arXiv subjects

Noam Soker

Publications and source records attributed to Noam Soker.

At least 433 records · Page 24Linked to original sources

arXiv2002

Local Circumstellar Magnetic Fields Around Evolved Stars

I argue that the presence of magnetic fields around evolved stars, e.g., asymptotic giant branch stars, and in PNe, does not necessarily imply that the magnetic field plays a global dynamical role in shaping the circumstellar envelope. Instead, I favor magnetic fields with small coherence lengths, which result from stellar magnetic spots or from jets blown by an accreting companion. Although the magnetic field does not play a global role in shaping the circumstellar envelope, it may enhance local motion (turbulent) via magnetic tension and reconnection. The locally strong magnetic tension may enforce coherence flow which may favor the masing process.

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arXiv2002

Planetary Nebulae in the Scheme of Binary Evolution

In this review I present the binary model for the shaping of planetary nebulae (PNe) as I view it, in the context of historical evolution of other models for the shaping of PNe over more than 30 years. In describing the binary model, I concentrate on works published since the last IAU meeting on PNe. I think stellar companions are behind the shaping of bipolar PNe, i.e., having two lobes with an equatorial waist between them, and extreme elliptical PNe, e.g., having a dense equatorial ring, but no lobes. The question of whether a planet is required to spin the progenitor in order to form a moderate elliptical PN, or whether a single stars can form moderate elliptical PNe, I consider to be an open question.

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arXiv2002

Hot Bubbles in Cooling Flow Clusters

As more cooling flow clusters of galaxies with central radio sources are observed with the Chandra and XMM-Newton X-ray Observatories, more examples of "bubbles" (low-emission regions in the X-ray coincident with radio emission) are being found. These bubbles are surrounded by bright shells of X-ray emission, and no evidence of current strong shocks has yet been found. Using an analytic approach and some simplifying assumptions, we derive expressions relating the size and location of a bubble, as well as the density contrast between the bubble and the ambient medium, with the shock history of the bubble. These can be applied straightforwardly to new observations. We find that existing observations are consistent with a mild shock occurring in the past, and with the bulk of the cool material in the X-ray shells being cooled at the cluster center and then pushed outward by the radio source. Strong shocks are ruled out unless they occurred more than 1 Gyr ago. We also discuss Rayleigh-Taylor instabilities as well as the case of a bubble expanding into an older bubble produced from a previous cycle of radio activity.

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arXiv2002

Formation of Double Rings Around Evolved Stars

I propose a scenario for the formation of double-ring systems, as observed in some planetary nebulae and in the outer rings of SN 1987A. In this scenario, two jets, one jet on each side of the equatorial plane, expand into a thin, dense shell. Such a flow is expected in binary systems where the mass-losing primary undergoes an impulsive mass loss episode which forms a thin, dense shell. I assume that a small fraction of that mass is accreted onto a companion which blows the jets. Each jet accelerates shell's material it hits sideways, forming a higher density ring. Using several simplifying assumptions, I derive an expression for the radius of the ring, which depends relatively weakly on the jets and impulsive mass loss episode properties. This shows that such a scenario is feasible. If there are several such impulsive mass loss episodes, more double rings will be formed, as observed in the planetary nebulae MyCn 18 (the Hourglass nebula). Because of the binary interaction and orbital motion, the double-ring system is displaced from the symmetry axis of the main nebula, as observed.

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arXiv2002

The Early Interaction of the Planetary Nebula NGC 40 with the Interstellar Medium

We report the detection of an Halpha emission-line structure in the upstream side of the planetary nebula NGC 40, which is predicted by numerical simulations, and which is attributed to Rayleigh-Taylor instability. Such a Rayleigh-Taylor instability is expected to occur at early stages of the interaction process between the interstellar medium (ISM) and a fast moving planetary nebula, as is the case for NGC 40. We resolved the Rayleigh-Taylor instability `tongues', as well as the flatness of the nebula around the `tongues', which results from the deceleration by the ISM.

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arXiv2001

A Superwind from Early Post-Red Giant Stars?

We suggest that the gap observed at 20,000 K in the horizontal branches of several Galactic globular clusters is caused by a small amount of extra mass loss which occurs when stars start to "peel off" the red giant branch (RGB), i.e., when their effective temperature starts to increase, even though they may still be on the RGB. We show that the envelope structure of RGB stars which start to peel off is similar to that of late asymptotic giant branch stars known to have a super-wind phase. An analogous super-wind in the RGB peel-off stars could easily lead to the observed gap in the distribution of the hottest HB stars.

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arXiv2001

X-Ray Emission from Central Binary Systems of Planetary Nebulae

We study the conditions under which a main sequence binary companion to the central ionizing star of a planetary nebula (PN) might become magnetically active and thereby display strong X-ray luminosity. Since most PNe are older than few billion years, any main sequence companion will rotate too slowly to have magnetic activity and hence bright X-ray emission, unless it is spun-up. We demonstrate that if the orbital separation during the AGB phase of the PN progenitor is less than 30-60 AU, main sequence companions in the spectral type range F7 to M4 will accrete enough angular momentum from the AGB wind to rotate rapidly, become magnetically active, and exhibit strong X-ray luminosities. Lower mass M stars and brown dwarfs can also become magnetically active, but they should have small orbital separations and hence are less likely to survive the AGB phase of the progenitor. We estimate that 20-30 per cent of elliptical PNe and 30-50 per cent of bipolar PN are likely to have magnetically active companions which will reveal themselves in X-ray observations. Re-analysis of Chandra X-ray Observatory spectroscopy of the compact central source of NGC 7293 indicates that the emitting region of this object possesses abundance anomalies similar to those of coronally active main-sequence stars.

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arXiv2001

Formation of Planetary Nebula Lobes by Jets

I conduct an analytical study of the interaction of jets, or a collimated fast wind (CFW), with a previously blown asymptotic giant branch (AGB) slow wind. Such jets (or CFWs) are supposedly formed when a compact companion, a main sequence star or a white dwarf, accretes mass from the AGB star, forms an accretion disk, and blows two jets. This type of flow, which is thought to shape bipolar planetary nebulae (PNe), requires 3-dimensional gas dynamical simulations, which are limited in the parameter space they can cover. By imposing several simplifying assumptions, I derive simple expressions which reproduce some basic properties of lobes in bipolar PNe, and which can be used to guide future numerical simulations. I quantitatively apply the results to two proto-PNe. I show that the jet interaction with the slow wind can form lobes which are narrow close to, and far away from, the central binary system, and which are wider somewhere in between. Jets that are recollimated and have constant cross section can form cylindrical lobes with constant diameter, as observed in several bipolar PNe. Close to their source, jets blown by main sequence companions are radiative; only further out they become adiabatic, i.e., they form high-temperature low-density bubbles that inflate the lobes. This implies that radiative cooling must be incorporated in numerical codes intended to study the formation of lobes in PNe.

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arXiv2001

Collisions of Free Floating Planets with Evolved Stars in Globular Clusters

We estimate the rate of collisions between stars and free-floating planets (FFPs) in globular clusters, in particular the collision of FFPs with red giant branch (RGB) stars. Recent dynamical simulations imply that the density of such objects could exceed million per cubic parsec near the cores of rich globular clusters. We show that in these clusters 5-10 per cents of all RGB stars near the core would suffer a collision with a FFP, and that such a collision can spin up the RGB star's envelope by an order of magnitude. In turn, the higher rotation rates may lead to enhanced mass-loss rates on the RGB, which could result in bluer horizontal branch (HB) stars. Hence, it is plausible that the presence of a large population of FFPs in a globular cluster can influence the distribution of stars on the HB of that cluster to a detectable degree.

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arXiv2001

On the Formation of Multiple-Shells Around Asymptotic Giant Branch Stars

Two types of models for the formation of semi-periodic concentric multiple shells (M-shells) around asymptotic giant branch (AGB) stars and in planetary nebulae are compared against observations. Models that attribute the M-shells to processes in an extended wind acceleration zone around AGB stars result in an optically thick acceleration zone, which reduces the acceleration efficiency in outer parts of the extended acceleration zone. This makes such models an unlikely explanation for the formation of M-shells. Models which attribute the M-shell to semi-periodic variation in one or more stellar properties are most compatible with observations. The only stellar variation models on time scales of 50-1500 years that have been suggested are based on an assumed solar-like magnetic cycle. Although ad-hoc, the magnetic cycle assumption fits naturally into the increasingly popular view that magnetic activity plays a role in shaping the wind from upper AGB stars.

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arXiv2001

Why Every Bipolar Planetary Nebula is `Unique'

We present the many evolutionary routes that progenitors of bipolar planetary nebulae (BPNe) can take. Overall, there are about a hundred qualitatively different evolutionary routes, hence about a hundred qualitatively different types of BPNe. Within each type there are quantitative differences as well. Adding the dependence of the appearance on inclination, we find that the number of different apparent structures of BPN is about equal to, or even larger than the number of known BPNe and proto-BPNe. Accordingly we argue that every BPN is a `unique' object in its appearance, but all can be explained within the binary model paradigm. Therefore, we request a stop to the attaching of adjectives such as `unique', `peculiar', and `unusual' to BPNe and proto-BPNe, thereby removing the need to invoke a new model for almost every `unusual' BPN. As a case study we try to build a binary model for the proto-BPN OH231.8+4.2. In our preferred model the AGB Mira-type star has a main sequence companion of mass 1 Mo, orbital period of five years, and eccentricity of 0.1.

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arXiv2001

Classification of Planetary Nebulae by their Departure from Axisymmetry

We propose a scheme to classify planetary nebulae (PNe) according to their departure from axisymmetric structure. We consider only departure along and near the equatorial plane, i.e., between the two sides perpendicular to the symmetry axis of the nebula. We consider 6 types of departure from axisymmetry: (1) PNe whose central star is not at the center of the nebula; (2) PNe having one side brighter than the other; (3) PNe having unequal size or shape of the two sides; (4) PNe whose symmetry axis is bent, e.g., the two lobes in bipolar PNe are bent toward the same side; (5) PNe whose main departure from axisymmetry is in the outer regions, e.g., an outer arc; (6) PNe which show no departure from axisymmetry, i.e., any departure, if it exists, is on scales smaller than the scale of blobs, filaments, and other irregularities in the nebula. We discuss the connection between departure types and the physical mechanisms that may cause them, mainly due to the influence by a stellar binary companion. We find that about 50 percents of all PNe possess large-scale departure from axisymmetry. This number is larger than that expected from the influence of binary companions, namely 25-30 percents. We argue that this discrepancy comes from many PNe whose departure from axisymmetry, mainly unequal size, shape, or intensity, results from the presence of long-lived and large, hot or cool, spots on the surface of their AGB progenitors. Such spots locally enhance mass loss rate, leading to a deparure from axisymmetry, mainly near the equator, in the descendant PN.

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arXiv2001

Turbulent Dynamo in Asymptotic Giant Branch Stars

Using recent results on the operation of turbulent dynamos, we show that a turbulent dynamo can amplify a large scale magnetic field in the envelopes of asymptotic giant branch (AGB) stars. We propose that a slow rotation of the AGB envelope can fix the symmetry axis, leading to the formation of an axisymmetric magnetic field structure. Unlike solar-type alpha-omega dynamos, the rotation has only a small role in amplifying the toroidal component of the magnetic field. The large-scale magnetic field is strong enough for the formation of magnetic cool spots on the AGB stellar surface. The spots can regulate dust formation, hence mass loss rate, leading to axisymmetric mass loss and the formation of elliptical planetary nebulae (PNe). Despite its role in forming cool spots, the large scale magnetic field is too weak to play a dynamic role and directly influence the wind from the AGB star. We find other problems in models where the magnetic field plays a dynamic role in shaping the AGB winds, and argue that they cannot explain the formation of nonspherical PNe.

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arXiv2001

A Model for the Strings of Eta Carinae

We propose a model based on ionization shadows to explain the formation of the long and narrow strings of Eta Carinae. Five strings are known, all located along the symmetry axis outside the Homunculus. The model assumes that each string is formed in a shadow behind a dense clump near the symmetry axis. The surrounding gas is ionized first, becomes much hotter, and compresses the gas in the shadow. This leads to the formation of a radial, dense, long, and narrow region, i.e., a string. Later the neutral material in the strings is ionized, and becomes brighter. Still later it re-expands, and we predict that in about 200 years the strings will fade. The condition for the model to work is that the ionization front, due to the diffuse ionizing recombination radiation of the surrounding gas, proceeds into the shadow at a velocity slower than the compression speed. From that we get a condition on the mass loss rate of the mass loss episode that formed the strings, which should be less than 10^{-4} Mo/year. The model can also explain the strings in the planetary nebula NGC 6543.

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arXiv2001

Spherical Planetary Nebulae

By examining their mass loss history and their distribution in the galaxy I argue that spherical planetary nebulae (PNe) form a special group among all planetary nebulae. The smooth surface brightness of most spherical PNe suggests that their progenitors did not go through a final intensive wind (FIW, also termed superwind) phase. While 70 per cent of the PNe of all other PNe groups are closer to the galactic center than the sun is, only 30 per cent of spherical PNe are. These, plus the well known high scale height above the galactic plane of spherical PNe, suggest that the progenitors of spherical PNe are low mass stars having low metallicity. Although many stars have these properties, only about 10 per cent of all PNe are spherical. By comparing the galactic distribution of spherical PNe to the metallicity evolution in the galaxy, I find that the critical metallicity above which no spherical PNe are formed is [Fe/H] ~ -0.4. I explain this as well as other properties of spherical PNe in the context of the companion model for shaping PNe, arguing that spherical PNe are formed from stars which had no close companion, stellar or substellar, orbiting them.

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arXiv2001

Backflow in Post-Asymptotic Giant Branch Stars

We derive the conditions for a backflow toward the central star(s) of circumstellar material to occur during the post-asymptotic giant branch (AGB) phase. The backflowing material may be accreted by the post-AGB star and/or its companion, if such exists. Such a backflow may play a significant role in shaping the descendant planetary nebula, by, among other things, slowing down the post-AGB evolution, and by forming an accretion disk which may blow two jets. We consider three forces acting on a slowly moving mass element: the gravity of the central system, radiation pressure, and fast wind ram pressure. We find that for a significant backflow to occur, a slow dense flow should exsist, such that the relation between the total mass in the slow flow, M, and the solid angle it covers, Omega, is given by (4*pi*M/Omega)>0.1Mo. The requirement for both high mass loss rate per unit solid angle and a very slow wind, such that it can be decelerated and flow back, probably requires close binary interaction.

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arXiv2001

The Departure of Eta Carinae from Axisymmetry and the Binary Hypothesis

I argue that the large scale departure from axisymmetry of the Eta Carinae nebula can be explained by the binary stars model of Eta Carinae. The companion diverts the wind blown by the primary star, by accreting from the wind and possibly by blowing its own collimated fast wind (CFW). The effect of these processes depends on the orbital separation, hence on the orbital phase of the eccentric orbit. The variation of the mass outflow from the binary system with the orbital phase leads to a large-scale departure from axisymmetry along the equatorial plane, as is observed in Eta Car. I further speculated that such a companion may have accreted a large fraction of the mass that was expelled in the Great Eruption of 1850 and the Lesser Eruption of 1890. The accretion process was likely to form an accretion disk, with the formation of a CFW, or jets, on the two sides of the accretion disk. The CFW may have played a crucial role in the formation of the two lobes.

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arXiv2001

Discovery of Extended X-ray Emission from the Planetary Nebula NGC 7027 by the Chandra X-ray Observatory

We report the discovery of X-ray emission from NGC 7027, a prototypical object for the study of the formation and evolution of planetary nebulae (PNs). Observations with the Advanced CCD Imaging Spectrometer (ACIS) aboard the Chandra X-ray Observatory show that the X-ray emission from NGC 7027 is extended and is bipolar in morphology. The ACIS spectrum displays strong emission from highly ionized Ne and weaker emission features which we attribute to O, Mg, and Si. Model fits to this spectrum suggest a characteristic temperature T_x ~ 3x10^6 K and an intrinsic (unabsorbed) X-ray luminosity of L_x ~ 1.3x10^32 ergs/s. The intranebular absorption of X-ray emission is highly nonuniform, but the modeling indicates an average column density N_H ~ 6x10^21 cm^-2, consistent with previous measurements of relatively large visual extinction within the nebula. We suggest that the X-ray emission from NGC 7027 is or was generated by a hitherto undetected fast wind from the central star of NGC 7027, or from a companion to this star. Chandra's detection of extended, high-temperature X-ray emission from BD +30 3639, NGC 6543, and now NGC 7027 suggests that such emission is a common feature of young planetary nebulae.

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