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Stellar mergers are a brief but common phase in the evolution of binary star systems.sup.1,2. These events have many astrophysical implications; for example, they may lead to the creation of atypical stars (such as magnetic stars.sup.3, blue stragglers.sup.4 and rapid rotators.sup.5), they play an important part in our interpretation of stellar populations.sup.6 and they represent formation channels of compact-object mergers.sup.7. Although a handful of stellar mergers have been observed directly.sup.8,9, the central remnants of these events were shrouded by an opaque shell of dust and molecules.sup.10, making it impossible to observe their final state (for example, as a single merged star or a tighter, surviving binary.sup.11). Here we report observations of an unusual, ring-shaped ultraviolet ('blue') nebula and the star at its centre, TYC 2597-735-1. The nebula has two opposing fronts, suggesting a bipolar outflow of material from TYC 2597-735-1. The spectrum of TYC 2597-735-1 and its proximity to the Galactic plane suggest that it is an old star, yet it has abnormally low surface gravity and a detectable long-term luminosity decay, which is uncharacteristic for its evolutionary stage. TYC 2597-735-1 also exhibits H[alpha] emission, radial-velocity variations, enhanced ultraviolet radiation and excess infrared emission--signatures of dusty circumstellar disks.sup.12, stellar activity.sup.13 and accretion.sup.14. Combined with stellar evolution models, the observations suggest that TYC 2597-735-1 merged with a lower-mass companion several thousand years ago. TYC 2597-735-1 provides a look at an unobstructed stellar merger at an evolutionary stage between its dynamic onset and the theorized final equilibrium state, enabling the direct study of the merging process. Observations and stellar evolution models of a blue ring nebula and its central star (TYC 2597-735-1) suggest that the remnant star merged with a lower-mass companion several thousand years ago., Author(s): Keri Hoadley [sup.1] , D. Christopher Martin [sup.1] , Brian D. Metzger [sup.2] [sup.3] , Mark Seibert [sup.4] , Andrew McWilliam [sup.4] , Ken J. Shen [sup.5] , James [...]

The planetary nebula stage is the ultimate fate of stars with masses one to eight times that of the Sun ([M.sub.[dot encircle]]). The origin of their complex morphologies is poorly [...]

High-redshift Lyman-α (Lyα) blobs (1,2) are extended, luminous but rare structures that seem to be associated with the highest peaks in the matter density of the Universe (3-6). Their energy [...]

Although it is well known that a massive planet opens a gap in a protoplanetary gaseous disk, there is no analytic description of the surface density profile in and near the gap. The simplest approach, which is based upon the balance between the torques due to the viscosity and the gravity of the planet and assumes local damping, leads to gaps with overestimated width, especially at low viscosity. Here, we take into account the fraction of the gravity torque that is evacuated by pressure supported waves. With a novel approach, which consists of following the fluid elements along their trajectories, we show that the flux of angular momentum carried by the waves corresponds to a pressure torque. The equilibrium profile of the disk is then set by the balance between gravity, viscous and pressure torques. We check that this balance is satisfied in numerical simulations, with a planet on a fixed circular orbit. We then use a reference numerical simulation to get an ansatz for the pressure torque, that yields gap profiles for any value of the disk viscosity, pressure scale height and planet to primary mass ratio. Those are in good agreement with profiles obtained in numerical simulations over a wide range of parameters. Finally, we provide a gap opening criterion that simultaneously involves the planet mass, the disk viscosity and the aspect ratio. Keywords: Planetary formation; Jovian planets; Solar nebula; Origin, Solar System

In the absence of global turbulence, solid particles in the solar nebula tend to settle into a thin layer in the central plane. Shear between this layer and pressure-supported gas produces localized turbulence in the midplane; the thickness of the particle layer is determined by balance between settling and turbulent diffusion. A numerical model is described, which allows computation of the vertical structure of a layer of particles of arbitrary size, with self-consistent distributions of particle density, turbulent velocity, and radial fluxes of particles and gas. Effects of varying particle size and the abundances of solids and gas are evaluated. If the surface density of solids is increased by an order of magnitude over nominal solar abundance, the peak density within a layer of small particles can approach the critical value needed for gravitational instability. However, depletion of the nebular gas is much less effective for raising the density of such a layer to the critical value, due to decreased coupling of particles to the gas as the density of the gas decreases. The variation of radial particle flux with surface density of the particle layer is not consistent with secular instability of the layer driven by gas drag. Keywords: Solar nebula; Planetesimals

The high concentration and sorting of chondrules, sub-mm sized spherules found in undifferentiated meteorites, is one of the great unsolved mysteries in planetology. Here we present a unifying explanation for these phenomena based on the assumption that chondrules were present when the Solar Nebula was optically thin but had a significant amount of gas. An immediate consequence is that chondrules feel a force known as photophoresis. Photophoresis is based on a temperature gradient over the surface of a particle resulting from absorption of radiation and non-uniform interaction with its gaseous environment. In comparison to well-known forces originating from starlight, i.e. radiation pressure, Poynting--Robertson drag, or the Yarkovski effect, photophoresis call be stronger by many orders of magnitude in gaseous environments. In the application discussed here photophoresis concentrates chondrules and CAIs, which are both found in chondrites, in the region of the asteroid belt. Chondrules from any place in the Solar Nebula will be dragged to the asteroid belt region, while smaller dust particles and their aggregates will be removed from this region at the same time. This leads to a high relative concentration of chondrules, sorted with respect to their thermal conductivity, density, and size, for building chondrite parent bodies. Furthermore, photophoresis prevents CAIs from being lost to the Sun. Keywords: Meteorites; Solar Nebula; Asteroids; Origin, Solar System

Recent R-matrix calculations of electron impact excitation rates in K v are used to derive the nebular emission line ratio R = I(4122.6 [Angstrom])/I(4163.3 [Angstrom]) as a function of electron density ([N.sub.e]). This ratio is found to be very sensitive to changes in [N.sub.e] over the density range [10.sup.3] to [10.sup.6] [cm.sup.-3] but does not vary significantly with electron temperature, and hence in principle should provide an excellent optical [N.sub.e] diagnostic for the high-excitation zones of nebulae. The observed value of R for the planetary nebula NGC 7027, measured from a spectrum obtained with the Hamilton Echelle spectrograph on the 3-m Shane Telescope, implies a density in excellent agreement with that derived from [Ne IV], formed in the same region of the nebula as [K v]. This observation provides observational support for the accuracy of the theoretical [K v] line ratios, and hence the atomic data on which they are based. However, the analysis of a high-resolution spectrum of the symbiotic star RR Telescopii, obtained with the University College london Echelle Spectrograph on the 3.9-m Anglo-Australian Telescope, reveals that the [K v] 4122.6 [Angstrom] line in this object is badly blended with Fe || 4122.6 [Angstrom]. Hence, the [K v] diagnostic may not be used for astrophysical sources that show a strong Fe || emission line spectrum.

The rotational velocity of a fluid element around the midplane of the solar nebula increased as dust settled toward the midplane. The Kelvin and Helmholtz instability due to velocity difference of a dust-rich region and a dust-poor region should have occurred and the dust layer became turbulent when the Richardson number decreased below the critical value. Then, dust aggregations stirred up due to turbulent diffusion and were prevented to settle further. In this paper, the sizes of dust aggregations are assumed to be equal to or smaller than the typical radius of chondrules ([approximately]0.3 mm). In this case, even very weak turbulence stirs up dust aggregations. Therefore a dust density distribution is considered to be self regulated so that the Richardson number is nearly equal to the critical value. The quasi-equilibrium dust density distribution is derived analytically by assuming that the Richardson number is equal to the critical value. The derived dust density at the midplane is much smaller than the critical density of the gravitational stability, if the solar composition of dust to gas ratio is assumed. On the other hand, the dust aggregations concentrate around the midplane and the dust layer becomes gravitationally unstable, if more than 97% (at 1 AU from the Sun) of the gaseous components have been dissipated from the nebula, leaving dusty components. Two alternative scenarios of planetesimal formation are proposed: planetesimals were formed by (1) mutual sticking of dust aggregations by nongravitational forces or by (2) gravitational instabilities in the nebula where the dust to gas ratio is much larger than the ratio with solar elemental abundance. Case (2) might be realized due to dissipation of the nebular gas and/or addition of dust by the bipolar outflow. In case (1), chondrule sizes do not indicate the maximum size of dust aggregations in the solar nebula. Key Words: planetary formation; planetesimals; solar nebula.

We investigated the long-term evolution of the spatial and velocity distribution of a planetesimal swarm around a protoplanet, calculating gas drag force and gravitational force of the protoplanet. Mutual scattering between planetesimals is taken into account, which removes the effect of mean motion resonances with the protoplanet. We simulated orbital evolution of 2000 planetesimals around a protoplanet for [approximately][10.sup.5] years and found that scattering by a protoplanet, coupled with the gas drag, cleans up planetesimals in the feeding zone of the protoplanet. Our results show that planetesimals are trapped outside the feeding zone without the help of resonances. Sufficiently small planetesimals, on which the gas drag effectively acts, can remain in the feeding zone. We obtained the trapping condition in this nonresonance case. It is almost the same as the condition obtained by Kary et al. (1993, Icarus 106, 288307), who considered 'resonance trapping.' Our results mean that the trapping of planetesimals does not depend on the strength of resonances against mutual scattering between planetesimals. Furthermore, the cleanup of the planetesimals occurs rapidly, so that the protoplanet cannot feed most of the planetesimals before the cleanup and growth of the protoplanet will slow down. That is, the effect of gas drag halts rather than prompts protoplanet growth by supply of material due to radial migration. We also examined the multiple-protoplanet case and considered the effect of gas on the formation of protoplanets.