arXiv subjects

Noam Soker

Publications and source records attributed to Noam Soker.

459 records · Page 26Linked to original sources

arXiv1998

Dust Formation Above Cool Magnetic Spots in Evolved Stars

We examine the structure of cool magnetic spots in the photospheres of evolved stars, specifically asymptotic giant branch (AGB) stars and R Coronae Borealis stars. We find that the photosphere of a cool magnetic spot will be above the surrounding photosphere of AGB stars, opposite to the situation in the sun. This results from the behavior of the opacity, which increases with decreasing temperature, opposite to the behavior of the opacity near the effective temperature of the sun. We analyze the formation of dust above the cool magnetic spots, and suggest that the dust formation is facilitated by strong shocks, driven by stellar pulsations, which run through and around the spots. The presence of both the magnetic field and cooler temperatures make dust formation easier as the shock passes above the spot. We review some observations supporting the proposed mechanism, and suggest further observations to check the model.

astro-ph↗

arXiv1998

A Model for the Outer Rings of SN 1987A

I propose a model for the formation of the two outer rings of SN 1987A. The main new ingredient is a short-lived, few times 10 years, intermediate wind, of velocity of about 100 km/s and total mass of about 10^{-3} Mo - few times 10^{-2} Mo\odot which is concentrated near the equatorial plane. This intermediate wind was formed during the few orbital periods when a binary companion entered the envelope of the progenitor of SN 1987a, when the latter was a red supergiant. The intermediate wind formed a hollowed torus with a horseshoe-shaped cross section. The inner regions at the ends of the horseshoe torus are the two outer rings of SN 1987A. The other regions of the torus were cleaned by the fast wind blown after the progenitor turns into a blue giant. The proposed scenario accounts for: (i) slow motion of the outer rings; (ii) the high density contrast of the outer rings to their surroundings; (iii) the presence of only two outer rings; (iv) the displacement of the rings relative to the central star; (v) the fact that no planetary nebulae have been observed to have a single pair of outer rings.

astro-ph↗

arXiv1998

Magnetic Field, Dust, and Axisymmetrical Mass Loss on the AGB

I propose a mechanism for axisymmetrical mass loss on the asymptotic giant branch, that may account for the axially symmetric structure of elliptical planetary nebulae. The proposed model operates for slowly rotating AGB stars, having angular velocities of 0.0001 the Keplerian velocity. Such angular velocities could be gained from a planet companion of mass 0.1M(Jupiter), or even from single stars which are fast rotators on the main sequence. The model assumes that dynamo magnetic activity results in the formation of cool spots, above which dust forms much easily. The enhanced magnetic activity toward the equator results in a higher dust formation rate there, and hence higher mass loss rate. As the star ascends the AGB, both the mass loss rate and magnetic activity increase rapidly, and hence the mass loss becomes more asymmetrical, with higher mass loss rate closer to the equatorial plane.

astro-ph↗

arXiv1998

Detecting Planets in Planetary Nebulae

We examine the possibility of detecting signatures of surviving Uranus-Neptune-like planets inside planetary nebulae. Planets that are not too close to the stars, orbital separation larger than about 5 AU, are likely to survive the entire evolution of the star. As the star turns into a planetary nebula, it has a fast wind and a strong ionizing radiation. The interaction of the radiation and wind with a planet may lead to the formation of a compact condensation or tail inside the planetary nebula, which emits strongly in Halpha, but not in [OIII]. The position of the condensation (or tail) will change over a time of about 10 years. Such condensations might be detected with currently existing telescopes.

astro-ph↗

arXiv1998

Radiation from a Uniformly Accelerated Charge

The emission of radiation by a uniformly accelerated charge is analyzed. According to the standard approach, a radiation is observed whenever there is a relative acceleraion between the charge and the observer. Analyzing difficulties that arose in the standard approach, we propose that a radaition is created whenever a relative acceleration between the charge and its own electric field exists. The electric field induced by a charge accelerated by an external (nongravitational) force, is not accelerated with the charge. Hence the electric field is curved in the instantanous rest frame of the accelerated charge. This curvature gives rise to a stress force, and the work done to overcome the stress force is the source of the energy carried by the radiation. In this way, the "energy balance paradox" finds its solution.

gr-qc↗

arXiv1998

Axisymmetrical Structures of Planetary Nebulae and SN 1987A

I summarize some recent models and ideas for the formation of axisymmetrical structures of planetary nebulae and the three rings of SN 1987A, as follows. (a) I review the general role of binary companions, including brown dwarfs and planets. (b) I propose a mechanism for axisymmetrical mass loss on the AGB that may account for the axially symmetric structures of elliptical planetary nebulae and that operates for slowly rotating AGB stars. (c) I propose a model for the formation of the two outer rings of SN 1987A, which is based on the numerical simulation of Soker (1989), and discuss a mechanism for their displacement from the exploding star.

astro-ph↗

arXiv1998

Nonthermal Radio Emission from Planetary Nebulae

In a recent analysis of the radio emission from the planetary nebula A30, Dgani, Evans & White (1998) claim that the emission, located in the inner region, is probably dominated by nonthermal emission. We propose a model to explain this. We assume that the fast wind, blown by the central star of A30 carries a very weak magnetic field. The interaction of this wind with a cluster of dense condensations traps the magnetic field lines for a long time and stretches them, leading to a strong magnetic field. If relativistic particles are formed as the fast wind is shocked, then the enhanced magnetic field will result in nonthermal radio emission. The typical nonthermal radio flux at 1 GHz can be up to several milli-Jansky. In order to detect the nonthermal emission, the emitting region should be spatially resolved from the main optical nebula. We list other planetary nebulae which may possess nonthermal radio emission.

astro-ph↗

arXiv1998

Can Planets Influence the Horizontal Branch Morphology?

As stars which have planetary systems evolve along the red giant branch and expand, they interact with the close planets. The planets deposit angular momentum and energy into the red giant stars' envelopes, both of which are likely to enhance mass loss on the red giant branch. The enhanced mass loss causes the star to become bluer as it turns to the horizontal branch. I propose that the presence of planetary systems, through this mechanism, can explain some anomalies in horizontal branch morphologies. In particular, planetary systems may be related to the ``second parameter'', which determines the distribution of horizontal branch stars on the Hertzsprung-Russel diagram. The proposed scenario predicts that surviving massive planets or brown dwarfs orbit many of the extreme blue horizontal branch stars, at orbital periods of tens days.

astro-ph↗

arXiv1997

The Number of Planets Around Stars

Based on the large number of elliptical planetary nebulae I argue that about 55 per cent of all progenitors of planetary nebulae have planets around them. The planets spin up the stars when the later evolve along the red giant branch or along the asymptotic giant branch. The arguments, which were presented in several of my earlier works, and are summarized in the paper, suggest that the presence of four gas-giant planets in the solar system is the generality rather than the exception. I here continue and: (1) examine the possibility of detecting signatures of surviving Saturn-like planets inside planetary nebulae, and, (2) propose a model by which the second parameter of the horizontal branch, which determines the distribution of horizontal branch stars in the HR diagram, is the presence of planets. A red giant branch star that interacts with a planet will lose a large fraction of its envelope and will become a blue horizontal branch star.

astro-ph↗