High Orbital Velocity Research Articles

RAM: Rapid Advection Algorithm on Arbitrary Meshes

The study of many astrophysical flows requires computational algorithms that can capture high Mach number flows, while resolving a large dynamic range in spatial and density scales. In this paper we present a novel method, RAM: Rapid Advection Algorithm on Arbitrary Meshes. RAM is a time-explicit method to solve the advection equation in problems with large bulk velocity on arbitrary computational grids. In comparison with standard upwind algorithms, RAM enables advection with larger time steps and lower truncation errors. Our method is based on the operator splitting technique and conservative interpolation. Depending on the bulk velocity and resolution, RAM can decrease the numerical cost of hydrodynamics by more than one order of magnitude. To quantify the truncation errors and speed-up with RAM, we perform one- and two-dimensional hydrodynamics tests. We find that the order of our method is given by the order of the conservative interpolation and that the effective speed-up is in agreement with the relative increment in time step. RAM will be especially useful for numerical studies of disk−satellite interaction, characterized by high bulk orbital velocities and nontrivial geometries. Our method dramatically lowers the computational cost of simulations that simultaneously resolve the global disk and potential well inside the Hill radius of the secondary companion.

Swift and XMM–Newton observations of an RS CVn-type eclipsing binary SZ Psc: superflare and coronal properties

ABSTRACT We present an in-depth study of a large and long duration (>1.3 d) X-ray flare observed on an RS CVn-type eclipsing binary system SZ Psc using observations from Swift observatory. In the 0.35–10 keV energy band, the peak luminosity is estimated to be 4.2 × 1033 $\rm {erg}~\rm {s}^{-1}$. The quiescent corona of SZ Psc was observed ∼5.67 d after the flare using Swift observatory, and also ∼1.4 yr after the flare using the XMM–Newton satellite. The quiescent corona is found to consist of three temperature plasma: 4, 13, and 48 MK. High-resolution X-ray spectral analysis of the quiescent corona of SZ Psc suggests that the high first ionization potential (FIP) elements are more abundant than the low-FIP elements. The time-resolved X-ray spectroscopy of the flare shows a significant variation in the flare temperature, emission measure, and abundance. The peak values of temperature, emission measure, and abundances during the flare are estimated to be 199 ± 11 MK, 2.13 ± 0.05 × 1056 cm−3, 0.66 ± 0.09 $\rm {Z}_{\odot }$, respectively. Using the hydrodynamic loop modelling, we derive the loop length of the flare as 6.3 ± 0.5 × 1011 cm, whereas the loop pressure and density at the flare peak are derived to be 3.5 ± 0.7 × 103 dyn cm−2 and 8 ± 2 × 1010 cm−3, respectively. The total magnetic field to produce the flare is estimated to be 490 ± 60 G. The large magnetic field at the coronal height is supposed to be due to the presence of an extended convection zone of the subgiant and the high orbital velocity.

Chandra Observations of Spikey: A Possible Self-lensing Supermassive Black Hole Binary System

Abstract This work examines the recent X-ray observations of the active galactic nucleus KIC 11606854 (nicknamed “Spikey”) by the Chandra space telescope. Based on previous observations of a symmetric flare in the system’s light curve by the Kepler space telescope, Spikey has been proposed to be a self-lensing supermassive black hole binary system in which the more massive black hole gravitationally lenses the accretion flow of its smaller companion. The recent Chandra observations (2020 March–May) correspond to the time when the next pulse was expected to occur and were separated in enough time to observe the apparent relativistic Doppler boosting effect from the high orbital velocities of the black holes. We model the expected self-lensing plus Doppler boosting light curve using our wavelength-dependent extended source self-lensing model combined with our relativistic orbital motion code. This orbital motion code is capable of modeling the expected apsidal precession for Spikey, which can be used to predict future pulses. We show that the expected signal was undetectable in the Chandra data as the intrinsic X-ray variability associated with the system was large relative to the changes expected by self-lensing and Doppler boosting. Expected flux increases in more favorable wavelengths were also calculated using our wavelength-dependent self-lensing model, revealing a relationship between the observing wavelength and measured orbital inclination.

Observational signatures of the surviving donor star in the double-detonation model of Type Ia supernovae

The sub-Chandrasekhar-mass double-detonation (DDet) scenario is a contemporary model for Type Ia supernovae (SNe Ia). The donor star in the DDet scenario is expected to survive the explosion and to be ejected at the high orbital velocity of a compact binary system. For the first time, we consistently perform 3D hydrodynamical simulations of the interaction of supernova ejecta with a helium (He) star companion within the DDet scenario. We map the outcomes of 3D impact simulations into 1D stellar evolution codes and follow the long-term evolution of the surviving He-star companions. Our main goal is to provide the post-impact observable signatures of surviving He-star companions of DDet SNe Ia, which will support the search for such companions in future observations. Such surviving companions are ejected with high velocities of up to about 930 km s−1. We find that our surviving He-star companions become significantly overluminous for about 106 yr during the thermal re-equilibration phase. After the star re-establishes thermal equilibrium, its observational properties are not sensitive to the details of the ejecta-donor interaction. We apply our results to the hypervelocity star US 708, which is one of the fastest unbound stars in our Galaxy; it travels with a velocity of about 1200 km s−1, making it a natural candidate for an ejected donor remnant of a DDet SN Ia. We find that a He-star donor with an initial mass of ≳0.5 M⊙ is needed to explain the observed properties of US 708. Based on our detailed binary evolution calculations, however, a progenitor system with such a massive He-star donor cannot get close enough at the moment of the SN explosion to explain the high velocity of US 708. Instead, if US 708 is indeed the surviving He-star donor of a DDet SN Ia, it would require the entire pre-supernova progenitor binary to travel at a velocity of about 400 km s−1. It could, for example, have been ejected from a globular cluster in the direction of the current motion of the surviving donor star.

Lagrangian modeling of water circulation in the Lofoten Basin

Lagrangian particle tracking is implemented for the Lofoten Basin of the Norwegian Sea. The ocean dynamic fields are obtained from the GLORYS 12V1 reanalysis available by the Copernicus Marine Environment Monitoring Service. Spatial distributions of the Lagrangian particles during May-November 2014 are analyzed for two depth layers: the sea-surface (0.5 m) and 266 m. The results show a significant impact of the Norwegian Coastal Current (NCC) on the thermohaline structure of the upper Lofoten Basin, underestimated previously. The NCC penetrates deep into the central Lofoten basin as far as the longitude 0°. In the subsurface layer, the area over which the NCC influences water structure is comparable to the area of the Norwegian Atlantic Slope Current (NASC), as well as to that of the Norwegian Atlantic Frontal Current (NAFC). The NCC maximum influence on the surface water of the Basin is reached in August. The inflow of the NCC is associated with relatively fresh water intrusions (0.5–2‰ fresher than the surrounding waters) moving from the coast to the central part of the Basin. The NASC and NAFC form two main sources of the Atlantic Water in the Lofoten Basin. At 266 m level, the NASC and NAFC waters dominate water structure in the basin. Herewith the NASC influence prevails over that of the NAFC, the latter being limited to the western periphery of the Basin. At this level, the NCC is observed only along a narrow band following the eastern coast. During summer, the core of the Lofoten Vortex (LV) at 266 m is mainly composed of the NAFC water. This fact contradicts the previous point of view of the dominance of the NASC in the LV core at all depth levels. Using two types of Lagrangian maps, we highlighted the summer and the autumn periods in the LV annual lifecycle. The summer LV is characterized by high orbital velocities, which are several times higher than those of the currents along the basin boundaries. The monthly mean orbital velocities in the LV reach 35 сm s−1. To the end of autumn, the LV weakens with the monthly mean orbital velocities below 10 cm s−1.

Modelling atmospheric escape and Mg ii near-ultraviolet absorption of the highly irradiated hot Jupiter WASP-12b

We present two-dimensional multi-fluid numerical modelling of the upper atmosphere of the hot Jupiter WASP-12b. The model includes hydrogen chemistry, and self-consistently describes the expansion of the planetary upper atmosphere and mass loss due to intensive stellar irradiation, assuming a weakly magnetized planet. We simulate the planetary upper atmosphere and its interaction with the stellar wind (SW) with and without the inclusion of tidal force and consider different XUV irradiation conditions and SW parameters. With the inclusion of tidal force, even for a fast SW, the escaping planetary material forms two streams, propagating towards and away from the star. The atmospheric escape and related mass loss rate reaching the value of 10^12 gs^-1 appear to be mostly controlled by the stellar gravitational pull. We computed the column density and dynamics of MgII ions considering three different sets of SW parameters and XUV fluxes. The simulations enable to compute the absorption at the position of the Mg h line and to reproduce the times of ingress and egress. In case of a slow SW and without accounting for tidal force, the high orbital velocity leads to the formation of a shock approximately in the direction of the planetary orbital motion. In this case, mass loss is proportional to the stellar XUV flux. At the same time, ignoring of tidal effects for WASP-12b is a strong simplification, so the scenario with a shock, altogether is an unrealistic one.

Re-Entry of Space Objects from Low Eccentricity Orbits

This paper deals with the re-entry predictions of the space objects from the low eccentric orbit. Any re-entering object re-enters the Earth’s atmosphere with a high orbital velocity. Due to the aerodynamic heating the object tends to break into multiple fragments which later pose a great risk hazard to the population. Here a satellite is considered as the space object for which the re-entry prediction is made. This prediction is made with a package where the trajectory path, the time of re-entry and the survival rate of the fragments is done. The prediction is done using DRAMA 2.0—ESA’s Debris Risk Assessment and Mitigation Analysis Tool suite, MATLAB and Numerical Prediction of Orbital Events software. The predicted re-entry time of OSIRIS 3U was found to be on 7th March 2019, 7:25 (UTC), whereas the actual re-entry time was on 7th March 2019, 7:03 (UTC). The trajectory path found was 51.5699 deg. (Lat), −86.5738 deg. (Long.) with an altitude of 168.643 km. But the actual trajectory was 51.76 deg. (Lat), −89.01deg. (Long.) with an altitude of 143.5 km.

Resource Allocation in Space Multiaccess Systems

Currently, most Landsat satellites are deployed in the low earth orbit (LEO) to obtain high-resolution data of the Earth surface and atmosphere. However, the return channels of LEO satellites are unstable and discontinuous intrinsically, resulting from the high orbital velocity, long revisit interval, and limited ranges of ground-based radar receivers. Space-based information networks, in which data can be delivered by the cooperative transmission of relay satellites, can greatly expand the spatial transport connection ranges of LEO satellites. While different types of these relay satellites deployed in orbits of different altitudes represent distinctive performances when they are participating in forwarding. In this paper, we consider the cooperative mechanism of relay satellites deployed in the geosynchronous orbit (GEO) and LEO according to their different transport performances and orbital characteristics. To take full advantage of the transmission resource of different kinds of cooperative relays, we propose a multiple access and bandwidth resource allocation strategy for GEO relay, in which the relay can receive and transmit simultaneously according to channel characteristics of space-based systems. Moreover, a time-slot allocation strategy that is based on the slotted time division multiple access is introduced for the system with LEO relays. Based on the queueing theoretic formulation, the stability of the proposed systems and protocols is analyzed and the maximum stable throughput region is derived as well, which provides the guidance for the design of the system optimal control. Simulation results exhibit multiple factors that affect the stable throughput and verify the theoretical analysis.

Vegetation-wave interactions in salt marshes under storm surge conditions

Vegetation-wave interactions are critical in determining the capacity of coastal salt marshes to reduce wave energy (wave dissipation), enhance sedimentation and protect the shoreline from erosion. While vegetation-induced wave dissipation is increasingly recognized in low wave energy environments, little is known about: (i) the effect of vegetation on wave dissipation during storms when wave heights and water levels are highest; and (ii) the ability of different plant species to dissipate waves and to maintain their integrity under storm surge conditions. Experiments undertaken in one of the world’s largest wave flumes allowed, for the first time, the study of vegetation-wave interactions at near-field scale, under wave heights ranging from 0.1–0.9m (corresponding to orbital velocities of 2–91cms−1) and water depths up to 2m, in canopies of two typical NW European salt marsh grasses: Puccinellia maritima (Puccinellia) and Elymus athericus (Elymus). Results indicate that plant flexibility and height, as well as wave conditions and water depth, play an important role in determining how salt marsh vegetation interacts with waves. Under medium conditions (orbital velocity 42–63cms−1), the effect of Puccinellia and Elymus on wave orbital velocities varied with water depth and wave period. Under high water levels (2m) and long wave periods (4.1s), within the flexible, low-growing Puccinellia canopy orbital velocity was reduced by 35% while in the more rigid, tall Elymus canopy deflection and folding of stems occurred and no significant effect on orbital velocity was found. Under low water levels (1m) and short wave periods (2.9s) by contrast, Elymus reduced near-bed velocity more than Puccinellia. Under high orbital velocities (≥74cms−1), flattening of the canopy and an increase of orbital velocity was observed for both Puccinellia and Elymus. Stem folding and breakage in Elymus at a threshold orbital velocity≥42cms−1 coincided with a levelling-off in the marsh wave dissipation capacity, while Puccinellia survived even extreme wave forces without physical damage. These findings suggest a species-specific control of wave dissipation by salt marshes which can potentially inform predictions of the wave dissipation capacity of marshes and their resilience to storm surge conditions.

Multidimensional extension of the continuity equation method for debris clouds evolution

As the debris spatial density increases due to recent collisions and inoperative spacecraft, the probability of collisions in space grows. Even a collision involving small objects may produce thousands of fragments due to the high orbital velocity and the high energy released. The propagation of the trajectories of all the objects produced by a breakup would be prohibitive in terms of computational time; therefore, simplified models are needed to describe the consequences of a collision with a reasonable computational effort. The continuity approach can be applied to this purpose as it allows switching the point of view from the analysis of each single fragment to the study of the evolution of the debris cloud globally. Previous formulations of the continuity equation approach focussed on the representation of the drag effect on the fragment spatial density. This work proposes how the continuity equation approach can be extended to multiple dimensions in the phase space defined by the relevant orbital parameters. This novel approach allows including in the propagation also the effect of the Earth’s oblateness and improving the description of the drag effect by considering the distribution of area-to-mass ratio and eccentricity among the fragments. Results for these three applications are shown and discussed in terms of accuracy compared to the numerical propagation and to the one-dimensional approach.

Angular momentum transport by thermal emission in black hole accretion disks

AbstractWe calculate the amount of angular momentum that thermal photons carry out of a viscous black hole accretion disk, due to the strong Doppler shift imparted to them by the high orbital velocity of the radiating disk material. While thermal emission can not drive accretion on its own, we show that along with disk heating it does nonetheless result in a loss of specific angular momentum, thereby contributing to an otherwise viscosity‐driven accretion flow. In particular, we show that the fraction of the angular momentum that is lost to thermal emission at a radius r in a standard, multi‐color disk is ∼0.4rs/r, where rs is the Schwarzschild radius of the black hole. We briefly highlight the key similarties between this effect and the closely related Poynting‐Robertson effect (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

High-frequency time-series of the dynamic sedimentation processes on the western shelf of the Mississippi River Delta

Multiple box cores were collected on the continental shelf in the Mississippi Deltaic Region adjacent to Southwest Pass and analyzed for particle reactive radionuclides 234Th and 7Be to examine seasonal sediment dynamics associated with variations of river discharge and hydrodynamics. Three stations located along a line west of Southwest Pass were cored and reoccupied in October, November, and December of 2003 and March, April, and May of 2004. High-frequency sampling (∼monthly) comparable to the short half-life of the radiotracers ( 234Th t 1/2=24.1 d; 7Be t 1/2=53.3) enabled us to isolate the relative influence that various forcing agents (river discharge, waves, currents) had on sediment inventories of 7Be and 234Th. In addition, the primary source of 7Be (fluvial) differs from 234Th (marine), providing further insight into processes affecting sediment transport and supply. Monthly 7Be inventories showed a significant positive relationship to river discharge ( P=0.03) proximal to Southwest Pass. Sites further from Southwest Pass exhibited little to no relationship between 7Be inventories and river flow. At these sites, monthly 7Be inventories demonstrated a significant positive relationship with average wave orbital velocity ( P<0.01). During our sampling period, the transport of 7Be-rich sediments to sites located on the middle to outer shelf were dependent on sea conditions not river discharge. Relatively high wave orbital velocities potentially allow particles to remain in suspension longer and travel further distances before initial deposition. In addition, 234Th inventories showed evidence of sediment focusing during periods of high wave orbital velocities.

Laboratory investigations into the threshold of movement of natural sand‐sized sediments under unidirectional, oscillatory and combined flows

The threshold of movement of sediment obtained from sandbanks within the Bristol Channel (UK) is investigated under unidirectional, oscillatory and combined flows. The experiments were undertaken in a recirculating, unidirectional laboratory flume containing an oscillating plate to simulate wave action, with movement along the same axis as the unidirectional flows. The sand samples consisted of cohesionless quartz grains with median grain sizes between 0·315 and 0·513 mm. The experiments were performed under flow velocities (measured at 2 cm above the bed) ranging between 0 and 24 cm s–1 and oscillatory currents (wave periods of 5, 12 and 15 s) ranging from 0 to 28 cm s–1. The critical conditions for the initiation of sediment movement were assessed, by visual observation, using the Yalin criterion. The results show that, under unidirectional flow, there is a slight overestimation of the threshold of naturally graded sediments derived on the basis of empirically derived threshold curves for artificially prepared sediments under similar flow conditions. In the case of oscillatory flows, the threshold for the natural sands is found to be higher than that predicted by previously derived empirical curves. Under combined flows, wave period is shown to control threshold conditions, with the unidirectional and oscillatory flow components combining in a linear fashion for long‐period (12 s and 15 s) waves. In contrast, in the presence of short‐period (5 s) waves, the unidirectional and oscillatory components of the flow appear to ‘decouple’. For high orbital velocities, in both cases, the effect of the wave period on threshold diminishes.

Storm-generated currents and offshore sediment transport on a sandy shoreface, Tibjak Beach, Canadian Beaufort Sea

Waves and currents were measured for three weeks in 3.4 and 5.0 m water depths on the sandy shoreface of Tibjak Beach, located on the eastern side of Kugmallit Bay in the Canadian Beaufort Sea. During the field study, five significant wind events resulted in strong shore-normal components of surface wind stress which induced sea level set-up at the coast. Because of the coastline configuration, storm surge conditions and horizontal pressure gradients varied along the eastern side of the bay as a function of small changes in wind direction. One of the westerly storms was responsible for raising the water level by 0.96 m above mean sea level in the nearshore zone. Northwesterly winds resulted in less significant storm surges at Tibjak Beach, but induced higher water levels to the south, near the head of the bay. During the northwesterly storms, strong seaward-directed near-bottom currents associated with downwelling conditions were observed. These currents, with mean velocity up to 0.49 m s −1, are believed to be primarily driven by wind-induced hydrostatic pressure gradients caused by the set-up of coastal waters. Once they began to develop, the seaward currents were directed offshore for periods of at least 6 to 9 consecutive hours; during one storm, near-bottom currents were flowing offshore for more than 18 hours. High wave orbital velocities were measured during these events which induced significant sediment remobilization as the threshold of sediment motion under oscillatory flow was largely exceeded. Because medium- to fine-grained sands prevail on the shoreface, a significant sediment transport load is probably moved down the shoreface by such currents during major events, thus contributing to offshore sediment dispersal.

Constraints on the Formation of Pliocene Hummocky Cross-stratification in Calabria (Southern Italy) from Consideration of Hydraulic and Dispersive Equivalence, Grain-Flow Theory, and Suspended-Load Fallout Rate

ABSTRACT Hummocky cross-stratification (HCS) occurs in two Pliocene progradational nearshore-marine to fluvial sequences in the town of Guardavalle, Calabria (southern Italy). The HCS is notable because of its shallow depth of deposition ( 2-5 m) and its coarse grain size (up to coarse sand). Individual units of HCS consist of, in ascending order, three subfacies which are characterized by distinctive internal structure: a massive subfacies; a centimeter-scale laminated (meso-laminated) subfacies; and a millimeterscale laminated (micro-laminated) subfacies. The massive subfacies consists of normally graded, coarse to fine sand ( = 1.98-1.85), in beds a few cm to 25 cm thick. These beds have flat to broadly undulatory bases and hummocky-swaley tops. The mesolaminated subfacies consists of sets of 0.8- to 1-cm-thick laminae of normally graded, medium to fine sand ( = 2.3) that drape the underlying massive subfacies, thicken gradually toward swales, and then flatten upward. The micro-laminated subfacies comprises sets of several, 2- to 8-ram-thick, planar to slightly undulatory laminae that are characterized by mica-rich and mica-poor zones. Individual micro-laminae are inversely graded, from silt and very fine sand at their hases to fine to medium sand at their tops ( = 2.84-2.95). The upper few tenths of individual lamin e are highly enriched (up to 56%) in biotite grains, most of which are aligned parallel to the lamination. The most likely mode of deposition of the massive and meso-laminated subfacies was rapid settling from suspension from increasingly unsteady flows. The bulk of the sediment was probably imported to the depositional site by unidirectional flows during major storms. Hummocky-swaley laminae did not develop during deposition of the massive subfacies because of the high rate of suspended-load fallout (), which probably was controlled by the relatively coarse grain size. The grains composing the microlaminae are in dispersive, not hydraulic, equivalence. The thickness/grain-size relationships within micro-laminae satisfy predictions of grain-flow theory and indicate that the laminae were produced by collapse of traction carpets under the influence of currents on the order of 80-150 cm/s. Deposition of the Guardavalle HCS took place in four stages: 1) initial plane bed; 2) growth of hummocky bedforms while was very high, probably under the influence of combined orbital and unidirectional currents; 3) draping of the hummocky bedforms by meso-laminae during a period of moderately high , high orbital velocity, but lower unidirectional-current velocity; and 4) reworking of the seafloor by powerful orbital currents under conditions of relatively low and minor unidirectional currents. Preservation of the Guardavalle HCS in a very shallow-marine environment was promoted by the microtidal, storm-wave-dominated chara ter of the Pliocene Ionian coast of Calabria.

Storm wave induced mortality of giant kelp, Macrocystis pyrifera, in Southern California

The storm-related mortality rates of adult Macrocystis pyrifera in a Southern California giant kelp forest were determined over several winter storm seasons and compared with the hydrodynamic attributes of the most energetic storms. The data include stormy and relatively benign years and an exceptional storm which resulted in almost total destruction of a major Macrocystis forest. High orbital velocities (associated with large, high frequency waves), the presence of breaking waves, and entanglement by drifters were found to increase mortality through stipe breakage or holdfast failure. Longshore variability in wave intensity was found to affect kelp mortality rates. The data suggest that wave breaking may be an important factor in determining the inner boundary of the kelp bed.

Broad emission lines from the mass-loss envelopes of giant stars in active galactic nuclei

view Abstract Citations (101) References (29) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Broad Emission Lines From the Mass-Loss Envelopes of Giant Stars in Active Galactic Nuclei Scoville, Nick ; Norman, Colin Abstract The X-ray and UV luminosity of an active galactic nucleus will irradiate and ionize the mass-loss envelopes of giant stars in the central star cluster of the host galaxy. For a star with mass loss at 10^-5^ M_sun_ yr^-1^ situated 10^18^ cm from the AGN, the inverted Stromgren sphere in the mass-loss envelope will extend down to a radius of 10^14^ cm at which point the density is 10^9^ cm^-3^. We propose that the broad emission-line clouds in active galaxies are the ionized mass-loss envelopes of giant stars with the line widths due to orbital motion in the massive central star cluster. Previous objections to a stellar origin for the broad line regions based on collisional destruction are circumvented by noting that although the giants do collide and their envelopes are removed, the envelopes are replenished within a few years. The required numbers of red giants could be a natural consequence of massive starbursts in the cores of galaxies (Norman and Scoville). Stellar density distributions yielding logarithmic line profiles are explored. The favored model has n_*_ is proportional to d^-3/2^, v_*_ is proportional to r^-1/2^, and M^dot^ is proportional to d^-1^ where d is the distance from the cluster center. These relations should be taken only as best estimates since the observed profiles are only approximately logarithmic. Since the stars which are closer to the central luminosity source are ionized to a greater depth and have high orbital velocities, the model predicts correlations of the line width and time scale for flux variations with the critical density of the observed transition {DELTA}v is proportional to n^0.1^ and τ is proportional to n^-2/3^). Publication: The Astrophysical Journal Pub Date: September 1988 DOI: 10.1086/166641 Bibcode: 1988ApJ...332..163S Keywords: Active Galactic Nuclei; Emission Spectra; Red Giant Stars; Star Formation; Stellar Envelopes; Stellar Mass Ejection; Density Distribution; Late Stars; Star Clusters; Stellar Luminosity; Astrophysics; GALAXIES: NUCLEI; STARS: FORMATION; STARS: LATE-TYPE; STARS: MASS LOSS full text sources ADS | data products SIMBAD (1)

Hyper-velocity and tidal stars from binaries disrupted by a massive Galactic black hole

A close but newtonian encounter between a tightly bound binary and a 106 M⊙ black hole causes one binary component to become bound to the black hole and the other to be ejected at up to 4,000 km−1. The discovery of even one such hyper-velocity star coming from the Galactic centre would be nearly definitive evidence for a massive black hole. The new companion of the black hole has a high orbital velocity which increases further as its orbit shrinks by tidal dissipation. The gravitational energy released by the orbit shrinkage of a such a tidal star can be comparable to its total nuclear energy release.

Air-Sea Momentum Transfer by Means of Short-Crested Wavelets

Abstract Recent laboratory studies of Okuda and others revealed shear stress “spikes” over wavelet crests as the principal instruments of air-sea momentum transfer. Examination of other laboratory evidence shows that sharp local intensifications of skin friction is a characteristic of reattaching flow in three-dimensional separation only. Flow reattachment over a downwind wavelet crest ensures the coincidence of high shear stress and downwind orbital velocity, resulting in energy input to the fluctuating motion, i.e., a coupling of the wavelets to the wind. Such coupling is shown to occur over short-crested wavelets at a specific waveheight-to-wavelength ratio. Under the shear stress spikes a thin vertical layer develops on the water surface, which rides over the essentially irrotational motion of the wavelets. Thickening of the vortical layer over the windward face of a wavelet causes a positive pressure perturbation in quadrature with downward vertical velocity, which tends to transfer energy to the wav...

A study of the dynamics of a marine sandwave

ABSTRACTThe movement of the crest of a sandwave was studied using cross‐sectional profiles obtained from lines of sea‐bed reference stakes. Measurements were made, over a six month period, before and after flood and ebb tides in relation to both spring and neap tides and surface wave conditions. Additional observations were obtained on a daily basis, over an equinoctial neap to spring to neap tidal period, in conjunction with boundary layer flow measurements. Tracer experiments were conducted to study the dispersion of sediment from the sandwave crest.The results showed that the sandwave was relatively stable at neap tides, whilst at higher tidal ranges, the crest position oscillated with successive flood and ebb tides. Net flank erosion occurred on the less steep, upstream slope during the dominant ebb tide. This, together with increased deposition on the lee slope, caused the crest to advance. It was not possible to extrapolate sandwave migration over long periods as the tidal dynamic trends were interrupted by wind stress and surface wave activity. High particle orbital velocities, generated at the sea‐bed by storm waves, caused major reductions in crestal heights.Calculated volumes of sediment eroded and accreted were used, with boundary layer flow measurements, to calculate threshold velocities for the movement of the sediment and sediment transport rates.