Distributions Of Orbital Parameters Research Articles

Probing the strength of radial migration via churning by using metal-rich red giant stars from APOGEE

ABSTRACT Making use of the APOGEE DR17 catalogue with high quality data for 143 509 red giant branch stars we explore the strength of different mechanisms that causes a star to radially migrate in the Milky Way stellar disc. At any position in the disc we find stars that are more metal-rich than the local interstellar medium. This is surprising and normally attributed to the migration of these stars after their formation inside their current Galactocentric radius. Such stars are prime candidates for studying the strength of different migratory processes. We specifically select two types of metal-rich stars: (i) super metal-rich stars ($\mathrm{[Fe/H]}\gt 0.2$) and (ii) stars that are more metal-rich than their local environment. For both, we explore the distribution of orbital parameters and ages as evidence of their migration history. We find that most super metal-rich stars have experienced some amount of churning as they have orbits with $R_g\gtrsim 5 \,{\rm kpc}$. Furthermore, about half of the super metal-rich stars are on non-circular orbits ($\mathrm{ecc} \gt 0.15$) and therefore also have experienced blurring. The metallicity of young stars in our sample is generally the same as the metallicity of the interstellar medium, suggesting they have not radially migrated yet. Stars with lower metallicity than the local environment have intermediate to old ages. We further find that super metal-rich stars have approximately the same age distribution at all Galactocentric radii, which suggests that radial migration is a key mechanism responsible for the chemical compositions of stellar populations in the Milky Way.

Kinematic origin of white dwarfs in the solar neighborhood

Context. White dwarfs are considered to be efficient cosmochronometers. Thanks to the recent space-borne mission Gaia, a nearly complete sample up to about 100 parsecs from the Sun has been compiled. However, the Galaxy, as a dynamic system, implies that these objects may have very diverse origins. It is therefore of paramount importance to characterize the origins of white dwarfs from the different Galactic structure components found in our solar neighborhood. Aims. We aim to compute the Galactic orbits for white dwarfs of the thin and thick disk, as well as the halo components observed in our solar neighborhood. On the basis of these determinations, we analyze the most probable regions of the Galaxy where they could have formed, along with the distribution of their orbital parameters and the observational biases introduced when constructing the local sample. Methods. We used a detailed Galactic orbit integration package, in conjunction with a detailed population synthesis code specifically designed to replicate the different Galactic components of the white dwarf population. Synthetic stars were generated based on the current observational sample and their orbital integration allowed for the reconstruction of the population’s history. Results. Our kinematic analysis of the white dwarf population reveals the ephemeral nature of the concept of the solar neighborhood, as the majority of thin-disk, thick-disk, and halo white dwarfs will have left our 100 pc neighborhood in approximately 3.30 Myr, 1.05 Myr, and 0.6 Myr, respectively. Moreover, the spatial distribution of the integrated thin-disk orbits suggests that 68% of these stars were formed at less than 1 kpc from the Sun, while most of the thick-disk members have undergone radial disk migration. Halo members are those stars that typically belong to the “inner halo”, given that their orbits mostly planar and do not extend beyond R = 20 − 25 kpc. Despite the observational bias, which mostly affects the oldest stars in the thick disk and halo, we show that the wider distribution of orbital parameters is well represented by the sample. Conclusions. The solar neighborhood is a transitory concept, whereby its current population of white dwarfs originates from larger regions of the Galaxy. This fact must be taken into account when analyzing the overall properties of such a population, such as its age distribution, metallicities, and formation history. Even so, the kinematic properties observed by recent missions such as Gaia are representative of the total population up to a distance of approximately 500 pc.

Free-floating Planets, Survivor Planets, Captured Planets, and Binary Planets from Stellar Flybys

In star clusters, close stellar encounters can strongly impact the architecture of a planetary system or even destroy it. We present a systematic study of the effects of stellar flybys on two-planet systems. When such a system experiences flybys, one or both planets can be ejected, forming free-floating planets (FFPs), captured planets (CPs) around the flyby star, and free-floating binary planets (BPs); the remaining single-surviving planets (SSPs) can have their orbital radii and eccentricities greatly changed. Through numerical experiments, we calculate the formation fractions (or branching ratios) of FFPs, SSPs, CPs, and BPs as a function of the pericenter distance of the flyby, and use them to derive analytical expressions for the formation rates of FFPs, SSPs, CPs and BPs in general cluster environments. We find that the production rates of FFPs and SSPs are similar (for the initial planet semimajor axis ratio a 1/a 2 = 0.6–0.8), while the rate for CPs is a few times smaller. The formation fraction of BPs depends strongly on a 1/a 2 and on the planet masses. For Jupiter-mass planets, the formation fraction of BPs is always less than 1% (for a 1/a 2 = 0.8) and typically much smaller (≲0.2% for a 1/a 2 ≲ 0.7). The fraction remains less than 1% when considering 4M J planets. Overall, when averaging over all flybys, the production rate of BPs is less than 0.1% of that of FFPs. We also derive the velocity distribution of FFPs produced by stellar flybys, and the orbital parameter distributions of SSPs, CPs, and BPs. These results can be used in future studies of exotic planets (including FFPs) and planetary systems.

Three-body binary formation in clusters: analytical theory

ABSTRACT Binary formation in clusters through triple encounters between three unbound stars, ‘three-body’ binary formation, is one of the main dynamical formation processes of binary systems in dense environments. In this paper, we use an analytical probabilistic approach to study the process for the equal mass case and calculate a probability distribution for the orbital parameters of three-body-formed binaries, as well as their formation rate. For the first time, we give closed-form analytical expressions to the full orbital parameter distribution, accounting for both energy and angular momentum conservation. This calculation relies on the sensitive dependence of the outcomes of three-body scatterings on the initial conditions: here we compute the rate of three-body binaries from ergodic interactions, which allow for an analytical derivation of the distribution of orbital parameters of the binaries thus created. We find that soft binaries are highly favoured in this process and that these binaries have a superthermal eccentricity distribution, while the few hard three-body binaries have an eccentricity distribution much closer to thermal. The analytical results predict and reproduce simulation results of three-body scattering experiments in the literature well.

Recalibration of the lunar chronology due to spatial cratering-rate variability

Cratering chronologies are used to derive the history of planetary bodies and assume an isotropic flux of impactors over the entire surface of the Moon. The impactor population is largely dominated by near-Earth-objects (NEOs) since ∼3.5 billion years ago. However, lunar impact probabilities from the currently known NEO population show an excess of impacts close to the poles compared to the equator as well as a latitudinal dependency of the approach angle of impactors. This is accompanied by a variation of the impact flux and speed with the distance from the apex due to the synchronicity of the lunar orbit around the Earth. Here, we compute the spatial dependency of the cratering rate produced by such variabilities and recalibrate the lunar chronology. We show that it allows to reconcile the crater density measured at mid-latitudes around the Chang'e-5 landing site with the age of the samples collected by this mission. Our updated chronology leads to differences in model ages of up to 30% compared to other chronology systems. The modeled cratering rate variability is then compared with the distribution of lunar craters younger than ∼1 Ma, 1 Ga and 4 Ga. The general trend of the cratering distribution is consistent with the one obtained from dynamical models of NEOs, thus potentially reflecting a nonuniform distribution of orbital parameters of ancient impactor populations, beyond 3.5 Ga ago, i.e., planetary leftovers and cometary bodies. If the nonuniformity of the cratering rate could be tested elsewhere in the Solar System, the recalibrated lunar chronology, corrected from spatial variations of the impact flux and approach conditions of impactors, could be extrapolated on other terrestrial bodies such as Mercury and Mars, at least over the last 3.5 billion years. The modeled cratering rate presented here has strong implications for interpreting results of the Artemis program, aiming to explore the South Pole of our satellite, in particular when it will come to link the radiometric age of the samples collected in this region and the crater density of the sampled units.

Exploring the chemodynamics of metal-poor stellar populations

Context. Metal-poor stars are key for studying the formation and evolution of the Galaxy. Evidence of the early mergers that built up the Galaxy remains in the distributions of abundances, kinematics, and orbital parameters of its stars. Several substructures resulting from these mergers have been tentatively identified in the literature. Aims. We conducted a global analysis of the chemodynamic properties of metal-poor stars. Our aim is to identify signs of accreted and in situ stars in different regions of the parameter space and investigate their differences and similarities. Methods. We selected a sample of about 6600 metal-poor stars with [Fe/H] ≤ −0.8 from DR3 of the GALAH survey. We used unsupervised machine learning to separate stars in a parameter space made of two normalised orbital actions, plus [Fe/H] and [Mg/Fe], without additional a priori cuts on stellar properties. Results. We divided the halo stars into four main groups. All groups exhibit a significant fraction of in situ contamination. Accreted stars of these groups have very similar chemical properties, except the group of stars with very retrograde orbits. This points to at most two main sources of accreted stars in the current sample, the major one related to Gaia-Enceladus and another possibly related to Thamnos and/or Sequoia. Gaia-Enceladus stars are r-process enriched at low metallicities, but a contribution of the s-process appears with increasing metallicity. A flat trend of [Eu/Mg] as a function of [Fe/H] suggests that only core-collapse supernovae contributed to r-process elements in Gaia-Enceladus. Conclusions. To better characterise accreted stars in the low-metallicity regime, high precision abundances and guidance from chemical evolution models are needed. It is possible that the in situ contamination in samples of accreted stars has been underestimated. This can have important consequences for attempts to estimate the properties of the original systems.

The thermodynamics of stellar multiplicity: an analytic model for the dynamical evolution of binary star populations in dense stellar environments due to single–binary interactions

ABSTRACT We recently derived, using the density-of-states approximation, analytic distribution functions for the outcomes of direct single-binary scatterings. Using these outcome distribution functions, we present in this paper a self-consistent statistical mechanics-based analytic model obtained using the Fokker–Planck limit of the Boltzmann equation. Our model quantifies the dominant gravitational physics, combining both strong and weak single–binary interactions, which drives the time evolution of binary orbital parameter distributions in dense stellar environments. We focus in particular the distributions of binary orbital energies and eccentricities. We find a novel steady-state distribution of binary eccentricities, featuring strong depletions of both the highest and the lowest eccentricity binaries. In energy space, we compare the predictions of our analytic model to the results of numerical N-body simulations, and find that the agreement is good for the initial conditions considered here. This work is a first step towards the development of a fully self-consistent semi-analytic model for dynamically evolving binary star populations in dense stellar environments due to direct few-body interactions.

A possible solution to the Milky Way’s binary-deficient retrograde stellar population

Context. The fraction of field binaries on retrograde orbits about the Milky Way is significantly lower compared to its prograde counterpart. Chemical and dynamical evidence suggests that the retrograde stellar population originates from ω Centauri, which is either the most massive globular cluster (GC) of the Milky Way or the putative core of a former dwarf galaxy. Aims. Star formation conditions required to produce the retrograde binary population are constrained assuming that the retrograde stellar population originates from ω Centauri’s progenitor. Methods. We match the observed low binary fraction with dynamical population synthesis models, including a universal initial binary population and dynamical processing in star clusters, making use of the publicly available binary population synthesis tool BIPOS1. Results. It is found that either the GC progenitor of ω Cen must have formed with a stellar density of ≈108 M⊙ pc−3 or the ω Centauri dwarf galaxy’s progenitor star cluster population must have formed in an extreme starburst with a star formation rate exceeding 1000 M⊙ yr−1 and probably a top-heavy embedded-cluster mass function with suppressed low-mass cluster formation. The separation and mass-ratio distribution for retrograde field binaries are predicted for comparison with future observations. Conclusions. A viable solution for the deficiency of binaries on retrograde orbits is presented, and star formation conditions for ω Centauri as well as orbital parameter distributions for the Milky Way’s retrograde binary population are predicted. The dwarf galaxy origin for ω Centauri is tentatively preferred within the present context.

The observed multiplicity properties of B-type stars in the Galactic young open cluster NGC 6231

Context. It is well known that massive O stars are frequently (if not always) found in binary or higher-order multiple systems, but this fact has been less robustly investigated for the lower mass range of the massive stars, represented by B-type stars. Obtaining the binary fraction and orbital parameter distributions of B-type stars is crucial to understand the impact of multiplicity on the archetypal progenitor of core-collapse supernovae as well as to properly investigate formation channels for gravitational wave progenitors. Aims. This work aims to characterise the multiplicity of the B star population of the young open cluster NGC 6231 through multi-epoch optical spectroscopy of 80 B-type stars. Methods. We analyse 31 FLAMES/GIRAFFE observations of 80 B-type stars, monitoring their radial velocities (RVs) and performing a least-squares spectral analysis (Lomb-Scargle) to search for periodicity in those stars with statistically significant variability in their RVs. Results. We constrained an observed spectroscopic binary fraction of 33 ± 5% for the B-type stars of NGC 6231, with a first order bias correction giving a true spectroscopic binary fraction of 52 ± 8%. Out of 27 B-type binary candidates, we obtained orbital solutions for 20 systems: 15 single-lined (SB1) and five double-lined spectroscopic binaries (SB2s). We present these orbital solutions and the orbital parameter distributions associated with them. Conclusions. Our results indicate that Galactic B-type stars are less frequently found in binary systems than their more massive O-type counterparts, but their orbital properties generally resemble those of B- and O-type stars in both the Galaxy and Large Magellanic Cloud.

Multiplicity of Galactic luminous blue variable stars

Context.Luminous blue variables (LBVs) are characterised by strong photometric and spectroscopic variability. They are thought to be in a transitory phase between O-type stars on the main sequence and the Wolf-Rayet stage. Recent studies also evoked the possibility that they might be formed through binary interaction. Only a few are known in binary systems so far, but their multiplicity fraction is still uncertain.Aims.We derive the binary fraction of the Galactic LBV population. We combine multi-epoch spectroscopy and long-baseline interferometry to probe separations from 0.1 to 120 mas around confirmed and candidate LBVs.Methods.We used a cross-correlation technique to measure the radial velocities of these objects. We identified spectroscopic binaries through significant radial velocity variability with an amplitude larger than 35 km s−1. We also investigated the observational biases to take them into account when we established the intrinsic binary fraction. We usedCANDIDto detect interferometric companions, derive their flux fractions, and their positions on the sky.Results.From the multi-epoch spectroscopy, we derive an observed spectroscopic binary fraction of 26−10+16%. Considering period and mass ratio ranges from log(Porb) = 0 − 3 (i.e. from 1 to 1000 days),q = 0.1 − 1.0, and a representative set of orbital parameter distributions, we find a bias-corrected binary fraction of 62−24+38%. Based on data of the interferometric campaign, we detect a binary fraction of 70 ± 9% at projected separations between 1 and 120 mas. Based on the derived primary diameters and considering the distances of these objects, we measure for the first time the exact radii of Galactic LBVs to be between 100 and 650 R⊙. This means that it is unlikely that short-period systems are included among LBV-like stars.Conclusions.This analysis shows for the first time that the binary fraction in the Galactic LBV population is large. If they form through single-star evolution, their orbit must be large initially. If they form through a binary channel, the implication is that either massive stars in short binary systems must undergo a phase of fully non-conservative mass transfer to be able to sufficiently widen the orbit to form an LBV, or that LBVs form through merging in initially binary or triple systems. Interferometric follow-up would provide the distributions of orbital parameters at more advanced stages and would serve to quantitatively test the binary evolution in massive stars.

Probabilistic orbits and dynamical masses of emission-line binaries

ABSTRACT The observed orbits of emission-line stars may be affected by systematics owing to their broad emission lines being formed in complex and extended environments. This is problematic when orbital parameter probability distributions are estimated assuming radial-velocity data are solely comprised of Keplerian motion plus Gaussian white noise, leading to overconfident and inaccurate orbital solutions, with implications for the inferred dynamical masses and hence evolutionary models. We present a framework in which these systems can be meaningfully analysed. We synthesize benchmark data sets, each with a different and challenging noise formulation, for testing the performance of different algorithms. We make these data sets freely available with the aim of making model validation an easy and standardized practice in this field. Next, we develop an application of Gaussian processes to model the radial-velocity systematics of emission-line binaries, named marginalized $\mathcal {GP}$. We benchmark this algorithm, along with current standardized algorithms, on the synthetic data sets and find our marginalized $\mathcal {GP}$ algorithm performs significantly better than the standard algorithms for data contaminated by systematics. Finally, we apply the marginalized $\mathcal {GP}$ algorithm to four prototypical emission-line binaries: Eta Carinae, GG Carinae, WR 140, and WR 133. We find systematics to be present in several of these case studies; and consequently, the predicted orbital parameter distributions, and dynamical masses, are modified from those previously determined.

Population Synthesis of Neutron Star X-Ray Binaries Associated with Supernova Remnants

Abstract Neutron star X-ray binaries (NS XRBs) associated with supernova remnants (SNRs) are youngest X-ray binaries that can provide insights into the early evolution of X-ray binaries and formation properties of neutron stars. There are an increasing number of NS XRBs that have been discovered within SNRs in our own and nearby galaxies. In this work, we perform binary population synthesis calculations to simulate the population of NS XRBs associated with SNRs for different types of companions, including Roche-lobe-overfilling main-sequence stars, Be stars, and supergiants. We estimate their birth rates and present the distributions of orbital parameters and companion mass for each type of companion. Our calculations show that the majority of the companions are Be X-ray binaries (BeXRBs) and that a few BeXRBs are expected to be associated with SNRs in a Milky Way–type galaxy.

The young massive SMC cluster NGC 330 seen by MUSE

Context.Observations of massive stars in open clusters younger than ∼8 Myr have shown that a majority of them are in binary systems, most of which will interact during their life. While these can be used as a proxy of the initial multiplicity properties, studying populations of massive stars older than ∼20 Myr allows us to probe the outcome of these interactions after a significant number of systems have experienced mass and angular momentum transfer and may even have merged.Aims.Using multi-epoch integral-field spectroscopy, we aim to investigate the multiplicity properties of the massive-star population in the dense core of the ∼40 Myr old cluster NGC 330 in the Small Magellanic Cloud in order to search for possible imprints of stellar evolution on the multiplicity properties.Methods.We obtained six epochs of VLT/MUSE observations operated in wide-field mode with the extended wavelength setup and supported by adaptive optics. We extracted spectra and measured radial velocities for stars brighter thanmF814W = 19. We identified single-lined spectroscopic binaries through significant RV variability with a peak-to-peak amplitude larger than 20 km s−1. We also identified double-lined spectroscopic binaries, and quantified the observational biases for binary detection. In particular, we took into account that binary systems with similar line strengths are difficult to detect in our data set.Results.The observed spectroscopic binary fraction among stars brighter thanmF814W = 19 (approximately 5.5M⊙on the main sequence) isfSBobs= 13.2 ± 2.0%. Considering period and mass ratio ranges from log(P) = 0.15−3.5 (about 1.4 to 3160 d),q = 0.1−1.0, and a representative set of orbital parameter distributions, we find a bias-corrected close binary fraction offcl= 34−7+8%. This fraction seems to decline for the fainter stars, which indicates either that the close binary fraction drops in the B-type domain, or that the period distribution becomes more heavily weighted toward longer orbital periods. We further find that both fractions vary strongly in different regions of the color-magnitude diagram, which corresponds to different evolutionary stages. This probably reveals the imprint of the binary history of different groups of stars. In particular, we find that the observed spectroscopic binary fraction of Be stars (fSBobs= 2 ± 2%) is significantly lower than that of B-type stars (fSBobs= 9 ± 2%).Conclusions.We provide the first homogeneous radial velocity study of a large sample of B-type stars at a low metallicity ([Fe/H] ≲ −1.0). The overall bias-corrected close binary fraction (log(P) < 3.5 d) of the B-star population in NGC 330 is lower than the fraction reported for younger Galactic and Large Magellanic Cloud clusters in previous works. More data are needed, however, to establish whether the observed differences are caused by an age or a metallicity effect.

The origin of the orbital parameter distribution of merging haloes

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

Three-body capture, ejection, and the demographics of bound objects in binary systems

ABSTRACT We study the capture of light objects of arbitrary velocity by binary systems. Extending results for the capture of comets in the Solar system, we develop a simple geometric characterization of the capture cross-section, leading directly to the distribution of orbital parameters of captured objects. We use the same framework to study the lifetimes of these bound orbits prior to ejection, and find that a simplified version of the Öpik–Arnold approach readily yields a closed-form estimate for the ejection rate that agrees well with numerical experiments. Without any detailed-balance assumptions, our results make manifest the characteristics of close encounters leading to capture and ejection. As an application of our results, we demonstrate the estimation of the equilibrium population of captured dark matter particles in a binary system.

Building and Calibrating the Binary Star Population Using Kepler Data

Abstract Modeling binary star populations is critical to linking the theories of star formation and stellar evolution with observations. In order to test these theories, we need accurate models of observable binary populations. The Kepler Eclipsing Binary Catalog (KEBC), with its estimated >90% completeness, provides an observational anchor on binary population models. In this work we present the results of a new forward model of the binary star population in the Kepler field. The forward model takes a single star population from a model of the Galaxy and pairs the stars into binaries by applying the constraints on the population from the results of observational binary population surveys such as Raghavan et al. and Duchêne & Kraus. A synthetic binary population is constructed from the initial distributions of orbital parameters. We identify the eclipsing binary sample from the generated binary star population and compare this with the observed sample of eclipsing binaries contained in the KEBC. Finally, we update the distributions of the synthetic population and repeat the process until the synthetic eclipsing binary sample agrees with the KEBC. The end result of this process is a model of the underlying binary star population that has been fit to observations. We find that for fixed flat mass ratio and eccentricity input distributions, the binary period distribution is logarithmically flat above ∼3.2 days. With additional constraints on distributions from observations, we can further adjust the synthetic binary population by relaxing other input constraints, such as mass ratio and eccentricity.

Age–metallicity dependent stellar kinematics of the Milky Way disc from LAMOST and Gaia

ABSTRACT We investigate the stellar kinematics of the Galactic disc in 7 < R < 13 kpc using a sample of 118 945 red giant branch (RGB) stars from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) and Gaia. We characterize the median, dispersion and skewness of the distributions of the three-dimensional stellar velocities, actions and orbital parameters across the age–metallicity and the disc R–Z plane. Our results reveal abundant but clear stellar kinematic patterns and structures in the age–metallicity and the disc R–Z plane. The most prominent features are the strong variations of the velocity, action and orbital parameter distributions from the young, metal-rich thin disc to the old, metal-poor thick disc; a number of smaller-scale structures (such as velocity streams, north–south asymmetries, and kinematic features of spiral arms) are clearly revealed. Particularly, the skewness of Vϕ and Jϕ reveals a new substructure at R ≃ 12 kpc and Z ≃ 0 kpc, possibly related to dynamical effects of spiral arms in the outer disc. We further study the stellar migration by analysing the stellar orbital parameters and stellar birth radii. The results suggest that the thick disc stars near the solar radii and beyond are mostly migrated from the inner disc of R ∼ 4−6 kpc as a result of their highly eccentrical orbits. Stellar migration resulting from dynamical processes with angular momentum transfer (churning) is prominent for both the old, metal-rich stars (outward migrators) and the young metal-poor stars (inward migrators). The spatial distribution in the R–Z plane for the inward migrators born at a Galactocentric radius of >12 kpc shows clear age stratifications, possibly evidence that these inward migrators are consequences of splashes triggered by merger events of satellite galaxies that have lasted the past few Gyr.

A random-walk model for dark matter halo spins

ABSTRACT We extend the random-walk model of Vitvitska et al. for predicting the spins of dark matter haloes from their merger histories. Using updated merger rates, orbital parameter distributions, and N-body constraints, we show that this model can accurately reproduce the distribution of spin parameters measured in N-body simulations when we include a weak correlation between the spins of haloes and the angular momenta of infalling subhaloes. We further show that this model is in approximate agreement with the correlation of the spin magnitude over time as determined from N-body simulations, while it slightly underpredicts the correlation in the direction of the spin vector measured from the same simulations. This model is useful for predicting spins from merger histories derived from non-N-body sources, thereby circumventing the need for very high resolution simulations to permit accurate measurements of spins. It may be particularly relevant to modelling systems that accumulate angular momentum from haloes over time (such as galactic discs) – we show that this model makes small but significant changes in the distribution of galactic disc sizes computed using the galacticus semi-analytic galaxy formation model.

Vector speckle grid: instantaneous incoherent speckle grid for high-precision astrometry and photometry in high-contrast imaging

Context. Photometric and astrometric monitoring of directly imaged exoplanets will deliver unique insights into their rotational periods, the distribution of cloud structures, weather, and orbital parameters. As the host star is occulted by the coronagraph, a speckle grid (SG) is introduced to serve as astrometric and photometric reference. Speckle grids are implemented as diffractive pupil-plane optics that generate artificial speckles at known location and brightness. Their performance is limited by the underlying speckle halo caused by evolving uncorrected wavefront errors. The speckle halo will interfere with the coherent SGs, affecting their photometric and astrometric precision. Aims. Our aim is to show that by imposing opposite amplitude or phase modulation on the opposite polarization states, a SG can be instantaneously incoherent with the underlying halo, greatly increasing the precision. We refer to these as vector speckle grids (VSGs). Methods. We derive analytically the mechanism by which the incoherency arises and explore the performance gain in idealised simulations under various atmospheric conditions. Results. We show that the VSG is completely incoherent for unpolarized light and that the fundamental limiting factor is the cross-talk between the speckles in the grid. In simulation, we find that for short-exposure images the VSG reaches a ∼0.3–0.8% photometric error and ∼3−10 × 10−3λ/D astrometric error, which is a performance increase of a factor ∼20 and ∼5, respectively. Furthermore, we outline how VSGs could be implemented using liquid-crystal technology to impose the geometric phase on the circular polarization states. Conclusions. The VSG is a promising new method for generating a photometric and astrometric reference SG that has a greatly increased astrometric and photometric precision.

On the Chemical and Kinematic Consistency between N-rich Metal-poor Field Stars and Enriched Populations in Globular Clusters

Abstract Interesting chemically peculiar field stars may reflect their stellar evolution history and their possible origin in a different environment from where they are found now; this is one of the most important research fields in Galactic archeology. To explore this further, we have used the CN–CH bands around 4000 Å to identify N-rich metal-poor field stars in LAMOST DR3. Here we expand our N-rich, metal-poor field star sample to ∼100 stars in LAMOST DR5, where 53 of them are newly found in this work. We investigate light elements of common stars between our sample and APOGEE DR14. While Mg, Al, and Si abundances generally agree with the hypothesis that N-rich metal-poor field stars come from enriched populations in globular clusters, it is still inconclusive for C, N, and O. After integrating the orbits of our N-rich field stars and a control sample of normal metal-poor field stars, we find that N-rich field stars have different orbital parameter distributions compared to the control sample—specifically, apocentric distances, maximum vertical amplitude (Z max), orbital energy, and z-direction angular momentum (Lz ). The orbital parameters of N-rich field stars indicate that most of them are inner-halo stars. The kinematics of N-rich field stars support their possible GC origin. The spatial and velocity distributions of our bona fide N-rich field star sample are important observational evidence to constrain simulations of the origin of these interesting objects.