arXiv subjects

Noam Soker

Publications and source records attributed to Noam Soker.

At least 181 records · Page 10Linked to original sources

arXiv2019

Intermediate Luminosity Optical Transients (ILOTs) from merging giants

We suggest and study the formation of intermediate luminosity optical transients (ILOTs) from the merger of two cool giant stars. For the two stars to merge when both are in their giant phases the stars must have close masses at their zero age main sequence, and the orbital separation must be in the right range. After the two giants merge, the two cores spiral-in toward each other within a common envelope. We study the energy sources of radiation in this process that include the ejection of mass that powers radiation by both recombination and by collision with previously ejected mass. This process includes no jets, unlike many other types of ILOTs, hence the event will not form a bipolar nebula. Using the stellar evolution numerical code mesa for two binary systems with stellar masses of (15Mo, 15.75Mo) and (31Mo, 31.5Mo), we find that the merger of the two cores releases gravitational energy that marginally ejects the entire common envelope. This implies that in many cases the two cores merge, i.e., a fatal common envelope evolution, leading to a somewhat more luminous ILOT. A typical ILOT from merger of two cool giant stars lasts for several months to several years and has a typical average luminosity of L(ILOT)~=10^6(M_{CE}/10Mo)Lo, where M_{CE} is the ejected common envelope mass. The merger-driven massive outflow forms dust, hence leading to a very red ILOT, possibly even infrared luminous and undetectable in the visible.

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arXiv2019

Amplifying magnetic fields of a newly born neutron star by stochastic angular momentum accretion in core collapse supernovae

I present a novel mechanism to boost magnetic field amplification of newly born neutron stars in core collapse supernovae. In this mechanism, that operates in the jittering jets explosion mechanism and comes on top of the regular magnetic field amplification by turbulence, the accretion of stochastic angular momentum in core collapse supernovae forms a neutron star with strong initial magnetic fields but with a slow rotation. The varying angular momentum of the accreted gas, which is unique to the jittering jets explosion mechanism, exerts a varying azimuthal shear on the magnetic fields of the accreted mass near the surface of the neutron star. This, I argue, can form an amplifying effect which I term the stochastic omega (S$ω$) effect. In the common $αω$ dynamo the rotation has constant direction and value, and hence supplies a constant azimuthal shear, while the convection has a stochastic behavior. In the S$ω$ dynamo the stochastic angular momentum is different from turbulence in that it operates on a large scale, and it is different from a regular rotational shear in being stochastic. The basic assumption is that because of the varying direction of the angular momentum axis from one accretion episode to the next, the rotational flow of an accretion episode stretches the magnetic fields that were amplified in the previous episode. I estimate the amplification factor of the S$ω$ dynamo alone to be $\approx 10$. I speculate that the S$ω$ effect accounts for a recent finding that many neutron stars are born with strong magnetic fields.

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arXiv2019

The strongly interacting binary scenarios of the enigmatic supernova iPTF14hls

We argue that some scenarios for the enigmatic supernova (SN) iPTF14hls and its progenitor require a strong binary interaction. We examine scenarios that attribute the extra power of iPTF14hls to a magnetar, to a late fallback on to the neutron star (NS) that launches jets, to an interaction of the ejecta with a circumstellar matter (CSM), or to a common envelope jets SN (CEJSN). For each of these four scenarios, we study the crucial process that supplies the extra energy and conclude that a binary companion to the progenitor must be present. For the magnetar scenario and late jets we claim that a companion should spin-up the pre-collapse core, in the ejecta-CSM scenario we find that the formation of the equatorial CSM requires a companion, and in the CEJSN where a NS spirals-in inside the giant envelope of the progenitor and launches jets the strong binary interaction is built-in. We argue that these types of strong binary interactions make the scenarios rare and explain the enigmatic nature of iPTF14hls. We further study processes that might accompany the binary interaction, in particular, the launching of jets before, during and after the explosion and their observational consequences. We do not consider the difficulties of the different scenarios and neither do we determine the best scenario for iPTF14hls. We rather focus on the binary nature of these scenarios that might as well explain other rare types of SNe.

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arXiv2019

Possible indications for jittering jets in core collapse supernova explosion simulations

I analyse recent three-dimensional hydrodynamical simulations of exploding core collapse supernovae (CCSNe) and find that a rapid increase in the diagnostic explosion energy occurs parallel to, or very shortly after, the accretion of gas with relatively large amount of angular momentum and/or specific angular momentum. The tendency towards a positive correlation between angular momentum accretion and the increase in the explosion energy appears despite the absence of magnetic fields in the simulations. I speculate that the inclusion of magnetic fields would make this positive correlation much stronger and would increase the explosion energy. At times, the high angular momentum accretion leads to a bipolar outflow that in some cases changes directions. I argue that there is a mutual influence between neutrino heating and stochastic angular momentum, and that both operate together to explode the star with bipolar outflows that change directions, namely, jittering jets. This study calls for a deep flow analysis of simulations of CCSNe, e.g., to present the flow structure, the density, and the angular momentum of the flow near the newly born neutron star (NS).

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arXiv2019

Companion-launched jets and their effect on the dynamics of common envelope interaction simulations

We conduct three-dimensional hydrodynamic simulations of the common envelope binary interaction and show that if the companion were to launch jets while interacting with the giant primary star's envelope, the jets would remove a substantial fraction of the envelope's gas. We use the setup and numerical code of an earlier common envelope study that did not include jets, with a 0.88Mo, 83Ro red giant star and a 0.3Mo companion. The assumption is that the companion star accretes mass via an accretion disk that is responsible for launching the jets which, in the simulations, are injected numerically. For the first time we conduct simulations that include jets as well as the gravitational energy released by the inspiraling core-companion system. We find that simulations with jets unbind approximately three times as much envelope mass than identical simulations that do not include jets, though the total fraction of unbound gas remains below 50 per cent for these particular simulations. The jets generate high velocity outflows in the polar directions. The jets also increase the final core-companion orbital separation and lead to a kick velocity of the core-companion binary system. Our results show that, if able to form, jets could play a crucial role in ejecting the envelope and in shaping the outflow.

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arXiv2019

Constraining Type Ia Supernova Asymmetry with the Gamma-Ray Escape Timescale

We calculate the effects of an asymmetric $^{56}\mathrm{Ni}$ distribution in Type Ia supernova (SN Ia) ejecta on the gamma-ray escape timescale ($t_0$) that characterizes the late light curve ($>40$ days after peak) and find the effect is modest compared to other possible variations in ejecta structure. We parameterize asymmetry in the $^{56}\mathrm{Ni}$ distribution and calculate $t_0$ for a grid of SN ejecta models spanning a large volume of the asymmetry parameter space. The models have spherical density profiles while the $^{56}\mathrm{Ni}$ distribution in them has various levels of asymmetry. By placing constraints based on the observational measurement of $t_0$ and other general properties of SN Ia ejecta, we find the range of allowed asymmetry in the $^{56}\mathrm{Ni}$ distribution. Considering these constraints we find that some level of asymmetry in the distribution is not ruled out. However, models with a single ejecta mass and varying $^{56}\mathrm{Ni}$ distributions cannot explain the full range of observed $t_0$ values. This strengthens the claim that both Chandrasekhar mass and sub-Chandrasekhar mass explosions are required to explain the diversity of SN Ia observations.

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arXiv2019

The common envelope jets supernova (CEJSN) r-process scenario

We study r-process feasibility inside jets launched by a cold neutron star (NS) spiralling-in inside the core of a giant star, and find that such common envelope jets supernova events might be a significant source of heavy r-process elements in the early Universe. We run the stellar evolution code MESA to follow the evolution of low metalicity giant stars that swallow NSs during their late expansion phases and find that in some of the cases the NSs penetrate the core. The Bondi-Hoyle-Lyttleton (BHL) mass accretion rate onto a NS as it spirals-in inside the core is sufficiently high to obtain a neutron rich ejecta as required for the heavy r-process where the second and third r-process elements are synthesized. Due to the small radius of the NS the accretion is through an accretion disk and the outflow is in jets (or bipolar disk winds). The r-process nucleosynthesis takes place inside the jets. To account for the r-process abundances in the Galaxy we require that one in ten cases of a NS entering the envelope of a giant star ends as a CEJSN r-process event.

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arXiv2019

Storing magnetic fields in pre-collapse cores of massive stars

We argue that the radiative zone above the iron core in pre-collapse cores of massive stars can store strong magnetic fields. To reach this conclusion we use the stellar evolutionary code MESA to simulate the evolution of two stellar models with initial masses of 15Mo and 25Mo, and reveal the entropy profile above the iron core just before core collapse. Just above the iron core there is a thin zone with convective shells. We assume that a dynamo in these convective shells amplifies magnetic fields and forms magnetic flux loops. By considering the buoyancy of magnetic flux loops we show that the steep entropy rise in the radiative zone above the dynamo can prevent buoyancy of flux loops with magnetic fields below about 10^{13}G. When this radiative zone collapses on to the newly born neutron star the converging inflow further amplifies the magnetic fields by a factor of about a hundred. After passing through the stalled shock at about a hundred kilometres from the center, these strong magnetic fields together with instabilities can facilitate the launching of jets that explode the star in the frame of the jittering jets explosion mechanism. Our study further supports the claim for the necessity to include magnetic fields in simulating the explosion of CCSNe.

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arXiv2019

The class of supernova progenitors that result from fatal common envelope evolution

I construct the class of supernovae and supernova progenitors that result from fatal common envelope evolution (CEE). The fatal CEE progenitors are stellar binary systems where a companion spirals-in inside the envelope of a giant star and merges with the core. The companion can be a neutron star (NS; or a black hole) that destroys the core and by that forms a common envelope jets supernova (CEJSN), a white dwarf (WD) that merges with the core to form a massive WD that later might explode as a Type Ia supernova (the core degenerate scenario), or a main sequence companion. In the latter case the outcome might be a core collapse supernova (CCSN) of a blue giant, a CCSN of type IIb or of type Ib. In another member of this class two giant stars merge and the two cores spiral-in toward each other to form a massive core that later explodes as a CCSN with a massive circumstellar matter (CSM). I discuss the members of this class, their characteristics, and their common properties. I find that fatal CEE events account for $\approx 6-10 \%$ of all CCSNe, and raise the possibility that a large fraction of peculiar and rare supernovae result from the fatal CEE. The study of these supernova progenitors as a class will bring insights on other types of supernova progenitors, as well as on the outcome of the CEE.

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arXiv2019

Diversity of common envelope jets supernovae and the fast transient AT2018cow

We propose a common-envelope jets supernova (CEJSN) scenario for the fast-rising blue optical transient AT2018cow. In a CEJSN a neutron star (NS) spirals-in inside the extended envelope of a massive giant star and enters the core. The NS accretes mass from the core through an accretion disc and launches jets. These jets explode the core and the envelope. In the specific polar CEJSN scenario that we propose here the jets clear the polar regions of the giant star before the NS enters the core. The jets that the NS launches after it enters the core expand almost freely along the polar directions that contain a small amount of mass. This, we suggest, explains the fast rise to maximum and the fast ejecta observed at early times of the enigmatic transient AT2018cow. The slower later time ejecta is the more massive equatorial outflow. We roughly estimate the accretion phase onto the NS during the explosion phase to last for a time of about 1000 seconds, during which the average mass accretion rate is about 0.0001 Mo/sec. We outline the possible diversity of CEJSNe by listing five other scenarios in addition to the polar CEJSN scenario.

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arXiv2019

Explaining the early excess emission of the Type Ia supernova 2018oh by the interaction of the ejecta with disk-originated matter (DOM)

We explain the early excess emission of the Type Ia supernova 2018oh by an interaction of the supernova ejecta with disk-originated matter (DOM). Such DOM can form in the merger process of two white dwarfs (WDs) in the double degenerate scenario of Type Ia supernovae (SNe Ia). We find that an ejecta-DOM interaction can fit the early light curve of SN 2018oh better than an ejecta-companion interaction in the single degenerate scenario. By composing the DOM from two components that were ejected in the merger process with two different velocities, we show that the ejecta-DOM interaction can account for the linear rise in the light curve, while the ejecta-companion interaction predicts too steep a rise. In addition, the ejecta-DOM interaction does not predict the presence of hydrogen and helium lines in nebular spectra, and hence does not suffer from this major drawback of the ejecta-companion model. We consider the ejecta-DOM interaction to be the most likely explanation for the early excess emission in SN 2018oh. By that we show that the double degenerate scenario can account for early excess emission in SNe Ia.

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arXiv2019

Reviving the stalled shock by jittering jets in core collapse supernovae: jets from the standing accretion shock instability

I present a scenario by which an accretion flow with alternating angular momentum sense on to a newly born neutron star in core collapse supernovae (CCSNe) efficiently amplifies magnetic fields and by that launches jets. The accretion flow of a collapsing core on to the newly born neutron star suffers the spiral standing accretion shock instability (SASI). This instability leads to a stochastically variable angular momentum of the accreted gas, that in turn forms an accretion flow with alternating directions of the angular momentum, hence alternating shear, at any given time. I study the shear in this alternating-shear sub-Keplerian inflow in published simulations, and present a new comparison with Keplerian accretion disks. From that comparison I argue that it might be as efficient as Keplerian accretion disks in amplifying magnetic fields by a dynamo. I suggest that although the average specific angular momentum of the accretion flow is small, namely, sub-Keplerian, this alternating-shear accretion flow can launch jets with varying directions, namely, jittering jets. Neutrino heating is an important ingredients in further energizing the jets. The jittering jets locally revive the stalled accretion shock in the momentarily polar directions, and by that they explode the star. I repeat again my call for a paradigm shift from a neutrino-driven explosion of CCSNe to a jet-driven explosion mechanism that is aided by neutrino heating.

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arXiv2018

Variable jets at the termination of the common envelope evolution

I propose that at the termination of the common envelope evolution (CEE) the companion to the giant star might launch jets that have variable directions and intensities, hence the jets shape the inner zones of the descendant nebula causing it to lose any type of symmetry. This might account for some of the chaotic departures from axisymmetric structures in some planetary nebulae. I base my study on the assumption that at the termination of the CEE the binary interaction forms a circumbinary thick disk that feeds the companion via an accretion disk that launches opposite jets. I consider two processes that might lead to the development of Rayleigh-Taylor instabilities that lead to chaotic accretion flow on to the companion. The chaotic accretion flow leads to a variable accretion disk around the companion, hence to variable jets. In the first process the jets inflate low-density hot bubbles in the inner regions of the envelope. In the second process the inner boundary of the thick circumbinary disk, that faces the center, cools by radiation thus leading to a temperature, and hence also a pressure, increase with radius. The opposite pressure and density gradients lead to Rayleigh-Taylor instabilities. This study further underlines the importance of exploring the termination phase of the CEE to the understanding of the entire CEE and its outcomes.

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arXiv2018

Common envelope jets supernova (CEJSN) impostors resulting from a neutron star companion

We propose a new type of repeating transient outburst initiated by a neutron star (NS) entering the envelope of an evolved massive star, accreting envelope material and subsequently launching jets which interact with their surroundings. This interaction is the result of either a rapid expansion of the primary star due to an instability in its core near the end of its nuclear evolution, or due to a dynamical process which rapidly brings the NS into the primary star. The ejecta can reach velocities of 10,000 km/s despite not being a supernova, and might explain such velocities in the 2011 outburst of the luminous blue variable progenitor of SN 2009ip. The typical transient duration and kinetic energy are weeks to months, and up to approximately 1 foe, respectively. The interaction of a NS with a giant envelope might be a phase in the evolution of the progenitors of most NS-NS binary systems that later undergo a merger event. If the NS spirals in all the way to the core of the primary star and brings about its complete disruption we term this a `common envelope jets supernova' (CEJSN), which is a possible explanation for the peculiar supernova iPTF14hls. For a limited interaction of the NS with the envelope we get a less luminous transient, which we term a CEJSN impostor.

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arXiv2018

Pre-supernova outbursts of massive stars in the presence of a neutron star companion

We study the pre-explosion outbursts (PEOs) of massive stars that might result from a rapid expansion of the massive star in the presence of a close companion. We assume that activity in the core of the massive star, an initial mass of 15Mo, about two years before explosion energizes the envelope, and with the stellar evolutionary code MESA follow the inflated envelope as a result of energy deposition to the envelope. We examine the conditions for a companion star to accrete mass from the inflated envelope. We find that for the general conditions that we assume, bright PEOs require a neutron star companion at an orbital separation of about 1000-2000Ro. We assume that the mass-accreting neutron star launches jets. These jets shape the circumstellar matter to highly non-spherical structures, such that the explosions of core collapse supernovae (CCSNe) that follow PEOs might lack an axial (cylindrical) symmetry. In some case main sequence star companions can also energize PEOs, but much weaker ones. This study adds another scenario by which neutron stars can power the radiation of PEOs. Another scenario is the common envelope jets supernova (CEJSN) impostor where a neutron star enters the envelope of the massive star.

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arXiv2018

Reviving the stalled shock by jittering jets in core collapse supernovae: The key role of magnetic fields

I find that an ingredient that was added in a recent study to facilitate the delayed neutrino explosion mechanism of core collapse supernovae, namely, large scale perturbations in the pre-collapse core, has a larger positive influence on the jittering jets explosion mechanism. By following the specific angular momentum of the accreted mass on to the newly born neutron star, I find that the accreted mass is likely to form intermittent accretion belts and disks, although they might lack axisymmetrical structure. These accretion belts and disks are likely to launch jets, but this can be simulated only if magnetic fields are included in the numerical code, as well as high numerical resolution that follows the rotation of the newly born neutron star and the shear in the accretion flow. I also discuss the possibility that the rotation of the pre-collapse core is important in increasing the shear in the accretion flow, hence the amplification of the magnetic fields. I call for a paradigm shift from a neutrino-driven explosion mechanism of massive stars to a jet-driven explosion mechanism aided by neutrino heating. Such a paradigm shift will bring the recognition that to simulate core collapse supernovae one must use magneto-hydrodynamical numerical codes.

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arXiv2018

On the requirement for mixing-heating to utilize bubble cosmic rays to heat the intracluster medium

I conduct simple analytical estimates and conclude that mixing by vortices is a more efficient process to transfer the cosmic ray energy of jet-inflated bubbles to the intracluster medium (ICM) than streaming of cosmic rays along magnetic field lines. Jets and the bubbles they inflate transfer heat to the ambient gas in cooling flows in cluster of galaxies and in galaxies. The internal energy of the jet-inflated bubbles is dominate by very hot thermal gas and/or cosmic rays. Cosmic rays that stream along magnetic field lines that connect the bubbles with the ICM heat the ICM as their energy is dissipated there. I find that about half of the cosmic ray energy is dissipated in the bubbles themselves. I also find that the ICM volume that the cosmic ray streaming process heats is only about five times as large as the volume of the bubbles. The outcome of heating by streaming only is that the cosmic rays form a larger bubble filled with very hot thermal gas. Therefore, there is a requirement for a more efficient process to transfer the internal energy of the bubbles to the ICM. I suggest that this process is heating by mixing that operates very well for both cosmic rays and the very hot thermal gas inside the bubbles. This leaves mixing-heating to be the dominant heating process of cooling flows.

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arXiv2018

The formation of `columns crowns' by jets interacting with a circumstellar dense shell

We conduct three-dimensional hydrodynamical simulations of two opposite jets that interact with a spherical slow wind that includes a denser shell embedded within it, and obtain a bipolar nebula where each of the two lobes is composed of two connected bubbles and Rayleigh-Taylor instability tongues that protrude from the outer bubble and form the `columns crown'. The jets are launched for a short time of 17 years and inflate a bipolar nebula inside a slow wind. When the bipolar structure encounters the dense shell, the interaction causes each of the two lobes to split to two connected bubbles. The interaction is prone to Rayleigh-Taylor instabilities that form tongues that protrude as columns from the outer bubble. The bases of the columns form a ring on the surface of the outer bubble, and the structure resemble a crown that we term the columns crown. This structure resembles, but is not identical to, the many filaments that protrude from the lobes of the bipolar planetary nebula Menzel~3. We discuss our results in comparison to the structure of Menzel~3 and the ways by which the discrepancies can be reconciled, and possibly turn our failure to reproduce the exact structure of Menzel~3 to a success with jets-shell interaction simulations that include more ingredients.

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