Orbital Velocity Research Articles

The excited spin state of Dimorphos resulting from the DART impact

The NASA Double Asteroid Redirection Test (DART) mission is a planetary defense-driven test of a kinetic impactor on Dimorphos, the satellite of the binary asteroid 65803 Didymos. DART will intercept Dimorphos at a relative speed of ∼6.5km s−1, perturbing Dimorphos’s orbital velocity and changing the binary orbital period. We present three independent methods (one analytic and two numerical) to investigate the post-impact attitude stability of Dimorphos as a function of its axial ratios, a/b and b/c (a≥b≥c), and the momentum transfer efficiency β. The first method uses a novel analytic approach in which we assume a circular orbit and a point-mass primary that identifies four fundamental frequencies of motion corresponding to the secondary’s mean motion, libration, precession, and nutation frequencies. At resonance locations among these four frequencies, we find that attitude instabilities are possible. Using two independent numerical codes, we recover many of the resonances predicted by the analytic model and indeed show attitude instability. With one code, we use fast Lyapunov indicators to show that the secondary’s attitude can evolve chaotically near the resonance locations. Then, using a high-fidelity numerical model, we find that Dimorphos enters a chaotic tumbling state near the resonance locations and is especially prone to unstable rotation about its long axis, which can be confirmed by ESA’s Hera mission arriving at Didymos in late 2026. We also show that a fully coupled treatment of the spin and orbital evolution of both bodies is crucial to accurately model the long-term evolution of the secondary’s spin state and libration amplitude. Finally, we discuss the implications of a post-impact tumbling or rolling state, including the possibility of terminating BYORP evolution if Dimorphos is no longer in synchronous rotation.

Revisiting the archetypical wind accretor Vela X-1 in depth

Context. The Vela X-1 system is one of the best-studied X-ray binaries because it was detected early, has persistent X-ray emission, and a rich phenomenology at many wavelengths. The system is frequently quoted as the archetype of wind-accreting high-mass X-ray binaries, and its parameters are referred to as typical examples. Specific values for these parameters have frequently been used in subsequent studies, however, without full consideration of alternatives in the literature, even more so when results from one field of astronomy (e.g., stellar wind parameters) are used in another (e.g., X-ray astronomy). The issues and considerations discussed here for this specific, very well-known example will apply to various other X-ray binaries and to the study of their physics. Aims. We provide a robust compilation and synthesis of the accumulated knowledge about Vela X-1 as a solid baseline for future studies, adding new information where available. Because this overview is targeted at a broader readership, we include more background information on the physics of the system and on methods than is usually done. We also attempt to identify specific avenues of future research that could help to clarify open questions or determine certain parameters better than is currently possible. Methods. We explore the vast literature for Vela X-1 and on modeling efforts based on this system or close analogs. We describe the evolution of our knowledge of the system over the decades and provide overview information on the essential parameters. We also add information derived from public data or catalogs to the data taken from the literature, especially data from the Gaia EDR3 release. Results. We derive an updated distance to Vela X-1 and update the spectral classification for HD 77518. At least around periastron, the supergiant star may be very close to filling its Roche lobe. Constraints on the clumpiness of the stellar wind from the supergiant star have improved, but discrepancies persist. The orbit is in general very well determined, but a slight difference exists between the latest ephemerides. The orbital inclination remains the least certain factor and contributes significantly to the uncertainty in the neutron star mass. Estimates for the stellar wind terminal velocity and acceleration law have evolved strongly toward lower velocities over the years. Recent results with wind velocities at the orbital distance in the range of or lower than the orbital velocity of the neutron star support the idea of transient wind-captured disks around the neutron star magnetosphere, for which observational and theoretical indications have emerged. Hydrodynamic models and observations are consistent with an accretion wake trailing the neutron star. Conclusions. With its extremely rich multiwavelength observational data and wealth of related theoretical studies, Vela X-1 is an excellent laboratory for exploring the physics of accreting X-ray binaries, especially in high-mass systems. Nevertheless, much room remains to improve the accumulated knowledge. On the observational side, well-coordinated multiwavelength observations and observing campaigns addressing the intrinsic variability are required. New opportunities will arise through new instrumentation, from optical and near-infrared interferometry to the upcoming X-ray calorimeters and X-ray polarimeters. Improved models of the stellar wind and flow of matter should account for the non-negligible effect of the orbital eccentricity and the nonspherical shape of HD 77581. There is a need for realistic multidimensional models of radiative transfer in the UV and X-rays in order to better understand the wind acceleration and effect of ionization, but these models remain very challenging. Improved magnetohydrodynamic models covering a wide range of scales are required to improve our understanding of the plasma-magnetosphere coupling, and they are thus a key factor for understanding the variability of the X-ray flux and the torques applied to the neutron star. A full characterization of the X-ray emission from the accretion column remains another so far unsolved challenge.

Study on wave attenuation in following and opposing currents due to rigid vegetation

The regular wave attenuation in following and opposing currents due to rigid, emergent vegetation has been studied by physical and numerical simulation in this study. Firstly, laboratory experiments were conducted to investigate the effect of currents on wave attenuation due to vegetation. Then, to overcome the limitation of the experimental conditions, a numerical model, based on the Boussinesq equations and validated by the experimental results, was applied to further investigate the effects of current velocity, vegetation density and water depth on wave attenuation due to vegetation in currents with larger α (the ratio of current velocity to the amplitude of the horizontal wave orbital velocity) range. The results show that both following and opposing currents can enhance or suppress wave attenuation due to vegetation depending on α. For the studied vegetation densities and water depths, the effect of wave attenuation due to vegetation in currents first decreases and then increases with the increase of α. Moreover, wave attenuation due to vegetation in currents is increased with the increase of vegetation density and decreased with the increase of water depth, and both of them have no influence on the relationship between relative wave height decay and α.

An orbital release model for the Orion BN/KL fingers

ABSTRACT We present a simple model in which the bullets that produce the ‘Orion fingers’ (ejected by the BN/KL object) are interpreted as protoplanets or low-mass protostars in orbit around a high-mass star that has a supernova explosion. As the remnant of the SN explosion has only a small fraction of the mass of the pre-supernova star, the orbiting objects then move away in free trajectories, preserving their orbital velocity at the time of release. We show that a system of objects arranged in approximately coplanar orbits results in trajectories with morphological and kinematical characteristics resembling the Orion fingers. We show that, under the assumption of constant velocity motions, the positions of the observed heads of the fingers can be used to reconstruct the properties of the orbital structure from which they originated, resulting in a compact disc with an outer radius of ∼2.4 au.

The Theoretical Value of the Hubble Constant Ho and Unification of the Fundamental Forces of Nature

The Hubble constant Ho represents the speed of expansion of the universe and various cosmological observations and modeling methods were utilized by astronomers for a century to pin down its exact value. Determining Ho from cosmological observations is a long and tedious process requiring highly accurate datasets. To circumvent this need, a simple theoretical approach is introduced in this study which uses the concept of gravitational weakening and seismic-induced recession. As tremors occur among celestial objects, their gravitational fields would also change. This resulted in a fundamental relation of Ho and the computed rate of recession that gives a theoretical value for Ho=69.921 Km/s/Mpc. Using the newly discovered seismic-induced gravitational weakening and time dilation, it is possible that various astrophysical methods using different measurement methods would converge to this theoretical Ho value when cosmological distances and time delay measurements are corrected with the simple formulas we derived. The new model assumes that, as quakes occur in celestial objects, luminosity-induced acceleration and high-energy collision of protons and electrons may produce a massive number of neutrinos, quarks and other subatomic particles. Furthermore, the fine structure constant was found to be inversely proportional to Ho-squared and that the fine-structure constant obtained in this study gives a new physical interpretation of α. New relations for the speed of light, orbital velocity, gravitational force and the Hubble constant were further derived from the new recession constant using approximate relations for the Newtonian and electric force constant. This resulted in a modified gravitational law that is both repulsive and attractive and a theoretical explanation of the phenomenon of light-induced gravitation analogous to the electromagnetic force where photon is the force-carrier. Finally, the fundamental forces of gravitation, electromagnetism and strong nuclear force are now unified.

Laboratory experiments on CO2 gas exchange with wave breaking

AbstractTheCO2gas transfer velocity () at air-sea interface is usually parameterized with the wind speed, but to a great extent is defined by waves and wave breaking. To investigate the direct relationship betweenand waves, laboratory experiments are conducted in a wind-wave flume. Three types of waves are forced in the flume: modulational wave trains generated by a wave maker, wind waves with 10-meter wind speed ranging from 4.5 m/s to 15.5 m/s, and (mechanically-generated) modulational wave trains coupled with superimposed wind force. The wave height and wave orbital velocity are found to be well correlated withwhile wind speed alone can not adequately describe. To reconcile the measurements, non-dimensional equations are established in which gas transfer velocity is expressed as a main function of wave parameters and an additional secondary factor to account for influence of the wind.

Conformal Higgs model: Gauge fields can produce a 125 GeV resonance

Recent cosmological observations and compatible theory offer an understanding of long-mysterious dark matter and dark energy. The postulate of universal conformal local Weyl scaling symmetry, without dark matter, modifies action integrals for both Einstein–Hilbert gravitation and the Higgs scalar field by gravitational terms. Conformal theory accounts for both observed excessive external galactic orbital velocities and for accelerating cosmic expansion. SU(2) symmetry-breaking is retained by the conformal scalar field, which does not produce a massive Higgs boson, requiring an alternative explanation of the observed LHC 125 GeV resonance. Conformal theory is shown here to be compatible with a massive neutral particle or resonance [Formula: see text] at 125 GeV, described as binary scalars [Formula: see text] and [Formula: see text] interacting strongly via quark exchange. Decay modes would be consistent with those observed at LHC. Massless scalar field [Formula: see text] is dressed by the [Formula: see text] field to produce Higgs Lagrangian term [Formula: see text] with the empirical value of [Formula: see text] known from astrophysics.

Numerical inside view of hypermassive remnant models for GW170817

The first multimessenger observation attributed to a merging neutron star binary provided an enormous amount of observational data. Unlocking the full potential of this data requires a better understanding of the merger process and the early post-merger phase, which are crucial for the later evolution that eventually leads to observable counterparts. In this work, we perform standard hydrodynamical numerical simulations of a system compatible with GW170817. We focus on a single equation of state (EOS) and two mass ratios, while neglecting magnetic fields and neutrino radiation. We then apply newly developed postprocessing and visualization techniques to the results obtained for this basic setting. The focus lies on understanding the three-dimensional structure of the remnant, most notably the fluid flow pattern, and its evolution until collapse. We investigate the evolution of mass and angular momentum distribution up to collapse, as well as the differential rotation along and perpendicular to the equatorial plane. For the cases that we studied, the remnant cannot be adequately modeled as a differentially rotating axisymetric NS. Further, the dominant aspect leading to collapse is the GW radiation and not internal redistribution of angular momentum. We relate features of the gravitational wave signal to the evolution of the merger remnant, and make the waveforms publicly available. Finally, we find that the three-dimensional vorticity field inside the disk is dominated by medium-scale perturbances and not the orbital velocity, with potential consequences for magnetic field amplification effects.

Potential for New Constraints on Tropical Cyclone Surface-Exchange Coefficients through Simultaneous Ensemble-Based State and Parameter Estimation

AbstractThe tropical cyclone (TC) surface-exchange coefficients of enthalpy (Ck) and momentum (Cd) at high wind speeds have been notoriously challenging to estimate. This difficulty arises from many factors, including the difficulties in collecting observations within the turbulent TC boundary layer, and the complex coupled physical interactions between the TC boundary layer and ocean surface, which are challenging to accurately model. Motivated by recent studies highlighting the limited practical predictability of TC intensity as a result of uncertainty in the physical representation of the air–sea fluxes of momentum and enthalpy at high wind speeds, we investigate the potential to estimate the surface enthalpy and momentum exchange coefficients through ensemble data assimilation. Significant ensemble correlations between tangential wind, radial wind, and simulated infrared brightness temperatures with parameters controlling the enthalpy and momentum exchange coefficients suggest potential to use all-sky satellite and/or airborne radial velocity observations to estimate these unknown parameters. Using a series of observing system simulation experiments (OSSEs), simulated infrared brightness temperature observations, and a known truth, we demonstrate some potential for simultaneous state and parameter estimation with an ensemble-based data assimilation system to converge toward the correct known parameter values. In all OSSEs with either one or multiple unknown parameters, the initial parameter errors are reduced through simultaneous model state and parameter estimation. However, challenges still exist in converging to the precise true parameter values, as state errors during rapid intensification can project onto the parameter estimates.

Enhanced Doppler Beaming for Dust-enshrouded Objects and Pulsars in the Galactic Center

Abstract Stars within the innermost part of the nuclear star cluster can reach orbital velocities up to a few percent of the light speed. As analyzed by Rafikov, Doppler boosting of stellar light may be of relevance at the pericenter of stellar orbits, especially with the upcoming high-precision photometry in the near- and mid-infrared bands. Here we analyze the previously neglected effect of the infrared spectral index of monitored objects on the Doppler-boosted continuum emission in a narrow photometric band. In contrast to main-sequence stars, the detected compact infrared-excess dust-enshrouded objects have an enhanced Doppler-boosting effect by as much as an order of magnitude, with the variability amplitude of the order of 10% for the most eccentric orbits. In a similar way, pulsars dominated by nonthermal synchrotron emission are also expected to exhibit a stronger Doppler-boosted signal by a factor of at least 4 in comparison with canonical S stars. In case the stellar orbit is robustly determined, the relative flux variation can thus provide hints about the nature of the objects. For extended dust-enshrouded objects, such as G1, that are variable due to tidal, ellipsoidal, bow-shock, and irradiation effects, the subtraction of the expected Doppler-boosting variations will help to better comprehend their internal physics. In addition, the relative flux variability due to higher-order relativistic effects is also modified for different negative spectral indices in a way that it can obtain both positive and negative values with the relative variability of the order of 1%.

Cross-Shore Sediment Transport for Modeling Long-Term Shoreline Evolution

A process-based parametric closure model for estimating cross-shore sediment transport (CST) rate has been developed for simulating long-term shoreline evolution. The formulations of this closure model include calculations of various subaqueous cross-shore transport components induced by waves, currents, and gravity. To better calculate asymmetrical near-bed orbital velocity that is a key to predict the cross-shore transport rate in the nearshore zone, a nonlinear wave-shape model is adopted. By formulating wave nonlinearity to cross-shore transport, this closure model can predict the net on- and offshore transport rates in response to shoreline accretion and erosion driven by waves and currents during low- and high-energetic conditions. This new capability for cross-shore shoreline change has been implemented into a one-line model, primarily driven by longshore sediment transport (LST). Calibration of model parameters and model validation were performed by simulating shoreline changes for a 5 km section of coastline in Duck, North Carolina, over a 14-year period from 2000 to 2013. The simulation results reproduced seasonal shoreline recoveries during low-energetic fair weather and also captured rapid retreats during storms. This shoreline model with inclusion of LST and CST has significantly improved the prediction accuracy of long-term shoreline changes at the study site. Impact analysis of a pier located within the domain reveals that this shoreline model with CST can better simulate sediment bypassing through the structure that depends on multiple local variables such as shoreline positions, updrift longshore transport, waves, and water levels. Therefore, this new shoreline model will facilitate long-term management of sediments and shoreline erosion and impact assessment of coastal structures.

Eulerian Two‐Phase Model Reveals the Importance of Wave Period in Ripple Evolution and Equilibrium Geometry

AbstractThe evolution of ripple geometries and their equilibrium states due to different wave forcing parameters are investigated by a Reynolds‐averaged two‐phase model, SedFoam, in a two‐dimensional domain. Modeled ripple geometries, for a given uniform grain diameter, show a good agreement with ripple predictors that include the wave period effect explicitly, in addition to the wave orbital excursion length (or wave orbital velocity amplitude). Furthermore, using a series of numerical experiments, the ripple's response to a step‐change in the wave forcing is studied. The model is capable of simulating “splitting,” “sliding,” “merging,” and “protruding” as the ripples evolve to a new equilibrium state. The model can also simulate the transition to sheet flow in energetic wave conditions and ripple reformation from a nearly flat bed condition. Simulation results reveal that the equilibrium state is such that the “primary” vortices reach half of the ripple length. Furthermore, an analysis of the suspended load and near‐bed load ratio in the equilibrium state indicates that in the orbital ripple regime, the near‐bed load is dominant while the suspended load is conducive to the ripple decaying regime (suborbital ripples) and sheet flow condition.

Seasonality of turbulence characteristics and wave-current interaction in two prospective tidal energy sites

Velocity fluctuations caused by unsteady flows substantially increase mechanical loads on tidal turbine blades and represent a major challenge to tidal energy converters. Overcoming the challenges posed by high levels of turbulence and large waves is a significant concern for turbine developers. Here we quantify turbulence and characterize wave-turbulence interaction in two prospective tidal energy sites in Australia: Banks Strait and Clarence Strait. Turbulence characterization was derived from deployments lasting up to 3 months. Our datasets represent the longest turbulence measurements at tidal sites published to date, setting a benchmark in tidal energy site characterization. Banks Strait data was affected by wave orbital velocities at mid-water column for nearly the entire period, whilst Clarence Strait data rarely presented significant wave-turbulence interactions. Turbulence intensities varied mostly between 12% and 17% in both sites. Higher turbulent kinetic energy density was observed in Banks Strait, although one of Clarence Strait stations revealed faster current speeds exceeding 2.8 m/s. Investigation of strong wind events suggests turbulence parameters were not majorly affected. Our findings are valuable to obtain more realistic turbine performance and loadings estimates from numerical models and considerably contribute towards efforts to establish best-practice turbulence assessment methods in tidal energy sites.

A new measurement of the spin and orbital parameters of the high mass X-ray binary Centaurus X-3 using AstroSat

We present the timing results of out-of-eclipse observations of Centaurus X-3 spanning half a binary orbit, performed on 12-13 December, 2016 with the Large Area X-ray Proportional Counter (LAXPC) on-board AstroSat. The pulse profile was confirmed to exhibit a prominent pulse peak with a secondary inter-pulse. The systemic spin period of the pulsar was found to be $4.80188 \pm 0.000085$ s in agreement with its spin up trend. The spin up timescale seems to have increased to $7709 \pm 58$ yr that points to negative torque effects in the inner accretion disk. We also report the derived values of projected semi-major axis and orbital velocity of the neutron star.

Strong Modulation of Short Wind Waves and Ka-Band Radar Return Due to Internal Waves in the Presence of Surface Films. Theory and Experiment

Strong variability of Ka-band radar backscattering from short wind waves on the surface of water covered with surfactant films in the presence of internal waves (IW) was studied in wave tank experiments. It has been demonstrated that modulation of Ka-band radar return due to IW strongly depends on the relationship between the phase velocity of IW and the velocity of drifting surfactant films. An effect of the strong increase in surfactant concentration was revealed in convergent zones, associated with IW orbital velocities in the presence of a “resonance” surface steady current, the velocity of which was close to the IW phase velocity. A phenomenological model of suppression and modulations in the spectrum of small-scale wind waves due to films and IW was elaborated. It has been shown that backscatter modulation could not be explained by the modulation of free (linear) millimeter-scale Bragg waves, but was associated with the modulation of bound (parasitic) capillary ripples generated by longer, cm–dm-scale waves—a “cascade” modulation mechanism. Theoretical analysis based on the developed model was found to be consistent with experiments. Field observations which qualitatively illustrated the effect of strong modulation of Ka-band radar backscatter due to IW in the presence of resonance drift of surfactant films are presented.

Primary Velocity and Orbital Phase Effects on Planetary Detectability from Small Epoch Number Data Sets

Abstract Cross-correlation analyses of high-resolution spectroscopic data have recently shown great success in directly detecting planetary signals and enabling the characterization of their atmospheres. One such technique aims to observe a system at multiple epochs and combine the measured planetary radial velocities from each epoch into a measurement of the planetary Keplerian orbital velocity K p , constituting a direct detection of the planetary signal. Recent work has shown that in few-epoch (∼5) data sets, unintended structure can arise at a high level, obscuring the planetary detection. In this work, we look to simulations to examine whether there are ways to reduce this structured noise in few-epoch data sets by careful planning of observations. The choice of observation date allows observers to select the primary (stellar) velocity through a set systemic velocity and chosen barycentric velocity and the planetary orbital phase so we focus on the effects of these two parameters. We find that epochs taken when the primary velocity is near zero and the stellar lines remain relatively fixed to the telluric rest-frame greatly reduce the level of structured noise and allow for much stronger planetary detections, on average more than twice the significance of detections made with epochs using randomly selected primary velocities. Following these results, we recommend that observers looking to build up high-resolution multi-epoch data sets target nights when their system has a near-zero primary velocity.

Calibration of a frontal ablation parameterisation applied to Greenland's peripheral calving glaciers

AbstractWe calibrate the calving parameterisation implemented in the Open Global Glacier Model via two methods (velocity constraint and surface mass balance (SMB) constraint) and assess the impact of accounting for frontal ablation on the ice volume estimate of Greenland tidewater peripheral glaciers (PGs). We estimate an average regional frontal ablation flux of 7.38±3.45 Gta−1 after calibrating the model with two different satellite velocity products, and of 0.69±0.49 Gta−1 if the model is constrained using frontal ablation fluxes derived from independent modelled SMB averaged over an equilibrium reference period (1961–90). This second method makes the assumption that most PGs during that time have an equilibrium between mass gain via SMB and mass loss via frontal ablation. This assumption serves as a basis to assess the order of magnitude of dynamic mass loss of glaciers when compared to the SMB imbalance. The differences between results from both methods indicate how strong the dynamic imbalance might have been for PGs during that reference period. Including frontal ablation increases the estimated regional ice volume of PGs, from 14.47 to 14.64±0.12 mm sea level equivalent when using the SMB method and to 15.84±0.32 mm sea level equivalent when using the velocity method.

A numerical and theoretical study of wind over fast-propagating water waves

Effects of fast-propagating water waves on the overlying wind are investigated using simulation and theoretical analysis. By performing a large eddy simulation (LES) of turbulent wind over water waves with high wave age, we observe that the perturbation to wind velocity and pressure by the waves, or the wave-induced airflow, is mainly induced by the vertical movement of the wave surface. We perform scaling analysis to show that the turbulent stress effects on fast wave-induced airflow are negligible. Moreover, we find that the curvilinear model developed for opposing wave effects on wind by Caoet al.(J. Fluid Mech., vol. 901, 2020, p. A27) provides predictions that agree well with the present LES results of wind following fast waves. Our analyses of the results indicate that the fast wave-induced airflow is a quasilinear process. To elucidate the mechanisms for fast wave effects, we split the curvilinear model into two equations corresponding to wave kinematics and forcing by wave elevation, respectively. Using these equations, we illustrate that the vertical component of wave orbital velocity induces a strong airflow perturbation, which produces the dominant components of fast wave-induced airflow and determines its overall spatial structure. Furthermore, we discover that the weak components of fast wave-induced airflow are forced by the dominant components via viscous stress and by the forcing induced by wave elevation, and generate the form drag on the wave surface.

The origin of the orbital parameter distribution of merging haloes

ABSTRACT We describe a simple model that explains the qualitative and (approximate) quantitative features of the distribution of orbital velocities of merging pairs of dark matter haloes. Our model considers a primary dark matter halo as a perturber in a background of secondary haloes with velocities described by linear theory. By evaluating the ensemble of secondary haloes on orbits within the perturbing halo’s ‘loss cone’ we derive the distribution of orbital parameters of these captured haloes. This model is able provide qualitative explanations for the features of this distribution as measured from N-body simulations, and is in approximate quantitative agreement with those measurements. As the velocity dispersion of the background haloes is larger on smaller scales our model predicts an overall increase in the characteristic velocities of merging haloes, relative to the virial velocities of those haloes, in lower mass systems. Our model also provides a simple explanation for the measured independence of the orbital velocity distribution function on redshift when considered at fixed peak height. By connecting the orbital parameter distribution to the underlying power spectrum our model also allows for estimates to be made of the effect of modifying that power spectrum, for example by including a truncation at large wavenumber. For plausible warm dark matter models, we find that this truncation has only a small effect on the predicted distributions.

How to Obtain a SAR Image of the Ocean Surface That Is Actually Free from The Impact of Orbital Velocities

As is known, the main problem in interpreting images of the ocean surface formed by microwave synthetic aperture radar (SAR) is the distortions introduced by the orbital movements of the small-scale (centimeter and decimeter) ripples in the field of large waves. The point is that the standard aperture synthesis procedure is a matched filtering operation aimed at extracting from the reflected signal a part that has a phase that changes according to a known scenario corresponding to a stationary reflecting surface.