Kinematic Trends Research Articles

Expansion kinematics of young clusters. I. Lambda Ori

Most stars form in clusters or associations, but only a small number of these groups are expected to remain bound for longer than a few megayears. Once star formation has ended and the molecular gas around young stellar objects has been expelled via feedback processes, most initially bound young clusters lose the majority of their binding mass and begin to disperse into the Galactic field. This process can be investigated by analysing the structure and kinematic trends in nearby young clusters, particularly by analyzing the trend of expansion, which is a tell-tale sign that a cluster is no longer gravitationally bound and dispersing into the field. We combined Gaia DR3 five-parameter astrometry with calibrated RVs for members of the nearby young cluster lambda Ori (Collinder 69). We characterised the plane-of-sky substructure of the cluster using the $Q$-parameter and the angular dispersion parameter. We find evidence that the cluster contains a significant substructure but that this is preferentially located away from the central cluster core, which is smooth and likely remains bound. We found strong evidence for expansion in lambda Ori in the plane of sky by using a number of metrics, but we also found that the trends are asymmetric at the 5sigma significance level, with the maximum rate of expansion being directed nearly parallel to the Galactic plane. We subsequently inverted the maximum rate of expansion of $0.144^ $ kms$^ $pc$^ $ to give an expansion timescale of $6.944^ \:$Myr, which is slightly larger than the typical literature age estimates for the cluster. We also found asymmetry in the velocity dispersion as well as signatures of cluster rotation, and we calculated the kinematic ages for individual cluster members by tracing their motion back in time to their closest approach to the cluster centre.

Spectroscopic age estimates for APOGEE red-giant stars: Precise spatial and kinematic trends with age in the Galactic disc

Over the last few years, many studies have found an empirical relationship between the abundance of a star and its age. Here we estimate spectroscopic stellar ages for 178 825 red-giant stars observed by the APOGEE survey with a median statistical uncertainty of 17%. To this end, we use the supervised machine learning techniqueXGBoost, trained on a high-quality dataset of 3060 red-giant and red-clump stars with asteroseismic ages observed by both APOGEE andKepler. After verifying the obtained age estimates with independent catalogues, we investigate some of the classical chemical, positional, and kinematic relationships of the stars as a function of their age. We find a very clear imprint of the outer-disc flare in the age maps and confirm the recently found split in the local age-metallicity relation. We present new and precise measurements of the Galactic radial metallicity gradient in small age bins between 0.5 and 12 Gyr, confirming a steeper metallicity gradient for ∼2 − 5 Gyr old populations and a subsequent flattening for older populations mostly produced by radial migration. In addition, we analyse the dispersion about the abundance gradient as a function of age. We find a clear power-law trend (with an exponentβ ≈ 0.15) for this relation, indicating a relatively smooth radial migration history in the Galactic disc over the past 7 − 9 Gyr. Departures from this power law may possibly be related to the Gaia Enceladus merger and passages of the Sagittarius dSph galaxy. Finally, we confirm previous measurements showing a steepening in the age-velocity dispersion relation at around ∼9 Gyr, but now extending it over a large extent of the Galactic disc (5 kpc <RGal < 13 kpc). To establish whether this steepening is the imprint of a Galactic merger event, however, detailed forward modelling work of our data is necessary. Our catalogue of precise stellar ages and the source code to create it are publicly available.

Doppler Lidar and Mobile Radiosonde Observation-Based Evaluation of Warn-on-Forecast System Predicted Near-Supercell Environments during TORUS 2019

Abstract Observational data collection is extremely hazardous in supercell storm environments, which makes for a scarcity of data used for evaluating the storm-scale guidance from convection allowing models (CAMs) like the National Oceanic and Atmospheric Administration (NOAA) Warn-on-Forecast System (WoFS). The Targeted Observations with UAS and Radar of Supercells (TORUS) 2019 field mission provided a rare opportunity to not only collect these observations, but to do so with advanced technology: vertically pointing Doppler lidar. One standing question for WoFS is how the system forecasts the feedback between supercells and their near-storm environment. The lidar can observe vertical profiles of wind over time, creating unique datasets to compare to WoFS kinematic predictions in rapidly evolving severe weather environments. Mobile radiosonde data are also presented to provide a thermodynamic comparison. The five lidar deployments (three of which observed tornadic supercells) analyzed show WoFS accurately predicted general kinematic trends in the inflow environment; however, the predicted feedback between the supercell and its environment, which resulted in enhanced inflow and larger storm-relative helicity (SRH), were muted relative to observations. The radiosonde observations reveal an overprediction of CAPE in WoFS forecasts, both in the near and far field, with an inverse relationship between the CAPE errors and distance from the storm. Significance Statement It is difficult to evaluate the accuracy of weather prediction model forecasts of severe thunderstorms because observations are rarely available near the storms. However, the TORUS 2019 field experiment collected multiple specialized observations in the near-storm environment of supercells, which are compared to the same near-storm environments predicted by the National Oceanic and Atmospheric Administration (NOAA) Warn-on-Forecast System (WoFS) to gauge its performance. Unique to this study is the use of mobile Doppler lidar observations in the evaluation; lidar can retrieve the horizontal winds in the few kilometers above ground on time scales of a few minutes. Using lidar and radiosonde observations in the near-storm environment of three tornadic supercells, we find that WoFS generally predicts the expected trends in the evolution of the near-storm wind profile, but the response is muted compared to observations. We also find an inverse relationship of errors in instability to distance from the storm. These results can aid model developers in refining model physics to better predict severe storms.

Trends in Stroke Kinematics, Reynolds Number, and Swimming Mode in Shrimp-Like Organisms.

Metachronal propulsion is commonly seen in organisms with the caridoid facies body plan, i.e. shrimp-like organisms, as they beat their pleopods in an adlocomotory sequence. These organisms exist across length scales ranging several orders of Reynolds number magnitude, from 10 to 104, during locomotion. Further, by altering their stroke kinematics, these organisms achieve three distinct swimming modes. To better understand the relationship between Reynolds number, stroke kinematics, and resulting swimming mode, Euphausia pacifica stroke kinematics were quantified using high-speed digital recordings and compared to the results for the larger E. superba. Euphausia pacifica consistently operate with a greater beat frequency and smaller stroke amplitude than E. superba for each swimming mode, suggesting that length scale may affect the kinematics needed to achieve similar swimming modes. To expand on this observation, these euphausiid data are used in combination with previously-published stroke kinematics from mysids and stomatopods to identify broad trends across swimming mode and length scale in metachrony. Principal component analysis (PCA) reveals trends in stroke kinematics and Reynolds number as well as the variation among taxonomic order. Overall, larger beat frequencies, stroke amplitudes, between-cycle phase lags, and Reynolds numbers are more representative of the fast forward swimming mode compared to the slower hovering mode. Additionally, each species has a unique combination of kinematics that result in metachrony, indicating that there are other factors, perhaps morphological, which affect the overall metachronal characteristics of an organism. Finally, uniform phase lag, in which the timing between power strokes of all pleopods is equal, in 5-paddle systems is achieved at different Reynolds numbers for different swimming modes, highlighting the importance of taking into consideration stroke kinematics, length scale, and the resulting swimming mode.

Phase-Variable Control of a Powered Knee-Ankle Prosthesis over Continuously Varying Speeds and Inclines.

Most controllers for lower-limb robotic prostheses require individually tuned parameter sets for every combination of speed and incline that the device is designed for. Because ambulation occurs over a continuum of speeds and inclines, this design paradigm requires tuning of a potentially prohibitively large number of parameters. This limitation motivates an alternative control framework that enables walking over a range of speeds and inclines while requiring only a limited number of tunable parameters. In this work, we present the implementation of a continuously varying kinematic controller on a custom powered knee-ankle prosthesis. The controller uses a phase variable derived from the residual thigh angle, along with real-time estimates of ground inclination and walking speed, to compute the appropriate knee and ankle joint angles from a continuous model of able-bodied kinematic data. We modify an existing phase variable architecture to allow for changes in speeds and inclines, quantify the closed-loop accuracy of the speed and incline estimation algorithms for various references, and experimentally validate the controller by observing that it replicates kinematic trends seen in able-bodied gait as speed and incline vary.

Validation of a portable marker-based motion analysis system

BackgroundThe Opti_Knee system, a marker-based motion capture system, tracks and analyzes the 6 degrees of freedom (6DOF) motion of the knee joint. However, the validation of the accuracy of this gait system had not been previously reported. The objective of this study was to validate and the system. Two healthy subjects were recruited for the study.MethodsThe 6DOF kinematics of the knee during flexion–extension and level walking cycles of the knee were recorded by Opti_Knee and compared to those from a biplanar fluoroscopy system. The root mean square error (RMSE) of knee kinematics in flexion–extension cycles were compared between the two systems to validate the accuracy at which they detect basic knee motions. The RMSE of kinematics at key events of gait cycles (level walking) were compared to validate the accuracy at which the systems detect functional knee motion. Pearson correlation tests were conducted to assess similarities in knee kinematic trends between the two systems.ResultsIn flexion–extension cycles, the average translational accuracy (RMSE) was between 2.7 and 3.7 mm and the average rotational accuracy was between 1.7 and 3.8°. The Pearson correlation of coefficients for flexion–extension cycles was between 0.858 and 0.994 for translation and 0.995-0.999 for angles. In gait cycles, the RMSEs of angular knee kinematics were 2.3° for adduction/abduction, 3.2° for internal/external rotation, and 1.4° for flexion/extension. The RMSEs of translational kinematics were 4.2 mm for anterior/posterior translation, 3.3 mm for distal/proximal translation, and 3.2 mm for medial/lateral translation. The Pearson correlation of coefficients values was between 0.964 and 0.999 for angular kinematics and 0.883 and 0.938 for translational kinematics.ConclusionThe Opti_Knee gait system exhibited acceptable accuracy and strong correlation strength compared to biplanar fluoroscopy. The Opti _Knee may serve as a promising portable clinical system for dynamic functional assessments of the knee.

Estimating the age–metallicity distribution of a stellar sample from the probability distributions of the individual stars

ABSTRACT Estimating age distributions, or star formation histories, of stellar populations in the Milky Way is important in order to study the evolution of trends in elemental abundances and kinematics. We build on previous work to develop an algorithm for estimating the age–metallicity distribution which uses the full age–metallicity probability density functions (PDFs) of individual stars. No assumptions are made about the shape of the underlying distribution, and the only free parameter of the algorithm is used to ensure a smooth solution. In this work, we use individual age–metallicity PDFs from isochrone fitting of stars with known metallicities. The method is tested with synthetic samples and is found to recover the input age–metallicity distribution more accurately than the distribution of individually estimated ages and metallicities. The recovered sample age distribution (SAD) is always more accurate than the distribution of individual ages, also when restricted to the most precise individual ages. By applying the method to the stars in the Geneva–Copenhagen survey, we detect a possible minimum in the star formation history of the Solar neighbourhood at an age of 10 Gyr which is not seen in the distribution of individual ages. Although we apply the method only to age–metallicity distributions, the algorithm is described more generally and can in principle be applied in other parameter spaces. It is also not restricted to individual parameter distributions from isochrone fitting, meaning that an SAD can be estimated based on individual age PDFs from other methods such as asteroseismology or gyrochronology.

Patellofemoral kinematics in healthy older adults during gait activities.

The patellofemoral (PF) joint is susceptible to many pathologies resulting from acute injury, chronic disease and complications following surgical treatment of the knee. The objectives of this study were to describe case series measurements of patellar motion in healthy older adults as they performed three gait activities, determine patellar tendon angle and moment arm, and show if these quantities were activity dependent. A stereo radiography system was utilized to obtain the 3D PF kinematics of seventeen healthy people over 55years of age (8F/9M, 66±7.9years old, 75.7±20.5kg) as they performed level walking, a step down, and a pivot turn. For a similar portion of the gait cycle, patellar flexion (6.2°±5.8) and average range of motion (ROM) (11.0°±5.9°) for walking with a step down was greater compared to the other gait activities (gait ROM 6.9°±4.3°, pivot ROM 5.7°±3.3°), while the average range of motion for patella tilt was greater during walking with a pivot turn (8.6°±3.9°). However, each subject displayed distinct PF kinematic trends during all activities with a few notable exceptions. Importantly, the knee extensor mechanism characteristics of patellar tendon angle and moment arm showed considerable variation across subjects but were largely unaltered by changing activities. The variation between subjects and the different behavior of the patella during the step down and pivot emphasized the need for analysis of a range of activities to reveal individual response to pathology and treatment in patellar maltracking and osteoarthritis.

Quantifying Coastal Fluvial Morphodynamics Over the Last 100 Years on the Lower Rio Grande, USA and Mexico

AbstractDownstream changes in fluvial channel morphology are commonly observed in association with the backwater zone, where rivers transition from quasi‐uniform flow with normal‐flow depth to gradually varying flow. This transition is linked to changes in channel morphology and mobility and resulting fluvial stratigraphy. However, the majority of systems studied to date are perennial rivers with relatively consistent flow conditions. Here we investigate the evolution of a large river with significant flood‐to‐baseflow variability as it transverses and builds a large delta. We provide the first comprehensive study of the morphology and morphodynamics of the lower Rio Grande, a major continental drainage system that enters the western Gulf of Mexico. We quantify the morphology of the current Rio Grande channel and document spatial trends in channel geometry and kinematics using lidar, historical surveys, and hydrographic analysis. The modern Rio Grande channel morphology does not significantly vary toward the coast. Rather, the channel width, levee, and bed slopes remain nearly constant for ~200 river km. We find historical migration rates between 10 and 100 m/yr with no significant reduction toward the coast in contrast to previously studied systems. We propose that this invariant channel geometry and sustained high migration rates are signatures of the channel not requiring adjustment within the lower coastal reach to accommodate baseflow conditions, and the channel remains continuously adjusted solely to peak flow conditions.

Kinematic analysis of dance-based exergaming: effect of song pace on center of mass and joint mobility.

[Purpose] To establish a quantitative kinematic assessment method for examining postural stability and physical mobility during dance-based exergaming. [Participants and Methods] Fifteen young adults participated in one session of dance-based exergaming assessment using segments from Kinect ‘Just Dance 3’ consisting of slow-, medium-, and fast-paced songs. A motion capture system was used to record full body kinematics, and a customized MATLAB code was used to compute the variables of interest, such as center of mass sway area, excursion, and peaks, as well as step count and joint excursions. [Results] Total center of mass sway area and excursion were significantly greater for slow-paced (total sway area=1,077.6 ± 209.9 cm2; total excursion=629.8 ± 380.5 cm) and fast-paced (total sway area=314.1 ± 133.6 cm2; total excursion=478.5 ± 149.0 cm) songs than for the medium-paced song (total sway area=212.9 ± 346.0 cm2; total excursion=311.2 ± 119.5 cm). Joint excursion was greater for the knee (ranging from: (slow to fast-paced songs: 55.5°–83.1°) and hip (slow to fast-paced: 40.6°–50.4°) than for the ankle (slow to fast-paced: 32.2°–46.7°) for all three dance paces. Additionally, step count was significantly, positively correlated with song pace (r=0.52). [Conclusion] The current study was able to quantify and provide normative values for postural control and joint mobility among healthy young adults during dance-based exergaming with 3 hip-hop songs of different paces from the Microsoft Kinect ‘Just Dance 3’. The results indicated that different paces (intensities) of dance songs corresponded to distinct movement kinematic trends, particularly with slow-paced song having the maximum center of mass excursion and lowest joint excursion, while fast-paced song exhibits the reverse, along with medium-paced song exhibiting the highest hip and ankle joint angle excursions, while the fast-paced song had increased knee joint angle excursions.

Slam Loads and Kinematics of Wave-Piercing Catamarans During Bow Entry Events in Head Seas

Centre bow (CB) design for wave-piercing catamarans (WPCs) is a critical compromise between minimization of slamming and protection against deck diving. To inform the design process, this article investigates the slam loads and kinematics during bow entry events in regular head seas for a 112-m WPC with systematic variations to its CB and wet-deck geometry. Model tests using a 2.5-m hydroelastic segmented catamaran considered five different CB configurations, designated as the parent, high, low, long, and short CB. The results indicated that changes in the CB length had little effect on the general kinematic trends obtained for the pitch, heave, and bow vertical displacement at the instant of slamming, but increasing the wet-deck height resulted in an increase in heave (but not pitch) at slamming. Two new design ratios are proposed. The CB immersion depth to arch height ratio showed slamming occurring in the range of 0.3–0.6 depending on the wave encounter frequency and the CB configuration. The CB buoyancy in the encountered waves was estimated by considering both immersion depth and area along the CB in waves through relative motion analyses. It was found that the buoyancy to slam force ratio increased with increasing wet-deck height but not with increasing CB length. This suggests that an optimal CB configuration could be achieved by first modifying the arched cross-structure to reduce the arch filling effect on slamming severity and then maximizing the CB buoyancy to slam force ratio by increasing either the wet-deck height or the CB length. 1. Introduction An above-water centre bow (CB) for improving seakeeping is a feature of modern wave-piercing catamarans (WPCs) (Soars 1993; Boulton 1998; Fang & Chan 2007; Dubrovsky 2014). Figure 1 shows a 112-m Incat WPC with the CB located between the two demihulls. There are several important factors to be considered when designing such a central bow. First, the reserve buoyancy offered by the CB is the primary design factor for providing a pitchrestoring moment and eliminating deck diving in the following waves (Davis & Whelan 2007). Second, the CB configuration can influence the slamming loads in WPCs during bow entry in waves (Lavroff et al. 2013). This is due to the complex fluid-structure interaction in the CB area. When the CB enters waves, the water gradually fills the spaces between the CB and demihulls, referred to here as archways, and may result in complete closure of the archways and slamming in excessive pitch conditions. Finally, the frequency of slamming occurrence is, to some extent, related to the CB design as it contributes to lateral jet flow during the CB entry. As a result, slamming may occur in even partial water entrapment below the arch wet-deck cross-structure, which could be the case in small pitch motions (Lavroff & Davis 2015).

Correlations between age, kinematics, and chemistry as seen by the RAVE survey

We explore the connections between stellar age, chemistry, and kinematics across a Galactocentric distance of $7.5 < R\,(\mathrm{kpc}) < 9.0$, using a sample of $\sim 12\,000$ intermediate-mass (FGK) turnoff stars observed with the RAdial Velocity Experiment (RAVE) survey. The kinematics of this sample are determined using radial velocity measurements from RAVE, and parallax and proper motion measurements from the Tycho-Gaia Astrometric Solution (TGAS). In addition, ages for RAVE stars are determined using a Bayesian method, taking TGAS parallaxes as a prior. We divide our sample into young ($0 < \tau < 3$ Gyr) and old ($8 < \tau < 13$ Gyr) populations, and then consider different metallicity bins for each of these age groups. We find significant differences in kinematic trends of young and old, metal-poor and metal-rich, stellar populations. In particular, we find a strong metallicity dependence in the mean Galactocentric radial velocity as a function of radius ($\partial {V_{\rm R}}/\partial R$) for young stars, with metal-rich stars having a much steeper gradient than metal-poor stars. For $\partial {V_{\phi}}/\partial R$, young, metal-rich stars significantly lag the LSR with a slightly positive gradient, while metal-poor stars show a negative gradient above the LSR. We interpret these findings as correlations between metallicity and the relative contributions of the non-axisymmetries in the Galactic gravitational potential (the spiral arms and the bar) to perturb stellar orbits.

The fall of the Northern Unicorn: tangential motions in the Galactic anticentre with SDSS and Gaia

We present the first detailed study of the behaviour of the stellar proper motion across the entire Galactic Anti-centre area visible in the Sloan Digital Sky Survey data. We use recalibrated SDSS astrometry in combination with positions from {\it Gaia} DR1 to provide tangential motion measurements with a systematic uncertainty $<$5 kms$^{-1}$ for the Main Sequence stars at the distance of the Monoceros Ring. We demonstrate that Monoceros members rotate around the Galaxy with azimuthal speeds of ~230 kms$^{-1}$, only slightly lower than that of the Sun. Additionally, both vertical and azimuthal components of their motion are shown to vary considerably but gradually as a function of Galactic longitude and latitude. The stellar over-density in the Anti-centre region can be split into two components, the narrow, stream-like ACS and the smooth Ring. According to our analysis, these two structures show very similar but clearly distinct kinematic trends, which can be summarised as follows: the amplitude of the velocity variation in $v_{\phi}$ and $v_z$ in the ACS is higher compared to the Ring, whose velocity gradients appear to be flatter. Currently, no model available can explain the entirety of the data in this area of the sky. However, the new accurate kinematic map introduced here should provide strong constraints on the genesis of the Monoceros Ring and the associated sub-structure.

Validation of a computational knee joint model using an alignment method for the knee laxity test and computed tomography.

Computational models have been identified as efficient techniques in the clinical decision-making process. However, computational model was validated using published data in most previous studies, and the kinematic validation of such models still remains a challenge. Recently, studies using medical imaging have provided a more accurate visualization of knee joint kinematics. The purpose of the present study was to perform kinematic validation for the subject-specific computational knee joint model by comparison with subject's medical imaging under identical laxity condition. The laxity test was applied to the anterior-posterior drawer under 90° flexion and the varus-valgus under 20° flexion with a series of stress radiographs, a Telos device, and computed tomography. The loading condition in the computational subject-specific knee joint model was identical to the laxity test condition in the medical image. Our computational model showed knee laxity kinematic trends that were consistent with the computed tomography images, except for negligible differences because of the indirect application of the subject's in vivo material properties. Medical imaging based on computed tomography with the laxity test allowed us to measure not only the precise translation but also the rotation of the knee joint. This methodology will be beneficial in the validation of laxity tests for subject- or patient-specific computational models.

Kinematic trends in young and old stars

AbstractUsing RAVE DR5, we explore the age, kinematic, and chemical correlations of a sample of 30,000 FGK stars. We separate a sample of turnoff stars into two age groups: young and old. For each of the two age groups, we calculate kinematic trends as a function of Galactocentric radius (R), for different metallicity ([Fe/H]) bins. For both young and old stars, we measure a negative gradient in ∂〈VR〉/∂R. In addition, for young stars we find a correlation between the magnitude of the slope and metallicity, with the most metal-rich bins having the steepest gradient and the most metal-poor bins having a flatter trend.

Rediscovering the disc origin of the Milky Way bulge

AbstractWe explore morphological, kinematic and chemical trends of boxy/peanut (b/p) bulges of Milky Way (MW)-type galaxies, to better understand the formation history of the MW’s bulge. We show, using N-body simulations with both a kinematically cold and a kinematically hot disc, that colder populations develop a more prominent bar and X-shaped peanut as compared to their hotter counterpart. Colder discs also exhibit lower line-of-sight velocities (when viewed edge-on) at the edges of the b/p compared to hot discs, in agreement with what is seen for the MW bulge. Furthermore, we explore an N-body model which has three co-spatial discs with metallicities which correspond to the stellar populations of the inner Milky Way, where the α-enhanced thick disc populations are massive and centrally concentrated. The metallicity trends seen in observations of the Bulge can be reproduced in the model without the need of adding any additional components, which hints to the disc origin of the MW’s bulge.

Evaluation of Clinical Gait Analysis parameters in patients affected by Multiple Sclerosis: Analysis of kinematics

BackgroundClinical Gait Analysis is commonly used to evaluate specific gait characteristics of patients affected by Multiple Sclerosis. The aim of this report is to present a retrospective cross-sectional analysis of the changes in Clinical Gait Analysis parameters in patients affected by Multiple Sclerosis. MethodsIn this study a sample of 51 patients with different levels of disability (Expanded Disability Status Scale 2–6.5) was analyzed. We extracted a set of 52 parameters from the Clinical Gait Analysis of each patient and used statistical analysis and linear regression to assess differences among several groups of subjects stratified according to the Expanded Disability Status Scale and 6-Minutes Walking Test. The impact of assistive devices (e.g. canes and crutches) on the kinematics was also assessed in a subsample of patients. FindingsSubjects showed decreased range of motion at hip, knee and ankle that translated in increased pelvic tilt and hiking. Comparison between the two stratifications showed that gait speed during 6-Minutes Walking Test is better at discriminating patients' kinematics with respect to Expanded Disability Status Scale. Assistive devices were shown not to significantly impact gait kinematics and the Clinical Gait Analysis parameters analyzed. InterpretationWe were able to characterize disability-related trends in gait kinematics. The results presented in this report provide a small atlas of the changes in gait characteristics associated with different disability levels in the Multiple Sclerosis population. This information could be used to effectively track the progression of MS and the effect of different therapies.

Search for Galactic warp signal in Gaia DR1 proper motions

AbstractThe nature and origin of the Galactic warp represent one of the open questions posed by Galactic evolution. Thanks to Gaia high precision absolute astrometry, steps towards the understanding of the warp's dynamical nature can be made. Indeed, proper motions for long-lived stable warp are expected to show measurable trends in the component vertical to the galactic plane. Within this context, we search for the kinematic warp signal in the first Gaia data release (DR1). By analyzing distant spectroscopically-identified OB stars in the Hipparcos subset in Gaia DR1, we find that the kinematic trends cannot be explained by a simple model of a long-lived warp. We therefore discuss possible scenarios for the interpretation of the obtained results. We also present current work in progress to select a larger sample of OB star candidates from the Tycho-Gaia Astrometric Solution (TGAS) subsample in DR1, and delineate the points that we will be addressing in the near future.

ACL substitution may improve kinematics of PCL-retaining total knee arthroplasty.

One of the key factors responsible for altered kinematics and joint stability following contemporary total knee arthroplasty (TKA) is resection of the anterior cruciate ligament (ACL). However, ACL retention can present several technical challenges, and in some cases may not be viable due to an absent or nonfunctional ACL. Therefore, the goal of this research was to investigate whether substitution of the ACL through an anterior post mechanism could improve kinematic deficits of contemporary posterior cruciate ligament (PCL) retaining implants. Kinematic analysis of different implant types was done using KneeSIM, a previously established dynamic simulation tool. Walking, stair-ascent, chair-sit, and deep knee bend were simulated for an ACL-substituting (PCL-retaining) design, a bi-cruciate-retaining and ACL-sacrificing (PCL-retaining) implant, as well as the native knee. The motion of the femoral condyles relative to the tibia was recorded for kinematic comparisons. The ACL-substituting and ACL-retaining implants provided similar kinematic improvements over the ACL-sacrificing implant, by reducing posterior femoral shift in extension and preventing paradoxical anterior sliding. During all simulated activities, the ACL-sacrificing implant showed between 7 and 8mm of posterior shift in extension in contrast to the ACL-retaining implant and the ACL-substituting design, which showed overall kinematic trends similar to the native knee. The absence of ACL function has been linked to abnormal kinematics and joint stability in patients with contemporary TKA. ACL-substituting implants could be a valuable treatment option capable of overcoming the limitations of contemporary TKA, particularly when retaining the native ACL is not feasible or is challenging.

Chemical separation of disc components using RAVE

We present evidence from the RAdial Velocity Experiment (RAVE) survey of chemically separated, kinematically distinct disc components in the solar neighbourhood. We apply probabilistic chemical selection criteria to separate our sample into $\alpha$-low (`thin disc') and $\alpha$-high (`thick disc') sequences. Using newly derived distances, which will be utilized in the upcoming RAVE DR5, we explore the kinematic trends as a function of metallicity for each of the disc components. For our $\alpha$-low disc, we find a negative trend in the mean rotational velocity ($V_{\mathrm{\phi}}$) as a function of iron abundance ([Fe/H]). We measure a positive gradient $\partial V_{\mathrm{\phi}}$/$\partial$[Fe/H] for the $\alpha$-high disc, consistent with results from high-resolution surveys. We also find differences between the $\alpha$-low and $\alpha$-high discs in all three components of velocity dispersion. We discuss the implications of an $\alpha$-low, metal-rich population originating from the inner Galaxy, where the orbits of these stars have been significantly altered by radial mixing mechanisms in order to bring them into the solar neighbourhood. The probabilistic separation we propose can be extended to other data sets for which the accuracy in [$\alpha$/Fe] is not sufficient to disentangle the chemical disc components a priori. For such datasets which will also have significant overlap with Gaia DR1, we can therefore make full use of the improved parallax and proper motion data as it becomes available to investigate kinematic trends in these chemical disc components.