arXiv subjects

Noam Soker

Publications and source records attributed to Noam Soker.

At least 397 records · Page 22Linked to original sources

arXiv2005

Interaction of young stellar object jets with their accretion disk

I reexamine recent observations of velocity profiles across jets blown by young stellar objects, and argue that the observations do not support the interpretation of jets rotating around their symmetry axes. Instead, I propose that the interaction of the jets with a twisted-tilted (wrapped) accretion disk can form the observed asymmetry in the jets' line of sight velocity profiles. The proposed scenario is based on two plausible assumptions. (1) There is an inclination between the jet and the outer parts of the disk; the jet is perpendicular to the inner part of the disk; Namely, there is a twisted-tilted (wrapped) disk. (2) The disk-jet interaction slows down the jet as the jet entrains mass from the disk, with larger decelaration of jet segments closer to the tilted disk. The proposed scenario can account for the basic properties of the observed velocity profiles, while having the advantage that there is no need to refer to any magnetic jet launching model, and there is no need to invoke jet rotation with a huge amount of angular momentum.

astro-ph↗

arXiv2004

Feedback Heating with Slow Jets in Cooling Flow Clusters

We propose a scenario in which a large fraction, or even most, of the gas cooling to low temperatures of T<10^4 K in cooling flow clusters, directly gains energy from the central black hole. Most of the cool gas is accelerated to non-relativistic high velocities, v ~ 10^3-10^4 km/sec, after flowing through, or close to, an accretion disk around the central black hole. A poorly collimated wind (or double not-well collimated opposite jets) is formed. According to the proposed scenario, this gas inflates some of the X-ray deficient bubbles, such that the average gas temperature inside these bubbles (cavities) in cooling flow clusters is kT_b ~< 100 keV. A large fraction of these bubbles will be very faint, or not detectable, in the radio. The bright rims of these weak smaller bubbles will appear as ripples. We suggest that the X-ray ripples observed in the Perseus cluster, for example, are not sound waves, but rather the rims of radio-faint weak bubbles which are only slightly hotter than their environment. This scenario is incorporated into the moderate cooling flow model; although not a necessary ingredient in that model, it brings it to better agreement with observations. In the moderate cooling flow model a cooling flow does exist, but the mass cooling rate is ~<10% of that in old versions of cooling flow models.

astro-ph↗

arXiv2004

The Binarity of Eta Carinae and its Similarity to Related Astrophysical Objects

I examine some aspects of the interaction between the massive star Eta Carinae and its companion, in particular during the eclipse-like event, known as the spectroscopic event or the shell event. The spectroscopic event is thought to occur when near periastron passages the stellar companion induces much higher mass loss rate from the primary star, and/or enters into a much denser environment around the primary star. I find that enhanced mass loss rate during periastron passages, if it occurs, might explain the high eccentricity of the system. However, there is not yet a good model to explain the presumed enhanced mass loss rate during periastron passages. In the region where the winds from the two stars collide, a dense slow flow is formed, such that large dust grains may be formed. Unlike the case during the 19th century Great Eruption, the companion does not accrete mass during most of its orbital motion. However, near periastron passages short accretion episodes may occur, which may lead to pulsed ejection of two jets by the companion. The companion may ionize a non-negligible region in its surrounding, resembling the situation in symbiotic systems. I discuss the relation of some of these processes to other astrophysical objects, by that incorporating Eta Car to a large class of astrophysical bipolar nebulae.

astro-ph↗

arXiv2004

Why there are no jets from cataclysmic variable stars

We argue that the recent thermal model of jet launching by young stellar objects, when applied to system containing disk-accreting white dwarfs naturally explain the otherwise astonishing absence of jets in cataclysmic variable stars. Thermal launching is possible when the accreted material is strongly shocked due to large gradients of physical quantities in the boundary layer (or at the inner boundary of a truncated disk) and then cools on a time scale longer than its ejection time from the disk. In our framework the magnetic fields are weak, and serve only to recollimate the outflow at large distances from the source, or to initiate the shock, but not as a jet-driving agent. Using criteria for shock formation and mass ejection, we find the mass accretion rate above which jets can be launched from boundary layers around accreting white dwarfs to be ~10^{-6} Mo/y, which explains the absence of jets in cataclysmic variable stars and their presence in other white-dwarf accreting systems such as super-soft X-ray sources, symbiotic stars and classical novae.

astro-ph↗

arXiv2004

The Shaping of the Red Rectangle Proto-Planetary Nebula

I argue that the slowly expanding biconical structure of the Red Rectangle--a nebula around the post asymptotic giant branch binary stellar system HD 44179--can be formed by intermittent jets blown by the accreting companion. The bright biconical structure of the Red Rectangle nebula can be understood to be composed of a multiple double-ring system. In the proposed shaping process, one among several processes through which a companion can shape the circumbinary gas, the companion accretes mass from the slow wind blown by the evolved mass losing star. An accretion disk is formed, and if mass accretion is larger than a critical value, two jets, or a collimated fast wind (CFW) are blown. If the high mass loss rate duration is long, bipolar lobes are formed. If, on the other hand, mass loss rate is intermittent, and during one orbital period the slow wind fills a region which does not extend much beyond the binary system, then only a fraction of the double-lobe structure is formed, namely, rings. This, I propose, was the case with the progenitor of the Red Rectangle, where intermittent episodes of enhanced mass loss rate led to the formation of a multiple double-ring system.

astro-ph↗

arXiv2004

X-rays from the Mira AB Binary System

We present the results of XMM-Newton X-ray observations of the Mira AB binary system, which consists of a pulsating, asymptotic giant branch primary and nearby (~0.6'' separation) secondary of uncertain nature. The EPIC CCD (MOS and pn) X-ray spectra of Mira AB are relatively soft, peaking at ~1 keV, with only very weak emission at energies > 3 keV; lines of Ne IX, Ne X, and O VIII are apparent. Spectral modeling indicates a characteristic temperature T_X ~ 10^7 K and intrinsic luminosity L_X ~ 5x10^29 erg s^{-1}, and suggests enhanced abundances of O and, possibly, Ne and Si in the X-ray-emitting plasma. Overall, the X-ray spectrum and luminosity of the Mira AB system more closely resemble those of late-type, pre-main sequence stars or late-type, magnetically active main sequence stars than those of accreting white dwarfs. We conclude that Mira B is most likely a late-type, magnetically active, main-sequence dwarf, and that X-rays from the Mira AB system arise either from magnetospheric accretion of wind material from Mira A onto Mira B, or from coronal activity associated with Mira B itself, as a consequence of accretion-driven spin-up. One (or both) of these mechanisms also could be responsible for the recently discovered, point-like X-ray sources within planetary nebulae.

astro-ph↗

arXiv2004

Cooling of X-ray Emitting Gas by Heat Conduction in the Center of Cooling Flow Clusters

We study the possibility that a large fraction of the gas at temperatures of \~10^7 K in cooling flow clusters cools by heat conduction to lower temperatures, rather than by radiative cooling. We argue that this process, when incorporated into the so-called "moderate cooling flow model", where the effective age of the intracluster medium is much lower than the age of the cluster, reduces substantially the expected X-ray luminosity from gas residing at temperatures of <10^7 K. In this model, the radiative mass cooling rate of gas at ~10^7 K inferred from X-ray observations, which is <20 % of the mass cooling rates cited in the past, is easily met. The heat conduction is regulated by reconnection between the magnetic field lines in cold (~10^4 K) clouds and the field lines in the intracluster medium. A narrow conduction front is formed, which, despite the relatively low temperature, allows efficient heat conduction from the hot ICM to the cold clouds. The reconnection between the field lines in cold clouds and those in the intracluster medium occurs only when the magnetic field in the ICM is strong enough. This occurs only in the very inner regions of cooling flow clusters, at r~10-30 kpc. The large ratio of the number of Hαphotons to the number of cooling hydrogen atoms is explained by this scenario.

astro-ph↗

arXiv2004

Why a Single-Star Model Cannot Explain the Bipolar Nebula of Eta Carinae

I examine the angular momentum evolution during the 1837-1856 Great Eruption of the massive star Eta Carinae. I find that the new estimate of the mass blown during that eruption implies that the envelope of Eta Car substantially spun-down during the 20 years eruption. Single-star models, most of which require the envelope to rotate close to the break-up velocity, cannot account for the bipolar nebula (the Homunculus) formed from matter expelled in that eruption. The kinetic energy and momentum of the Homunculus further constrains single-star models. I discuss how Eta Car can fit into a unified model for the formation of bipolar lobes where two oppositely ejected jets inflate two lobes (or bubbles). These jets are blown by an accretion disk, which requires stellar companions in the case of bipolar nebulae around stellar objects.

astro-ph↗

arXiv2004

The absence of jets in cataclysmic variable stars

We show that the recently developed thermal model which successfully describes how jets are launched by young stellar objects, when applied to system containing disk-accreting white dwarfs naturally explain the otherwise surprising absence of jets in cataclysmic variable stars. Our main argument uses the crucial element of the thermal model, namely that the accreted material is strongly shocked due to large gradients of physical quantities in the boundary layer, and then cools on a time scale longer than its ejection time from the disk. In our scenario the magnetic fields are weak, and serve only to recollimate the outflow at large distances from the source, or to initiate the shock, but not as a jet-driving agent. Using two criteria in that model, for the shock formation and for the ejection of mass, we find the mass accretion rate above which jets could be blown from accretion disks around young stellar objects and white dwarfs. We find that these accretion mass rates are ~10^{-7} Mo/yr, and ~10^{-6} Mo/yr for young stellar objects and white dwarfs respectively. Considering the uncertainties of the model, these limits could overestimate the critical value by a factor of ~10.

astro-ph↗

arXiv2004

Constraining the X-ray Luminosities of Asymptotic Giant Branch Stars: TX Cam and T Cas

To probe the magnetic activity levels of asymptotic giant branch (AGB) stars, we used XMM-Newton to search for X-ray emission from two well-studied objects, TX Cam and T Cas. The former star displays polarized maser emission indicating magnetic field strengths of B ~ 5 G; the latter is one of the nearest known AGB stars. Neither star was detected by XMM-Newton. We use the upper limits on EPIC (CCD detector) count rates to constrain the X-ray luminosities of these stars, and derive L_X < 10^{31} erg s^{-1} (<10^{30} erg s^{-1}) for an assumed X-ray emission temperature T_X = 3x10^6 K (10^7 K). These limits represent <~ 10% (<~ 1%) of the X-ray luminosity expected under models in which AGB magnetic fields are global and potentially play an important role in collimating and/or launching AGB winds. We suggest, instead, that the B field strengths inferred from maser observations are representative of localized, magnetic clouds.

astro-ph↗

arXiv2004

Wind Accretion by a Binary Stellar System and Disk Formation

I calculate the specific angular momentum of mass accreted by a binary system embedded in the dense wind of a mass losing asymptotic giant branch star. The accretion flow is of the Bondi-Hoyle-Lyttleton type. For most of the relevant parameters space the flow is basically an isothermal high Mach number accretion flow. I find that when the orbital plane of the accreting binary system and the orbital plane of the triple system are not parallel to each other, the accreted mass onto one or two of the binary system components has high specific angular momentum. For a large fraction of triple-star systems, accretion disks will be formed around one or two of the stars in the binary system, provided that the mass ratio of the two stars in the accreting binary system is >0.5. Such disks may blow jets which shape the descendant planetary nebula. The jets' axis will be almost parallel to the orbital plane of the triple-star system. One jet is blown outward relative to the wind, while the other jet pass near the mass losing star, and is more likely to be slowed down or deflected. I find that during the final asymptotic giant branch phase, when mass loss rate is very high, accretion disk may form for orbital separation between the accreting binary systems and the mass losing star of up to 400-800 AU. I discuss the implications for the shape of the descendant planetary nebula, and list several planetary nebulae which may have been shaped by an accreting binary star system, i.e., by a triple star system.

astro-ph↗

arXiv2003

Bubbles in Planetary Nebulae and Clusters of Galaxies: Instabilities at Bubble Fronts

I study the stability of off-center low-density more or less spherical (fat) bubbles in clusters of galaxies and in planetary nebulae (PNs) to Rayleigh-Taylor (RT) instability. As the bubble expands and decelerates, the interface between the low-density bubble's interior and the dense shell formed from the accreted ambient medium is RT-stable. If, however, in a specific direction the density decreases such that this segment is accelerated by the pressure inside the bubble, then this accelerated region is RT-unstable. The outermost region, relative to the center of the system, is the most likely to become unstable because there the density gradient is the steepest. Using simple analytical analysis, I find that off-center fat bubbles in PNs are much less stable than in clusters. In PNs bubbles become unstable when they are very small relative to their distance from the center; they can be stabilized somewhat if the mass loss rate from the stellar progenitor decreases for a time, such that the negative density gradient is much shallower. In clusters fat bubbles become unstable when their size is comparable to their distance from the center. I discuss some implications of this instability in clusters and in PNs.

astro-ph↗

arXiv2003

Shaping Planetary Nebulae and Related Objects

I review some open questions and other aspects concerning the shaping of planetary nebulae (PNs) and related objects. I attribute the non-spherical structures of PNs to binary companions, stellar or substellar. I emphasize the role of jets (or collimated fast wind: CFW) blown by an accreting stellar companion in shaping PNs and some related objects, and discuss the limited role of magnetic fields. I speculate that some stars which are leaving the asymptotic giant branch, i.e., becoming hotter, possess long-term (10-1000 years) oscillations; these may be related to semi-periodic concentric rings. I end with a list of objects whose shaping is related to the shaping processes of PNs, from young stellar objects to clusters of galaxies.

astro-ph↗

arXiv2003

Energy and Angular Momentum Deposition During Common Envelope Evolution

I consider three processes which enhance mass loss rate from a common envelope of a giant star with a main sequence or a white dwarf companion spiraling-in inside its envelope. I consider deposition of orbital energy and orbital angular momentum to the giant's envelope, and in more detail the formation of jets by an accreting companion and their propagation in the envelope. I find that in many cases the deposition of orbital angular momentum to the envelope may be more important to the mass loss process than the deposition of orbital energy. Jets blown by an accreting companion, in particular a white dwarf, orbiting inside the outer regions of the giant's envelope may also dominate over orbital energy deposition at early stage of the common envelope evolution. These imply that studies which ignore the deposition of angular momentum to the envelope and the effects of the accreting companion may reach wrong conclusions.

astro-ph↗

arXiv2003

Cooling by Heat Conduction Inside Magnetic Flux Loops and the Moderate Cluster Cooling Flow Model

I study non-radiative cooling of X-ray emitting gas via heat conduction along magnetic field lines inside magnetic flux loops in cooling flow clusters of galaxies. I find that such heat conduction can reduce the fraction of energy radiated in the X-ray band by a factor of 1.5-2. This non-radiative cooling joins two other proposed non-radiative cooling processes, which can be more efficient. These are mixing of cold and hot gas, and heat conduction initiated by magnetic fields reconnection between hot and cold gas. These processes when incorporated into the moderate cooling flow model lead to a general cooling flow model with the following ingredients. (1) Cooling flow does occur, but with a mass cooling rate about 10 times lower than in old versions of the cooling flow model. Namely, heating occurs such that the effective age of the cooling flow is much below the cluster age, but the heating can't prevent cooling altogether. (2) The cooling flow region is in a non-steady state evolution. (3) Non-radiative cooling of X-ray emitting gas can bring the model to a much better agreement with observations. (4) The general behavior of the cooling flow gas, and in particular the role played by magnetic fields, make the intracluster medium in cooling flow clusters similar in some aspects to the active solar corona.

astro-ph↗

arXiv2003

The Egg Nebula (AFGL 2688): Deepening Enigma

Recent observations of the Egg Nebula (AFGL 2688), obtained at ever-increasing spatial and spectral resolution, have revealed a perplexing array of phenomena. Many of these phenomena present challenges to our understanding of this object as an emerging, bipolar planetary nebula. Here, we consider two particularly intriguing aspects of the Egg: the peculiar structure and kinematics of its equatorial regions, and the nature of an apparent widely separated companion to the central star. In the first case, we use recently acquired Hubble Space Telescope images to demonstrate that the H2 emission distributed east and west of the central star is spatially coincident with a dusty, equatorial disk or torus. The H2 is thus constrained to lie near the equatorial plane, casting doubt on pure radial outflow models for the equatorial kinematics. In the second case, we show that the apparent companion (``Peak A'') may be an accreting white dwarf that has undergone one or more thermonuclear bursts.

astro-ph↗

arXiv2003

Influence of Planets on Parent Stars: Angular Momentum

I review some possible processes by which planets and brown dwarfs can influence the evolution of their parent evolved stars. As sun-like stars evolve on the red giant branch (RGB) and then on the asymptotic giant branch (AGB), they will interact with their close planets (if exist). The interaction starts with tidal interaction: this will lead the planets to deposit most of their angular momentum to the envelope of the giant, and then spiral-in to the envelope. (Too many papers dealing with close planets [less than about 3-6 AU] around evolved stars neglect tidal interaction, hence their results are questionable.) They may spin-up their parent stars by up to several orders of magnitude. The interaction of substellar objects with evolved star may enhance the mass loss rate, mainly in the equatorial plane. Possible outcomes are: (i) Planetary systems interacting with their parent AGB star may lead to the formation of moderate elliptical planetary nebulae. (ii) RGB stars which lose more mass turn to bluer horizontal branch (HB) stars. Therefore, planets may explain the formation of blue HB stars. This may explain the presence of many blue HB stars in many globular clusters (the planets be the second parameter), and some hot HB stars in the galaxy (sdB stars). (The 8.3 days use of the Hubble Space Telescope in search of planets in a globular clusters with no blue HB stars was a wrong move.) (iii) Most known stars with planets will not form planetary nebulae, because they will lose most of their envelope already on the RGB.

astro-ph↗

arXiv2003

Bubbles in Planetary Nebulae and Clusters of Galaxies: Jet Properties

I derive constraints on jet properties for inflating pairs of bubbles in planetary nebulae and clusters of galaxies. This work is motivated by the similarity in morphology and some non-dimensional quantities between X-ray-deficient bubbles in clusters of galaxies and the optical-deficient bubbles in planetary nebulae, which was pointed out in an earlier work. In the present paper I find that for inflating fat bubbles, the opening angle of the jets must be large, i.e., the half opening angle measured from the symemtry axis of the jets should typically be larger than 40 degrees. Narrow jets will form elongated lobes rather than fat bubbles. I emphasize the need to include jets with large opening angle in simulating bubble inflation in both planetary nebulae and (cooling flow) clusters of galaxies.

astro-ph↗