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At least 1,333 records · Page 74
All sourcesNASA NTRSDOE OSTI
DOE OSTI2011

DEEP MIXING IN EVOLVED STARS. II. INTERPRETING Li ABUNDANCES IN RED GIANT BRANCH AND ASYMPTOTIC GIANT BRANCH STARS

We reanalyze the problem of Li abundances in red giants of nearly solar metallicity. After outlining the problems affecting our knowledge of the Li content in low-mass stars (M {<=} 3 M{sub sun}), we discuss deep-mixing models for the red giant branch stages suitable to account for the observed trends and for the correlated variations of the carbon isotope ratio; we find that Li destruction in these phases is limited to masses below about 2.3 M{sub sun}. Subsequently, we concentrate on the final stages of evolution for both O-rich and C-rich asymptotic giant branch (AGB) stars. Here, the constraints on extra-mixing phenomena previously derived from heavier nuclei (from C to Al), coupled to recent updates in stellar structure models (including both the input physics and the set of reaction rates used), are suitable to account for the observations of Li abundances below A(Li) {identical_to} log {epsilon}(Li) {approx_equal} 1.5 (and sometimes more). Also, their relations with other nucleosynthesis signatures of AGB phases (like the abundance of F, and the C/O and {sup 12}C/{sup 13}C ratios) can be explained. This requires generally moderate efficiencies (M-dot < or approx. 0.3-0.5 x 10{sup -6} M{sub sun} yr{sup -1}) for non-convective mass transport. At such rates, slow extra mixing does not remarkably modify Li abundances in early AGB phases; on the other hand, faster mixing encounters a physical limit in destroying Li, set by the mixing velocity. Beyond this limit, Li starts to be produced; therefore, its destruction on the AGB is modest. Li is then significantly produced by the third dredge up. We also show that effective circulation episodes, while not destroying Li, would easily bring the {sup 12}C/{sup 13}C ratios to equilibrium, contrary to the evidence in most AGB stars, and would burn F beyond the limits shown by C(N) giants. Hence, we do not confirm the common idea that efficient extra mixing drastically reduces the Li content of C stars with respect to K-M giants. This misleading appearance is induced by biases in the data, namely: (1) the difficulty of measuring very low Li abundances in O-rich AGB stars due to the presence of TiO bands and (2) the fact that many, relatively massive (M > 3 M{sub sun}) K- and M-type giants may remain Li-rich, not evolving to the C-rich stages. Efficient extra mixing on the AGB is instead typical of very low masses (M {approx}< 1.5 M{sub sun}). It also characterizes CJ stars, where it produces Li and reduces F and the carbon isotope ratio, as observed in these peculiar objects.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2011

DUSTY EXPLOSIONS FROM DUSTY PROGENITORS: THE PHYSICS OF SN 2008S AND THE 2008 NGC 300-OT

SN 2008S and the 2008 NGC 300-OT were explosive transients of stars self-obscured by very dense, dusty stellar winds. An explosive transient with an unobserved shock breakout luminosity of order 10{sup 10} L{sub sun} is required to render the transients little obscured and visible in the optical at their peaks. Such a large breakout luminosity then implies that the progenitor stars were cool, red supergiants, most probably {approx}9 M{sub sun} extreme asymptotic giant branch stars. As the shocks generated by the explosions propagate outward through the dense wind, they produce a shock luminosity in soft X-rays that powers the long-lived luminosity of the transients. Unlike typical cases of transients exploding into a surrounding circumstellar medium, the progenitor winds in these systems are optically thick to soft X-rays, easily absorb radio emission, and rapidly reform dust destroyed by the peak luminosity of the transients. As a result, X-rays are absorbed by the gas and the energy is ultimately radiated by the reformed dust. Three years post-peak, both systems are still significantly more luminous than their progenitor stars, but they are again fully shrouded by the reformed dust and only visible in the mid-IR. The high luminosity and heavy obscuration may make it difficult to determine the survival of the progenitor stars for {approx}10 years. However, our model indicates that SN 2008S, but not the NGC 300-OT, should now be a detectable X-ray source. SN 2008S has a higher estimated shock velocity and a lower density wind, so the X-rays begin to escape at a much earlier phase.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

RADIATION FROM ELECTRON PHASE SPACE HOLES AS A POSSIBLE SOURCE OF JOVIAN S-BURSTS

Radio-frequency short burst emissions (10–40 MHz), known as Jovian S-bursts, have been observed from the Jovian aurora for over fifty years. These emissions, associated with Io’s motion, have a rapidly declining frequency and an exceptionally narrow bandwidth. While it is widely believed that S-bursts are generated by the electron cyclotron maser instability, the mechanism responsible for the rapidly declining frequency and narrow bandwidth currently is not well established. We explore a hypothesis that electron phase space holes radiate or stimulate radiation in the Jovian aurora plasma environment as a possible source of S-burst emissions. Electron phase-space holes (EHs) are ubiquitous in an auroral environment and travel at the implied speeds (∼20,000 km s{sup −1}) of the structures creating the Jovian S-bursts. Furthermore, EHs have the proper physical size to create the observed bandwidth, have sufficient energy content, and can create an environment whereby X mode emissions can be excited. If verified, these findings imply that EHs may be an important source of radiation from strongly magnetized or relativistic astrophysical plasmas.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

RESONANT ABSORPTION OF TRANSVERSE OSCILLATIONS AND ASSOCIATED HEATING IN A SOLAR PROMINENCE. I. OBSERVATIONAL ASPECTS

Transverse magnetohydrodynamic waves have been shown to be ubiquitous in the solar atmosphere and can, in principle, carry sufficient energy to generate and maintain the Sun’s million-degree outer atmosphere or corona. However, direct evidence of the dissipation process of these waves and subsequent heating has not yet been directly observed. Here we report on high spatial, temporal, and spectral resolution observations of a solar prominence that show a compelling signature of so-called resonant absorption, a long hypothesized mechanism to efficiently convert and dissipate transverse wave energy into heat. Aside from coherence in the transverse direction, our observations show telltale phase differences around 180° between transverse motions in the plane-of-sky and line-of-sight velocities of the oscillating fine structures or threads, and also suggest significant heating from chromospheric to higher temperatures. Comparison with advanced numerical simulations support a scenario in which transverse oscillations trigger a Kelvin–Helmholtz instability (KHI) at the boundaries of oscillating threads via resonant absorption. This instability leads to numerous thin current sheets in which wave energy is dissipated and plasma is heated. Our results provide direct evidence for wave-related heating in action, one of the candidate coronal heating mechanisms.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

KINETIC ALFVÉN WAVE GENERATION BY LARGE-SCALE PHASE MIXING

One view of the solar wind turbulence is that the observed highly anisotropic fluctuations at spatial scales near the proton inertial length d{sub p} may be considered as kinetic Alfvén waves (KAWs). In the present paper, we show how phase mixing of large-scale parallel-propagating Alfvén waves is an efficient mechanism for the production of KAWs at wavelengths close to d{sub p} and at a large propagation angle with respect to the magnetic field. Magnetohydrodynamic (MHD), Hall magnetohydrodynamic (HMHD), and hybrid Vlasov–Maxwell (HVM) simulations modeling the propagation of Alfvén waves in inhomogeneous plasmas are performed. In the linear regime, the role of dispersive effects is singled out by comparing MHD and HMHD results. Fluctuations produced by phase mixing are identified as KAWs through a comparison of polarization of magnetic fluctuations and wave-group velocity with analytical linear predictions. In the nonlinear regime, a comparison of HMHD and HVM simulations allows us to point out the role of kinetic effects in shaping the proton-distribution function. We observe the generation of temperature anisotropy with respect to the local magnetic field and the production of field-aligned beams. The regions where the proton-distribution function highly departs from thermal equilibrium are located inside the shear layers, where the KAWs are excited, this suggesting that the distortions of the proton distribution are driven by a resonant interaction of protons with KAW fluctuations. Our results are relevant in configurations where magnetic-field inhomogeneities are present, as, for example, in the solar corona, where the presence of Alfvén waves has been ascertained.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

Kinetic theory of particle-in-cell simulation plasma and the ensemble averaging technique

Abstract We derive the kinetic theory of fluctuations in physically and numerically stable particle-in-cell (PIC) simulations of electrostatic plasmas. The starting point is the single-time correlation at the start of the simulation between the statistical fluctuations of the weighted densities of macroparticle centers in the plasma particle phase-space. The fluctuations are associated with different initial conditions, typically due to the random initial conditions (in velocity space) of the macroparticles/simulation plasma, assigned according to their initial distribution of probability. The single-time correlations at all time steps and in each spatial grid cell are then determined from the Laplace–Fourier transforms of the discretized Klimontovich-like equation for the macroparticles and Maxwell’s equations for the fields, as computed by modern PIC codes. We recover the expressions for the electrostatic field and the plasma particle density fluctuation autocorrelation spectra as well as the kinetic equations describing the average evolution of PIC-simulated plasma particles, first derived by Langdon (1970b Proc. 4th Conf. Numerical Simulation of Plasmas ) using a test macroparticle approach perturbing a discretized Vlasovian plasma and then averaging the obtained physical quantity over the initial macroparticle velocity distribution. We generalize and extend these results to the modern algorithms in PIC codes using arbitrary macroparticle weights. Analytical estimates of statistical fluctuation amplitudes are derived as a function of the plasma simulation parameters, using the central limit theorem in the limit of a large number of macroparticles per cell. The theory is then used to analyze the ensemble averaging technique of PIC simulations where statistical averages are performed over ensembles of PIC simulations, modeling the same plasma physics problem but using different statistical realizations of the initial distribution functions of the macroparticles. This method is illustrated by linear Landau damping uncovering (from noise, which is usually considered numerical) the physical fluctuations driven by a single small amplitude electrostatic wave perturbing a PIC simulation plasma in equilibrium.

fluctuations correlations↗
DOE OSTI2014

THE MILKY WAY AS A HIGH-REDSHIFT GALAXY: THE IMPORTANCE OF THICK DISK FORMATION IN GALAXIES

We compare the star formation history and dynamics of the Milky Way (MW) with the properties of distant disk galaxies. During the first ∼4 Gyr of its evolution, the MW formed stars with a high star formation intensity (SFI), Σ{sub SFR} ≈ 0.6 M {sub ☉} yr{sup –1} kpc{sup –2} and as a result, generated outflows and high turbulence in its interstellar medium (ISM). This intense phase of star formation corresponds to the formation of the thick disk. The formation of the thick disk is a crucial phase that enables the MW to have formed approximately half of its total stellar mass by z ∼ 1 which is similar to ''MW progenitor galaxies'' selected by abundance matching. This agreement suggests that the formation of the thick disk may be a generic evolutionary phase in disk galaxies. Using a simple energy injection-kinetic energy relationship between the one-dimensional velocity dispersion and SFI, we can reproduce the average perpendicular dispersion in stellar velocities of the MW with age. This relationship, its inferred evolution, and required efficiency are consistent with observations of galaxies from z ≈ 0-3. The high turbulence generated by intense star formation naturally resulted in a thick disk, a chemically well-mixed ISM, and is the mechanism that links the evolution of MW to the observed characteristics of distant disk galaxies.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

DUST FROM COMET 209P/LINEAR DURING ITS 2014 RETURN: PARENT BODY OF A NEW METEOR SHOWER, THE MAY CAMELOPARDALIDS

We report a new observation of the Jupiter family comet 209P/LINEAR during its 2014 return. The comet is recognized as a dust source of a new meteor shower, the May Camelopardalids. 209P/LINEAR was apparently inactive at a heliocentric distance r{sub h} = 1.6 AU and showed weak activity at r{sub h} ≤ 1.4 AU. We found an active region of <0.001% of the entire nuclear surface during the comet's dormant phase. An edge-on image suggests that particles up to 1 cm in size (with an uncertainty of factor 3-5) were ejected following a differential power-law size distribution with index q = –3.25 ± 0.10. We derived a mass-loss rate of 2-10 kg s{sup –1} during the active phase and a total mass of ≈5 × 10{sup 7} kg during the 2014 return. The ejection terminal velocity of millimeter- to centimeter-sized particles was 1-4 m s{sup –1}, which is comparable to the escape velocity from the nucleus (1.4 m s{sup –1}). These results imply that such large meteoric particles marginally escaped from the highly dormant comet nucleus via the gas drag force only within a few months of the perihelion passage.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

Numerical Calculation of Neoclassical Distribution Functions and Current Profiles in Low Collisionality, Axisymmetric Plasmas

A new code, the Neoclassical Ion-Electron Solver (NIES), has been written to solve for stationary, axisymmetric distribution functions ( f ) in the conventional banana regime for both ions and electrons using a set of drift-kinetic equations (DKEs) with linearized Fokker-Planck-Landau collision operators. Solvability conditions on the DKEs determine the relevant non-adiabatic pieces of f (called h ). We work in a 4D phase space in which Ψ defines a flux surface, θ is the poloidal angle, v is the total velocity referenced to the mean flow velocity, and λ is the dimensionless magnetic moment parameter. We expand h in finite elements in both v and λ . The Rosenbluth potentials, φ and ψ, which define the integral part of the collision operator, are expanded in Legendre series in cos χ, where χ is the pitch angle, Fourier series in cos θ , and finite elements in v . At each ψ, we solve a block tridiagonal system for hi (independent of fe), then solve another block tridiagonal system for he (dependent on f i ). We demonstrate that such a formulation can be accurately and efficiently solved. NIES is coupled to the MHD equilibrium code JSOLVER [J. DeLucia, et al., J. Comput. Phys. 37 , pp 183-204 (1980).] allowing us to work with realistic magnetic geometries. The bootstrap current is calculated as a simple moment of the distribution function. Results are benchmarked against the Sauter analytic formulas and can be used as a kinetic closure for an MHD code (e.g., M3D-C1 [S.C. Jardin, et al., Computational Science & Discovery, 4 (2012).]).

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

MULTICOLOR LIGHT CURVE SIMULATIONS OF POPULATION III CORE-COLLAPSE SUPERNOVAE: FROM SHOCK BREAKOUT TO {sup 56}CO DECAY

The properties of the first generation of stars and their supernova (SN) explosions remain unknown due to the lack of actual observations. Recently, many transient surveys have been conducted and the feasibility of detecting supernovae (SNe) of Pop III stars is growing. In this paper, we study the multicolor light curves for a number of metal-free core-collapse SN models (25–100 M{sub ⊙}) to determine the indicators for the detection and identification of first generation SNe. We use mixing-fallback supernova explosion models that explain the observed abundance patterns of metal-poor stars. Numerical calculations of the multicolor light curves are performed using the multigroup radiation hydrodynamic code stella. The calculated light curves of metal-free SNe are compared with non-zero-metallicity models and several observed SNe. We have found that the shock breakout characteristics, the evolution of the photosphere’s velocity, the luminosity, and the duration and color evolution of the plateau, that is, all of the SN phases from shock breakout to {sup 56}Co decay, are helpful for estimating the parameters of the SN progenitor: the mass, the radius, the explosion energy, and the metallicity. We conclude that the multicolor light curves could potentially be used to identify first-generation SNe in current (Subaru/HSC) and future transient surveys (LSST, James Webb Space Telescope). They are also suitable for identifying low-metallicity SNe in the nearby universe (PTF, Pan-STARRS, Gaia).

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

GLOBAL SAUSAGE OSCILLATION OF SOLAR FLARE LOOPS DETECTED BY THE INTERFACE REGION IMAGING SPECTROGRAPH

An observation from the Interface Region Imaging Spectrograph reveals coherent oscillations in the loops of an M1.6 flare on 2015 March 12. Both the intensity and Doppler shift of Fe xxi 1354.08 Å show clear oscillations with a period of ∼25 s. Remarkably similar oscillations were also detected in the soft X-ray flux recorded by the Geostationary Operational Environmental Satellites ( GOES ). With an estimated phase speed of ∼2420 km s{sup −1} and a derived electron density of at least 5.4 × 10{sup 10} cm{sup −3}, the observed short-period oscillation is most likely the global fast sausage mode of a hot flare loop. We find a phase shift of ∼ π /2 (1/4 period) between the Doppler shift oscillation and the intensity/ GOES oscillations, which is consistent with a recent forward modeling study of the sausage mode. The observed oscillation requires a density contrast between the flare loop and coronal background of a factor ≥42. The estimated phase speed of the global mode provides a lower limit of the Alfvén speed outside the flare loop. We also find an increase of the oscillation period, which might be caused by the separation of the loop footpoints with time.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

Oxidation of SiC cladding under Loss of Coolant Accident (LOCA) conditions in LWRs

An experimental assessment of Silicon Carbide (SiC) cladding oxidation rate in steam under conditions representative of Loss of Coolant Accidents (LOCA) in light water reactors (LWRs) was conducted. SiC oxidation tests were performed with monolithic alpha phase tubular samples in a vertical quartz tube at a steam temperature of 1140 deg. C and steam velocity range of 1 to 10 m/sec, at atmospheric pressure. Linear weight loss of SiC samples due to boundary layer controlled reaction of silica scale (SiO{sub 2} volatilization) was experimentally observed. The weight loss rate increased with increasing steam flow rate. Over the range of test conditions, SiC oxidation rates were shown to be about 3 orders of magnitude lower than the oxidation rates of zircaloy 4. A SiC volatilization correlation for developing laminar flow in a vertical channel is formulated. (authors)

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗
DOE OSTI2012

HUBBLE SPACE TELESCOPE FUV SPECTRA OF THE POST-COMMON-ENVELOPE HYADES BINARY V471 Tauri

We have carried out an analysis of the Hubble Space Telescope (HST)/STIS archival spectra of the magnetic white dwarf (WD) in the Hyades eclipsing-spectroscopic, post-common-envelope binary V471 Tauri, time resolved on the orbit and on the X-ray rotational phase of the magnetic WD. An HST/STIS spectrum obtained during primary eclipse reveals a host of transition region/chromospheric emission features including N V (1238, 1242), Si IV (1393, 1402), C IV (1548, 1550), and He II (1640). The spectroscopic characteristics and emission line fluxes of the transition region/chromosphere of the very active, rapidly rotating, K2V component of V471 Tauri are compared with the emission characteristics of fast rotating K dwarfs in young open clusters. We have detected a number of absorption features associated with metals accreted onto the photosphere of the magnetic WD from which we derive radial velocities. All of the absorption features are modulated on the 555 s rotation period of the WD with maximum line strength at rotational phase 0.0 when the primary magnetic accretion region is facing the observer. The photospheric absorption features show no clear evidence of Zeeman splitting and no evidence of a correlation between their variations in strength and orbital phase. We report clear evidence of a secondary accretion pole. We derive C and Si abundances from the Si IV and C III features. All other absorption lines are either interstellar or associated with a region above the WD and/or with coronal mass ejection events illuminated as they pass in front of the WD.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2024

High Velocity Joining of Multi-material Stacks Containing 3rd Generation Advanced High-Strength Stainless Steel and Aluminum Extrusion and Casting (Abstract)

Advanced lightweight materials, including multi-phase steels, aluminum extrusions and aluminum castings, have increasingly replaced mild steel to meet original equipment manufacturer (OEM) demand for increased fuel economy and crash-safety requirements. Third Generation (Gen 3) AHSS provides a superior combination of strength and ductility for crash energy management components in the vehicle body-in-white (BIW). Aluminum extrusions are widely used for bumper, door rocker panel, cradle and battery frame applications. Aluminum castings are increasingly being used in electric vehicles to provide part consolidation, weight saving and increased driving range. Rocker panels are, for example, an important component of the battery electric vehicle (BEV) body structure. The rocker panel serves several purposes, including structural support, crash resistance, protection for the vehicle's underbody and battery tray, and contributing to the overall aesthetics. In order to reduce weight, rocker panels are usually constructed using outer panels consisting of 3rd generation advanced high strength steels (AHSS) and an inner multicell aluminum extrusion structure for energy absorption which protects the batteries in a crash event. It is well known, however, that these material combinations provide significant challenges for joining and are susceptible to liquid metal embrittlement and loss of strength. The overall goal is to find a solution that addresses these challenges in order to produce multi-material vehicle components with reduced weight and minimal galvanic and recyclability issues. The approach being proposed here is to utilize a novel high velocity (HiVe) riveting and clinching technique recently demonstrated by Pacific Northwest National Laboratory (PNNL) to join (2T/3T) sheets of aluminum alloys and steel. This process will not only address the aforementioned challenges but also help significantly reduce the joining cycle time. This work will also demonstrate use of HiVe assemblies to be retrofitted to existing body-shop robots thus having minimal impact to existing body shop operations.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Benchmarking of massively parallel phase-field codes for directional solidification

We present a detailed benchmark comparing two state-of-the-art phase-field implementations for simulating alloy solidification under experimentally relevant conditions. The study investigates the directional solidification of Al-3wt%Cu under high-velocity solidification conditions and SCN-0.46wt% camphor under microgravity conditions from National Aeronautics and Space Administration (NASA) DECLIC-DSI-R experiments. Both codes, one employing finite-difference discretization with uniform mesh and GPU-acceleration (GPU-PF) and the other one employing finite-element discretization with adaptive-mesh and CPU-parallelization (PRISMS-PF), solve the same quantitative phase-field formulation that incorporates an anti-trapping current for the solidification of dilute alloys. We evaluate the predictions of each code for dendritic morphology, primary spacing, and tip dynamics in both 2D and 3D, as well as their numerical convergence and computational performance. While existing benchmark problems have primarily focused on simplified or small-scale simulations, they do not reflect the computational and modeling challenges posed by employing experimentally relevant time and length scales. Our results provide a practical framework for assessing phase-field code performance as well as validating and facilitating their application in integrated computational materials engineering (ICME) workflows that require integration with realistic experimental data.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Subluminal and superluminal parametric doppler effects in the case of light reflection from a moving smooth medium inhomogeneity

The reflection of test radiation from a smooth inhomogeneity of medium characteristics propagating with a subluminal or superluminal velocity is analyzed. The equations describing the propagation of the forward- and counter-propagating waves in such an inhomogeneous medium are derived. Quasi-phase conjugation is demonstrated in the case of superluminal inhomogeneities. The Bragg resonance conditions are formulated and the conditions for increasing the reflection coefficient of radiation from an inhomogeneity are discussed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Constraining sub-parsec binary supermassive black holes in quasars with multi-epoch spectroscopy. II. The population with kinematically offset broad Balmer emission lines

A small fraction of quasars have long been known to show bulk velocity offsets (of a few hundred to thousands of km s{sup –1}) in the broad Balmer lines with respect to the systemic redshift of the host galaxy. Models to explain these offsets usually invoke broad-line region gas kinematics/asymmetry around single black holes (BHs), orbital motion of massive (∼sub-parsec (sub-pc)) binary black holes (BBHs), or recoil BHs, but single-epoch spectra are unable to distinguish between these scenarios. The line-of-sight (LOS) radial velocity (RV) shifts from long-term spectroscopic monitoring can be used to test the BBH hypothesis. We have selected a sample of 399 quasars with kinematically offset broad Hβ lines from the Sloan Digital Sky Survey (SDSS) Seventh Data Release quasar catalog, and have conducted second-epoch optical spectroscopy for 50 of them. Combined with the existing SDSS spectra, the new observations enable us to constrain the LOS RV shifts of broad Hβ lines with a rest-frame baseline of a few years to nearly a decade. While previous work focused on objects with extreme velocity offset (>10{sup 3} km s{sup –1}), we explore the parameter space with smaller (a few hundred km s{sup –1}) yet significant offsets (99.7% confidence). Using cross-correlation analysis, we detect significant (99% confidence) radial accelerations in the broad Hβ lines in 24 of the 50 objects, of ∼10-200 km s{sup –1} yr{sup –1} with a median measurement uncertainty of ∼10 km s{sup –1} yr{sup –1}, implying a high fraction of variability of the broad-line velocity on multi-year timescales. We suggest that 9 of the 24 detections are sub-pc BBH candidates, which show consistent velocity shifts independently measured from a second broad line (either Hα or Mg II) without significant changes in the broad-line profiles. Combining the results on the general quasar population studied in Paper I, we find a tentative anti-correlation between the velocity offset in the first-epoch spectrum and the average acceleration between two epochs, which could be explained by orbital phase modulation when the time separation between two epochs is a non-negligible fraction of the orbital period of the motion causing the line displacement. We discuss the implications of our results for the identification of sub-pc BBH candidates in offset-line quasars and for the constraints on their frequency and orbital parameters.

79 ASTRONOMY AND ASTROPHYSICS↗