3D Smoothed Particle Hydrodynamics Simulations Research Articles

Dust formation in common envelope binary interaction – I: 3D simulations using the Bowen approximation

ABSTRACT We carried out 3D smoothed particle hydrodynamics simulations of the common envelope binary interaction using the approximation of Bowen to calculate the dust opacity in order to investigate the resulting dust-driven accelerations. We have simulated two types of binary star: a 1.7 and a 3.7 M⊙ thermally pulsating, asymptotic giant branch stars with a 0.6 M⊙ companion. We carried out simulations using both an ideal gas and a tabulated equations of state, with the latter considering the recombination energy of the envelope. We found that the dust-driven wind leads to a relatively small increase in the unbound gas, with the effect being smaller for the tabulated equation of state simulations. Dust acceleration does contribute to envelope expansion with only a slightly elongated morphology, if we believe the results from the tabulated equation of state as more reliable. The Bowen opacities in the outer envelopes of the two models, at late times, are large enough that the photosphere of the post-in spiral object is about ten times larger compared to the same without accounting for the dust opacities. As such, the prediction of the appearance of the transient would change substantially if dust is included.

Sub-stellar engulfment by a main-sequence star: Where is the lithium?

Context. Since the discovery of exoplanetary systems, questions have been raised as to the sub-stellar companions that can survive encounters with their host star, and how this interaction may affect the internal structure and evolution of the hosting star, and particularly its surface chemical composition. Aims. We study whether the engulfment of a brown dwarf (BD) by a solar-like main-sequence (MS) star can significantly alter the structure of the star and the Li content on its surface. Methods. We performed 3D smoothed particle hydrodynamics simulations of the engulfment of a BD with masses 0.01 and 0.019 M⊙, on an MS star of 1 M⊙ and solar composition, in three different scenarios: a head-on collision, a grazing collision with an impact parameter η = 0.5 R⊙, and a merger. We studied the dynamics of the interaction in detail, and the relevance of the type of interaction and the mass of the BD on the final fate of the sub-stellar object and the host star in terms of mass loss of the system, angular momentum transfer, and changes in the Li abundance on the surface of the host star. Results. In all the studied scenarios, most of the BD mass is diluted in the denser region of the MS star. Only in the merger scenario a significant fraction (∼40%) of the BD material would remain in the outer layers. We find a clear increase in the surface rotational velocity of the host star after the interaction, ranging between 25 km s−1 (grazing collision) to 50 km s−1 (merger). We also find a significant mass loss from the system (in the range 10−4 − 10−3 M⊙) due to the engulfment, which in the case of the merger may form a circumstellar disk-like structure. Assuming that neither the depth of the convective envelope of the host star nor its mass content are modified during the interaction, a small change in the surface Li abundance in the head-on and grazing collisions is found. However, in the merger we find large Li enhancements, by factors of 20 − 30, depending on the BD mass. Some of these features could be detected observationally in the host star, provided they remained for a long enough time. Conclusions. In our 3D simulations, a sizable fraction of the BD survives long enough to be mixed with the inner core of the MS star. This is at odds with previous suggestions based on 1D simulations. In some cases the final surface rotational velocity is very high, coupled with enough mass loss that may form a circumstellar disk. Merger scenarios tend to dilute considerably more BD material on the surface of the MS star, which could be detected as a Li-enhancement. The dynamic of the simulated scenarios suggests the development of asymmetries in the structure of the host star that can only be tackled with 3D codes, including the long-term evolution of the system.

Continuing to hide signatures of gravitational instability in protoplanetary discs with planets

ABSTRACT We carry out 3D smoothed particle hydrodynamics simulations to study the impact of planet–disc interactions on a gravitationally unstable protoplanetary disc. We find that the impact of a planet on the disc’s evolution can be described by three scenarios. If the planet is sufficiently massive, the spiral wakes generated by the planet dominate the evolution of the disc and gravitational instabilities are completely suppressed. If the planet’s mass is too small, then gravitational instabilities are unaffected. If the planet’s mass lies between these extremes, gravitational instabilities are weakened. We present mock Atacama Large Millimeter/submillimeter Array (ALMA) continuum observations showing that the observability of large-scale spiral structures is diminished or completely suppressed when the planet is massive enough to influence the disc’s evolution. Our results show that massive discs that would be expected to be gravitationally unstable can appear axisymmetric in the presence of a planet. Thus, the absence of observed large-scale spiral structures alone is not enough to place upper limits on the disc’s mass, which could have implications on observations of young Class I discs with rings and gaps.

Common envelope binary interaction simulations between a thermally pulsating AGB star and a low mass companion.

At least one in five of all planetary nebulae are the product of a common envelope (CE) interaction, where the companion in-spirals into the envelope of an asymptotic giant branch (AGB) star ejecting the nebula and leaving behind a compact binary. In this work we carry out 3D smoothed particle hydrodynamics simulations of the CE interaction between a 1.7M⊙ AGB star and a 0.6M⊙ companion. We model the AGB structure using a 1D stellar model taken at the seventh thermal pulse. The interaction takes place when the giant is on the expanding phase of the seventh thermal pulse and has a radius of 250R⊙. The post-CE orbital separations varies between 20 and 31R⊙, with the inclusion of recombination energy resulting in wider separations. Based on the observed short in-spiral time-scales, we suggest that thermal pulses can trigger CEs, extending the ability of AGB stars to capture companions into CEs, that would lead to the prediction of a larger population of post-AGB, post-CE binaries. Simulations that include a tabulated equationof state unbind a great deal more gas, likely unbinding the entire envelope on short time-scales. The shape of the CE after the in-spiral is more spherical for AGB than red giant branch stars, and even more so if recombination energy is included. We expect that the planetary nebula formed from this CE will have different features from those predicted by Zou etal. 2020.

Clumpy Accretion in Pre-main-sequence Stars as a Source of Perturbations in Circumstellar Disks

The development of perturbations in the circumstellar disks of pre-main-sequence stars caused by clumpy accretion was investigated. Here we perform 3D smoothed particle hydrodynamics simulations of disks perturbed by a recent clump accretion event. These simulations are further explored by radiative transfer calculations to quantify the observational appearance of such disks. It was shown that the density waves in the disks were formed at the fall of the clump. After several revolutions they can transform into spirals and ring structures. Their images in millimeter wavelengths are very similar to those observed with Atacama Large Millimeter/submillimeter Array in some protoplanetary disks. We assume that clumpy accretion may be the source of such structures.

Binary companions triggering fragmentation in self-gravitating discs

ABSTRACT Observations of systems hosting close-in (<1 au) giant planets and brown dwarfs (M ≳ 7 MJup) find an excess of binary-star companions, indicating that stellar multiplicity may play an important role in their formation. There is now increasing evidence that some of these objects may have formed via fragmentation in gravitationally unstable discs. We present a suite of 3D smoothed particle hydrodynamics simulations of binary-star systems with circumprimary self-gravitating discs, which include a realistic approximation to radiation transport, and extensively explore the companion’s orbital parameter space for configurations that may trigger fragmentation. We identify a ‘sweet spot’ where intermediate separation binary companions (100 au ≲ a ≲ 400 au) can cause a marginally stable disc to fragment. The exact range of ideal binary separations is a function of the companion’s eccentricity, inclination, and mass. Heating is balanced by efficient cooling, and fragmentation occurs inside a spiral mode driven by the companion. Short separation, disc-penetrating binary encounters (a ≲ 100 au) are prohibitive to fragmentation, as mass stripping and disc heating quench any instability. This is also true of binary companions with high orbital eccentricities (e ≳ 0.75). Wide separation companions (a ≳ 500 au) have little effect on the disc properties for the set-up parameters considered here. The sweet spot found is consistent with the range of binary separations that display an excess of close-in giant planets and brown dwarfs. Hence, we suggest that fragmentation triggered by a binary companion may contribute to the formation of these substellar objects.

Circumbinary disc self-gravity governing supermassive black hole binary mergers

Abstract Understanding the interaction of massive black hole binaries with their gaseous environment is crucial since at sub-parsec scales the binary is too wide for gravitational wave emission to take over and to drive the two black holes to merge. We here investigate the interaction between a massive black hole binary and a self-gravitating circumbinary disc using 3D smoothed particle hydrodynamics simulations. We find that, when the disc self-gravity regulates the angular momentum transport, the binary semi-major axis decreases regardless the choice of disc masses and temperatures, within the range we explored. In particular, we find that the disc initial temperature (hence the disc aspect ratio) has little effect on the evolution of the binary since discs with the same mass self-regulate towards the same temperature. Initially warmer discs cause the binary to shrink on a slightly shorter timescale until the disc has reached the self-regulated equilibrium temperature. More massive discs drive the binary semi-major axis to decrease at a faster pace compared to less massive discs and result in faster binary eccentricity growth even after the initial-condition-dependent transient evolution. Finally we investigate the effect that the initial cavity size has on the binary-disc interaction and we find that, in the self-gravitating regime, an initially smaller cavity leads to a much faster binary shrinking, as expected. Our results are especially important for very massive black hole binaries such as those in the PTA band, for which gas self gravity cannot be neglected.

Atmospheric Erosion by Giant Impacts onto Terrestrial Planets: A Scaling Law for any Speed, Angle, Mass, and Density

Abstract We present a new scaling law to predict the loss of atmosphere from planetary collisions for any speed, angle, impactor mass, target mass, and body composition, in the regime of giant impacts onto broadly terrestrial planets with relatively thin atmospheres. To this end, we examine the erosion caused by a wide range of impacts, using 3D smoothed particle hydrodynamics simulations with sufficiently high resolution to directly model the fate of low-mass atmospheres around 1% of the target’s mass. Different collision scenarios lead to extremely different behaviors and consequences for the planets. In spite of this complexity, the fraction of lost atmosphere is fitted well by a power law. Scaling is independent of the system mass for a constant impactor mass ratio. Slow atmosphere-hosting impactors can also deliver a significant mass of atmosphere, but always accompanied by larger proportions of their mantle and core. Different Moon-forming impact hypotheses suggest that around 10%–60% of a primordial atmosphere could have been removed directly, depending on the scenario. We find no evident departure from the scaling trends at the extremes of the parameters explored. The scaling law can be incorporated readily into models of planet formation.

The evolution of large cavities and disc eccentricity in circumbinary discs

ABSTRACT We study the mutual evolution of the orbital properties of high-mass ratio, circular, co-planar binaries and their surrounding discs, using 3D Smoothed Particle Hydrodynamics simulations. We investigate the evolution of binary and disc eccentricity, cavity structure, and the formation of orbiting azimuthal overdense features in the disc. Even with circular initial conditions, all discs with mass ratios q > 0.05 develop eccentricity. We find that disc eccentricity grows abruptly after a relatively long time-scale (∼400–700 binary orbits), and is associated with a very small increase in the binary eccentricity. When disc eccentricity grows, the cavity semimajor axis reaches values $a_{\rm cav}\approx 3.5\, a_{\rm bin}$. We also find that the disc eccentricity correlates linearly with the cavity size. Viscosity and orbit crossing appear to be responsible for halting the disc eccentricity growth – eccentricity at the cavity edge in the range ecav ∼ 0.05–0.35. Our analysis shows that the current theoretical framework cannot fully explain the origin of these evolutionary features when the binary is almost circular (ebin ≲ 0.01); we speculate about alternative explanations. As previously observed, we find that the disc develops an azimuthal overdense feature in Keplerian motion at the edge of the cavity. A low-contrast overdensity still co-moves with the flow after 2000 binary orbits; such an overdensity can in principle cause significant dust trapping, with important consequences for protoplanetary disc observations.

Planet migration, resonant locking, and accretion streams in PDS 70: comparing models and data

ABSTRACTThe disc surrounding PDS 70, with two directly imaged embedded giant planets, is an ideal laboratory to study planet–disc interaction. We present 3D smoothed particle hydrodynamics simulations of the system. In our simulations, planets, which are free to migrate and accrete mass, end up in a locked resonant configuration that is dynamically stable. We show that features observed at infrared (scattered light) and millimetre (thermal continuum) wavelengths are naturally explained by the accretion stream on to the outer planet, without requiring a circumplanetary disc around Planet c. We post-processed our near-infrared synthetic images in order to account for observational biases known to affect high-contrast images. Our successful reproduction of the observations indicates that planet–disc dynamical interactions alone are sufficient to explain the observations of PDS 70.

Atmospheric Erosion by Giant Impacts onto Terrestrial Planets

Abstract We examine the mechanisms by which the atmosphere can be eroded by giant impacts onto Earth-like planets with thin atmospheres, using 3D smoothed particle hydrodynamics simulations with sufficient resolution to directly model the fate of low-mass atmospheres. We present a simple scaling law to estimate the fraction lost for any impact angle and speed in this regime. In the canonical Moon-forming impact, only around 10% of the atmosphere would have been lost from the immediate effects of the collision. There is a gradual transition from removing almost none to almost all of the atmosphere for a grazing impact as it becomes more head-on or increases in speed, including complex, nonmonotonic behavior at low impact angles. In contrast, for head-on impacts, a slightly greater speed can suddenly remove much more atmosphere. Our results broadly agree with the application of 1D models of local atmosphere loss to the ground speeds measured directly from our simulations. However, previous analytical models of shock-wave propagation from an idealized point-mass impact significantly underestimate the ground speeds and hence the total erosion. The strong dependence on impact angle and the interplay of multiple nonlinear and asymmetrical loss mechanisms highlight the need for 3D simulations in order to make realistic predictions.

Fragmentation favoured in discs around higher mass stars

ABSTRACT We investigate how a protoplanetary disc’s susceptibility to gravitational instabilities and fragmentation depends on the mass of its host star. We use 1D disc models in conjunction with 3D smoothed particle hydrodynamics simulations to determine the critical disc-to-star mass ratios at which discs become unstable against fragmentation, finding that discs become increasingly prone to the effects of self-gravity as we increase the host star mass. The actual limit for stability is sensitive to the disc temperature, so if the disc is optically thin stellar irradiation can dramatically stabilize discs against gravitational instability. However, even when this is the case we find that discs around 2 M⊙ stars are prone to fragmentation, which will act to produce wide-orbit giant planets and brown dwarfs. The consequences of this work are twofold: that low-mass stars could in principle support high disc-to-star mass ratios, and that higher mass stars have discs that are more prone to fragmentation, which is qualitatively consistent with observations that favour high-mass wide-orbit planets around higher mass stars. We also find that the initial masses of these planets depends on the temperature in the disc at large radii, which itself depends on the level of stellar irradiation.

Influence of interfacial bonding conditions on the anti-penetration performance of ceramic/metal composite targets

This study analyzes the influence of bonded and unbonded interface conditions on the anti-penetration performance of a ceramic/metal composite target and determines the associated mechanism. The 3D finite element and 3D smoothed particle hydrodynamics simulation results revealed that a bonded ceramic/metal target exhibited better anti-penetration performance than an unbonded target, and the associated mechanism was determined. Notably, the bond strength between the ceramic and metal backplate plays an important role in the formation of the ceramic conoid, and the ceramic conoid that formed in the bonded target effectively consumed the kinetic energy of the projectile, thereby improving the anti-penetration performance of ceramic composite armor. To verify this conclusion, we also compare and analyze the anti-penetration performance of interface bonded and unbonded metal/metal composite targets. The results show that due to the absence of the ceramic conoid, the interfacial bonding conditions have little influence on the anti-penetration performance of a metal/metal composite target.

On the Ultra-relativistic Prompt Emission, the Hard and Soft X-Ray Flares, and the Extended Thermal Emission in GRB 151027A

Abstract We analyze GRB 151027A within the binary-driven hypernova approach, with a progenitor of a carbon–oxygen core on the verge of a supernova (SN) explosion and a binary companion neutron star (NS). The hypercritical accretion of the SN ejecta onto the NS leads to its gravitational collapse into a black hole (BH), to the emission of the gamma-ray burst (GRB), and to a copious e + e − plasma. The impact of this e + e − plasma on the SN ejecta explains the early soft X-ray flare observed in long GRBs. Here, we apply this approach to the ultra-relativistic prompt emission (UPE) and to the hard X-ray flares. We use GRB 151027A as a prototype. From the time-integrated and the time-resolved analysis, we identify a double component in the UPE and confirm its ultra-relativistic nature. We confirm the mildly relativistic nature of the soft X-ray flare, of the hard X-ray flare, and of the extended thermal emission (ETE). We show that the ETE identifies the transition from an SN to a hypernova (HN). We then address the theoretical justification of these observations by integrating the hydrodynamical propagation equations of the e + e − into the SN ejecta, with the latter independently obtained from 3D smoothed particle hydrodynamics simulations. We conclude that the UPE, the hard X-ray flare, and the soft X-ray flare do not form a causally connected sequence. Within our model, they are the manifestation of the same physical process of the BH formation as seen through different viewing angles, implied by the morphology and the ∼300 s rotation period of the HN ejecta.

Flybys in protoplanetary discs: I. Gas and dust dynamics

We present 3D smoothed particle hydrodynamics simulations of protoplanetary discs undergoing a flyby by a stellar perturber on a parabolic orbit lying in a plane inclined relative to the disc mid-plane. We model the disc as a mixture of gas and dust, with grains ranging from 1 {\mu}m to 10 cm in size. Exploring different orbital inclinations, periastron distances and mass ratios, we investigate the disc dynamical response during and after the flyby. We find that flybys induce evolving spiral structure in both gas and dust which can persist for thousands of years after periastron. Gas and dust structures induced by the flyby differ because of drag-induced effects on the dust grains. Variations in the accretion rate by up to an order of magnitude occur over a time-scale of order 10 years or less, inducing FU Orionis-like outbursts. The remnant discs are truncated and warped. The dust disc is left more compact than the gas disc, both because of disc truncation and accelerated radial drift of grains induced by the flyby.

3D SPH Simulation of Dynamic Water Surface and Its Interaction with Underlying Flow Structure for Turbulent Open Channel Flows Over Rough Beds

In this study, a fully 3D numerical model based on the Smoothed Particle Hydrodynamics (SPH) approach has been developed to simulate turbulent open channel flows over a fixed rough bed. The model focuses on the study of dynamic free surface behavior as well as its interaction with underlying flow structures near the rough bed. The model is improved from the open source code SPHysics ( http://www.sphysics.org ) by adding more advanced turbulence and rough bed treatment schemes. A modified sub-particle-scale (SPS) eddy viscosity model is proposed to reflect the turbulence transfer mechanisms and a modified drag force equation is included into the momentum equations to account for the existence of roughness elements on the bed as well as on the sidewalls. The computed results of various free surface patterns have been compared with the laboratory measurements of the fluctuating water surface elevations in the streamwise and spanwise directions of a rectangular open-channel flow under a range of flow conditions. The comparison has demonstrated that the proposed 3D SPH model can simulate well the complex free surface flows over a fixed rough bed.

High-resolution Hydrodynamic Simulation of Tidal Detonation of a Helium White Dwarf by an Intermediate Mass Black Hole

Abstract We demonstrate tidal detonation during a tidal disruption event (TDE) of a helium (He) white dwarf (WD) with 0.45 M ⊙ by an intermediate mass black hole using extremely high-resolution simulations. Tanikawa et al. have shown tidal detonation in results of previous studies from unphysical heating due to low-resolution simulations, and such unphysical heating occurs in three-dimensional (3D) smoothed particle hydrodynamics (SPH) simulations even with 10 million SPH particles. In order to avoid such unphysical heating, we perform 3D SPH simulations up to 300 million SPH particles, and 1D mesh simulations using flow structure in the 3D SPH simulations for 1D initial conditions. The 1D mesh simulations have higher resolutions than the 3D SPH simulations. We show that tidal detonation occurs and confirm that this result is perfectly converged with different space resolution in both 3D SPH and 1D mesh simulations. We find that detonation waves independently arise in leading parts of the WD, and yield large amounts of 56Ni. Although detonation waves are not generated in trailing parts of the WD, the trailing parts would receive detonation waves generated in the leading parts and would leave large amounts of Si group elements. Eventually, this He WD TDE would synthesize 56Ni of 0.30 M ⊙ and Si group elements of 0.08 M ⊙, and could be observed as a luminous thermonuclear transient comparable to SNe Ia.

Does Explosive Nuclear Burning Occur in Tidal Disruption Events of White Dwarfs by Intermediate-mass Black Holes?

Abstract We investigate nucleosynthesis in tidal disruption events (TDEs) of white dwarfs (WDs) by intermediate-mass black holes. We consider various types of WDs with different masses and compositions by means of three-dimensional (3D) smoothed particle hydrodynamics (SPH) simulations. We model these WDs with different numbers of SPH particles, N, from a few 104 to a few 107 in order to check mass resolution convergence, where SPH simulations with N > 107 (or a space resolution of several 106 cm) have unprecedentedly high resolution in this kind of simulation. We find that nuclear reactions become less active with increasing N and that these nuclear reactions are excited by spurious heating due to low resolution. Moreover, we find no shock wave generation. In order to investigate the reason for the absence of a shock wave, we additionally perform one-dimensional (1D) SPH and mesh-based simulations with a space resolution ranging from 104 to 107 cm, using a characteristic flow structure extracted from the 3D SPH simulations. We find shock waves in these 1D high-resolution simulations, one of which triggers a detonation wave. However, we must be careful of the fact that, if the shock wave emerged in an outer region, it could not trigger the detonation wave due to low density. Note that the 1D initial conditions lack accuracy to precisely determine where a shock wave emerges. We need to perform 3D simulations with ≲106 cm space resolution in order to conclude that WD TDEs become optical transients powered by radioactive nuclei.

3D printing meets computational astrophysics: deciphering the structure of η Carinae's inner colliding winds

We present the first 3D prints of output from a supercomputer simulation of a complex astrophysical system, the colliding stellar winds in the massive (≳120 M⊙), highly eccentric (e ∼ 0.9) binary star system η Carinae. We demonstrate the methodology used to incorporate 3D interactive figures into a PDF (Portable Document Format) journal publication and the benefits of using 3D visualization and 3D printing as tools to analyse data from multidimensional numerical simulations. Using a consumer-grade 3D printer (MakerBot Replicator 2X), we successfully printed 3D smoothed particle hydrodynamics simulations of η Carinae's inner (r ∼ 110 au) wind–wind collision interface at multiple orbital phases. The 3D prints and visualizations reveal important, previously unknown ‘finger-like’ structures at orbital phases shortly after periastron (ϕ ∼ 1.045) that protrude radially outwards from the spiral wind–wind collision region. We speculate that these fingers are related to instabilities (e.g. thin-shell, Rayleigh–Taylor) that arise at the interface between the radiatively cooled layer of dense post-shock primary-star wind and the fast (3000 km s−1), adiabatic post-shock companion-star wind. The success of our work and easy identification of previously unrecognized physical features highlight the important role 3D printing and interactive graphics can play in the visualization and understanding of complex 3D time-dependent numerical simulations of astrophysical phenomena.

On the Bardeen–Petterson effect in black hole accretion discs

We investigate the effect of black hole spin on warped or misaligned accretion discs - in particular i) whether or not the inner disc edge aligns with the black hole spin and ii) whether the disc can maintain a smooth transition between an aligned inner disc and a misaligned outer disc, known as the Bardeen-Petterson effect. We employ high resolution 3D smoothed particle hydrodynamics simulations of $\alpha$-discs subject to Lense-Thirring precession, focussing on the bending wave regime where the disc viscosity is smaller than the aspect ratio $\alpha \lesssim H/R$. We first address the controversy in the literature regarding possible steady-state oscillations of the tilt close to the black hole. We successfully recover such oscillations in 3D at both small and moderate inclinations ($\lesssim 15^{\circ}$), provided both Lense-Thirring and Einstein precession are present, sufficient resolution is employed, and provided the disc is not so thick so as to simply accrete misaligned. Second, we find that discs inclined by more than a few degrees in general steepen and break rather than maintain a smooth transition, again in contrast to previous findings, but only once the disc scale height is adequately resolved. Finally, we find that when the disc plane is misaligned to the black hole spin by a large angle, the disc 'tears' into discrete rings which precess effectively independently and cause rapid accretion, consistent with previous findings in the diffusive regime ($\alpha \gtrsim H/R$). Thus misalignment between the disc and the spin axis of the black hole provides a robust mechanism for growing black holes quickly, regardless of whether the disc is thick or thin.