Bimodal Velocity Distribution Research Articles

Spin–Orbit Alignment of Early-type Astrometric Binaries and the Origin of Slow Rotators

The spin–orbit alignment of binary stars traces their formation and accretion history. Previous studies of spin–orbit alignment have been limited to small samples, slowly rotating solar-type stars, and/or wide visual binaries that not surprisingly manifest random spin–orbit orientations. We analyze 917 Gaia astrometric binaries across periods P = 100–3000 days (a = 0.5–5 au) that have B8-F1 IV/V primaries (M 1 = 1.5–3 M ⊙) and measured projected rotational velocities v sin i. The primary stars in face-on orbits exhibit substantially smaller v sin i compared to those in edge-on orbits at the 6σ level, demonstrating significant spin–orbit alignment. The primaries in our astrometric binaries are rotating more slowly than their single-star or wide-binary counterparts and therefore comprise the slow-rotator population in the observed bimodal rotational velocity distribution of early-type stars. We discuss formation models of close binaries where some of the disk angular momentum is transferred to the orbit and/or secondary spin, quenching angular momentum flow to the primary spin. The primaries in astrometric binaries with small mass ratios q = M 2/M 1 < 0.3 possess even smaller v sin i, consistent with model predictions. Meanwhile, astrometric binaries with large eccentricities e > 0.4 do not display spin–orbit alignment or spin reduction. Using a Monte Carlo technique, we measure a spin–orbit alignment fraction of F align = 75% ± 5% and an average spin reduction factor of 〈S align〉 = 0.43 ± 0.04. We conclude that 75% of close A-type binaries likely experienced circumbinary disk accretion and probably formed via disk fragmentation and inward disk migration. The remaining 25%, mostly those with e > 0.4, likely formed via core fragmentation and orbital decay via dynamical friction.

VLBA Astrometry of the Fastest-spinning Magnetar Swift J1818.0−1607: A Large Trigonometric Distance and a Small Transverse Velocity

In addition to being the most magnetic objects in the known Universe, magnetars are the only objects observed to generate fast-radio-burst-like emissions. The formation mechanism of magnetars is still highly debated and may potentially be probed with the magnetar velocity distribution. We carried out a 3 yr long astrometric campaign on Swift J1818.0−1607, the fastest-spinning magnetar, using the Very Long Baseline Array. After applying the phase-calibrating 1D interpolation strategy, we obtained a small proper motion of 8.5 mas yr−1 mag and a parallax of 0.12 ± 0.02 mas (uncertainties at 1σ confidence throughout the Letter) for Swift J1818.0−1607. The latter is the second magnetar parallax and is among the smallest neutron star parallaxes ever determined. From the parallax, we derived the distance 9.4−1.6+2.0 kpc, which locates Swift J1818.0−1607 at the far side of the Galactic central region. Combined with the distance, the small proper motion leads to a transverse peculiar velocity v⊥=48−16+50 km s−1—a new lower limit to magnetar v ⊥. Incorporating previous v ⊥ estimates of seven other magnetars, we acquired v⊥=149−68+132 km s−1 for the sample of astrometrically studied magnetars, corresponding to the three-dimensional space velocity ∼190−87+168 km s−1, smaller than the average level of young pulsars. Additionally, we found that the magnetar velocity sample does not follow the unimodal young pulsar velocity distribution reported by Hobbs et al. at >2σ confidence, while loosely agreeing with more recent bimodal young pulsar velocity distributions derived from relatively small samples of quality astrometric determinations.

Discrete magnification lens model: A new hybrid multi-scale modelling method for fluid-particle systems

We present a novel hybrid multi-scale model, referred to as “discrete magnification lens” method, which utilizes a computational fluid dynamics coupled to discrete element method (CFD-DEM) approach within a specific region of interest in a two-fluid model (TFM) simulation. We applied the discrete magnification lens to the crossing jet problem, where the TFM fails due to lack of considering bi-modal velocity distribution in its formulation. It was observed that adopting the discrete magnification lens, can effectively alter the non-physical behavior of the TFM to obey the behavior seen from CFD-DEM simulations. Moreover, the ability of the discrete magnification lens in reproducing particle clusters and improving the TFM predictions in case of unbounded fluidization simulations was further shown. We found that the discrete magnification lens can effectively enhance the TFM predictions in a complex fluid-particle system while requiring less numerical costs compared to the full-scale CFD-DEM simulations.

Runaway OB Stars in the Small Magellanic Cloud. III. Updated Kinematics and Insights into Dynamical versus Supernova Ejections

We use the kinematics of field OB stars to estimate the frequencies of runaway stars generated by the dynamical ejection scenario (DES), the binary supernova scenario (BSS), and the combined two-step mechanism. We update the proper motions for field OB and OBe stars in the Small Magellanic Cloud (SMC) using Gaia DR3. Our sample now contains 336 stars from the Runaways and Isolated O-Type Star Spectroscopic Survey of the SMC, and we update our algorithm to calculate more accurate velocities compared to those obtained previously from DR2. We find a decrease in median velocity from 39 to 29 km s−1, implying that the proper motions from our previous work were systematically overestimated. We present the velocity distribution for OBe stars and quantitatively compare it to those of noncompact binaries and high-mass X-ray binaries. We confirm that OBe stars appear to be dominated by the BSS and are likely post-SN binary systems, further supporting the mass-transfer model to explain the origin of their emission-line disks. In contrast, normal OB stars may show a bimodal velocity distribution, which may be expected from different processes that occur with dynamical ejections. The kinematics of fast-rotating OB stars are similar to those of normal OB stars rather than OBe stars, suggesting that the origin of their high vrsini is different from that of OBe stars. We update our model parameters describing the kinematic origins of the SMC field population, still confirming that for runaway stars, the DES mechanism dominates, and two-step ejections seem comparable in frequency to pure BSS ejections.

Evidence of non-Maxwellian ion velocity distributions in spherical shock-driven implosions.

The ion velocity distribution functions of thermonuclear plasmas generated by spherical laser direct drive implosions are studied using deuterium-tritium (DT) and deuterium-deuterium (DD) fusion neutron energy spectrum measurements. A hydrodynamic Maxwellian plasma model accurately describes measurements made from lower temperature (<10keV), hydrodynamiclike plasmas, but is insufficient to describe measurements made from higher temperature more kineticlike plasmas. The high temperature measurements are more consistent with Vlasov-Fokker-Planck (VFP) simulation results which predict the presence of a bimodal plasma ion velocity distribution near peak neutron production. These measurements provide direct experimental evidence of non-Maxwellian ion velocity distributions in spherical shock driven implosions and provide useful data for benchmarking kinetic VFP simulations.

Capture of Dark Matter Particles by a Galaxy in the Case of a Bimodal Distribution of Their Velocities

We have analyzed the rate of capture of dark matter (DM) particles by the galaxy in the case of the existence of two different types of DM or a bimodal velocity distribution function for DM. It is shown that, in addition to the scenario considered in our previous work which is based on the assumption of an unimodal distribution, more complex scenarios are possible in which the transition to the state of intense capture and/or exit from it can occur in two stages. A detailed description is given of the change in the curve describing the rate of capture of dark matter particles as a function of the rate of increase in the baryon mass of the galaxy for various values of the rate of decrease of the DM density.

Experimental study of the spray characteristics of twin-fluid atomization: Focusing on the annular flow regime

The characteristics of twin-fluid atomization operating in the annular flow regime were studied experimentally under various gas-to-liquid ratios (GLRs) and injection pressures. The macroscopic morphology of the spray was obtained by shadowgraph, while the droplet size and velocity were measured using a phase-Doppler particle analyzer technique. It was found that the spray cone angle increases almost linearly with the GLR, and the axial distance required for droplet coalescence to outweigh the breakup decreases with increasing GLR. The Sauter mean diameter (SMD) first decreases and then increases along the axial direction due to the competition between turbulent breakup and droplet coalescence. The droplet size follows a lognormal distribution; the droplet velocity distribution is closer to a lognormal distribution under large GLRs, while it follows normal distribution with GLR = 3%. Regarding the radial distribution, low GLRs (3% and 5%) lead to a bimodal spatial velocity distribution, while for large GLRs, the droplet velocity decreases monotonically toward the far field. The spray tends to become more stable with increasing GLR and injection pressure Pinj, whereas the SMD increases with increasing Pinj. The underlying atomization mechanism in a twin-fluid injector in the annular flow state can be regarded as the disintegration of the initial liquid sheet by longitudinal Kelvin–Helmholtz instability followed by transverse Rayleigh–Taylor instability, which yields a direct proportionality of the droplet size to the initial liquid sheet thickness ΔL. Subsequently, for high Pinj, the gas core shrinks and ΔL increases, which results in an increased SMD but enhanced atomization efficiency ΔL/SMD.

The velocity and quiescence distributions in spontaneous locomotion across animals vary between modes of movement

Abstract Locomotion in animals follows simple mathematical rules: when active, most time is spent at lower velocities and exponentially decreasing time is spent at increasing velocities; when inactive, there is an abundance of short quiescence periods decreasing by an inverse power function towards longer quiescence periods. We investigated whether these mathematical rules are ‘universal’, i.e. followed by all animals independently of their modes of movement (walking, gliding/crawling, swimming) or their habitat (land, water). Using automated video tracking with high temporal and spatial resolution, we investigated the velocity and quiescence period distributions in ten species representing major divisions of the animal kingdom (snails, woodlice, springtail, beetle, fish, reptile, chicken and mouse). We confirm that the two functions are followed by most species, although we also found important exceptions: the snails and woodlice showed uni- or bimodal velocity distributions; the snails did not follow the inverse power law of quiescence periods. Thus, most animals running on legs or swimming follow the expected distributions, while other runners (isopods) and substrate gliders/crawlers (snails) do not. We suggest a difference in velocity distribution between animals that use running to escape predators and those that do not (snails and woodlice).

Imaging studies of the CH3 fragments formed in the ultraviolet photodissociation of dimethylamine: Role of the parent 3s and 3p Rydberg states

State-resolved ion-imaging of the CH3 product of dimethylamine photolysis in the 3s and 3p Rydberg bands (200–235 nm) revealed bimodal CH3 velocity distributions with a fast portion disappearing at the longer photolysis wavelength. The fast component is assigned to direct dissociation based on the calculated potential energy curves. The slow component is assigned to indirect dissociation via N–H channel conical intersection with subsequent CH3 dissociation. The origin of the threshold wavelength near 225 nm was attributed to the N–CH3 bond rupture energy barrier on the S1(3s) state, implying an increasing contribution of direct dissociation at the shorter photolysis wavelengths.

Long-term 3D MHD simulations of black hole accretion discs formed in neutron star mergers

ABSTRACT We examine the long-term evolution of accretion tori around black hole (BH) remnants of compact object mergers involving at least one neutron star, to better understand their contribution to kilonovae and the synthesis of r-process elements. To this end, we modify the unsplit magnetohydrodynamic (MHD) solver in flash 4.5 to work in non-uniform three-dimensional spherical coordinates, enabling more efficient coverage of a large dynamic range in length scales while exploiting symmetries in the system. This modified code is used to perform BH accretion disc simulations that vary the initial magnetic field geometry and disc compactness, utilizing a physical equation of state, a neutrino leakage scheme for emission and absorption, and modelling the BH’s gravity with a pseudo-Newtonian potential. Simulations run for long enough to achieve a radiatively inefficient state in the disc. We find robust mass ejection with both poloidal and toroidal initial field geometries, and suppressed outflow at high disc compactness. With the included physics, we obtain bimodal velocity distributions that trace back to mass ejection by magnetic stresses at early times, and to thermal processes in the radiatively inefficient state at late times. The electron fraction distribution of the disc outflow is broad in all models, and the ejecta geometry follows a characteristic hourglass shape. We test the effect of removing neutrino absorption or nuclear recombination with axisymmetric models, finding ${\sim} 50{{\ \rm per\ cent}}$ less mass ejection and more neutron-rich composition without neutrino absorption, and a subdominant contribution from nuclear recombination. Tests of the MHD and neutrino leakage implementations are included.

Characterisation of solid hydrodynamics in a three-phase stirred tank reactor with positron emission particle tracking (PEPT)

It is challenging to measure the hydrodynamics of stirred tank reactors when they contain multiphase flows comprising liquid, gas bubbles and particles. Radioactive particle tracking techniques such as positron emission particle tracking (PEPT) are the only established techniques to determine internal flow behaviour due to the inherent opacity and density of fluid and the vessel walls. The profiles of solids flow are an important tool for robust reactor design and optimisation and offer insight into underlying transport processes and particle–fluid–bubble interactions for applications such as froth flotation. In this work, measurements with PEPT were performed with two tracer particles differing in surface hydrophobicity to characterise the solids hydrodynamics in a baffled vessel agitated with a Rushton turbine. The location data from PEPT were averaged with time to estimate the probability density function (PDF) of particle velocity in individual voxels. The peaks of these voxel distributions were used to produce profiles of solids flow in different azimuthal and horizontal slices. Bimodal vertical velocity distributions were observed in the impeller radial jet which suggest the particles experienced trajectory crossing effects due to inertia. Statistical tests were performed to compare the velocity distributions of the hydrophilic and hydrophobic tracer particles, which indicated similar average flow behaviour in the liquid or pulp phase of the vessel and differences near the air inlet, in the impeller discharge stream and pulp–froth interface. With tracers designed to represent gangue and valuable mineral species, the differences in velocity reveal interactions such as bubble–particle attachment and entrainment. • Positron emission particle tracking (PEPT) measurements of multiphase flow. • Solid hydrodynamics in a three-phase stirred tank reactor of gas, liquid and solids. • Comparison of hydrophilic and hydrophobic particle flow behaviour. • Observations of particle interaction with the gas bubble flow field.

Pore-Scale Flow Fields of the Viscosity-Lost Partially Hydrolyzed Polyacrylamide Solution Caused by Sulfide Ion

The rheology of a partially hydrolyzed polyacrylamide (HPAM) solution plays an important role in its oil recovery during polymer flooding. However, multiple factors in brine, such as sulfide ions, cause a dramatic loss in the viscosity and oil recovery. To better understand the sulfide-induced viscosity loss and the consequent flow mechanisms in pore networks, the morphology of polymer solutions with and without sulfide ion was observed by scanning electron microscopy; and the variations of the pore scale flow fields were demonstrated by a microscopic visualization seepage experiment combined with Micro-PIV (Microscale Particle Image Velocimetry). The results showed that, with the presence of sulfide ion, the microstructure of the polymer changed from a uniform three-dimensional network structure to loose and uneven floccules, which resulted in viscosity loss (over 70% with 5-mg/L sulfide ion). Moreover, higher concentrations of sulfide ions (5 mg/L and 10 mg/L) resulted in earlier shear thinning characteristics than those with lower sulfide concentrations. Due to viscosity loss, the average flow velocity in the main stream of the microscopic seepage experiment increased more significantly than that without sulfide. However, the viscosity loss alone cannot independently explain the severe viscous fingering during the subsequent post-water flooding, which was about five times greater than that of the primary water flooding in terms of the velocity ratio between the mainstream and margin. A further pore-scale flow field analysis exhibited an eccentric and a bimodal velocity distribution in the throat along the radial and axial directions, respectively. The former distribution indicated that the adsorbed polymer on the pore wall was broken through by hydraulic shear due to the collapsed structure caused by sulfide ion. The latter suggested that another sulfide-induced impact was an earlier-occurring non-Newtonian characteristic with a low shear rate. Therefore, instead of viscosity loss, elastic loss is the dominant mechanism affecting the characteristics of the aggregate flow field under the action of sulfide. Microscopic flooding combined with Micro-PIV is a feasible and essential method to reveal pore scale flow mechanisms.

Retrieving Pulsatility in Ultrasound Localization Microscopy

Ultrasound localization microscopy (ULM) is a vascular imaging method that provides a 10-fold improvement in resolution compared to ultrasound Doppler imaging. Because typical ULM acquisitions accumulate large numbers of synthetic microbubble (MB) trajectories over hundreds of cardiac cycles, transient hemodynamic variations such as pulsatility get averaged out. Here we introduce two independent processing methods to retrieve pulsatile flow characteristics from MB trajectories sampled at kilohertz frame rates and demonstrate their potential on a simulated dataset. The first approach follows a Lagrangian description of the flow. We filter the MB trajectories to eliminate ULM localization grid artifacts and successfully recover the pulsatility fraction <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</sub> with a root mean square error (RMSE) of 3.3%. Our second approach follows a Eulerian description of the flow. It relies on the accumulation of MB velocity estimates as observed from a stationary observer. We show that pulsatile flow gives rise to a bimodal velocity distribution with peaks indicating the maximum and minimum velocities of the cardiac cycle. In this second method, we recovered the pulsatility fraction <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</sub> by measuring the location of these distribution peaks with a RMSE of 5.2%. We evaluated the impact of the MB localization precision σ on our ability to retrieve the bimodal signature of a pulsatile flow. Together, our results demonstrate that pulsatility can be retrieved from ULM acquisitions at kilohertz frame rate and that the estimation of the pulsatility fraction improves with localization precision.

Formation and evolution of binary neutron stars: mergers and their host galaxies

ABSTRACT In this paper, we investigate the properties of binary neutron stars (BNSs) and their mergers by combining population synthesis models for binary stellar evolution (BSE) with cosmological galaxy formation and evolution models. We obtain constraints on BSE model parameters by using the observed Galactic BNSs and local BNS merger rate density (R0) inferred from gravitational wave (GW) observations, and consequently estimate the host galaxy distributions of BNS mergers. We find that the Galactic BNS observations imply efficient energy depletion in the common envelope (CE) phase, a bimodal kick velocity distribution, and low mass ejection during the secondary supernova explosion. However, the inferred R0 does not necessarily require an extremely high CE ejection efficiency and low kick velocities, different from the previous claims, mainly because the latest inferred R0 is narrowed to a lower value ($320_{-240}^{+490}\, \rm Gpc^{-3}\, yr^{-1}$). The BNS merger rate density resulting from the preferred model can be described by R($z$) ∼ R0(1 + $z$)ζ at low redshift ($z$ ≲ 0.5), with R0 ∼ 316–$784\, \rm Gpc^{-3}\, yr^{-1}$ and ζ ∼ 1.34–2.03, respectively. Our results also show that R0 and ζ depend on settings of BSE model parameters, and thus accurate estimates of these parameters by future GW detections will put strong constraints on BSE models. We further estimate that the fractions of BNS mergers hosted in spiral and elliptical galaxies at $z$ ∼ 0 are ∼81–84 and ∼16–19 per cent, respectively. The BNS merger rate per galaxy can be well determined by the host galaxy stellar mass, star formation rate, and metallicity, which provides a guidance in search for most probable candidates of BNS host galaxies.

Presence of red giant population in the foreground stellar substructure of the Small Magellanic Cloud

ABSTRACT The eastern region of the Small Magellanic Cloud (SMC) is found to have a foreground stellar substructure, which is identified as a distance bimodality (∼12 kpc apart) in the previous studies using red clump (RC) stars. Interestingly, studies of red giant branch (RGB) stars in the eastern SMC indicate a bimodal radial velocity (RV) distribution. In this study, we investigate the connection between these two bimodal distributions to better understand the nature and origin of the foreground stellar substructure in the eastern SMC. We use the Gaia Early Data Release 3 astrometric data and archival RV data of RGB stars for this study. We find a bimodal RV distribution of RGB stars (separated by ∼35–45 km s−1) in the eastern and south-western (SW) outer regions. The observed proper motion values of the lower and higher RV RGB components in the eastern regions are similar to those of the foreground and main-body RC stars, respectively. This suggests that the two RGB populations in the eastern region are separated by a similar distance to those of the RC stars, and the RGB stars in the lower RV component are part of the foreground substructure. Based on the differences in the distance and RV of the two components, we estimate an approximate time of formation of this substructure as 307 ± 65 Myr ago. This is comparable with the values predicted by simulations for the recent epoch of tidal interaction between the Magellanic Clouds. Comparison of the observed properties of RGB stars, in the outer SW region, with N-body simulations shows that the higher RV component in the SW region is at a farther distance than the main body, indicating the presence of a stellar counter-bridge in the SW region of the SMC.

Exploring the role of binarity in the origin of the bimodal rotational velocity distribution in stellar clusters

ABSTRACT Many young- and intermediate-age massive stellar clusters host bimodal distributions in the rotation rates of their stellar populations, with a dominant peak of rapidly rotating stars and a secondary peak of slow rotators. The origin of this bimodal rotational distribution is currently debated and two main theories have been put forward in the literature. The first is that all/most stars are born as rapid rotators and that interacting binaries break a fraction of the stars, resulting in two populations. The second is that the rotational distribution is a reflection of the early evolution of pre-main sequence stars, in particular, whether they are able to retain or lose their protoplanetary discs during the first few Myr. Here, we test the binary channel by exploiting multi-epoch Very Large Telescope/MUSE observations of NGC 1850, an ∼100 Myr massive cluster in the Large Magellanic Cloud, to search for differences in the binary fractions of the slow- and fast-rotating populations. If binarity is the cause of the rotational bimodality, we would expect that the slowly rotating population should have a much larger binary fraction than the rapid rotators. However, in our data we detect similar fractions of binary stars in the slow and rapidly rotating populations (5.9 ± 1.1 and 4.5 ± 0.6 per cent, respectively). Hence, we conclude that binarity is not a dominant mechanism in the formation of the observed bimodal rotational distributions.

Bimodal velocity and size distributions of pulsed superfluid helium droplet beams.

We report detailed measurements of velocities and sizes of superfluid helium droplets produced from an Even-Lavie pulse valve at stagnation pressures of 20-60 atm and temperatures between 5.7 and 18.0 K. By doping neutral droplets with Rhodamine 6G cations produced from an electrospray ionization source and detecting the positively charged droplets at two different locations along the beam path, we determine the velocities of the different groups of droplets. By subjecting the doped droplet beam to a retardation field, size distributions can then be analyzed. We discover that at stagnation temperatures above 8.0 K, a single group of droplets is observed at both locations, but at 8.0 K and below, two different groups of droplets with different velocities are detectable. The slower group, considered from fragmentation of liquid helium, cannot be deterred by the retardation voltage at 9 kV, implying an exceedingly large size. The faster group, considered from condensation of gaseous helium, has a bimodal distribution when the stagnation temperatures are below 12.3 K at 20 and 40 atm, or 16.1 K at 60 atm. We also report similar size measurements using low energy electrons for impact ionization, and this latter method can be used for facile in situ characterization of pulsed droplet beams. The mechanism of the bimodal size distribution of the condensation group and the reason for the coexistence of both the condensation and fragmentation groups remain elusive.

Hybrid sideways/longitudinal swimming in the monoflagellate Shewanella oneidensis: from aerotactic band to biofilm.

Shewanella oneidensis MR-1 are facultative aerobic electroactive bacteria with an appealing potential for sustainable energy production and bioremediation. They gather around air sources, forming aerotactic bands and biofilms. Here, we experimentally follow the evolution of the band around an air bubble, and we find good agreement with the numerical solutions of the pertinent transport equations. Video microscopy reveals a transition between motile and non-motile MR-1 upon oxygen depletion, preventing further development of the biofilm. We discover that MR-1 can alternate between longitudinal fast and sideways slow swimming. The resulting bimodal velocity distributions change in response to different oxygen concentrations and gradients, supporting the biological functions of aerotaxis and confinement.

Flow around a scoured bridge pier: a stereoscopic PIV analysis

We performed stereoscopic particle image velocimetry of the turbulent flow inside a scour hole around a cylinder in a sandy bed. At two planes, symmetry plane and 45^circ with respect to the approach flow, the flow and its turbulence structure were investigated. We used two Reynolds numbers (20, 000 and 39, 000) based on the cylinder diameter and the depth-averaged velocity in the symmetry plane. The flow is characterized by a strong down-flow in front of the cylinder, a large horseshoe vortex inside the scour, and an upstream directed wall jet underneath. The values of vorticity in the horseshoe vortex and of the velocity in the wall jet are larger than in a comparable configuration on a flat bed. Enhanced levels of turbulent kinetic energy are found around the horseshoe vortex and in the shear layer detaching from the rim. The orientation of the main axis of the velocity fluctuations changes when the flow enters the scour hole: from about wall-parallel in the detaching shear layer to vertical at the horseshoe vortex. The production of turbulent kinetic energy shows a maximum upstream of the horseshoe vortex centre with considerable production in the shear layer and in the wall jet underneath the horseshoe vortex. Furthermore, strong wall-parallel velocity fluctuations are visible in this region, and bimodal velocity distributions are found, but not anywhere else. The time-averaged wall-shear stresses are largest under the horseshoe vortex and most likely larger than in a corresponding flat-bed configuration.Graphic abstract

On the origin of the bimodal rotational velocity distribution in stellar clusters: rotation on the pre-main sequence

ABSTRACT We address the origin of the observed bimodal rotational distribution of stars in massive young and intermediate age stellar clusters. This bimodality is seen as split main sequences at young ages and also has been recently directly observed in the Vsini distribution of stars within massive young and intermediate age clusters. Previous models have invoked binary interactions as the origin of this bimodality, although these models are unable to reproduce all of the observational constraints on the problem. Here, we suggest that such a bimodal rotational distribution is set-up early within a cluster’s life, i.e. within the first few Myr. Observations show that the period distribution of low-mass ($\lesssim\! 2 \, \mathrm{M}_\odot$) pre-main-sequence (PMS) stars is bimodal in many young open clusters, and we present a series of models to show that if such a bimodality exists for stars on the PMS that it is expected to manifest as a bimodal rotational velocity (at fixed mass/luminosity) on the main sequence for stars with masses in excess of ∼1.5 M⊙. Such a bimodal period distribution of PMS stars may be caused by whether stars have lost (rapid rotators) or been able to retain (slow rotators) their circumstellar discs throughout their PMS lifetimes. We conclude with a series of predictions for observables based on our model.