arXiv subjects

Ronald E. Taam

Publications and source records attributed to Ronald E. Taam.

At least 19 recordsLinked to original sources

arXiv2022

Common Envelope Shaping of Planetary Nebulae. IV. From Proto-planetary to Planetary Nebula

We present 2D hydrodynamical simulations of the transition of a proto-planetary nebula to a planetary nebula for central stars in binary systems that have undergone a common envelope event. After 1,000 yr of magnetically driven dynamics (proto-planetary nebula phase), a line-driven stellar wind is introduced into the computational domain and the expansion of the nebula is simulated for another 10,000 yr, including the effects of stellar photoionization. In this study we consider central stars with main sequence (final) masses of 1 (0.569) and 2.5 (0.677) \Mo, together with a 0.6 \Mo ma in sequence companion. Extremely bipolar, narrow-waisted proto-planetary nebulae result in bipolar planetary nebulae, while the rest of the shapes mainly evolve into elliptical planetary nebulae. The initial magnetic field's effects on the collimated structures, such as jets, tend to disappear in most of the cases, leaving behind the remnants of those features in only a few cases. Equatorial zones fragmented mainly by photoionization ( 1 \Mo progenitors), result in ``necklace'' structures made of cometary clumps aligned with the radiation field. On the other hand, fragmentation by photoionization and shocked wind ( 2.5 \Mo progenitors) give rise to the formation of multiple clumps in the latitudinal direction, which remain within the lobes, close to the center, which are immersed and surrounded by hot shocked gas, not necessarily aligned with the radiation field. These results reveal that the fragmentation process has a dependence on the stellar mass progenitor. This fragmentation is made possible by the distribution of gas in the previous post-common envelope proto-planetary nebula as sculpted by the action of the jets.

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arXiv2021

Common Envelope Shaping of Planetary Nebulae. III. The Launching of Jets in Proto-Planetary Nebulae

We compute successfully the launching of two magnetic winds from two circumbinary disks formed after a common envelope event. The launching is produced by the increase of magnetic pressure due to the collapse of the disks. The collapse is due to internal torques produced by a weak poloidal magnetic field. The first wind can be described as a wide jet, with an average mass-loss rate of $\sim 1.3 \times 10^{-7}$ \Moy\ and a maximum radial velocity of $\sim 230$ \kms. The outflow has a half-opening angle of $\sim 20^{\circ}$. Narrow jets are also formed intermittently with velocities up to 3,000 \kms, with mass-loss rates of $\sim 6 \times 10^{-12} $ \Moy\ during short periods of time. The second wind can be described as a wide X-wind, with an average mass-loss rate of $\sim 1.68 \times 10^{-7}$ \Moy\ and a velocity of $\sim 30$ \kms. A narrow jet is also formed with a velocity of 250 \kms, and a mass-loss rates of $\sim 10^{-12} $ \Moy. The computed jets are used to provide inflow boundary conditions for simulations of proto-planetary nebulae. The wide jet evolves into a molecular collimated outflow within a few astronomical units, producing proto-planetary nebulae with bipolar, elongated shapes, whose kinetic energies reach $\sim 4 \times 10^{45}$ erg at 1,000 years. Similarities with observed features in W43A, OH231.8+4.2, and Hen 3-1475 are discussed. The computed wide X-wind produces proto-planetary nebulae with slower expansion velocities, with bipolar and elliptical shapes, and possible starfish type and quadrupolar morphology.

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arXiv2021

The role of mass transfer and common envelope evolution in the formation of merging binary black holes

As the number of observed merging binary black holes (BHs) grows, accurate models are required to disentangle multiple formation channels. In models with isolated binaries, important uncertainties remain regarding the stability of mass transfer (MT) and common-envelope (CE) evolution. To study some of these uncertainties, we have computed simulations using MESA of a $30M_\odot$, low metallicity ($Z_\odot/10$) star with a BH companion. We developed a prescription to compute MT rates including possible outflows from outer Lagrangian points, and a method to self-consistently determine the core-envelope boundary in the case of CE evolution. We find that binaries survive a CE only if unstable MT happens after the formation of a deep convective envelope, resulting in a narrow range (0.2 dex) in period for envelope ejection. All cases where interaction is initiated with a radiative envelope have large binding energies ($\sim 10^{50}$ erg), and merge during CE even under the assumption that all the internal and recombination energy of the envelope, as well as the energy from an inspiral, is used for ejection. This is independent of core helium ignition for the donor, a condition under which various rapid-population synthesis calculations assume a successful ejection is possible. Moreover, we find that the critical mass ratio for instability is such that for periods between $\sim 1-1000$ days merging binary BHs can be formed via stable MT. A large fraction of these systems overflow their L$_2$ equipotential, in which case we find stable MT produces merging binary BHs even under extreme assumptions of mass and angular momentum outflows. Our conclusions are limited to the study of one donor star, but suggest that population synthesis calculations overestimate the formation rate of merging binary BHs produced by CE evolution, and that stable MT could dominate the rate from isolated binaries.

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arXiv2020

Pulsational pair-instability supernovae in very close binaries

Pair-instability and pulsational pair-instability supernovae (PPISN) have not been unambiguously observed so far. They are, however, promising candidates for the progenitors of the heaviest binary black hole (BBH) mergers detected. If these BBHs are the product of binary evolution, then PPISNe could occur in very close binaries. Motivated by this, we discuss the implications of a PPISN happening with a close binary companion, and what impact these events have on the formation of merging BBHs through binary evolution. For this, we have computed a set of models of metal-poor ($Z_\odot/10$) single helium stars using the \texttt{MESA} software instrument. For PPISN progenitors with pre-pulse masses $>50M_\odot$ we find that, after a pulse, heat deposited throughout the layers of the star that remain bound cause it to expand to more than $100R_\odot$ for periods of $10^2-10^4\;$~yrs depending on the mass of the progenitor. This results in long-lived phases of Roche-lobe overflow or even common-envelope events if there is a close binary companion, leading to additional electromagnetic transients associated to PPISN eruptions. If we ignore the effect of these interactions, we find that mass loss from PPISNe reduces the final black hole spin by $\sim 30\%$, induces eccentricities below the threshold of detectability of the LISA observatory, and can produce a double-peaked distribution of measured chirp masses in BBH mergers observed by ground-based detectors.

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arXiv2020

Wind accretion in Cygnus X-1

Context: Cygnus X-1 is a black hole X-ray binary system in which the black hole captures and accretes gas from the strong stellar wind emitted by its supergiant O9.7 companion star. The irradiation of the supergiant star essentially determines the flow properties of the stellar wind and the X-ray luminosity from the system. The results of three-dimensional hydrodynamical simulations of wind-fed X-ray binary systems reported in recent work reveal that the ionizing feedback of the X-ray irradiation leads to the existence of two stable states with either a soft or a hard spectrum. Aims: We discuss the observed radiation of Cygnus X-1 in the soft and hard state in the context of mass flow in the corona and disk, as predicted by the recent application of a condensation model. Methods: The rates of gas condensation from the corona to the disk for Cygnus X-1 are determined, and the spectra of the hard and soft radiation are computed. The theoretical results are compared with the MAXI observations of Cygnus X-1 from 2009 to 2018. In particular, we evaluate the hardness-intensity diagrams (HIDs) for its ten episodes of soft and hard states which show that Cygnus X-1 is distinct in its spectral changes as compared to those found in the HIDs of low-mass X-ray binaries. Results: The theoretically derived values of photon counts and hardness are in approximate agreement with the observed data in the HID. However, the scatter in the diagram is not reproduced. Improved agreement could result from variations in the viscosity associated with clumping in the stellar wind and corresponding changes of the magnetic fields in the disk. The observed dipping events in the hard state may also contribute to the scatter and to a harder spectrum than predicted by the model.

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arXiv2020

Common Envelope Shaping of Planetary Nebulae. II. Magnetic Solutions and Self-Collimated Outflows

Magnetic fields of order $10^1-10^2$ gauss that are present in the envelopes of red giant stars are ejected in common envelope scenarios. These fields could be responsible for the launching of magnetically driven winds in proto-planetary nebulae. Using 2D simulations of magnetized winds interacting with an envelope drawn from a 3D simulation of the common envelope phase, we study the confinement, heating, and magnetic field development of post-common envelope winds. We find that the ejected magnetic field can be enhanced via compression by factors up to $\sim 10^4$ in circumbinary disks during the self-regulated phases. We find values for the kinetic energy of the order of $10^{46}$ erg that explain the large values inferred in proto-planetary nebula outflows. We show that the interaction of the formed circumbinary disk with a spherical, stellar wind produces a "tapered" flow that is almost indistinguishable from an imposed tapered flow. This increases the uncertainty of the origin of proto-planetary nebula winds, which could be either stellar, circumstellar (stellar accretion disk), circumbinary (circumbinary accretion disk), or a combination of all three. Within this framework, a scenario for self-collimation of weakly magnetized winds is discussed, which can explain the two objects where the collimation process is observationally resolved, HD 101584 and Hen 3-1475. An explanation for the equatorial, molecular hydrogen emission in CRL 2688 is also presented.

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arXiv2019

The Complete Evolution of a Neutron-Star Binary through a Common Envelope Phase Using 1D Hydrodynamic Simulations

Over forty years of research suggests that the common envelope phase, in which an evolved star engulfs its companion upon expansion, is the critical evolutionary stage forming short-period, compact-object binary systems, such as coalescing double compact objects, X-ray binaries, and cataclysmic variables. In this work, we adapt the one-dimensional hydrodynamic stellar evolution code, MESA, to model the inspiral of a 1.4M$_{\odot}$ neutron star (NS) inside the envelope of a 12$M_{\odot}$ red supergiant star. We self-consistently calculate the drag force experienced by the NS as well as the back-reaction onto the expanding envelope as the NS spirals in. Nearly all of the hydrogen envelope escapes, expanding to large radii ($\sim$10$^2$ AU) where it forms an optically thick envelope with temperatures low enough that dust formation occurs. We simulate the NS orbit until only 0.8M$_{\odot}$ of the hydrogen envelope remains around the giant star's core. Our results suggest that the inspiral will continue until another $\approx$0.3M$_{\odot}$ are removed, at which point the remaining envelope will retract. Upon separation, a phase of dynamically stable mass transfer onto the NS accretor is likely to ensue, which may be observable as an ultraluminous X-ray source. The resulting binary, comprised of a detached 2.6M$_{\odot}$ helium-star and a NS with a separation of 3.3-5.7R$_{\odot}$, is expected to evolve into a merging double neutron-star, analogous to those recently detected by LIGO/Virgo. For our chosen combination of binary parameters, our estimated final separation (including the phase of stable mass transfer) suggests a very high $α_{\rm CE}$-equivalent efficiency of $\approx$5.

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arXiv2019

Templates of binary-induced spiral-shell patterns around mass-losing post-main sequence stars

The morphological properties of the outflowing circumstellar envelopes surrounding mass-losing stars in eccentric binary systems are presented from a set of three-dimensional hydrodynamical model simulations. Based on four template models of the envelope viewed for a range of inclination angles of the systems, we implement interpretative tools for observations at high spectral/angular resolutions (as illustrated via velocity channel maps as well as position-velocity, radius-velocity, and angle-radius diagrams). Within this framework, the image and kinematical structures can be used to place constraints on the orbital parameters of the system. Specifically, three unique characteristic patterns in the envelopes are found that distinguish these systems from those in binary systems in circular orbits. Bifurcation of the spiral pattern, asymmetry in the interarm density depression, and a concurrent spiral/ring appearance all point to a binary system with an eccentric orbit. The methodology presented in this paper is illustrated in an analysis of recent radio observations of several asymptotic giant branch stars.

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arXiv2019

Gas Flows Within Cavities of Circumbinary Discs in Eccentric Binary Protostellar Systems

The structure and evolution of gas flows within the cavity of a circumbinary disk (CBD) surrounding the stellar components in eccentric binaries are examined via two-dimensional hydrodynamical simulations. The degree to which gas fills the cavity between the circumstellar disks (CSDs) and the CBD is found to be greater for highly eccentric systems, in comparison to low-eccentricity systems, reflecting the spatial extent over which mass enters into the cavity throughout the orbit. The pattern of the gas flow in the cavity differs for eccentric binaries from that of binaries in a circular orbit. In particular, the former reveals tightly wound gas streams and figure-eight-like structures for systems characterized by eccentricities, $e \ge 0.4$, whereas the latter only reveal relatively loosely bent streams from the CBD to the CSDs. Hence, the description of the stream structures can be a probe of sufficient non-circularity of the binary orbital motion. Given that the inner edge of the CBD is not very well defined for highly eccentric systems due to the complex gas structures, it is suggested that the area of the cavity for high-sensitivity imaging observations may prove to be a more useful diagnostic for probing the effectiveness of CBD clearing in the future.

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arXiv2018

Common Envelope Evolution of Massive Stars

The discovery via gravitational waves of binary black hole systems with total masses greater than $60M_\odot$ has raised interesting questions for stellar evolution theory. Among the most promising formation channels for these systems is one involving a common envelope binary containing a low metallicity, core helium burning star with mass $\sim 80-90M_\odot$ and a black hole with mass $\sim 30-40M_\odot$. For this channel to be viable, the common envelope binary must eject more than half the giant star's mass and reduce its orbital separation by as much as a factor of 80. We discuss issues faced in numerically simulating the common envelope evolution of such systems and present a 3D AMR simulation of the dynamical inspiral of a low-metallicity red supergiant with a massive black hole companion.

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arXiv2018

Time-dependent Pattern Speeds in Barred Galaxies

Based on a high quality $N$-body simulation of a double bar galaxy model, we investigate the evolution of the bar properties, including their size, strength and instantaneous pattern speed derived by using three distinct methods: the Fourier, Jacobi integral, and moment of inertia methods. The interaction of the two bars, which rotate at distinct speeds, primarily affects the size, strength and pattern speed of the inner bar. When the two bars are perpendicular to each other, the size and the pattern speed of the inner bar decrease and its strength increases. The emergence of a strong Fourier $m=1$ mode increases the oscillation amplitude of the size, strength and pattern speed of the inner bar. On the other hand, the characteristics of the outer bar are substantially influenced by its adjacent spiral structure. When the spiral structure disappears, the size of the outer bar increases and its strength and pattern speed decrease. Consequently, the ratio of the pattern speed of the outer bar with respect to the inner bar is not constant and increases with time. Overall, the double bar and disk system displays substantial high frequency semi-chaotic fluctuations of the pattern strengths and speeds both in space and time, superposed on the slow secular evolution, which invalidates the assumption that the actions of individual stars should be well conserved in barred galaxies, such as the Milky Way.

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arXiv2018

The growth of stellar mass black hole binaries trapped in an accretion disk of active galactic nuclei

Among the four black hole binary merger events detected by LIGO, six progenitor black holes have masses greater than 20\,$M_\odot$. The existence of such massive BHs calls for extreme metal-poor stars as the progenitors. An alternative possibility that a pair of stellar mass black holes each with mass $\sim7\,M_\odot$ increases to $>20\,M_\odot$ via accretion from a disk surrounding a super massive black hole in an active galactic nucleus is considered. The growth of mass of the binary and the transfer of orbital angular momentum to the disk accelerates the merger. Based on the recent numerical work of Tang et al. (2017), it is found that, in the disk of a low mass AGN with mass $\sim10^6\,M_\odot$ and Eddington ratio $>0.01$, the mass of an individual BH in the binary can grow to $>20\,M_\odot$ before coalescence provided that accretion takes place at a rate more than 10 times the Eddington value. The mechanism predicts a new class of gravitational wave sources involving the merger of two extreme Kerr black holes associated with active galactic nuclei and a possible electromagnetic wave counterpart.

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arXiv2018

A Model for Spectral States and Their Transition in Cyg X-1

A new accretion picture based on a small disk surrounding a black hole is developed for the wind-fed source Cyg X-1. The hard and soft spectral states of Cyg X-1 are interpreted in terms of co-spatial two component flows for the innermost region of an accretion disk. The state transitions result from the outward expansion and inward recession of this inner disk for the hard to soft and soft to hard transition respectively. The theoretical framework for state transitions in black hole X-ray binaries with high mass companions involving a change in the inner disk size, thus, differs from systems with low mass companions involving the change in the outer disk size. This fundamental difference stems from the fact that matter captured and supplied to the black hole in wind-fed systems has low specific angular momentum and is hot essentially heated in the bow and spiral shocks, whereas it has high specific angular momentum and is cool in Roche lobe overflow systems. The existence of a weak cool disk around the ISCO region in the hard state allows for the presence of a relativistically broadened Fe K line. The small disk fed by gas condensation forms without an extensive outer disk, precluding thermal instabilities and large outbursts, resulting in the lack of large amplitude outbursts and hysteresis effects in the light curve of high mass black hole X-ray binaries. Their relatively persistent X-ray emission is attributed to their wind-fed nature.

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arXiv2018

Common Envelope Shaping of Planetary Nebulae

The morphology of planetary nebulae emerging from the common envelope phase of binary star evolution is investigated. Using initial conditions based on the numerical results of hydrodynamical simulations of the common envelope phase it is found that the shapes and sizes of the resulting nebula are very sensitive to the effective temperature of the remnant core, the mass-loss rate at the onset of the common envelope phase, and the mass ratio of the binary system. These parameters are related to the efficiency of the mass ejection after the spiral-in phase, the stellar evolutionary phase (i.e., RG, AGB or TP-AGB), and the degree of departure from spherical symmetry in the stellar wind mass loss process itself respectively. It is found that the shapes are mostly bipolar in the early phase of evolution, but can quickly transition to elliptical and barrel-type shapes. Solutions for nested lobes are found where the outer lobes are usually bipolar and the inner lobes are elliptical, bipolar or barrel-type, a result due to the flow of the photo-evaporated gas from the equatorial region. It is found that the lobes can be produced without the need for two distinct mass ejection events. In all the computations, the bulk of the mass is concentrated in the orbital or equatorial plane, in the form of a large toroid, which can be either neutral (early phases) or photoionized (late phases), depending of the evolutionary state of the system.

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arXiv2017

Centrally Concentrated X-ray Radiation from an Extended Accreting Corona in Active Galactic Nuclei

The X-ray emission from bright active galactic nuclei (AGNs) is believed to originate in a hot corona lying above a cold, geometrically thin accretion disk. A highly concentrated corona located within $\sim10$ gravitational radii above the black hole is inferred from observations. Based on the accretion of interstellar medium/wind, a disk corona model has been proposed in which the corona is well coupled to the disk by radiation, thermal conduction, as well as by mass exchange \citep{Liu2015, Qiao2017}. Such a model avoids artificial energy input to the corona and has been used to interpret the spectral features observed in AGN. In this work, it is shown that the bulk emission size of the corona is very small for the extended accretion flow in our model. More than 80\% of the hard X-ray power is emitted from a small region confined within 10 Schwarzschild radii around a non-spinning black hole, which is expected to be even smaller accordingly for a spinning black hole. Here, the corona emission is more extended at higher Eddington ratios. The compactness parameter of the corona, $l={L\over R}{σ_{\rm T}\over m_{\rm e} c^3}$, is shown to be in the range of 1-33 for Eddington ratios of 0.02 - 0.1. Combined with the electron temperature in the corona, this indicates that electron--positron pair production is not dominant in this regime. A positive relation between the compactness parameter and photon index is also predicted. By comparing the above model predictions with observational features, we find that the model is in agreement with observations.

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arXiv2017

The large-scale nebular pattern of a superwind binary in an eccentric orbit

Preplanetary nebulae and planetary nebulae are evolved, mass-losing stellar objects that show a wide variety of morphologies. Many of these nebulae consist of outer structures that are nearly spherical (spiral/shell/arc/halo) and inner structures that are highly asymmetric (bipolar/multipolar). The coexistence of such geometrically distinct structures is enigmatic because it hints at the simultaneous presence of both wide and close binary interactions, a phenomenon that has been attributed to stellar binary systems with eccentric orbits. Here we report new high-resolution molecular-line observations of the circumstellar spiral-shell pattern of AFGL 3068, an asymptotic giant branch star transitioning to the preplanetary nebula phase. The observations clearly reveal that the dynamics of the mass loss is influenced by the presence of an eccentric-orbit binary. This quintessential object opens a new window on the nature of deeply embedded binary stars through the circumstellar spiral-shell patterns that reside at distances of several thousand Astronomical Units from the stars.

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arXiv2016

Time-dependent Corotation Resonance in Barred Galaxies

The effective potential neighboring the corotation resonance region in barred galaxies is shown to be strongly time-dependent in any rotating frame because of the competition of nearby perturbations of similar strengths with differing rotation speeds. Contrary to the generally adopted assumption, that in the bar rotating frame the corotation region should possess four stationary equilibrium points (Lagrange points), with high quality N-body simulations we localize the instantaneous equilibrium points and find that they circulate or oscillate broadly in azimuth with respect to the pattern speeds of the inner or outer perturbations. This implies that at the particle level the Jacobi integral is not well conserved around the corotation radius. That is, angular momentum exchanges decouple from energy exchanges, enhancing the chaotic diffusion of stars through the corotation region.

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arXiv2016

Self-Gravitational Force Calculation of Infinitesimally Thin Gaseous Disks on Nested Grids

We extend the work of Yen et al. (2012) and develop 2nd order formulae to accommodate a nested grid discretization for the direct self-gravitational force calculation for infinitesimally thin gaseous disks. This approach uses a two-dimensional kernel derived for infinitesimally thin disks and is free of artificial boundary conditions. The self-gravitational force calculation is presented in generalized convolution forms for a nested grid configuration. A numerical technique derived from a fast Fourier transform is employed to reduce the computational complexity to be nearly linear. By comparing with analytic potential-density pairs associated with the generalized Maclaurin disks, the extended approach is verified to be of second order accuracy using numerical simulations. The proposed method is accurate, computationally fast and has the potential to be applied to the studies of planetary migration and the gaseous morphology of disk galaxies.

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