Be star discs by successfully combining two numerical codes: a non-local thermodynamic equilibrium (non-LTE) radiative transfer code which calculates the level populations and disc temperature distribution self-consistently, and a hydrodynamical code. The output of one code is used as input to the other code, and hence evolving density and thermal structures may be examined. The temperatures, disc density and velocity distribution are used to investigate the outflowing viscous disc model for Be stars. We find that these simulations place constraints on the power-law density decrease in the disc with increasing distance from the star. We find that the power-law index for the line-forming region of the disc lies between 3 and 3.5 with a small dispersion.     

Heating of the circumstellar medium by gamma-ray burst prompt emission

We describe the technique and results of our numerical simulation of the effects related to the heating of the circumstellar medium by hard gamma-ray burst radiation using a modified STELLA radiation-hydrodynamics code. The code modification allows the processes of nonstationary heating and change in the state of the matter to be taken into account. We present the computed light curves and emergent gamma-ray, X-ray, and optical spectra for several models of the circumstellar medium (shells) with different geometrical sizes, densities, density profiles, chemical compositions, and temperatures. Depending on the model parameters, the total thermal and optical luminosities of the heated shell can reach 10⁴⁷ and 10⁴³ erg s⁻¹, respectively. The presence of bumps in the X-ray and optical GRB afterglow light curves can be explained by such effects.     

Measuring galaxy potentials using shell kinematics

We show that the kinematics of the shells seen around some elliptical galaxies provide a new, independent means for measuring the gravitational potentials of elliptical galaxies out to large radii. A numerical simulation of a set of shells formed in the merger between an elliptical and a smaller galaxy reveals that the shells have a characteristic observable kinematic structure, with the maximum line-of-sight velocity increasing linearly as one moves inward from a shell edge. A simple analytic calculation shows that this structure provides a direct measure of the gradient of the gravitational potential at the shell radius. In order to extract this information from attainable data, we have also derived a complete distribution of line-of-sight velocities for material within a shell; comparing the observed spectra of a shell to a stellar template convolved with this distribution will enable us to measure the gradient of the potential at this radius. Repeating the analysis for a whole series of nested shells in a galaxy allows the complete form of the gravitational potential as a function of radius to be mapped out. The requisite observations lie within reach of the up-coming generation of large telescopes.     

Bursting and quenching in massive galaxies without major mergers or AGNs

In this work, we study the formation and evolution of dark matter haloes by means of the spherical infalling model with shell-crossing. We present a framework to tackle this effect properly based on the numerical follow-up, with time, of that individual shell of matter that contains always the same fraction of mass with respect to the total mass. In this first step, we do not include angular momentum, velocity dispersion or triaxiality. Within this framework – named as the spherical shell tracker – we investigate the dependence of the evolution of the halo with virial mass, with the adopted mass fraction of the shell, and for different cosmologies. We find that our results are very sensitive to a variation of the halo virial mass or the mass fraction of the shell that we consider. However, we obtain a negligible dependence on cosmology. Furthermore, we show that the effect of shell-crossing plays a crucial role in the way that the halo reaches the stabilization in radius and the virial equilibrium. We find that the values currently adopted in the literature for the actual density contrast at the moment of virialization,  δ_vir  , may not be accurate enough. In this context, we stress the problems related to the definition of a virial mass and a virial radius for the halo. The question of whether the results found here may be obtained by tracking the shells with an analytic approximation remains to be explored.     

A New Model for Propagating Parts of EIT Waves: A Current Shell in a CME

EIT waves are observed in EUV as bright fronts. Some of these bright fronts propagate across the solar disk. EIT waves are all associated with a flare and a CME and are commonly interpreted as fast-mode magnetosonic waves. Propagating EIT waves could also be the direct signature of the gradual opening of magnetic field lines during a CME. We quantitatively addressed this alternative interpretation. Using two independent 3D MHD codes, we performed nondimensional numerical simulations of a slowly rotating magnetic bipole, which progressively result in the formation of a twisted magnetic flux tube and its fast expansion, as during a CME. We analyse the origins, the development, and the observability in EUV of the narrow electric currents sheets that appear in the simulations. Both codes give similar results, which we confront with two well-known SOHO/EIT observations of propagating EIT waves (7 April and 12 May 1997), by scaling the vertical magnetic field components of the simulated bipole to the line of sight magnetic field observed by SOHO/MDI and the sign of helicity to the orientation of the soft X-ray sigmoids observed by Yohkoh/SXT. A large-scale and narrow current shell appears around the twisted flux tube in the dynamic phase of its expansion. This current shell is formed by the return currents of the system, which separate the twisted flux tube from the surrounding fields. It intensifies as the flux tube accelerates and it is co-spatial with weak plasma compression. The current density integrated over the altitude has the shape of an ellipse, which expands and rotates when viewed from above, reproducing the generic properties of propagating EIT waves. The timing, orientation, and location of bright and faint patches observed in the two EIT waves are remarkably well reproduced. We conjecture that propagating EIT waves are the observational signature of Joule heating in electric current shells, which separate expanding flux tubes from their surrounding fields during CMEs or plasma compression inside this current shell. We also conjecture that the bright edges of halo CMEs show the plasma compression in these current shells.