Disk Dynamics Research Articles

Active chiral dynamics and boundary accumulation phenomenon in confined camphor particles.

In this work, we perform experimental and numerical investigations on the dynamics of camphor-infused discs, well-established as active particles in their behavior. Our analysis focuses on examining the individual dynamics of these discs within a confined circular domain, revealing that they exhibit characteristics akin to active chiral particles. To characterize this behavior effectively, we introduce a methodology for estimating key model parameter values from our experiments, including linear velocity, angular velocity, and angular noise intensity. To validate our findings, we compare our experimental results with numerical simulations of the model. Our results demonstrate a striking phenomenon associated with camphor-infused discs: a pronounced accumulation of particles along the boundary. This intriguing observation suggests the occurrence of an attractive interaction between the active particles and the boundary, resulting in a kind of adsorption effect. The latter results in the confinement of the camphor disc along the Petri dish wall, which we refer to as sliding dynamics. We empirically determine the velocity of the particle along the Petri dish wall as well as its fluctuations, properties whose behavior notably deviates from the bulk dynamics.

Future Prospects for Constraining Black Hole Spacetime: Horizon-scale Variability of Astrophysical Jets

The Event Horizon Telescope (EHT) Collaboration has recently published the first horizon-scale images of the supermassive black holes M87* and Sgr A* and provided some first information on the physical conditions in their vicinity. The comparison between the observations and the three-dimensional general relativistic magnetohydrodynamic (GRMHD) simulations has enabled the EHT to set initial constraints on the properties of these black hole spacetimes. However, accurately distinguishing the properties of the accretion flow from those of the spacetime, most notably, the black hole mass and spin, remains challenging because of the degeneracies the emitted radiation suffers when varying the properties of the plasma and those of the spacetime. The next-generation EHT (ngEHT) observations are expected to remove some of these degeneracies by exploring the complex interplay between the disk–jet dynamics, which represents one of the most promising tools for extracting information on the black hole spin. By using GRMHD simulations of magnetically arrested disks and general relativistic radiative transfer (GRRT) calculations of the emitted radiation, we have studied the properties of the jet and the accretion disk dynamics on spatial scales that are comparable with the horizon. In this way, we are able to highlight that the radial and azimuthal dynamics of the jet are well correlated with the black hole spin. Based on the resolution and image reconstruction capabilities of the ngEHT observations of M87*, we can assess the detectability and associated uncertainty of this correlation. Overall, our results serve to assess the prospects for constraining the black hole spin with future EHT observations.

Dynamics of freely falling perforated disks

The dynamics of free-falling perforated disks within the moderate Reynolds number Rem regime (700<Rem<1400) are experimentally investigated in this paper, including translation and rotation dynamics. Four falling styles are analyzed, including spiral motion, Hula-Hoop motion, helical precession motion and spiral irregular motion. In contrast to spiral irregular motion, we group the other three motions as regular motions. The relative contribution of different dynamical effects (impulsive effect, Magnus effect and viscous effect respectively) is quantitatively determined. The contribution of impulsive effect is commonly ignorable during the descent, and Magnus effect plays a crucial role in horizontal oscillations. Vortex loads always dominate this dynamical process, and balance the gravity along the vertical direction. In the Frenet–Serret coordinate system, the contribution resulting from Magnus effect is relatively small to that of vorticity-induced fluid force. Hence, an alternative way to model this involved system is presented, which is based on the extended Kelvin-Kirchhoff equations. A Fast Fourier Transform (FFT) is performed on total hydrodynamics forces FHx and FHy. For regular motions with the dimensionless inertia ratio I∗≤1×10−3, sub-harmonic force components are identified in the frequency domain. However, the frequency ratio of high-frequency components and the dominant peak is no longer integer in spiral irregular motion. The azimuth distribution of angular velocity is counted in the Frenet–Serret coordinate system. For spiral irregular motion with the dimensionless inertia ratio I∗>1×10−3, no preferential alignment is observed and its distribution scatters, while those of regular motions are rather concentrated. The probability density functions (PDFs) of angular velocity components in spiral and Hula-Hoop motions share similar distributions, which indicates angular dynamics may decouple from translation dynamics in high Reynolds number regime under certain circumstances.

The formation of bars and warps in rotating haloes

ABSTRACT We investigate the effects of halo kinematics on the dynamics of stellar discs by simulating the evolution of isolated disc–halo systems from equilibrium initial conditions. Our main results come from four simulations where the initial disc is identical and the halo is either treated as a rigid potential or is ‘live’ with isotropic orbits or orbits that preferentially rotate with or counter to the disc. We confirm previous results that bar formation is more vigorous in models with a live halo than a rigid one and is further enhanced when halo orbits preferentially rotate with the disc. We discuss two types of buckling events with different symmetries about the mid-plane, one that occurs just as the bar is forming and the other well after the bar has been established. We also show that warps are most easily excited and maintained when the halo is counter-rotating with the disc, in agreement with theoretical predictions. Our most novel result is the discovery of a rotating halo instability, which causes the disc and halo cusp to spiral outward from the centre-of-mass of the system whether the halo rotates with the disc or counter to it and also occurs in a disc–bulge–halo system that does not form a bar. We provide a heuristic linear model that captures the essential dynamics of the instability.

Dynamical Analysis of the Maclaurin Disk with Velocity Dispersion and Its Influence on the Formation of the Bar

We investigate the influence of Toomre’s Q parameter on the bar-forming dynamics of the Maclaurin disk using N-body simulations. According to Toomre’s criterion, the local velocity dispersion parametrized by Q ≥ 1 is required to suppress the local axisymmetric instability but, in turn, it deviates particle orbits from a nearly circular limit in which particle natural frequencies are calculated. We resolve this by including the effect of velocity dispersion, as the pressure potential, into the effective potential with the gravitational potential. With this formulation, a circular orbit approximation is retrieved. The effective potential hypothesis can describe the Q dependence of angular and epicyclic motions of the processes of the formation of a bar and the established bars reasonably well provided that Q ≥ 1. This indicates the influence of the initial Q imprinted on the entire disk dynamics and not only that Q serves as the indicator of stability. In addition, we perform a stability test for the disk-in-halo systems. With the presence of a halo, disks are more susceptible to the formation of a bar as seen by the elevated critical Q than that for the isolated disk. This is attributed to the differential rotation that builds the unstable non-axisymmetric spiral modes more efficiently, which are the ingredients of the bar instability.

Scattered disc dynamics: the mapping approach

ABSTRACT We derive, and discuss the properties of, a symplectic map for the dynamics of bodies on nearly parabolic orbits. The orbits are perturbed by a planet on a circular, coplanar orbit interior to the pericentre of the parabolic orbit. The map shows excellent agreement with direct numerical integrations and elucidates how the dynamics depends on perturber mass and pericentre distance. We also use the map to explore the onset of chaos, statistical descriptions of chaotic transport, and sticking in mean-motion resonances. We discuss implications of our mapping model for the dynamical evolution of the Solar system’s scattered disc and other highly eccentric trans-Neptunian objects.

A random 2D walk of a submerged free-floating disc in a convective layer

Free motions of a thermally insulating disc submerged in a liquid layer heated from below are studied experimentally. The disc is fixed at a certain height from the bottom of the convective cell, but can move freely in horizontal directions. Blocking of vertical motion and heat flux leads to the appearance of large-scale vortices, which in turn, drags the disc. The interplay of the large-scale circulation and the disc produces complex two-dimensional disc motions. The disc dynamics is mainly determined by large-scale modes: toroidal vortex with descending fluid in the centre and large-scale vortices along each coordinate, existing on the background of intense, but relatively short-lived small-scale convective cells. The wandering disc forms a cold spot above itself, which affects large-scale circulation but has little effect on cell convection at scales on the order of the layer thickness, which provide the main heat transport and dominate the upper part of the layer. Despite the essentially three-dimensional flow structure, which is fundamentally different from that of the convective flow in an elongated tank with one-dimensional disc motions, the travels along each coordinate look similar to one-dimensional motions.

Multimission view of the low-luminosity ‘obscured’ phase of GRS 1915+105

ABSTRACT GRS 1915+105 has been observed in an ‘obscured’ phase since 2019 May, exhibiting steady and low X-ray luminosities with sporadic re-brightenings. In this paper, we perform a comprehensive and wide-band analysis of the spectral and timing properties of the source during the period 2019–2021 using observations from AstroSat (SXT: 0.5–8 keV; LAXPC: 3–60 keV), the Neutron star Interior Composition Explorer (NICER) (0.5–12 keV), and the Nuclear Spectroscopic Telescope Array (NuSTAR) (3–60 keV). Spectral analysis reveals the presence of a highly variable obscurer ($N_{\rm H_{\rm 1}}\sim$ 1022–1024 atoms cm−2) throughout the observation period. The source is detected in the low/hard state for most of the time, with the spectra being described by a Comptonized component (Γ ∼ 1.16–1.79, kTe ∼ 2–31 keV). The source spectra steepen (Γ ∼ 2.5), indicating a softening of the spectrum, during the rise of the re-brightenings. Various emission and absorption lines corresponding to the neutral Fe Kα, Fe xxv Kα, Fe xxvi Kα, and the Ni xxviii Kα were detected, with the equivalent widths varying in the range 70 eV–3.5 keV. The column density of the absorbing plasma varied in the range 1016–1018 atoms cm−2 at a distance ≤2 × 1010 cm. Interestingly, the source is also seen to exhibit various variability classes (ρ, λ, δ, χ) at relatively low luminosities (∼0.01 LEdd) during the re-brightening phases. Different variability classes show the signature of quasi-periodic oscillations (νQPO: 20–180 mHz, rmsQPO: 7.5 per cent–16 per cent). The source showed a maximum bolometric luminosity (Lbol) of ∼0.01 LEdd (re-brightening phases) and a minimum Lbol of 0.004 LEdd (quiet phase) during the period. We discuss the possible disc dynamics around the black hole during this low-luminosity ‘obscured’ phase.

A new 2D stochastic methodology for simulating variable accretion discs: propagating fluctuations and epicyclic motion

ABSTRACT Accretion occurs across a large range of scales and physical regimes. Despite this diversity in the physics, the observed properties show remarkable similarity. The theory of propagating fluctuations, in which broad-band variability within an accretion disc travel inwards and combine, has long been used to explain these phenomena. Recent numerical work has expanded on the extensive analytical literature but has been restricted to using the 1D diffusion equation for modelling the disc behaviour. In this work we present a novel numerical approach for 2D (vertically integrated), stochastically driven α-disc simulations, generalizing existing 1D models. We find that the theory of propagating fluctuations translates well to 2D. However, the presence of epicyclic motion in 2D (which cannot be captured within the diffusion equation) is shown to have an important impact on local disc dynamics. Additionally, there are suggestions that for sufficiently thin discs the log-normality of the light curves changes. As in previous work, we find that the break frequency in the luminosity power spectrum is strongly dependent on the driving time-scale of the stochastic perturbations within the disc, providing a possible observational signature for probing the magnetorotational instability dynamo. We also find that thinner discs are significantly less variable than thicker ones, providing a compelling explanation for the greater variability seen in the hard state versus the soft state of X-ray binaries. Finally, we consider the wide-ranging applications of our numerical model for use in other simulations.

KURVS: the outer rotation curve shapes and dark matter fractions of z ∼ 1.5 star-forming galaxies

ABSTRACT We present first results from the KMOS Ultra-deep Rotation Velocity Survey (KURVS), aimed at studying the outer rotation curves shape and dark matter content of 22 star-forming galaxies at z ∼ 1.5. These galaxies represent ‘typical’ star-forming discs at z ∼ 1.5, being located within the star-forming main sequence and stellar mass-size relation with stellar masses 9.5 ≤ log(M⋆/M⊙) ≤ 11.5. We use the spatially resolved H α emission to extract individual rotation curves out to 4 times the effective radius, on average, or ∼10–15 kpc. Most rotation curves are flat or rising between three and six disc scale radii. Only three objects with dispersion-dominated dynamics (vrot/σ0 ∼ 0.2) have declining outer rotation curves at more than 5σ significance. After accounting for seeing and pressure support, the nine rotation-dominated discs with vrot/σ0 ≥ 1.5 have average dark matter fractions of $50 \pm 20{{\ \rm per\ cent}}$ at the effective radius, similar to local discs. Together with previous observations of star-forming galaxies at cosmic noon, our measurements suggest a trend of declining dark matter fraction with increasing stellar mass and stellar mass surface density at the effective radius. Measurements of simulated EAGLE galaxies are in quantitative agreement with observations up to log$(M_{\star } R_{\rm eff}^{-2} /\mathrm{M_{\odot } kpc^{-2}}) \sim 9.2$, and overpredict the dark matter fraction of galaxies with higher mass surface densities by a factor of ∼3. We conclude that the dynamics of typical rotationally-supported discs at z ∼ 1.5 is dominated by dark matter from effective radius scales, in broad agreement with cosmological models. The tension with observations at high stellar mass surface density suggests that the prescriptions for baryonic processes occurring in the most massive galaxies (such as bulge growth and quenching) need to be reassessed.

Dynamic Gardner cross-over in a simple glass.

The criticality of the jamming transition responsible for amorphous solidification has been theoretically linked to the marginal stability of a thermodynamic Gardner phase. While the critical exponents of jamming appear independent of the preparation history, the pertinence of Gardner physics far from equilibrium is an open question. To fill this gap, we numerically study the nonequilibrium dynamics of hard disks compressed toward the jamming transition using a broad variety of protocols. We show that dynamic signatures of Gardner physics can be disentangled from the aging relaxation dynamics. We thus define a generic dynamic Gardner cross-over regardless of the history. Our results show that the jamming transition is always accessed by exploring increasingly complex landscape, resulting in anomalous microscopic relaxation dynamics that remains to be understood theoretically.

Protoplanetary Disk Chemistry

Planets form in disks of gas and dust around young stars. The disk molecular reservoirs and their chemical evolution affect all aspects of planet formation, from the coagulation of dust grains into pebbles to the elemental and molecular compositions of the mature planet. Disk chemistry also enables unique probes of disk structures and dynamics, including those directly linked to ongoing planet formation. We review the protoplanetary disk chemistry of the volatile elements H, O, C, N, S, and P; the associated observational and theoretical methods; and the links between disk and planet chemical compositions. Three takeaways from this review are: ▪The disk chemical composition, including the organic reservoirs, is set by both inheritance and in situ chemistry.▪Disk gas and solid O/C/N/H elemental ratios often deviate from stellar values due to a combination of condensation of molecular carriers, chemistry, and dynamics.▪Chemical, physical, and dynamical processes in disks are closely linked, which complicates disk chemistry modeling, but these links also present an opportunity to develop chemical probes of different aspects of disk evolution and planet formation.

Calcium–aluminum‐rich inclusions in non‐carbonaceous chondrites: Abundances, sizes, and mineralogy

AbstractAs the Sun was forming, calcium–aluminum‐rich inclusions (CAIs) were the first rocks to have condensed in the hottest regions of the solar nebula disk. Carbonaceous chondrites (CCs) contain abundant CAIs but are thought to have accreted in the outer Solar System, requiring that CAIs must have been transported outward. Curiously, CAIs are rare in ordinary, enstatite, rumuruti, and kakangari chondrites, non‐carbonaceous chondrites (NCs), that likely formed in the inner Solar System. Thus, CAI abundances and characteristics can provide constraints on the early dynamical evolution of the disk. In this work, we address whether the hypothesis of an early‐formed proto‐Jupiter “opening a gap” in the disk can explain the dichotomy in the relative abundance of CAIs in CC and NC chondrites. We searched 76 NC meteorite sections to find 232 CAIs which have an average apparent diameter of 46 μm and comprise 0.01 area%, about half the size of and ~200 times less abundant than CC CAIs on average. Unlike CC CAIs, only 4% of the NC CAIs contain melilite and most contain alteration features suggesting that NC CAIs underwent pervasive fluid‐assisted thermal metamorphism on asteroidal parent bodies. However, based on NC CAI populations correlating with meteorite metamorphic grade, we argue that disk dynamics is likely the primary reason behind the existence of small (&lt;100 μm) and rare NC CAIs. Our data support astrophysical models which suggest that, after outward transport of CAIs, formation of a gap in the disk trapped CAIs in the outer Solar System.

The phase spiral in Gaia DR3

Aims. We aim to study the phase spiral in the Milky Way (MW) disc with data from the third data release of Gaia (DR3) and use it as an inference tool to decipher the late-time evolution of the Galaxy. Methods. We used an edge-detection algorithm to find the border of the phase spiral, allowing us to robustly quantify its shape at different positions and for different selections. We calculated the time of onset of the phase-mixing by determining the different turns of the phase spiral and using the vertical frequencies from commonly used models of the gravitational potential of the MW. Results. We find that the phase spiral extends down to −1.2 kpc in height below the plane (about 3–5 scale heights of the thin disc) and beyond ±50 km s−1 in VZ. We see a secondary branch mostly at positive vertical velocities when coloured by azimuthal velocity and in the counts projection. We also find complex variations of the phase spirals with angular momentum and azimuth. All these findings are possible evidence of multiple perturbations (from different times or from different perturbers) and/or of the complexity of the phase-mixing process. We detect the phase spiral from 6 to 11 kpc from the Galactic centre and find signatures of vertical asymmetries 1–2 kpc beyond this range. We measure small but clear variations with azimuth. When we determine the phase mixing times from the phase spiral at different angular momenta and using the different spiral turns (at different Z), we obtain inconsistent times with systematic differences (times increasing with |LZ| and with |Z|). Our determinations are mostly in the range of [0.3–0.9] Gyr, with an average of 0.5 Gyr. The inconsistencies do not change when using different commonly used potential models for the MW, different stellar distances, or frequencies for different kinetic temperatures; they could stem from the inconsistency of the assumed gravitational potentials with the true MW, and from oversimplification of the modelling, in particular where self-gravity is neglected or where multiple perturbations and/or interference with other processes are not considered. Conclusions. The wealth of information provided by the new Gaia DR3 data should encourage us to make progress in crucial modelling aspects of the disc dynamics, such as non-equilibrium, self-gravity, propagation of different types of bending waves, and interactions between different mechanisms. Such advancements could finally enable us to establish the origin of the phase spiral and its relation to the Sagittarius dwarf galaxy.

Dynamics of Molecular Gas in the Central Region of the Quasar I Zwicky 1

We present a study of the molecular gas distribution and kinematics in the cicumnuclear region (radii ≲2 kpc) of the z ≈ 0.061 quasar I Zwicky 1 using a collection of available Atacama Large Millimeter/submillimeter Array observations of the carbon monoxide (CO) emission. With an angular resolution of ∼0.″36 (corresponding to ∼400 pc), the host-galaxy substructures including the nuclear molecular gas disk, spiral arms, and a compact bar-like component are resolved. We analyzed the gas kinematics based on the CO image cube and obtained the rotation curve and radial distribution of velocity dispersion. The velocity dispersion is about 30 km s−1 in the outer CO disk region and rises up to ≳100 km s−1 at radius ≲1 kpc, suggesting that the central region of the disk is dynamically hot. We constrain the CO-to-H2 conversion factor, α CO, by modeling the cold gas disk dynamics. We find that, with prior knowledge about the stellar and dark matter components, the α CO value in the circumnuclear region of this quasar host galaxy is , which is between the value reported in ultraluminous infrared galaxies and in the Milky Way. The central 1 kpc region of this quasar host galaxy has significant star formation activity, which can be identified as a nuclear starburst. We further investigate the high-velocity dispersion in the central region. We find that the interstellar medium (ISM) turbulent pressure derived from the gas velocity dispersion is in equilibrium with the weight of the ISM. This argues against extra power from active galactic nuclei feedback that significantly affects the kinematics of the cold molecular gas.

Falling styles of perforated disks

We experimentally investigate the dynamics of thin perforated disks freely falling under gravity in still water. With a stereo vision system, three-dimensional paths are reconstructed. Three motions corresponding to different paths are identified, namely spiral motion, Hula-Hoop motion and helical precession motion. Compared with previous studies, it is found that the lateral displacements of perforated disks are much smaller than those of the other regular particles under the same number of Re and I*. Flow visualizations for freely falling perforated disks are performed using sodium fluorescein dye, and significant differences can be found in the wake. Hairpin vortices can be clearly distinguished in Hula-Hoop motion, which are hardly found in the other motions. Due to the presence of vortices shed from holes, there are complex vortex interactions in the wake, such as adjacent vortices fusion and the secondary induced vortex. Mechanisms behind different wake patterns are analyzed. And different three-dimensional paths under the same I* and Re are observed due to different wake patterns.

Parametric instability in warped astrophysical discs: growth, saturation, and feedback

ABSTRACT Attempts to understand the dynamics of warped astrophysical discs have garnered significant attention, largely motivated by the growing catalogue of observed distorted systems. Previous studies have shown that the evolution of the warp is crucially regulated by the internal flow fields established by the undulating geometry. These are typically modelled as laminar horizontal, shearing flows which oscillate back and forth at approximately the orbital frequency. However this shearing motion is known to be susceptible to a hydrodynamic, parametric instability of inertial waves which might modify the warped dynamics. Whilst the linear growth phase is well understood, the subsequent non-linear saturation combined with the self-consistent feedback onto the warp has not been studied. In this work, we implement a novel numerical setup using the recent ring model framework of Fairbairn and Ogilvie, within the Lagrangian code gizmo. We formally identify several locally growing modes in the simulation, as predicted by a three-mode coupling analysis of the instability, and find decent agreement with the theoretical growth rates. We understand the saturation mechanism as a wave breaking process which suppresses the growth of shorter wavelength parametric couplings first, whilst allowing the longest mode to dominate the final quasi-steady, wave-like turbulence. The Reynolds stresses, transporting energy from the warp to the small scales, can be effectively modelled using a time-dependent, anisotropic viscous alpha model which closely captures the amplitude and phase evolution of the warp. Consequently, this model might help inform future global studies which are commonplace but typically do not resolve the parametric instability.

The Bardeen–Petterson effect, disc breaking, and the spin orientations of supermassive black hole binaries

ABSTRACT Supermassive black hole binaries are driven to merger by dynamical friction, loss-cone scattering of individual stars, disc migration, and gravitational wave emission. Two main formation scenarios are expected. Binaries that form in gas-poor galactic environments do not experience disc migration and likely enter the gravitational wave-dominated phase with roughly isotropic spin orientations. Comparatively, binaries that evolve in gas-rich galactic environments might experience prominent phases of disc accretion, where the Bardeen–Petterson effect acts to align the spins of the black holes with the orbital angular momentum of the disc. However, if the accretion disc breaks, alignment is expected to be strongly suppressed – a phenomenon that was recently shown to occur in a large portion of the parameter space. In this paper, we develop a semi-analytical model of joint gas-driven migration and spin alignment of supermassive black hole binaries taking into account the impact of disc breaking for the first time. Our model predicts the occurrence of distinct subpopulations of binaries depending on the efficiency of spin alignment. This implies that future gravitational wave observations of merging black holes could potentially be used to (i) discriminate between gas-rich and gas-poor hosts and (ii) constrain the dynamics of warped accretion discs.

On the crown formation and cavity dynamics of free-falling thick disks

The water entry, crown formation, and cavity dynamics of gravity-driven thick solid and annular disks were studied by conducting a series of detailed laboratory experiments. Three different release conditions were examined to study the fully guarded and partially guarded release conditions on the cavity dynamics. The effects of aspect ratio, geometry, and density of disks on crown formation, interface impact, seal development, and air entrainment were investigated. Four crown shapes were identified, and the crown structure was classified based on the dimensionless moment of inertia, I*, and the impact Froude number, Fro. The effects of controlling parameters on variations of crown dimensions were investigated. It was found that the normalized crown diameter decreased with I* and crown geometries were found to be smaller in annular disks. Experimental observations have shown that high-density disks have larger pinch-off depth and form a deep seal in the ambient water. The normalized pinch-off depth increased with Fro and the normalized pinch-off depth in cylindrical thick disks were smaller than thin disks and spherical objects. The temporal variations of cavity indicated a non-linear correlation between the growth rate of normalized pinch-off depth and time. The disk's velocities in three different stages were measured, it was found that the settling velocities followed a linear relationship with I*, and it was affected by the release conditions. The velocity and vortex fields were extracted from the particle image velocimetry data. The velocity fields showed that the solid disks affected a greater surrounding ambient in comparison to the annular disks due to their higher initial momentum. Periodic vortex shedding was formed in the wake of annular disks, and the frequency of the vortex field was found to be proportional to the disk density.

Effect of speed fluctuations on the collective dynamics of active disks.

Numerical simulations are performed on the collective dynamics of active disks, whose self-propulsion speed (U) varies in time, and whose orientation evolves according to rotational Brownian motion. Two protocols for the evolution of speed are considered: (i) a deterministic one involving a periodic change in U at a frequency ω; and (ii) a stochastic one in which the speeds are drawn from a power-law distribution at time-intervals governed by a Poissonian process of rate β. In the first case, an increase in ω causes the disks to go from a clustered state to a homogeneous one through an apparent phase-transition, provided that the direction of self-propulsion is allowed to reverse. Similarly, in the second case, for a fixed value of β, the extent of cluster-breakup is larger when reversals in the self-propulsion direction are permitted. Motility-induced phase separation of the disks may therefore be avoided in active matter suspensions in which the constituents are allowed to reverse their self-propulsion direction, immaterial of the precise temporal nature of the reversal (deterministic or stochastic). Equally, our results demonstrate that phase separation could occur even in the absence of a time-averaged motility of an individual active agent, provided that the rate of direction reversals is smaller than the orientational diffusion rate.