arXiv subjects

Noam Soker

Publications and source records attributed to Noam Soker.

At least 451 records · Page 25Linked to original sources

arXiv2001

Departure from Axisymmetry in Planetary Nebulae

Many planetary nebulae (PNe) exhibit distinctly non-axisymmetric structure in either (i) the shape of the nebula, or (ii) in the off-centered position of the illuminating star. By examining a large number of well resolved images of PNe we estimate that about 30-50 percents of all PNe exhibit distinctly non-axisymmetric structure. In this paper, we discuss how such departures from axisymmetry can arise from the binary nature of the progenitors of the PNe. The scenarios include (a) relatively close binaries with eccentric orbits, and (b) longer orbital period systems with either circular or eccentric orbits. In order to assess the fraction of PNe whose non-axisymmetric morphologies are expected to arise in binary systems, we have carried out a detailed population synthesis study. The expected deviations from axisymmetry are classified for each binary and the results tabulated. We find that about 25 percents of elliptical and 30-50 percents of bipolar PNe are expected to acquire non-axisymmetric structure from binary interactions.

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arXiv2001

Collimated Fast Winds in Wide Binary Progenitors of Planetary Nebulae

We discuss the formation of planetary nebulae (PNe) having a pair of lobes, or multi-lobes, in their inner region surrounded by an elliptical or spherical shell or halo. We suggest that most of these PNe are formed by wide binary systems, with final orbital periods in the range of 40-10,000 years, such that there is no strong tidal interaction. The outer more spherical structure is formed from the early asymptotic giant branch (AGB) wind. Toward the end of the AGB the mass loss rate increases, and wind velocity possibly decreases, making the conditions for the formation of an accretion disk around the wide companion, more favorable. We assume that the accreting companion blows jets or a collimated fast wind (CFW), which lead to the formation of a pair of lobes in the inner region. In cases of a precessing accretion disk, a multi-lobes structure can be formed. We conduct a population synthesis study of such systems, and find that overall 5-20 percents of all PNe are formed by such binary systems. We also estimate that 20-40 percents of these systems possess observable departure from axisymmetry, e.g., the central star is not in the center of the nebula.

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arXiv2000

Planets and Axisymmetric Mass Loss

Bipolar planetary nebulae (PNe), as well as extreme elliptical PNe are formed through the influence of a stellar companion. But half of all PN progenitors are not influenced by any stellar companion, and, as I show here, are expected to rotate very slowly on reaching the upper asymptotic giant branch; hence they expect to form spherical PNe, unless they are spun-up. But since most PNe are not spherical, I argue that about 50 percents of AGB stars are spun-up by planets, even planets having a mass as low as 0.01 times the mass of Jupiter, so they form elliptical PNe. The rotation by itself will not deform the AGB wind, but may trigger another process that will lead to axisymmetric mass loss, e.g., weak magnetic activity, as in the cool magnetic spots model. This model also explains the transition from spherical to axisymmetric mass loss on the upper AGB. For such low mass planets to substantially spin-up the stellar envelope, they should enter the envelope when the star reaches the upper AGB. This "fine-tuning" can be avoided if there are several planets on average around each star, as is the case in the solar system, so that one of them is engulfed when the star reaches the upper AGB.

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arXiv2000

The "Twin Jet" Planetary Nebula M2-9

We present a model for the structure, temporal behavior, and evolutionary status of the bipolar nebula M2-9. According to this model the system consists of an AGB or post-AGB star and a hot white dwarf companion, with an orbital period of about 120 years. The white dwarf has undergone a symbiotic nova eruption about 1200 years ago, followed by a supersoft x-ray source phase. The positional shift of the bright knots in the inner nebular lobes is explained in terms of a revolving ionizing source. We show that the interaction between the slow, AGB star's wind, and a collimated fast wind from the white dwarf clears a path for the ionizing radiation in one direction, while the radiation is attenuated in others. This results in the mirror-symmetric (as opposed to the more common point-symmetric) shift in the knots. We show that M2-9 provides an important evolutionary link among planetary nebulae with binary central stars, symbiotic systems, and supersoft x-ray sources.

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arXiv2000

Chandra X-ray Observatory Detection of Extended X-ray Emission from the Planetary Nebula BD+303639

We report the detection of well resolved, extended X-ray emission from the young planetary nebula BD+303639 using the Advanced CCD Imaging Spectrometer (ACIS) aboard the Chandra X-ray Observatory. The X-ray emission from BD+303639 appears to lie within, but is concentrated to one side of, the interior of the shell of ionized gas seen in high-resolution optical and IR images. The relatively low X-ray temperature (Tx ~ 3x10^6 K) and asymmetric morphology of the X-ray emission suggests that conduction fronts are present and/or mixing of shock-heated and photoionized gas has occurred and, furthermore, hints at the presence of magnetic fields. The ACIS spectrum suggests that the X-ray emitting region is enriched in the products of helium burning. Our detection of extended X-ray emission from BD+303639 demonstrates the power and utility of Chandra imaging as applied to the study of planetary nebulae.

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arXiv2000

A Moderate Cluster Cooling Flow Model

We propose that the outer portions of cooling flows (CFs) in clusters of galaxies are frequently disrupted by radio jets, and that their effective ages are much shorter than the cluster ages. The inner regions, where the gas density is higher, are more difficult to disrupt, and may continue to harbor CFs even after disruption events. The main assumption of the proposed scenario is that, on a time scale of about 2-4 times 10^9 years the cD galaxies in CF clusters undergo powerful bursts of AGN activity that produce strong radio jets. The jets excite shock waves moving at several thousands km/sec and heat the CF region, hence terminating it in the outer regions. The total accreted mass due to the CF is an order of magnitude lower than the mass accreted according to the "standard" CF model (which assumes an undisturbed CF for a time equal to the age of the cluster). The scenario, therefore, brings to agreement the observations that a large fraction of clusters harbor CFs, that strong optical and radio activity are present only in the very inner regions of CFs, and the lack of a satisfactory reservoir of the expected (in the standard CF model) large mass that has been cooling over the life of the cluster.

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arXiv2000

The `Second Parameter': A Memory from the Globular Cluster Formation Epoch

We study the correlations between the distribution of stars on the horizontal branch (HB), the HB morphology, with some other properties of globular clusters (GCs) in a large sample of GCs. We strengthen previous results that a general correlation exists only between HB morphology and metallicity. Correlations with other properties, e.g., central density and core radius, exist only for GCs within a narrow metallicity range. We conjecture that the lack of correlations with present properties of GCs (besides metallicity), is because the variation of the HB morphologies between GCs having similar metallicities is caused by a process, or processes, whose effect was determined at the formation time of GCs. This process (or processes) is historically termed the `second parameter', metallicity being the `first parameter'. We then argue that the `planet second parameter' model, where the presence of planets and to a lesser degree brown dwarfs and low mass main sequence stars is the `second parameter', fits this conjecture.

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arXiv2000

Extrasolar Planets and the Rotation and Axisymmetrical mass loss of evolved stars

I examine the implications of the recently found extrasolar planets on the planet-induced axisymmetrical mass loss model for the formation of elliptical planetary nebulae (PNs). This model, which was developed in several earlier papers by the author and a few collaborators, attributes low departure from spherical mass loss of upper asymptotic giant branch (AGB) stars to envelope rotation which results from deposition of planet's orbital angular momentum. It was predicted that about 50 percent of all sun-like stars have Jupiter-like planets around them. In light of the new finding that only 5 percent of sun-like stars do have such planets I revise this prediction. I now predict that indeed about 50 percent of PNs progenitors do have close planets around them, but the planets can have much lower masses, as low as 0.01 times Jupiter's mass. To support this claim I follow the angular momentum evolution of single stars as they evolve to the post-AGB phase. The prediction that on average several such planets orbit each star, as in the solar system, still holds.

astro-ph↗

arXiv2000

Eccentric Orbits of close Companions to Asymptotic Giant Branch Stars

I propose that the relatively high eccentricity found in some tidally strongly interacting binary systems, where the mass-losing star is an evolved giant star, e.g., an asymptotic giant branch star, is caused by an enhanced mass loss rate during periastron passages. Tidal interaction by itself will circularize the orbits of these systems in a relatively short time, hence a mechanism which increases the eccentricity on a shorter time scale is required. The proposed scenario predicts that the nebula formed by the mass loss process possesses a prominent departure from axisymmetrical structure. Such a departure is observed in the Red Rectangle, which has a central binary system, HD 44179, with an orbital period of 318 days, and an eccentricity of e=0.38.

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arXiv2000

Solar-Like Cycle in Asymptotic Giant Branch Stars

I propose that the mechanism behind the formation of concentric semi-periodic shells found in several planetary nebulae (PNs) and proto-PNs, and around one asymptotic giant branch (AGB) star, is a solar-like magnetic activity cycle in the progenitor AGB stars. The time intervals between consecutive ejection events is about 200-1,000 years, which is assumed to be the cycle period (the full magnetic cycle can be twice as long, as is the 22-year period in the sun). The magnetic field has no dynamical effects; it regulates the mass loss rate by the formation of magnetic cool spots. The enhanced magnetic activity at the cycle maximum results in more magnetic cool spots, which facilitate the formation of dust, hence increasing the mass loss rate. The strong magnetic activity implies that the AGB star is spun up by a companion, via a tidal or common envelope interaction. The strong interaction with a stellar companion explains the observations that the concentric semi-periodic shells are found mainly in bipolar PNs.

astro-ph↗

arXiv1999

Asymmetry and Inhomogeneity in Proto and Young Planetary Nebulae

I study some effects of aspherical mass loss during the last stages of the asymptotic giant branch (AGB) on the appearance of proto planetary nebulae (PNs) and young PNs. The aspherical mass loss can be small scale inhomogeneities, and/or axially symmetric mass loss geometry. I first examine the role of the dust opacity in the optical band on the appearance of proto-PNs. I conclude that large optical depth will be found in proto-PNs which are descendant of AGB stars having high equatorial mass loss rates, which require a stellar binary companion for their existence. These proto-PNs will become bipolar-PNs, i.e., PNs with two lobes an an equatorial waist between them, or extreme elliptical PNs. I then derive the conditions for the enhancement of non-radial density inhomogeneity by the propagation of the ionization front at the early PN stages. The ionization will proceed faster in the radial direction along low density regions. The low density regions will be heated earlier, and they will expand due to their higher pressures, reducing further their densities. The opposite occurs in high density regions.

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arXiv1999

The Formation of Very Narrow Waist Bipolar Planetary Nebulae

We discuss the interaction of the slow wind blown by an asymptotic giant branch (AGB) star with a collimated fast wind (CFW) blown by its main sequence or white dwarf companion, at orbital separations in the range of several AU to about 200 AU. The CFW results from accretion of the AGB wind into an accretion disk around the companion. The fast wind is collimated by the accretion disk. We argue that such systems are the progenitors of bipolar planetary nebulae and bipolar symbiotic nebulae with a very narrow equatorial waist between the two polar lobes. We conduct a population synthesis study of the formation of planetary nebulae in binary systems which quantitatively supports the proposed model. We show the number of expected systems that blow a CFW is in accord with the number found from observations, to within the many uncertainties involved. Overall, we find that 5 per cent of all planetary nebulae are bipolars with very narrow waists. Our population synthesis not only supports the CFW model, but more generally supports the binary model for the formation of bipolar planetary nebulae.

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arXiv1999

Rotation, Planets, and the "Second Parameter" of the Horizontal Branch

We analyze the angular momentum evolution from the red giant branch (RGB) to the horizontal branch (HB) and along the HB. Using rotation velocities for stars in the globular cluster M13, we find that the required angular momentum for the fast rotators is up to 1-3 orders of magnitude (depending on some assumptions) larger than that of the sun. Planets of masses up to five times Jupiter's mass and up to an initial orbital separation of about 2 AU are sufficient to spin-up the RGB progenitors of most of these fast rotators. Other stars have been spun-up by brown dwarfs or low-mass main sequence stars. Our results show that the fast rotating HB stars must have been spun-up by companions while they evolved on the RGB. The angular momentum considerations presented in this paper further supports the "planet second parameter" model. In this model the "second parameter" process is interaction with low-mass companions, in most cases gas giant planets, and in a minority of cases with brown dwarfs or low-mass main sequence stars.

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arXiv1999

Radiation from a Charge in a Gravitational Field

When an electric charge is supported at rest in a static gravitational field, its electric field is not supported with the charge, and it falls freely in the gravitational field. Drawing the electric field lines continuously in time, we find that they always emerge from the charge, but the electric field is curved and there is a stress force between the freely falling (curved) field and the static charge. The charge radiates and the work done by the gravitational field to overcome the stress force is the source for the energy radiated by the supported (static) charge. {\it A static charge in a gravitational field radiates, as predicted by the principle of equivalence}. This mechanism is similar to the one applied to an electric charge accelerated in a free space. In this case, the electric field is not accelerated with the charge. The electric field is curved, and there is a stress force between the charge and its field. The work done in overcoming the stress force is the source of the energy radiated by the accelerated charge.

physics.class-ph↗

arXiv1999

The Transition to Axisymmetrical Mass Loss

Any model for the formation of elliptical planetary nebulae (PNs) should account for the positive correlation between the mass loss rate and the degree of departure from sphericity of the AGB progenitor's wind. I propose that this correlation results from dust formation above magnetic cool spots. The model deals with elliptical PNs, but not with bipolar PNs. The basic assumption is that a weak dynamo amplifies magnetic fields in AGB stars, such that magnetic cool spots are formed, mainly near the equatorial plane. Enhanced dust formation above these cool spots leads to a higher mass loss rate in the equatorial plane, resulting in the formation of an elliptical PN. The dust formation above cool spots becomes much more efficient when mass loss rate is high. In addition to explaining the correlation, the model has the advantage that it can operate for very slowly rotating AGB stars, having rotation velocities of less than $10^{-4}$ times the break-up velocity. Such velocities can be achieved by a planet companion of mass $\sim 0.1 M_{\rm Jupiter}$ which spins-up the envelope, or even from singly evolved stars which leave the main sequence with a high rotation velocity. The sporadic nature of magnetic cool spots also leads to the formation of filaments, arcs, and clumps in the descendant PN. The model cannot explain the presence of jets and ansae in elliptical PNs. These are attributed to the destruction of a planet or brown dwarf on the AGB core.

astro-ph↗

arXiv1999

Dust Formation and Inhomogeneous Mass Loss from AGB Stars

We examine the flow from asymptotic giant branch (AGB) stars when along a small solid angle the optical depth due to dust is very large. We consider two types of flows. In the first, small cool spots are formed on the surface of slowly rotating AGB stars. During the last AGB phase when mass loss rate is high, the dust shields the region above it from the stellar radiation. This leads to both further dust formation in the shaded region, and the convergence of the stream toward the shaded region. A concentration of magnetic cool spots toward the equator will lead to a density contrast of up to a few between the equatorial and polar directions. This process can explain the positive correlation between high mass loss rate and a larger departure from sphericity in progenitors of elliptical planetary nebulae. In the second type of flow, the high density in the equatorial plane is formed by a binary interaction, where the secondary star is close to, but outside the AGB envelope. The shielding of the radiation by dust results in a very slow and dense flow in the equatorial plane. We suggest this flow as an alternative explanation for the equatorial dense matter found at several hundred astronomical units around several post-AGB binary systems.

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arXiv1999

Stellar Structure and Mass Loss on the Upper AGB

We examine the envelope properties of asymptotic giant branch (AGB) stars as they evolve on the upper AGB and during the early post-AGB phase. Due to the high mass loss rate, the envelope mass decreases by more than an order of magnitude. This makes the density profile below the photosphere much shallower, and the entropy profile much steeper. We discuss the possible role of these changes in the profiles in the onset of the high mass loss rate (superwind) and the large deviation from spherical mass loss at the termination of the AGB. We concentrate on the idea that the shallower density profile and steeper entropy profile allow the formation of cool magnetic spots, above which dust forms much more easily.

astro-ph↗

arXiv1999

Visual Wide Binaries and the Structure of Planetary Nebulae

In a recent work Ciardullo et al. (1999) list 19 planetary nebulae surveyed by the Hubble Space Telescope for the presence of resolved visual binary companions of their central stars. For ten planetary nebulae they argue for probable physical association of the resolved stars with the central stars, while for nine the association is less likely. Such stellar companions, at orbital separations of hundreds to thousands of astronomical units, will cause the structures of these planetary nebulae to possess a non-axisymmetrical signatures. By using images from the literature of these 19 planetary nebulae, I demonstrate that the structures of the planetary nebulae are compatible in most cases with Ciardullo et al.'s arguments for an association, or not, of the resolved stars with the planetary nebulae central stars. This shows that the departure, or not, of a planetary nebula from having pure axisymmetrical structure can be used to strengthen an argument for an association, or a non-association, of a putative wide companion with the stellar progenitor of the nebula.

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