Distribution Of Rotational Velocities Research Articles

Chemical evolution of the Galactic bulge with different stellar populations

The metallicity distribution function (MDF) of the Galactic bulge is characterized by a multi-peak shape, with a metal-poor peak centered at $ Fe/H sim-0.3 $ dex and a metal-rich peak centered at $ Fe/H sim+0.3 $ dex. The bimodality of the MDF is also reflected in the alpha /Fe versus Fe/H abundance ratios, suggesting the presence of different stellar populations in the bulge. In this work we aim to reproduce the observed MDF of the Galactic bulge by testing a scenario in which the metal-poor component of the bulge is formed by stars formed in situ, during a strong burst of star formation, while the metal-rich population is formed by stars created in situ during a second burst of star formation and/or stars accreted from the innermost part of the Galactic disk as an effect of a growing bar. We adopted a chemical evolution model that is able to follow the evolution of several chemical species with detailed nucleosynthesis prescriptions. In particular, because of the importance of the production of Fe in constraining the MDF, close attention is paid to the production of this element in both Type Ia supernovae and massive stars. In particular, we included yields from rotating massive stars with different rotational velocity prescriptions. Our model also takes the infall and outflow of gas into account, as well as the effect of stellar migration. Results are compared to sim 13000 stars from the SDSS/APOGEE survey that belong to the region located at a Galactocentric distance $ R_ GC kpc We successfully reproduce the observed double-peak shape of the bulge MDF as well as the abundance trends of the alpha elements relative to Fe by assuming both (i) a multi-burst star formation history with a quenching of the first burst of $ 10^2\ Myr $ and (ii) migration of stars from the innermost part of the Milky Way disk, as an effect of a growing bar. According to our results, the fraction of the stellar mass of the bulge-bar that belongs to the inner disk is $ sim40<!PCT!> $. In terms of the nucleosynthesis, we conclude that models that assume either no rotation for massive stars or a distribution of rotational velocities that favors slow rotation at high metallicities best reproduce the observed MDF as well as the alpha /Fe and the Ce/Fe versus Fe/H abundance patterns.

The metal-poor edge of the Milky Way’s “thin disc”

Context. The emergence of the disc in our Galaxy and the relation between thick and thin disc formation and evolution is still a matter of debate. The chemo-dynamical characterization of disc stars is key to resolving this question, particularly at parameter regimes, where both disc components overlap, such as the region around [Fe/H] ∼ −0.7 corresponding to the thin disc’s metal-poor end. Aims. In this paper, we re-assess the recent detection of a metal-poor extension of stars moving with thin-disc-like rotational velocities between −2 < [Fe/H] < −0.7, carried out on the basis of metallicity estimates obtained from photometric data and their rotational velocity distributions. Methods. We explored the chemo-dynamical properties of metal-poor stars from the recent Gaia third data release (DR3), which includes the first catalog of metallicity estimates from the Radial Velocity Spectrometer (RVS) experiment. We complemented them with the two largest high-resolution (λ/dλ > 20 000) spectroscopic surveys available: the GALAH DR3 and the APOGEE DR17. Results. We confirm that there are high-angular-momentum stars moving in thin-disc-like orbits, that is, with a high angular momentum of Lz/Jtot > 0.95, and close to the Galactic plane, |Zmax|< 750 pc, reaching metallicity values down to [Fe/H] ∼ −1.5. We also find tentative evidence for stars moving on such orbits at lower metallicities, down to [Fe/H] ∼ −2.5, albeit in smaller numbers. Based on their chemical trends, thin-disc-like stars with [Fe/H] < −1 would have formed in a medium that is less chemically evolved than the bulk of the thick disc. Stars with chemical abundances typical of the thin disc appear at metallicities between −1 < [Fe/H] < −0.7.

Tracing the evolution of short-period binaries with super-synchronous fast rotators

Context. The initial distribution of rotational velocities of stars is still poorly known, and how the stellar spin evolves from birth to the various end points of stellar evolution is an actively debated topic. Binary interactions are often invoked to explain the existence of extremely fast-rotating stars (vsin i ≳ 200 km s−1). The primary mechanisms through which binaries can spin up stars are tidal interactions, mass transfer, and possibly mergers. However, fast rotation could also be primordial, that is, a result of the star formation process. To evaluate these scenarios, we investigated in detail the evolution of three known fast-rotating stars in short-period spectroscopic and eclipsing binaries, namely HD 25631, HD 191495, and HD 46485, with primaries of masses of 7, 15, and 24 M⊙, respectively, with companions of ∼1 M⊙ and orbital periods of less than 7 days. These systems belong to a recently identified class of binaries with extreme mass ratios, whose evolutionary origin is still poorly understood. Aims. We evaluated in detail three scenarios that could explain the fast rotation observed in these binaries: it could be primordial, a product of mass transfer, or the result of a merger within an originally triple system. We also discuss the future evolution of these systems to shed light on the impact of fast rotation on binary products. Methods. We computed grids of single and binary MESA models varying tidal forces and initial binary architectures to investigate the evolution and reproduce observational properties of these systems. When considering the triple scenario, we determined the region of parameter space compatible with the observed binaries and used a publicly available machine-learning model to determine the dynamical stability of the triple system. Results. We find that, because of the extreme mass-ratio between binary components, tides have a limited impact, regardless of the prescription used, and that the observed short orbital periods are at odds with post-mass-transfer scenarios. We also find that the overwhelming majority of triple systems compatible with the observed binaries are dynamically unstable and would be disrupted within years of formation, forcing a hypothetical merger to happen so close to a zero-age main-sequence that it could be considered part of the star formation process. Conclusions. The most likely scenario to form such young, rapidly rotating, and short-period binaries is primordial rotation, implying that the observed binaries are pre-interaction ones. Our simulations further indicate that such systems will subsequently go through a common envelope and likely merge. These binaries show that the initial spin distribution of massive stars can have a wide range of rotational velocities.

The young massive SMC cluster NGC 330 seen by MUSE

Context. The origin of the initial rotation rates of stars, and how a star’s surface rotational velocity changes during the evolution, either by internal angular momentum transport or due to interactions with a binary companion, remain open questions in stellar astrophysics. Aims. Here, we aim to derive the physical parameters and study the distribution of (projected) rotational velocities of B-type stars in the ∼35 Myr-old, massive cluster NGC 330 in the Small Magellanic Cloud. NGC 330 is in an age range where the number of post-interaction binaries is predicted to be high near the cluster turnoff (TO). Methods. We developed a simultaneous photometric and spectroscopic grid-fitting method adjusting atmosphere models on multiband Hubble Space Telescope (HST) photometry and Multi Unit Spectroscopic Explorer (MUSE) spectroscopy. This allowed us to homogeneously constrain the physical parameters of over 250 B and Be stars (i.e., B-type stars with emission lines), brighter than mF814W = 18.8 mag. Results. The rotational velocities of Be stars in NGC 330 are significantly higher than the ones of B-type stars. The rotational velocities vary as a function of the star’s position in the color-magnitude diagram, qualitatively following predictions of binary population synthesis. A comparison to younger clusters shows that stars in NGC 330 rotate more rapidly on average. Conclusions. The rotational velocities of the ∼35 Myr old population in NGC 330 quantitatively agree with predictions for a stellar population that underwent significant binary interactions: the majority of the B-type stars could be single stars or primaries in pre-interaction binaries. The rapidly spinning Be stars could be mass and angular momentum gainers in previous interactions, while those Be stars close to the TO may be spun-up single stars. The slowly rotating, apparently single stars above the TO could be merger products. The different v sin i characteristics of NGC 330 compared to younger populations can be understood in this framework.

Influence mechanism of particle density in a gas−solid fluidized bed

This paper presents a comprehensive analysis of gas−solid fluidized beds with different particle densities using a coupled computational fluid dynamics−discrete element method. The accuracy of the numerical method is experimentally verified, and the characterization parameters of the gas−solid fluidized bed are evaluated, including velocity distribution, bubble equivalent diameter, dead zone angle, particle force, bed pressure, and mixing index. The effects of five particle densities on the gas−solid fluidized bed were analyzed in detail while contact and drag models were fixed. The predicted results show that an increase in particle density will reduce the distribution of translational and rotational velocities, which will have an inhibitory effect on the bubbles in the bed, with a positive correlation with the angle of the dead zone. It is explained that the main forces on particles during fluidization are contact force, drag force, and pressure gradient force, and the normal contact force is two orders of magnitude larger than the drag force and the pressure gradient force. The phenomenon that the normal contact force is much larger than the tangential contact force is explained. The increase in particle density has an increasing effect on the bed pressure, takes more time to reach a good degree of mixing, and reduces the mixing performance. The study of the effect of particle density on gas−solid fluidized beds can provide theoretical guidance for the structural design as well as theoretical development of subsequent fluidized beds.

Fast-rotating Blue Straggler Stars in the Globular Cluster NGC 3201* *Based on observations collected at the Clay Magellan Telescope, located at Las Campanas Observatory (Chile).

We used high-resolution spectra acquired with the Magellan Telescope to measure radial and rotational velocities of approximately 200 stars in the Galactic globular cluster NGC 3201. The surveyed sample includes blue straggler stars (BSSs) and reference stars in different evolutionary stages (main-sequence turnoff, subgiant, red giant, and asymptotic giant branches). The average radial velocity value (〈V r 〉 = 494.5 ± 0.5 km s−1) confirms a large systemic velocity for this cluster and was used to distinguish 33 residual field interlopers. The final sample of member stars has 67 BSSs and 114 reference stars. Similarly to what is found in other clusters, the totality of the reference stars has negligible rotation (< 20 km s−1), while the BSS rotational velocity distribution shows a long tail extending up to ∼200 km s−1, with 19 BSSs (out of 67) spinning faster than 40 km s−1. This sets the percentage of fast-rotating BSSs to ∼28%. Such a percentage is roughly comparable to that measured in other loose systems (ω Centauri, M4, and M55) and significantly larger than that measured in high-density clusters (as 47 Tucanae, NGC 6397, NGC 6752, and M30). This evidence supports a scenario where recent BSS formation (mainly from the evolution of binary systems) is occurring in low-density environments. We also find that the BSS rotational velocity tends to decrease for decreasing luminosity and surface temperature, similarly to what is observed in main-sequence stars. Hence, further investigations are needed to understand the impact of BSS internal structure on the observed rotational velocities.

The role of tidal interactions in the formation of slowly rotating early-type stars in young star clusters

ABSTRACT The split main sequences found in the colour–magnitude diagrams of star clusters younger than ∼600 Myr are suggested to be caused by the dichotomy of stellar rotation rates of upper main-sequence stars. Tidal interactions have been suggested as a possible explanation of the dichotomy of the stellar rotation rates. This hypothesis proposes that the slow rotation rates of stars along the split main sequences are caused by tidal interactions in binaries. To test this scenario, we measured the variations in the radial velocities of slowly rotating stars along the split main sequence of the young Galactic cluster NGC 2422 (∼90 Myr) using spectra obtained at multiple epochs with the Canada–France–Hawai’i Telescope. Our results show that most slowly rotating stars are not radial velocity variables. Using the theory of dynamical tides, we find that the binary separations necessary to fully or partially synchronize our spectroscopic targets, on time-scales shorter than the cluster age, predict much larger radial velocity variations across multiple-epoch observations, or a much larger radial velocity dispersion at a single epoch, than the observed values. This indicates that tidal interactions are not the dominant mechanism to form slowly rotating stars along the split main sequences. As the observations of the rotation velocity distribution among B- and A-type stars in binaries of larger separations hint at a much stronger effect of braking with age, we discuss the consequences of relaxing the constraints of the dynamical tides theory.

Configuration-dependent dynamics of non-spherical particles in a gas–solid fluidized bed

Configuration-dependent dynamics of non-spherical particles are investigated experimentally in a quasi-two-dimensional gas–solid fluidized bed. We show that the vertical velocity profiles are sensitive to the aspect ratio at low gas velocity, but not sensitive at high gas velocity. The velocity distribution, especially the rotational velocity distribution, deviates from the Maxwellian distribution. The joint probability density distribution of vertical and rotational velocity is asymmetric. We identify two high-probability particle configurations, two particles in a parallel or vertical arrangement, by calculating the radial distribution function, nematic correlation function and coordination number. We further explain the cause of this order appearance through entropy theory. By comparing the drag force of the particle configurations at various angles and Re, we demonstrate that it is necessary to put forward the configuration-dependent drag force for non-spherical particle fluidization.

The initial spin distribution of B-type stars revealed by the split main sequences of young star clusters

Spectroscopic observations of stars in young open clusters have revealed evidence for a dichotomous distribution of stellar rotational velocities, with 10−30% of stars rotating slowly and the remaining 70−90% rotating fairly rapidly. At the same time, high-precision multiband photometry of young star clusters shows a split main sequence band, which is again interpreted as due to a spin dichotomy. Recent papers suggest that extreme rotation is required to retrieve the photometric split. Our new grids of MESA models and the prevalent SYCLIST models show, however, that initial slow (0−35% of the linear Keplerian rotation velocities) and intermediate (50−65% of the Keplerian rotation velocities) rotation are adequate to explain the photometric split. These values are consistent with the recent spectroscopic measurements of cluster and field stars, and are likely to reflect the birth spin distributions of upper main-sequence stars. A fraction of the initially faster-rotating stars may be able to reach near-critical rotation at the end of their main-sequence evolution and produce Be stars in the turn-off region of young star clusters. However, we find that the presence of Be stars up to two magnitudes below the cluster turnoff advocates for a crucial role of binary interaction in creating Be stars. We argue that surface chemical composition measurements may help distinguish these two Be star formation channels. While only the most rapidly rotating, and therefore nitrogen-enriched, single stars can evolve into Be stars, slow pre-mass-transfer rotation and inefficient accretion allows for mild or no enrichment even in critically rotating accretion-induced Be stars. Our results shed new light on the origin of the spin distribution of young and evolved B-type main sequence stars.

The Gaia-ESO survey: A spectroscopic study of the young open cluster NGC 3293

We present a spectroscopic analysis of the GIRAFFE and UVES data collected by the Gaia-ESO survey for the young open cluster NGC 3293. Archive spectra from the same instruments obtained in the framework of the ‘VLT-FLAMES survey of massive stars’ are also analysed. Atmospheric parameters, non-local thermodynamic equilibrium (LTE) chemical abundances for six elements (He, C, N, Ne, Mg, and Si), or variability information are reported for a total of about 160 B stars spanning a wide range in terms of spectral types (B1 to B9.5) and rotation rate (up to 350 km s−1). Our analysis leads to about a five-fold increase in the number of cluster members with an abundance determination and it characterises the late B-star population in detail for the first time. We take advantage of the multi-epoch observations on various timescales and a temporal baseline, sometimes spanning ∼15 years, to detect several binary systems or intrinsically line-profile variables. A deconvolution algorithm is used to infer the current, true (deprojected) rotational velocity distribution. We find a broad, Gaussian-like distribution peaking around 200–250 km s−1. Although some stars populate the high-velocity tail, most stars in the cluster appear to rotate far from critical. We discuss the chemical properties of the cluster, including the low occurrence of abundance peculiarities in the late B stars and the paucity of objects showing CN-cycle burning products at their surface. We argue that the former result can largely be explained by the inhibition of diffusion effects because of fast rotation, while the latter is generally in accord with the predictions of single-star evolutionary models under the assumption of a wide range of initial spin rates at the onset of main-sequence evolution. However, we find some evidence for a less efficient mixing in two quite rapidly rotating stars that are among the most massive objects in our sample. Finally, we obtain a cluster age of ∼20 Myr through a detailed, star-to-star correction of our results for the effect of stellar rotation (e.g., gravity darkening). This is significantly older than previous estimates from turn-off fitting that fully relied on classical, non-rotating isochrones.

Generalized statistical mechanics of stellar systems.

The observed distributions of stellar parameters, in particular, rotational and radial velocities, often depart from the Maxwellian (Gaussian) distribution. In the absence of a consistent statistical framework, these distributions are, in general, accounted for phenomenologically by employing power-law distributions, such as Tsallis or Kaniadakis distributions. Here we argue that the observed distributions correspond to locally Gaussian distributions, whose characteristic width is regarded as a statistical variable, in accordance with common knowledge that this parameter is mass dependent. The distributions arising within this picture correspond to superstatistics-a formalism emerging naturally in the context of self-gravitating media. We discuss in detail the distributions arising within this formalism and confront them with observational data of open clusters. We compute their moments and show that the Chandrasekhar-Münch relation remains invariant in this scenario. We also address the effect of these distributions on the thermalization of a massive body, e.g., a supermassive black hole, immersed in a stellar gas. We further discuss how the superstatistical picture clarifies certain ambiguities while offering a whole family of distributions (of which asymptotic power laws represent a special case), opening possibilities for fitting observational data.

Mechanical feedback from stellar winds with an application to galaxy formation at high redshift

ABSTRACT We compute different sets of stellar evolutionary tracks in order to quantify the energy, mass, and metals yielded by massive main-sequence and post-main-sequence winds. Our aim is to investigate the impact of binary systems and of a metallicity-dependent distribution of initial rotational velocities on the feedback by stellar winds. We find significant changes compared to the commonly used non-rotating, single-star scenario. The largest differences are noticeable at low metallicity, where the mechanical-energy budget is substantially increased. So as to establish the maximal (i.e. obtained by neglecting dissipation in the near circumstellar environment) influence of winds on the early stages of galaxy formation, we use our new feedback estimates to simulate the formation and evolution of a sub-L* galaxy at redshift 3 (hosted by a dark-matter halo with a mass of 1.8 × 1011 M⊙) and compare the outcome with simulations in which only supernova (SN) feedback is considered. Accounting for the continuous energy injection by winds reduces the total stellar mass, the metal content, and the burstiness of the star-formation rate as well as of the outflowing gas mass. However, our numerical experiment suggests that the enhanced mechanical feedback from the winds of rotating and binary stars has a limited impact on the most relevant galactic properties compared to the non-rotating single-star scenario. Eventually, we look at the relative abundance between the metals entrained in winds and those ejected by SNe and find that it stays nearly constant within the simulated galaxy and its surrounding halo.

Empirical Constraints on Core-collapse Supernova Yields Using Very Metal-poor Damped Lyα Absorbers

Abstract We place empirical constraints on the yields from zero- and low-metallicity core-collapse supernovae (CCSNe) using abundances measured in very metal-poor (VMP; [Fe/H] ≤ −2) damped Lyα absorbers (DLAs). For some abundance ratios ([N,Al,S/Fe]), VMP DLAs constrain the metal yields of the first SNe more reliably than VMP stars. We compile a large sample of high-S/N VMP DLAs from over 30 yr of literature, most with high-resolution spectral measurements. We infer the initial-mass-function-averaged CCSNe yield from the median values from the DLA abundance ratios of C, N, O, Al, Si, S, and Fe (over Fe and O). We assume that the DLAs are metal-poor enough that they represent galaxies in their earliest stages of evolution, when CCSNe are the only nucleosynthetic sources of the metals we analyze. We compare five sets of zero- and low-metallicity theoretical yields to the empirical yields derived in this work. We find that the five models agree with the DLA yields for ratios containing Si and S. Only one model (Heger &amp; Woosley 2010, hereafter HW10) reproduced the DLA values for N, and one other model (Limongi &amp; Chieffi 2018, hereafter LC18) reproduced [N/O]. We found little change in the theoretical yields with the adoption of an SN explosion landscape (where certain progenitor masses collapse into black holes, contributing no yields) onto HW10, but fixing explosion energy to progenitor mass results in wide disagreements between the predictions and DLA abundances. We investigate the adoption of a simple, observationally motivated initial distribution of rotational velocities for LC18 and find a slight improvement.

The Second Order Term in the Linearized Theory of General ‎Relativity, Dark Matter and Related ‎Cosmological Mysteries

The blackhole jets, Saturn ring, dark matter and GPB anomaly are generally considered unrelated physical mysteries that have no common causes that create them. In Newtonian mechanics, the center of mass of an object change only when there is an external force applied to the object. However, the longitudinally asymmetric and radially circular (LARC) rotating objects like cone, funnel, and hemisphere have a unique mechanical property of creating a finite shift of relativistic center of mass depending on the speed of the rotation due to the difference in the instant speed of the individual mass components of the object while in rotation (Jeong, 1996). This suggests that the LARC rotating object has a complex mechanical property that does not obey the conventional Newtonian mechanical principle. It turned out that in the weak field limit of general relativity there is a second order mathematical term that requires a finite shift of the center of mass to establish its physicality. This term was discarded as physically meaningless because spherical source does not develop a shift of the center of mass even in rotation due to the mathematical cancelation. It is shown that the relativistic shift of the center of mass from the rotating LARC object is the cause of the physically meaningful dipole gravity that reduces into Lense-Thirring force at the center of the rotating spherical shell. However, after careful examination we found out that the signs of Lense-Thirring forces were reversed and once the signs are corrected, dipole gravity predicts blackhole jets and the flat rotational velocity distribution curve which is the key evidence of the existence of the dark matter. We presented that the rings in Saturn, Jupiter, Neptune and Uranus, the GPB experimental anomaly are also the results of dipole gravity from the rotating spherical sources.

Spiral structure of the galactic disk and its influence on the rotational velocity curve

AbstractMost spiral galaxies have a flat rotational velocity curve, according to the different observational techniques used in several wavelengths domain. In this work, we show that nonlinear terms are able to balance the dispersive effect of the wave, thus reviving the observed rotational curve profiles without the inclusion of any other but baryonic matter concentrated in the bulge and disk. To prove that the considered model is able to restore a flat rotational curve, Milky Way has been chosen as the best‐mapped galaxy, to apply on. Using the gravitational N‐body simulations with up to particles, we test this dynamical model in the case of the Milky Way with two different approaches. Within the direct approach, as an input condition in the simulation runs, we set the spiral surface density distribution which is previously obtained as an explicit solution to nonlinear Schrödinger equation (instead of a widely used exponential disk approximation). In the evolutionary approach, we initialize the runs with different initial mass and rotational velocity distributions, in order to capture the natural formation of spiral arms and to determine their role in the disk evolution. In both cases, we are able to reproduce the stable and nonexpanding disk structures at the simulation end times of years, with no halo inclusion. Although the given model does not take into account the velocity dispersion of stars and finite disk thickness, the results presented here still imply that nonlinear effects can significantly alter the amount of dark matter which is required to keep the galactic disk in a stable configuration.

SmAvo: Packhouse optimization using smart avocadoes in South Africa

Cultivation of avocadoes serves as a key economic driver for many countries, including South Africa. Nearly all fresh produce produced forms part of a complex, interconnected transportation chain. Packhouses, representing a wide range of capacities and degrees of mechanization, serve to classify, treat and package the fresh produce prior to distribution by road, rail and ocean to its destination. Reducing the damage or postharvest stresses is paramount to ensure consistent quality of the ripened fruit. Packhouse operations represent the phase associated with the greatest degree of mechanical handling. Until now, quantification of packhouse activities exerted on the fruit has not been quantified with sufficient accuracy to identify characteristics of an optimized packhouse. This paper demonstrates the successful application of a novel sensor platform that measures the linear acceleration and rotational velocity of a typical avocado for 24 packhouses spread throughout South Africa. Unique statistics were developed pertaining to the distribution of contact forces, peak accelerations and rotational velocities, freefall events, freefall distance and impact energy experienced by avocadoes. A Damage Index Score (DIS) was developed to classify the relative performance of the sampled packhouses, alongside identification of key areas of concern where the greatest intensity and variation of postharvest stresses are observed. Improvement of avocado quality as a result of improving deficient practices and equipment ultimately reduces waste, prevalence of postharvest diseases and improves the income for farming communities.

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

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

Evidence for Multiple Accretion Events in the Gaia-Sausage/Enceladus Structures

Abstract We present evidence that multiple accretion events are required to explain the origin of the Gaia-Sausage and Enceladus (GSE) structures, based on an analysis of dynamical properties of main-sequence stars from the Sloan Digital Sky Survey Data Release 12 and Gaia Data Release 2. GSE members are selected to have eccentricity (e) &gt; 0.7 and [Fe/H] &lt; −1.0, and separated into low and high orbital-inclination (LOI/HOI) groups. We find that the LOI stars mainly have e &lt; 0.9 and are clearly separable into two groups with prograde and retrograde motions. The LOI stars exhibit prograde motions in the inner-halo region and strong retrograde motions in the outer-halo region (OHR). We interpret the LOI stars in these regions to be stars accreted from two massive dwarf galaxies with low-inclination prograde and retrograde orbits, affected to different extents by dynamical friction due to their different orbital directions. In contrast, the majority of the HOI stars have e &gt; 0.9, and exhibit a globally symmetric distribution of rotational velocities (V ϕ ) near zero, although there is evidence for a small retrograde motion for these stars (V ϕ ∼ −15 km s−1) in the OHR. We consider these stars to be stripped from a massive dwarf galaxy on a high-inclination orbit. We also find that the LOI and HOI stars on highly eccentric and tangential orbits with clear retrograde motions exhibit different metallicity peaks at [Fe/H] = −1.7 and −1.9, respectively, and argue that they are associated with two low-mass dwarf galaxies accreted in the OHR of the Galaxy.

Mapping the core of the Tarantula Nebula with VLT-MUSE

We present the spectroscopic analysis of 333 OB-type stars extracted from VLT-MUSE observations of the central 30 × 30 pc of NGC 2070 in the Tarantula Nebula on the Large Magellanic Cloud, the majority of which are analysed for the first time. The distribution of stars in the spectroscopic Hertzsprung-Russell diagram (sHRD) shows 281 stars in the main sequence. We find two groups in the main sequence, with estimated ages of 2.1 ± 0.8 and 6.2 ± 2 Myr. A subgroup of 52 stars is apparently beyond the main sequence phase, which we consider to be due to emission-type objects and/or significant nebular contamination affecting the analysis. As in previous studies, stellar masses derived from the sHRD are systematically larger than those obtained from the conventional HRD, with the differences being largest for the most massive stars. Additionally, we do not find any trend between the estimated projected rotational velocity and evolution in the sHRD. The projected rotational velocity distribution presents a tail of fast rotators that resembles findings in the wider population of 30 Doradus. We use published spectral types to calibrate the He Iλ4921/He IIλ5411 equivalent-width ratio as a classification diagnostic for early-type main sequence stars when the classical blue-visible region is not observed. Our model-atmosphere analyses demonstrate that the resulting calibration is well correlated with effective temperature.

Loss of peripheral vestibular input alters the statistics of head movement experienced during natural self-motion.

Sensory systems are adapted to the statistical structure of natural stimuli, thereby optimizing neural coding. Head motion during natural activities is first sensed and then processed by central vestibulo-motor pathways to influence subsequent behaviour, thereby establishing a feedback loop. To investigate the role of this vestibular feedback on the statistical structure of the head movements, we compared head movements in patients with unilateral vestibular loss and healthy controls. We show that the loss of vestibular feedback substantially alters the statistical structure of head motion for activities that require rapid online feedback control and predict this change by modelling the effects of increased movement variability. Our findings suggest that, following peripheral vestibular loss, changes in the reliability of the sensory input to central pathways impact the statistical structure of head motion during voluntary behaviours. It is widely believed that sensory systems are adapted to optimize neural coding of their natural stimuli. Recent evidence suggests that this is the case for the vestibular system, which senses head movement and contributes to essential functions ranging from the most automatic reflexes to voluntary motor control. During everyday behaviours, head motion is sensed by the vestibular system. In turn, this sensory feedback influences subsequent behaviour, raising the questions of whether and how real-time feedback provided by the vestibular system alters the statistical structure of head movements. We predicted that a reduction in vestibular feedback would alter head movement statistics, particularly for tasks reliant on rapid vestibular feedback. To test this proposal, we recorded six-dimensional head motion in patients with variable degrees of unilateral vestibular loss during standard balance and gait tasks, as well as dynamic self-paced activities. While distributions of linear accelerations and rotational velocities were comparable for patients and age-matched healthy controls, comparison of power spectra revealed significant differences during more dynamic and challenging activities. Specifically, consistent with our prediction, head movement power spectra were significantly altered in patients during two tasks that required rapid online vestibular feedback: active repetitive jumping and walking on foam. Using computational methods, we analysed concurrently measured torso motion and identified increases in head-torso movement variability. Taken together, our results demonstrate that vestibular loss significantly alters head movement statistics and further suggest that increased variability and impaired feedback to internal models required for accurate motor control contribute to the observed changes.