Search

JASMINE is a Japanese planned space mission that aims to reveal the formation history of our Galaxy and discover habitable exoEarths. For these objectives, the JASMINE satellite performs high-precision astrometric observations of the Galactic bulge and high-precision transit monitoring of M-dwarfs in the near-infrared (1.0-1.6 microns in wavelength). For feasibility studies, we develop an image simulation software named JASMINE-imagesim, which produces realistic observation images. This software takes into account various factors such as the optical point spread function (PSF), telescope jitter caused by the satellite's attitude control error (ACE), detector flat patterns, exposure timing differences between detector pixels, and various noise factors. As an example, we report a simulation for the feasibility study of astrometric observations using JASMINE-imagesim. The simulation confirms that the required position measurement accuracy of 4 mas for a single exposure of 12.5-mag objects is achievable if the telescope pointing jitter uniformly dilutes the PSF across all stars in the field of view. On the other hand, the simulation also demonstrates that the combination of realistic pointing jitter and exposure timing differences in the detector can significantly degrade accuracy and prevent achieving the requirement. This means that certain countermeasures against this issue must be developed. This result implies that this kind of simulation is important for mission planning and advanced developments to realize more realistic simulations help us to identify critical issues and also devise effective solutions., Comment: 13 pages, 7 figures, 1 table

A tracer sample in a gravitational potential, starting from a generic initial condition, phase-mixes towards a stationary state. This evolution is accompanied by an entropy increase, and the final state is characterized by a distribution function (DF) that depends only on integrals of motion (Jeans theorem). We present a method to constrain a gravitational potential where a sample is stationary by minimizing the entropy the sample would have if it were allowed to phase-mix in trial potentials. This method avoids assuming a known DF, and is applicable to any sets of integrals. We provide expressions for the entropy of DFs depending on energy, $f(E)$, energy and angular momentum, $f(E,L)$, or three actions, $f(\vec{J})$, and investigate the bias and fluctuations in their estimates. We show that the method correctly recovers the potential parameters for spherical and axisymmetric models. We also present a methodology to characterize the posterior probability distribution of the parameters with an Approximate Bayesian Computation, indicating a pathway for application to observational data. Using $N=10^4$ tracers with $20\%$-uncertainties in the 6D coordinates, we recover the flattening parameter $q$ of an axisymmetric potential with $\sigma_q/q\sim 10\%$., Comment: Significant changes in appendix with mathematical results. Final expressions and numerical results are the same. A couple of discussions subsections added, and a new appendix on the possibility of maximizing an entropy in angle-space. Submitted to ApJ

We present stellar parameters and chemical abundances of 47 elements detected in the bright (V = 11.63) very metal-poor ([Fe/H] = -2.20 +- 0.12) star 2MASS J22132050-5137385. We observed this star using the Magellan Inamori Kyocera Echelle spectrograph as part of ongoing work by the R-Process Alliance. The spectrum of 2MASS J22132050-5137385 exhibits unusually strong lines of elements heavier than the iron group, and our analysis reveals that these elements were produced by rapid neutron-capture (r-process) nucleosynthesis. We derive a europium enhancement, [Eu/Fe] = +2.45 +- 0.08, that is higher than any other r-process-enhanced star known at present. This star is only the eighth r-process-enhanced star where both thorium and uranium are detected, and we calculate the age of the r-process material, 13.6 +- 2.6 Gyr, from the radioactive decay of these isotopes. This star contains relatively large enhancements of elements that may be produced as transuranic fission fragments, and we propose a new method using this characteristic to assess the r-process yields and gas dilution in samples of r-process-enhanced stars. We conclude that 2MASS J22132050-5137385 exhibits a high level of r-process enhancement because it formed in an environment where the r-process material was less diluted than average. Assuming a canonical baryonic minihalo mass of 10^6 M_sun and a 1 percent metal retention rate, this star formed in a cloud of only ~ 600 M_sun., Comment: Accepted for publication in the Astrophysical Journal (23 pages, 9 figures)

We develop an efficient real-time simulation method for the spin-charge coupled system in the velocity gauge. This method enables us to compute the real-time simulation for the two-dimensional system with the complex spin texture. We focus on the effect of the collective excitation of the localized spins on the electronic transport properties of the non-trivial topological state in real space. To investigate this effect, we calculate the linear optical conductivity by calculating the real-time evolution of the Kondo lattice model on the triangular lattice, which hosts an all-in/all-out magnetic structure. In the linear conductivity spectra, we observe multiple peaks below the bandgap regime, attributed to the resonant contributions of collective modes similar to the skyrmionic system, alongside broadband modifications resulting from off-resonant spin dynamics. The result shows that the collective excitation, similar to the skyrmionic system, influences the optical response of the electron systems based on symmetry analysis. We elucidate the interference between the contributions from the different spin excitations to the optical conductivity in the multiple spin texture, pointing out the mode-dependent electrical activity. We show the complex interplay between the complex spin texture and the itinerant electrons in the two-dimensional spin-charge coupled system., Comment: 10 pages, 6 figures, 2 tables

We estimate ([M/H], [$\alpha$/M]) for 48 million giants and dwarfs in low dust extinction regions from the Gaia DR3 XP spectra by using tree-based machine-learning models trained on APOGEE DR17 and metal-poor star sample of Li et al. The root mean square error of our estimation is 0.0890 dex for [M/H] and 0.0436 dex for [$\alpha$/M], when we evaluate our models with the test data. Our estimation is most reliable for giants and metal-rich stars, because the training data are dominated by such stars. The high-[$\alpha$/M] stars and low-[$\alpha$/M] stars selected by our ([M/H], [$\alpha$/M]) show different kinematical properties for giants and low-temperature dwarfs. We further investigate how our machine-learning models extract information on ([M/H], [$\alpha$/M]). Intriguingly, our models seem to extract information on [$\alpha$/M] from Na D lines (589 nm) and Mg I line (516 nm). This result is understandable given the observed correlation between Na and Mg abundances in the literature. The catalog of ([M/H], [$\alpha$/M]) as well as their associated uncertainties will be publicly available online., Comment: 29 pages, 27 figures, 2 tables. To be submitted to ApJ. Comments are welcome. Catalog available at https://zenodo.org/records/10902172

Japan Astrometry Satellite Mission for INfrared Exploration (JASMINE) is a planned M-class science space mission by the Institute of Space and Astronautical Science, the Japan Aerospace Exploration Agency. JASMINE has two main science goals. One is the Galactic archaeology with Galactic Center Survey, which aims to reveal the Milky Way's central core structure and formation history from Gaia-level (~25 $\mu$as) astrometry in the Near-Infrared (NIR) Hw-band (1.0-1.6 $\mu$m). The other is the Exoplanet Survey, which aims to discover transiting Earth-like exoplanets in the habitable zone from NIR time-series photometry of M dwarfs when the Galactic center is not accessible. We introduce the mission, review many science objectives, and present the instrument concept. JASMINE will be the first dedicated NIR astrometry space mission and provide precise astrometric information of the stars in the Galactic center, taking advantage of the significantly lower extinction in the NIR. The precise astrometry is obtained by taking many short-exposure images. Hence, the JASMINE Galactic center survey data will be valuable for studies of exoplanet transits, asteroseismology, variable stars and microlensing studies, including discovery of (intermediate mass) black holes. We highlight a swath of such potential science, and also describe synergies with other missions., Comment: 50 pages, 22 figures, accepted for publication in PASJ

Visual explanation is an approach for visualizing the grounds of judgment by deep learning, and it is possible to visually interpret the grounds of a judgment for a certain input by visualizing an attention map. As for deep-learning models that output erroneous decision-making grounds, a method that incorporates expert human knowledge in the model via an attention map in a manner that improves explanatory power and recognition accuracy is proposed. In this study, based on a deep-learning model that incorporates the knowledge of experts, a method by which a learner "learns from AI" the grounds for its decisions is proposed. An "attention branch network" (ABN), which has been fine-tuned with attention maps modified by experts, is prepared as a teacher. By using an interactive editing tool for the fine-tuned ABN and attention maps, the learner learns by editing the attention maps and changing the inference results. By repeatedly editing the attention maps and making inferences so that the correct recognition results are output, the learner can acquire the grounds for the expert's judgments embedded in the ABN. The results of an evaluation experiment with subjects show that learning using the proposed method is more efficient than the conventional method., Comment: 12 pages, 5 figures

We report the discovery of a candidate binary system consisting of a black hole (BH) and a red giant branch star from the Gaia DR3. This binary system is discovered from 64108 binary solutions for which both astrometric and spectroscopic data are available. For this system, the astrometric and spectroscopic solutions are consistent with each other, making this system a confident candidate of a BH binary. The primary (visible) star in this system, Gaia DR3 5870569352746779008, is a red giant branch whose mass is quite uncertain. Fortunately, albeit the uncertainty of the primary's mass, we can estimate the mass of the secondary (dark) object in this system to be $>5.68$ $M_\odot$ with a probability of $99$ \%, based on the orbital parameters. The mass of the secondary object is much larger than the maximum neutron star mass ($\sim 2.0$ $M_\odot$), which indicates that the secondary object is likely a BH. We argue that, if this dark object is not a BH, this system must be a more exotic system, in which the primary red giant branch star orbits around a quadruple star system (or a higher-order multiple star system) whose total mass is more than $5.68$ $M_\odot$. If this is a genuine BH binary, this has the longest period ($1352.22 \pm 45.81$ days) among discovered so far. As our conclusion entirely relies on the Gaia DR3 data, independent confirmation with follow-up observations (e.g. long-term spectra) is desired., Comment: 16 pages, 9 figure, 2 tables. ApJ accepted

$R$-process enhanced stars with [Eu/Fe]$\geq+0.7$ (so-called $r$-II stars) are believed to have formed in an extremely neutron-rich environment in which a rare astrophysical event (e.g., a neutron star merger) occurred. This scenario is supported by the existence of an ultra-faint dwarf galaxy, Reticulum~II, where most of the stars are highly enhanced in $r$-process elements. In this scenario, some small fraction of dwarf galaxies around the Milky Way were $r$ enhanced. When each $r$-enhanced dwarf galaxy accreted to the Milky Way, it deposited many $r$-II stars in the Galactic halo with similar orbital actions. To search for the remnants of the $r$-enhanced systems, we analyzed the distribution of the orbital actions of $N=161$ $r$-II stars in the Solar neighborhood by using the Gaia EDR3 data. Since the observational uncertainty is not negligible, we applied a newly-developed {\it greedy optimistic clustering method} to the orbital actions of our sample stars. We found six clusters of $r$-II stars that have similar orbits and chemistry, one of which is a new discovery. Given the apparent phase-mixed orbits of the member stars, we interpret that these clusters are good candidates for remnants of completely disrupted $r$-enhanced dwarf galaxies that merged with the ancient Milky Way., Comment: Main text 21 pages, References 3 pages, Appendix 16 pages. 11 figures. 3 tables. ApJ accepted. (See arXiv:2204.08205 for the algorithm of our method.)

Massive stars born in star clusters terminate star cluster formation by ionizing the surrounding gas. This process is considered to be prevalent in young star clusters containing massive stars. The Orion Nebula is an excellent example associated with a forming star cluster including several massive stars (the Orion Nebula Cluster; ONC) and a 2-pc size H{\sc ii} region (ionized bubble) opening toward the observer; however, the other side is still covered with dense molecular gas. Recent astrometric data acquired by the Gaia satellite revealed the stellar kinematics in this region. By comparing this data with star cluster formation simulation results, we demonstrate that massive stars born in the ONC center were ejected via three-body encounters. Further, orbit analysis indicates that $\theta^2$ Ori A, the second massive star in this region, was ejected from the ONC center toward the observer and is now returning to the cluster center. Such ejected massive stars can form a hole in the dense molecular cloud and contribute to the formation of the 2-pc bubble. Our results demonstrate that the dynamics of massive stars are essential for the formation of star clusters and H{\sc ii} regions that are not always centered by massive stars., Comment: 13 pages, 12 figures, accepted for MNRAS

We present a nearly complete rapid neutron-capture process (r-process) chemical inventory of the metal-poor ([Fe/H] = -1.46 +/- 0.10) r-process-enhanced ([Eu/Fe] = +1.32 +/- 0.08) halo star HD 222925. This abundance set is the most complete for any object beyond the solar system, totaling 63 metals detected and 7 with upper limits. It comprises 42 elements from 31 <= Z <= 90, including elements rarely detected in r-process-enhanced stars, such as Ga, Ge, As, Se, Cd, In, Sn, Sb, Te, W, Re, Os, Ir, Pt, and Au. We derive these abundances from an analysis of 404 absorption lines in ultraviolet spectra collected using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope and previously analyzed optical spectra. A series of appendices discusses the atomic data and quality of fits for these lines. The r-process elements from Ba to Pb, including all elements at the third r-process peak, exhibit remarkable agreement with the Solar r-process residuals, with a standard deviation of the differences of only 0.08 dex (17%). In contrast, deviations among the lighter elements from Ga to Te span nearly 1.4 dex, and they show distinct trends from Ga to Se, Nb through Cd, and In through Te. The r-process contribution to Ga, Ge, and As is small, and Se is the lightest element whose production is dominated by the r-process. The lanthanide fraction, log(X_La) = -1.39 +/- 0.09, is typical for r-process-enhanced stars and higher than that of the kilonova from the GW170817 neutron-star merger event. We advocate adopting this pattern as an alternative to the Solar r-process-element residuals when confronting future theoretical models of heavy-element nucleosynthesis with observations., Comment: Accepted for publication in the Astrophysical Journal Supplement Series (40 pages, 10 figures; includes 43 appendices with detailed discussion of line detections and atomic data). Version 2 corrects a typo in the abstract

In this study, we examine a clustering problem in which the covariates of each individual element in a dataset are associated with an uncertainty specific to that element. More specifically, we consider a clustering approach in which a pre-processing applying a non-linear transformation to the covariates is used to capture the hidden data structure. To this end, we approximate the sets representing the propagated uncertainty for the pre-processed features empirically. To exploit the empirical uncertainty sets, we propose a greedy and optimistic clustering (GOC) algorithm that finds better feature candidates over such sets, yielding more condensed clusters. As an important application, we apply the GOC algorithm to synthetic datasets of the orbital properties of stars generated through our numerical simulation mimicking the formation process of the Milky Way. The GOC algorithm demonstrates an improved performance in finding sibling stars originating from the same dwarf galaxy. These realistic datasets have also been made publicly available., Comment: 20 pages, 27 figures

One of the most intriguing properties of the GD-1 stellar stream is the existence of three gaps. If these gaps were formed by close encounters with dark matter subhalos, the GD-1 stream opens an exciting window through which we can see the size, mass, and velocity distributions of the dark matter subhalos in the Milky Way. However, in order to use the GD-1 stream as a probe of the dark matter substructure, we need to disprove that these gaps are not due to the perturbations from baryonic components of the Milky Way. Here we ran a large number of test particle simulations to investigate the probability that each of the known globular clusters (GCs) can form a GD-1-like gap, by using the kinematical data of the GD-1 stream and GCs from Gaia EDR3 and by fully taking into account the observational uncertainty. We found that the probability that all of the three gaps were formed by GCs is as low as $1.7\times10^{-5}$ and the expected number of gaps formed by GCs is only $0.057$ in our fiducial model. Our result highly disfavors a scenario in which GCs form the gaps. Given that other baryonic perturbers (e.g., giant molecular clouds) are even less likely to form a gap in the retrograde-moving GD-1 stream, we conclude that at least one of the gaps in the GD-1 stream was formed by dark matter subhalos if the gaps were formed by flyby perturbations., Comment: Main text 15 pages, References 1 page, Appendix 4 pages. 9 figures. 3 tables. ApJ accepted

Context: The Helmi streams are a kinematic substructure whose progenitor is likely a dwarf galaxy. Although 20 years have passed since their discovery, it is still unclear whether their members are chemically distinguishable from other halo stars in the Milky Way. Aim: We aim to precisely characterize the chemical properties of the Helmi streams. Methods: We analyzed high-resolution, high signal-to-noise ratio spectra for 11 Helmi stream stars through a line-by-line abundance analysis. We compared the derived abundances to homogenized literature abundances of the other halo stars, including those belonging to other kinematic substructures, such as Gaia-Enceladus and Sequoia. Results: Compared to typical halo stars, the Helmi stream members clearly show low values of [X/Fe] in elements produced by massive stars, such as Na and $\alpha$-elements. This tendency is seen down to metallicities of at least [Fe/H]$\sim -2.2$, suggesting type~Ia supernovae already started to contribute to the chemical evolution at this metallicity. We find that the [$\alpha$/Fe] ratio does not evolve significantly with metallicity, making the Helmi stream stars less distinguishable from Gaia-Enceladus stars at [Fe/H]$\gtrsim -1.5$. The almost constant but low value of [$\alpha$/Fe] might be indicative of quiescent star formation with low efficiency at the beginning and bursty star formation at later times. We also find extremely low values of [Y/Fe] at low metallicity, providing further support for the claim that light neutron-capture elements are deficient in Helmi streams. While Zn is deficient at low metallicity, it shows a large spread at high metallicity. The origin of the extremely low Y abundances and Zn variations remains unclear. Conclusion: The Helmi stream stars are distinguishable from the majority of the halo stars if homogeneously derived abundances are compared., Comment: Accepted version

Photometric monitoring by the Transiting Exoplanet Survey Satellite (TESS) has discovered not only periodic signals by transiting exoplanets but also episodic or quasi-periodic dimming around young stellar objects. The mechanisms of the dimming of these objects, so-called "dippers", are thought to be related to the property of the accretion or the structure of protoplanetary disks especially in regions close to the host star. Recently, we have created the catalog of dippers from the one year of TESS Full Frame Image (FFI) data. In this paper, we report spectral features of four newly found dippers in that catalog and show that they potentially shed light on the dimming mechanisms. We found that all of the targets exhibit the H alpha emission line, which is an indicator of an ongoing accretion. Based on its line profiles and/or their variability, we characterized the properties of the disks of each source, which can support the dimming mechanisms due to a dusty disk wind or an accretion warp. Also, we found an interesting dipper (TIC 317873721), "close-in binary dipper," showing the complex variability of the line profile and the large radial velocity variation. Since the dimming intervals are similar to the orbital period of the binary, we suggest that the dips are caused by dust in the warp of accretion from a circumbinary disk onto stars. Such a close-in ($<$ 0.1 au) binary dipper is rarely reported so far, further investigation will reveal the new aspect of the disk evolution and planetary formation., Comment: 15 pages, 10 figures, accepted for publication in ApJS

Metal-deficient stars are important tracers for understanding the early formation of the Galaxy. Recent large-scale surveys with both photometric and spectroscopic data have reported an increasing number of metal-deficient stars whose kinematic features are consistent with those of the disk stellar populations. We report the discovery of an RR~Lyrae variable (hereafter RRL) that is located within the thick disk and has an orbit consistent with the thick-disk kinematics. Our target RRL (HD 331986) is located at around 1 kpc from the Sun and, with V=11.3, is among the 130 brightest RRLs known so far. However, this object was scarcely studied because it is in the midplane of the Galaxy, the Galactic latitude around -1 deg. Its near-infrared spectrum (0.91-1.32 micron) shows no absorption line except hydrogen lines of the Paschen series, suggesting [Fe/H] less than -2.5. It is the most metal-deficient RRL, at least, among the RRLs whose orbits are consistent with the disk kinematics, although we cannot determine to which of the disk and the halo it belongs. This unique RRL would provide us with essential clues for studying the early formation of stars in the inner Galaxy with further investigations, including high-resolution optical spectroscopy., Comment: 13 pages, 7 figures, 6 tables, accepted for publication in ApJ

Context: Sequoia is a retrograde kinematic substructure in the nearby Galactic halo, whose properties are a matter of debate. For example, previous studies do not necessarily agree on the chemical abundances of Sequoia stars, which are important for understanding its nature. Aim: We characterize the chemical properties of a sample of stars from Sequoia by determining high-precision abundances. Methods: We measured abundances of Na, Mg, Si, Ca, Ti, Cr, Mn, Ni, Zn, Y, and Ba from a differential abundance analysis on high signal-to-noise ratio, high-resolution spectra from new observations and from archival data. We compared precisely measured chemical abundances of 12 Sequoia candidates with those of typical halo stars from the literature, which also includes stars from Gaia-Enceladus. This allowed us to characterize Sequoia and compare it to another Galactic building block. The comparison was made after putting all the abundances onto the same scale using standard stars. Results: There are significant differences in [Na/Fe], [Mg/Fe], [Ca/Fe], [Ti/Fe], [Zn/Fe], and [Y/Fe] between Sequoia and Gaia-Enceladus stars at $-1.8\lesssim [\mathrm{Fe/H}]\lesssim -1.4$ in the sense that these abundance ratios are lower in Sequoia. These differences are similar to those seen between Gaia-Enceladus and in situ stars at a higher metallicity, suggesting that Sequoia is affected by type~Ia supernovae at a lower metallicity than Gaia-Enceladus. We also confirm that the low [{Mg}/{Fe}] of Sequoia is seen in the literature and in surveys, namely APOGEE DR17 and GALAH DR3, if the stars are kinematically selected in the same way. Conclusion: Sequoia stars have a distinct chemical abundance pattern and can be chemically separated from in situ stars or Gaia-Enceladus stars if abundances are measured with sufficient precision, namely $\sigma([\mathrm{X/Fe}])\lesssim 0.07\,\mathrm{dex}$., Comment: Accepted version

Orbital characteristics based on Gaia Early Data Release 3 astrometric parameters are analyzed for ${\sim} 4000$ metal-poor stars ([Fe/H] $\leq -0.8$) compiled from the Best $\&$ Brightest survey. Selected as metal-poor candidates based on broadband near- and far-IR photometry, $43\%$ of these stars had medium-resolution ($1200 \lesssim R \lesssim 2000$) validation spectra obtained over a seven-year campaign from $2014$ to $2020$ with a variety of telescopes. The remaining stars were chosen based on photometric metallicity determinations from the Huang et al. recalibration of the Sky Mapper Southern Survey. Dynamical clusters of these stars are obtained from the orbital energy and cylindrical actions using the \HDBSCAN ~unsupervised learning algorithm. We identify $52$ Dynamically Tagged Groups (DTGs) with between $5$ and $21$ members; $18$ DTGs have at least $10$ member stars. Milky Way (MW) substructures such as Gaia-Sausage-Enceladus, the Metal-Weak Thick-Disk, Thamnos, the Splashed Disk, and the Helmi Stream are identified. Associations with MW globular clusters are determined for $8$ DTGs; no recognized MW dwarf galaxies were associated with any of our DTGs. Previously identified dynamical groups are also associated with our DTGs, with emphasis placed on their structural determination and possible new identifications. Chemically peculiar stars are identified as members of several DTGs, with $6$ DTGs that are associated with \textit{r}-process-enhanced stars. We demonstrate that the mean carbon and $\alpha$-element abundances of our DTGs are correlated with their mean metallicity in an understandable manner. Similarly, we find that the mean metallicity, carbon, and $\alpha$-element abundances are separable into different regions of the mean rotational-velocity space.

We estimate the 3D density profile of the Galactic dark matter (DM) halo within $r \lesssim 30$ kpc from the Galactic centre by using the astrometric data for halo RR Lyrae stars from Gaia DR2. We model both the stellar halo distribution function and the Galactic potential, fully taking into account the survey selection function, the observational errors, and the missing line-of-sight velocity data for RR Lyrae stars. With a Bayesian MCMC method, we infer the model parameters, including the density flattening of the DM halo $q$, which is assumed to be constant as a function of radius. We find that 99\% of the posterior distribution of $q$ is located at $q>0.963$, which strongly disfavours a flattened DM halo. We cannot draw any conclusions as to whether the Galactic DM halo at $r \lesssim 30$ kpc is prolate, because we restrict ourselves to axisymmetric oblate halo models with $q\leq1$. Our result is inconsistent with predictions from cosmological hydrodynamical simulations that advocate more oblate ($\langle{q}\rangle \sim0.8 \pm 0.15$) DM halos within $\sim 15\%$ of the virial radius for Milky-Way-sized galaxies. An alternative possibility, based on our validation tests with a cosmological simulation, is that the true value $q$ of the Galactic halo could be consistent with cosmological simulations but that disequilibrium in the Milky Way potential is inflating our measurement of $q$ by 0.1-0.2. As a by-product of our analysis, our model constrains the DM density in the Solar neighbourhood to be $\rho_{\mathrm{DM},\odot} = (9.01^{+0.18}_{-0.20})\times10^{-3}M_\odot \mathrm{pc}^{-3} = 0.342^{+0.007}_{-0.007}$ $\;\mathrm{GeV} \mathrm{cm}^{-3}$., Comment: MNRAS submitted. 15 pages (plus Appendix 7 pages). 6 figures (plus 10 figures in Appendix). Comments welcome

In this paper, we report 591 high velocity star candidates (HiVelSCs) selected from over 10 million spectra of the data release seven (DR7) of the Large Sky Area Multi-object Fiber Spectroscopic Telescope and the second Gaia data release, with three-dimensional velocities in the Galactic rest-frame larger than 445 km/s. We show that at least 43 HiVelSCs are unbound to the Galaxy with escape probabilities larger than 50%, and this number decreases to eight if the possible parallax zero-point error is corrected. Most of these HiVelSCs are metal-poor and slightly alpha-enhanced inner halo stars. Only 14% of them have [Fe/H] > -1, which may be the metal-rich "in situ" stars in halo formed in the initial collapse of the Milky Way or metal-rich stars formed in the disk or bulge but kinematically heated. The low ratio of 14% implies that the bulk of stellar halo was formed from the accretion and tidal disruption of satellite galaxies. In addition, HiVelSCs on the retrograde orbits have a slightly lower metallicities on average compared with these on the prograde orbits, meanwhile metal-poor HiVelSCs with [Fe/H] < -1 have an even faster mean retrograde velocity compared with metal-rich HiVelSCs. To investigate the origins of HiVelSCs, we perform orbit integrations and divide them into four types, i.e., hypervelocity stars, hyper-runaway stars, runaway stars and fast halo stars. A catalog for these 591 HiVelSCs, including radial velocities, atmospheric parameters, Gaia astrometric parameters, spatial positions, and velocities, etc., is available at http://paperdata.chinavo.org/LYB/lamostdr7_gaiadr2_hvs_591.csv., Comment: 28 pages, 13 pages, Accepted by ApJS

We present a comprehensive catalog of the dippers---young stellar objects that exhibit episodic dimming---derived from the one year's worth of data of Transiting Exoplanet Survey Satellite ($ TESS$) full-frame images. In the survey, we found 35 dippers using the convolutional neural network, most of them newly discovered. Although these dippers are widely distributed over the first half-hemisphere that $TESS$ surveyed, we identified the majority's membership with the nearest association Scorpius--Centaurus, Velorum OB2, and nearby Orion molecular cloud complex. However, several dippers are likely to be located in the field. We also found three old dippers whose age exceeds ten million year, which is considered as the disk dissipation time. The color-color diagram indicates that these old dippers are likely to have an extreme debris disk. In particular, we found a runaway old dipper having a large three-dimensional velocity of $72 \mathrm{km\ s}^{-1}$. The dippers in the field, which were probably escaping from their birth molecular clouds or were born outside the current area of star forming regions, are more common than previously considered., Comment: 18 pages, 20 figures; accepted for publication in The Astrophysical Journal Supplement

We present high-resolution Magellan/MIKE spectroscopy of 42 red giant stars in seven stellar streams confirmed by the Southern Stellar Stream Spectroscopic Survey (S5): ATLAS, Aliqa Uma, Chenab, Elqui, Indus, Jhelum, and Phoenix. Abundances of 30 elements have been derived from over 10,000 individual line measurements or upper limits using photometric stellar parameters and a standard LTE analysis. This is currently the most extensive set of element abundances for stars in stellar streams. Three streams (ATLAS, Aliqa Uma, and Phoenix) are disrupted metal-poor globular clusters, although only weak evidence is seen for the light element anticorrelations commonly observed in globular clusters. Four streams (Chenab, Elqui, Indus, and Jhelum) are disrupted dwarf galaxies, and their stars display abundance signatures that suggest progenitors with stellar masses ranging from $10^6-10^7 M_\odot$. Extensive description is provided for the analysis methods, including the derivation of a new method for including the effect of stellar parameter correlations on each star's abundance and uncertainty. This paper includes data gathered with the 6.5 meter Magellan Telescopes located at Las Campanas Observatory, Chile., Comment: Accepted to AJ. 39 pages, 13 figures, 7 tables

We present the first spectroscopic measurements of the ATLAS and Aliqa Uma streams from the Southern Stellar Stream Spectroscopic Survey ($S^5$), in combination with the photometric data from the Dark Energy Survey and astrometric data from $Gaia$. From the coherence of spectroscopic members in radial velocity and proper motion, we find out that these two systems are extremely likely to be one stream with discontinuity in morphology and density on the sky (the "kink" feature). We refer to this entire stream as the ATLAS-Aliqa Uma stream, or the AAU stream. We perform a comprehensive exploration of the effect of baryonic substructures and find that only an encounter with the Sagittarius dwarf $\sim 0.5$ Gyr ago can create a feature similar to the observed "kink". In addition, we also identify two gaps in the ATLAS component associated with the broadening in the stream width (the "broadening" feature). These gaps have likely been created by small mass perturbers, such as dark matter halos, as the AAU stream is the most distant cold stream known with severe variations in both the stream surface density and the stream track on the sky. With the stream track, stream distance and kinematic information, we determine the orbit of the AAU stream and find that it has been affected by the Large Magellanic Cloud, resulting in a misalignment between the proper motion and stream track. Together with the Orphan-Chenab Stream, AAU is the second stream pair that has been found to be a single stream separated into two segments by external perturbation., Comment: 33 pages, 22 figures (including 1 movie), 3 tables. Accepted for publication in ApJ

We report the discovery of J1521-3538, a bright (V=12.2), very metal-poor ([Fe/H]=-2.8) strongly r-process enhanced field horizontal branch star, based on a high-resolution, high signal-to-noise Magellan/MIKE spectrum. J1521-3538 shows the largest r-process element over-abundance in any known r-process-enhanced star, with [Eu/Fe]=+2.2, and its chemical abundances of 22 neutron-capture elements closely match the scaled solar r-process pattern. J1521-3538 is also one of few known carbon-enhanced metal-poor stars with r-process enhancement (CEMP-r stars), as found after correcting the measured C abundance for the star's evolutionary status. We propose to extend the existing classification of moderately enhanced (+0.3<=[Eu/Fe]<=+1.0) r-I and strongly r-process enhanced ([Eu/Fe]>+1.0) r-II stars to include an r-III class, for r-process stars such as J1521-3538, with [Eu/Fe]>+2.0 and [Ba/Eu]<-0.5, or >100 times the solar ratio of europium to iron. Using cosmochronometry, we estimate J1521-3538 to be 12.5+-5 Gyr and 8.9+-5 Gyr old, using two different sets of initial production ratios. These ages are based on measurements of the Th line at 4019 A and other r-process element abundances. This is broadly consistent with the old age of a low-mass metal-poor field red horizontal branch star. J1521-3538 likely originated in a low-mass dwarf galaxy that was later accreted by the Milky Way, as evidenced by its highly eccentric orbit., Comment: 24 pages, includes a long table, 4 figures

We search for dynamical substructures in the LAMOST DR3 very metal-poor (VMP) star catalog. After cross-matching with Gaia DR2, there are 3300 VMP stars with available high-quality astrometric information that have halo-like kinematics. We apply a method based on self-organizing maps to find groups clustered in the 4D space of orbital energy and angular momentum. We identify 57 dynamically tagged groups, which we label DTG-1 to DTG-57. Most of them belong to existing substructures in the nearby halo, such as the $Gaia$ Sausage or Sequoia. The stream identified by Helmi et al. is recovered, but the two disjoint portions of the substructure have distinct dynamical properties. The very retrograde substructure Rg5 found previously by Myeong et al. is also retrieved. We report 6 new DTGs with highly retrograde orbits, 2 with very prograde orbits, and 12 with polar orbits. By mapping other datasets (APOGEE halo stars, and catalogs of r-process-enhanced and CEMP stars) onto the trained neuron map, we can associate stars with detailed chemical abundances to the DTGs, and look for associations with chemically peculiar stars. The highly eccentric $Gaia$ Sausage groups contain representatives both of debris from the satellite itself (which is $\alpha$-poor) and the Splashed Disk, sent up into eccentric halo orbits from the encounter (and is $\alpha$-rich). The new prograde substructures also appear to be associated with the Splashed Disk. The DTGs belonging to the $Gaia$ Sausage host two relatively metal-rich $r$-II stars and six CEMP stars in different sub-classes, consistent with the idea that the $Gaia$ Sausage progenitor is a massive dwarf galaxy. Rg5 is dynamically associated with two highly $r$-process-enhanced stars with [Fe/H] $\sim -$3. This finding indicates that its progenitor might be an ultra-faint dwarf galaxy that has experienced $r$-process enrichment from neutron star mergers., Comment: ApJ, submitted

A recently discovered young, high-velocity giant star J01020100-7122208 is a good candidate of hypervelocity star ejected from the Galactic center, although it has a bound orbit. If we assume that this star was ejected from the Galactic center, it can be used to constrain the Galactic potential, because the deviation of its orbit from a purely radial orbit informs us of the torque that this star has received after its ejection. Based on this assumption, we estimate the flattening of the dark matter halo of the Milky Way by using the Gaia DR2 data and the circular velocity data from Eilers et al. (2019). Our Bayesian analysis shows that the orbit of J01020100-7122208 favors a prolate dark matter halo within $\sim$ 10 kpc from the Galactic center. The posterior distribution of the density flattening $q$ shows a broad distribution at $q \gtrsim1$ and peaks at $q \simeq 1.5$. Also, 98.5\% of the posterior distribution is located at $q>1$, highly disfavoring an oblate halo., Comment: Comments welcome. 4 pages. 2 figures. Proceedings for IAUS 353 Galactic Dynamics in the Era of Large Surveys

We report that LAMOST-HVS1 is a massive hyper-runaway subgiant star with mass of 8.3 Msun and super-Solar metallicity, ejected from the inner stellar disk of the Milky Way $\sim$ 33 Myr ago with the intrinsic ejection velocity of $568^{+19}_{-17}$ km/s (corrected for the streaming motion of the disk), based on the proper motion data from Gaia Data Release 2 (DR2) and high-resolution spectroscopy. The extremely large ejection velocity indicates that this star was not ejected by the supernova explosion of the binary companion. Rather, it was probably ejected by a 3- or 4-body dynamical interaction with more massive objects in a high-density environment. Such a high-density environment may be attained at the core region of a young massive cluster with mass of $\gtrsim 10^4$ Msun. The ejection agent that took part in the ejection of LAMOST-HVS1 may well be an intermediate mass black hole ($\gtrsim$ 100 Msun), a very massive star ($\gtrsim$ 100 Msun), or multiple ordinary massive stars ($\gtrsim$ 30 Msun). Based on the flight time and the ejection location of LAMOST-HVS1, we argue that its ejection agent or its natal star cluster is currently located near the Norma spiral arm. The natal star cluster of LAMOST-HVS1 may be an undiscovered young massive cluster near the Norma spiral arm., Comment: Minor revision after referee's comments (conclusion unchanged). ApJ submitted. 23 pages. 7 figures. 2 tables. Predicted location of IMBH (or the undiscovered young massive cluster) in the Milky Way is shown in Fig 7. High-resolution spectra (Magellan/MIKE) can be distributed upon request. Comments welcome

We present kinematics of 35 highly r-process-enhanced ([Eu/Fe] >= +0.7) metal-poor (-3.8 < [Fe/H] < -1.4) field stars. We calculate six-dimensional positions and velocities, evaluate energies and integrals of motion, and compute orbits for each of these stars using parallaxes and proper motions from the second Gaia data release and published radial velocities. All of these stars have halo kinematics. Most stars (66%) remain in the inner regions of the halo (< 13 kpc), and many (51%) have orbits that pass within 2.6 kpc of the Galactic center. Several stars (20%) have orbits that extend beyond 20 kpc, including one with an orbital apocenter larger than the Milky Way virial radius. We apply three clustering methods to search for structure in phase space, and we identify eight groups. No abundances are considered in the clustering process, but the [Fe/H] dispersions of the groups are smaller than would be expected by random chance. The orbital properties, clustering in phase space and metallicity, and lack of highly r-process-enhanced stars on disk-like orbits indicate that such stars likely were accreted from disrupted satellites. Comparison with the galaxy luminosity-metallicity relation suggests M_V > -9 for most of the progenitor satellites, characteristic of ultra-faint or low-luminosity classical dwarf spheroidal galaxies. Environments with low rates of star formation and Fe production, rather than the nature of the r-process site, may be key to obtaining the [Eu/Fe] ratios found in highly r-process-enhanced stars., Comment: Accepted for publication in the Astronomical Journal (19 pages, 10 figures, 6 tables)

We report the discovery of 30 stars with extreme space velocities ($>$ 480 km/s) in the Gaia-DR2 archive. These stars are a subset of 1743 stars with high-precision parallax, large tangential velocity ($v_{tan}>$ 300 km/s), and measured line-of-sight velocity in DR2. By tracing the orbits of the stars back in time, we find at least one of them is consistent with having been ejected by the supermassive black hole at the Galactic Center. Another star has an orbit that passed near the Large Magellanic Cloud (LMC) about 200 Myr ago. Unlike previously discovered blue hypervelocity stars, our sample is metal-poor (-1.5 $<$ [Fe/H] $<$ -1.0) and quite old ($>$ 1 Gyr). We discuss possible mechanisms for accelerating old stars to such extreme velocities. The high observed space density of this population, relative to potential acceleration mechanisms, implies that these stars are probably bound to the Milky Way (MW). If they are bound, the discovery of this population would require a local escape speed of around $\sim$ 600 km/s and consequently imply a virial mass of $M_{200} \sim 1.4 \times 10^{12} M_\odot$ for the MW., Comment: 14 pages, 5 figures. ApJ accepted. Dust correction using 3D dust map is explained in the Appendix A. ADQL queries are provided in Appendix C

Gravitational 3-body interaction among binary stars and the supermassive black hole (SMBH) at the center of the Milky Way occasionally ejects a hypervelocity star (HVS) with a velocity of ~1000 km/s. Due to the ejection location, such a HVS initially has negligible azimuthal angular momentum Lz ~ 0 kpc km/s. Even if the halo is mildly triaxial, Lz of a recently ejected nearby HVS remains negligible, since its flight time from the Galactic Center is too short to accumulate noticeable torque. However, if we make a wrong assumption about the Solar position and velocity, such a HVS would apparently have noticeable non-zero azimuthal angular momentum, due to the wrong reflex motion of the Sun. Conversely, with precise astrometric data for a nearby HVS, we can measure the Solar position and velocity by assuming that the HVS has zero azimuthal angular momentum. Based on this idea, here we propose a method to estimate the Galactocentric distance of the Sun R0 and the Galactocentric Solar azimuthal velocity Vsun by using a HVS. We demonstrate with mock data for a nearby HVS candidate that the Gaia astrometric data, along with the currently available constraint on Vsun/R0 from the proper motion measurement of Sgr A*, can constrain R0 and Vsun with uncertainties of ~0.27 kpc and ~7.8 km/s (or fractional uncertainties of 3 percent), respectively. Our method will be a promising tool to constrain (R0, Vsun), given that Gaia is expected to discover many nearby HVSs in the near future., Comment: ApJ accepted. 15 pages, 5 figures. We added Section 5.5, where the situation after this paper first appeared (on April 23, 2018) is described. Conclusion is unchanged. Comments welcome

The origin of the Hercules stream, the most prominent velocity substructure in the Solar neighbour disc stars, is still under debate. Recent accurate measurements of position, velocity, and metallicity provided by Tycho Gaia Astrometric Solution (TGAS) and RAdial Velocity Experiments (RAVE) have revealed that the Hercules stream is most clearly seen in the metal-rich region ([Fe/H] > 0), while it is not clearly seen in lower metallicity region ([Fe/H] < -0.25). By using a large number of chemo-dynamical 2D test-particle simulations with a rotating bar and/or spiral arms, we find that the observed [Fe/H] dependence of the Hercules stream is a natural consequence of the inside-out formation of the stellar disc and the existence of highly non-closed orbits in the rotating frame of the bar or spiral arms. Our successful models that reproduce the observed properties of the Hercules stream include not only fast-bar-only and fast-bar+spiral models, but also slow-bar+spiral models. This indicates that it is very difficult to estimate the pattern speed of the bar or spiral arms based only on the observations of the Hercules stream in the Solar neighbourhood. As a by-product of our simulations, we make some predictions about the locations across the Galactic plane where we can observe velocity bimodality that is not associated with the Hercules stream. These predictions can be tested by the Gaia Data Release 2, and such a test will improve our understanding of the evolution of the Milky Way stellar disc., Comment: MNRAS accepted. 25 pages, 14 figures. Comments welcome

Determining the velocity distribution of halo stars is essential for estimating the mass of the Milky Way and for inferring its formation history. Since the stellar halo is a dynamically hot system, the velocity distribution of halo stars is well described by the 3-dimensional velocity dispersions $(\sigma_r, \sigma_\theta, \sigma_\phi)$, or by the velocity anisotropy parameter $\beta=1-(\sigma_\theta^2+\sigma_\phi^2)/(2\sigma_r^2)$. Direct measurements of $(\sigma_r, \sigma_\theta, \sigma_\phi)$ consistently suggest $\beta =0.5$-$0.7$ for nearby halo stars. In contrast, the value of $\beta$ at large Galactocentric radius $r$ is still controversial, since reliable proper motion data are available for only a handful of stars. In the last decade, several authors have tried to estimate $\beta$ for distant halo stars by fitting the observed line-of-sight velocities at each radius with simple velocity distribution models (local fitting methods). Some results of local fitting methods imply $\beta<0$ at $r \gtrsim 20 \;\rm{kpc}$, which is inconsistent with recent predictions from cosmological simulations. Here we perform mock-catalogue analyses to show that the estimates of $\beta$ based on local fitting methods are reliable only at $r \leq 15 \;\rm{kpc}$ with the current sample size ($\sim10^3$ stars at a given radius). As $r$ increases, the line-of-sight velocity (corrected for the Solar reflex motion) becomes increasingly closer to the Galactocentric radial velocity, so that it becomes increasingly more difficult to estimate tangential velocity dispersion $(\sigma_\theta, \sigma_\phi)$ from line-of-sight velocity distribution. Our results suggest that the forthcoming Gaia data will be crucial for understanding the velocity distribution of halo stars at $r \geq 20\;\rm{kpc}$., Comment: [Deleted non-relevant figures] ApJ submitted. Comments welcome. 20 pages (14 pages + 6 pages for Appendix). 13 figures. Main result: Fig 7. Schematic diagram: Fig 9. Companion paper to Loebman et al. (2017)

The velocity anisotropy parameter, beta, is a measure of the kinematic state of orbits in the stellar halo which holds promise for constraining the merger history of the Milky Way (MW). We determine global trends for beta as a function of radius from three suites of simulations, including accretion only and cosmological hydrodynamic simulations. We find that both types of simulations are consistent and predict strong radial anisotropy (~0.7) for Galactocentric radii greater than 10 kpc. Previous observations of beta for the MW's stellar halo claim a detection of an isotropic or tangential "dip" at r~20 kpc. Using the N-body+SPH simulations, we investigate the temporal persistence, population origin, and severity of "dips" in beta. We find dips in the in situ stellar halo are long-lived, while dips in the accreted stellar halo are short-lived and tied to the recent accretion of satellite material. We also find that a major merger as early as z~1 can result in a present day low (isotropic to tangential) value of beta over a wide range of radii and angular expanse. While all of these mechanisms are plausible drivers for the beta dip observed in the MW, in the simulations, each mechanism has a unique metallicity signature associated with it, implying that future spectroscopic surveys could distinguish between them. Since an accurate knowledge of beta(r) is required for measuring the mass of the MW halo, we note significant transient dips in beta could cause an overestimate of the halo's mass when using spherical Jeans equation modeling., Comment: 14 pages, 7 figures, accepted to ApJ, companion paper to Hattori et al. (2017), appendix with Gaia specific predictions

The dynamics of stellar streams in rotating barred potentials is explained for the first time. Naturally, neighbouring stream stars reach pericentre at slightly different times. In the presence of a rotating bar, these neighbouring stream stars experience different bar orientations during pericentric passage and hence each star receives a different torque from the bar. These differing torques reshape the angular momentum and energy distribution of stars in the stream, which in turn changes the growth rate of the stream. For a progenitor orbiting in the same sense as the bar's rotation and satisfying a resonance condition, the resultant stream can be substantially shorter or longer than expected, depending on whether the pericentric passages of the progenitor occur along the bar's minor or major axis respectively. We present a full discussion of this phenomenon focusing mainly on streams confined to the Galactic plane. In stark contrast with the evolution in static potentials, which give rise to streams that grow steadily in time, rotating barred potentials can produce dynamically old, short streams. This challenges the traditional viewpoint that the inner halo consists of well phase-mixed material whilst the tidally-disrupted structures in the outer halo are more spatially coherent. We argue that this mechanism plays an important role in explaining the mysteriously short Ophiuchus stream that was recently discovered near the bulge region of the Milky Way., Comment: 17 pages, 13 figures. Submitted to MNRAS

We explored a method to reconstruct the distribution function of the Galactic thick disc within the action space where nearby thick-disc stars are distributed. By applying this method to 127 chemically-selected thick-disc stars in the Solar neighbourhood, we found that the vertical velocity dispersion that corresponds to the reconstructed distribution function declines approximately as $\exp (-R/R_s)$ at 4.5 kpc < R < 9.5 kpc, with $R_s$ = 8.3 $\pm$ 1.1 (rand.) $\pm$ 1.6 (sys.) kpc. Also, we found that the vertical velocity dispersion $\sigma_z$ of our local thick-disc stars shows only weak dependency on radial and azimuthal velocities $(v_R, v_\phi)$. We discuss possible implications of these results on the global structure of the Milky Way thick disc., Comment: 9 pages, 7 figures. MNRAS accepted. Comments welcome

The entire evolution of the Milky Way, including its mass-assembly and star-formation history, is imprinted onto the chemo-dynamical distribution function of its member stars, f(x, v, [X/H]), in the multi-dimensional phase space spanned by position, velocity, and elemental abundance ratios. In particular, the chemo-dynamical distribution functions for low-mass stars (e.g., G- or K-type dwarfs) are precious tracers of the earliest stages of the Milky Way's formation, since their main-sequence lifetimes approach or exceed the age of the universe. A basic tenet of essentially all previous analyses is that the stellar metallicity, usually parametrized as [Fe/H], is conserved over time for main-sequence stars (at least those that have not been polluted due to mass transfer from binary companions). If this holds true, any correlations between metallicity and kinematics for long-lived main-sequence stars of different masses, effective temperatures, or spectral types must strictly be the same, since they reflect the same mass-assembly and star-formation histories. By analyzing a sample of nearby metal-poor halo and thick-disk stars on the main sequence, taken from Data Release 8 of the Sloan Digital Sky Survey, we find that the median metallicity of G-type dwarfs is systematically higher (by about 0.2 dex) than that of K-type dwarfs having the same median rotational velocity about the Galactic center. If it can be confirmed, this finding may invalidate the long-accepted assumption that the atmospheric metallicities of long-lived stars are conserved over time., Comment: 12 pages, 7 figures, ApJ accepted, comments welcome

The orbital motions of halo stars in the Milky Way reflect the orbital motions of the progenitor systems in which they formed, making it possible to trace the mass-assembly history of the Galaxy. Direct measurement of three-dimensional velocities, based on accurate proper motions and line-of-sight velocities, has revealed that the majority of halo stars in the inner-halo region move on eccentric orbits. However, our understanding of the motions of distant, in-situ halo-star samples is still limited, due to the lack of accurate proper motions for these stars. Here we explore a model-independent analysis of the line-of-sight velocities and spatial distribution of a recent sample of 1865 carefully selected halo blue horizontal-branch (BHB) stars within 30 kpc of the Galactic center. We find that the mean rotational velocity of the very metal-poor ([Fe/H] < -2.0) BHB stars significantly lags behind that of the relatively more metal-rich ([Fe/H] > -2.0) BHB stars. We also find that the relatively more metal-rich BHB stars are dominated by stars with eccentric orbits, as previously observed for other stellar samples in the inner-halo region. By contrast, the very metal-poor BHB stars are dominated by stars on rounder, lower-eccentricity orbits. Our results indicate that the motion of the progenitor systems of the Milky Way that contributed to the stellar populations found within 30 kpc correlates directly with their metal abundance, which may be related to their physical properties such as gas fractions. These results are consistent with the existence of an inner/outer halo structure for the halo system, as advocated by Carollo et al. (2010)., Comment: 5 pages, 3 figures, ApJ Letter accepted, comments welcome

We present theoretical calculations for the differential distribution of stellar orbital eccentricity for a sample of solar-neighbour halo stars. Two types of static, spherical gravitational potentials are adopted to define the eccentricity e for given energy E and angular momentum L, such as an isochrone potential and a Navarro-Frenk-White potential that can serve as two extreme ends covering in-between any realistic potential of the Milky Way halo. The solar-neighbour eccentricity distribution \Delta N(e) is then formulated, based on a static distribution function of the form f(E,L) in which the velocity anisotropy parameter \beta monotonically increases in the radial direction away from the galaxy center, such that beta is below unity (near isotropic velocity dispersion) in the central region and asymptotically approaches \sim 1 (radially anisotropic velocity dispersion) in the far distant region of the halo. We find that \Delta N(e) sensitively depends upon the radial profile of \beta, and this sensitivity is used to constrain such profile in comparison with some observational properties of \Delta N_{obs}(e) recently reported by Carollo et al. (2010). Especially, the linear e-distribution and the fraction of higher-e stars for their sample of solar-neighbour inner-halo stars rule out a constant profile of \beta, contrary to the opposite claim by Bond et al. (2010). Our constraint of \beta \lesssim 0.5 at the galaxy center indicates that the violent relaxation that has acted on the inner halo is effective within a scale radius of \sim 10 kpc from the galaxy center. We discuss that our result would help understand the formation and evolution of the Milky Way halo., Comment: 13 pages, 6 figures, accepted for publication in MNRAS

We present theoretical calculations for the differential distribution of stellar orbital eccentricity in a galaxy halo, assuming that the stars constitute a spherical, collisionless system in dynamical equilibrium with a dark matter halo. In order to define the eccentricity e of a halo star for given energy E and angular momentum L, we adopt two types of gravitational potential, such as an isochrone potential and a Navarro-Frenk-White potential, that could form two ends covering in-between any realistic potential of dark matter halo. Based on a distribution function of the form f(E,L) that allows constant anisotropy in velocity dispersions characterized by a parameter \beta, we find that the eccentricity distribution is a monotonically increasing function of e for the case of highly radially anisotropic velocity dispersions (\beta > 0.6), while showing a hump-like shape for the cases from radial through tangential velocity anisotropy (\beta < 0.6). We also find that when the velocity anisotropy agrees with that observed for the Milky Way halo stars (\beta = 0.5-0.7), a nearly linear eccentricity distribution of N(e) \alpha e results at e < 0.7, largely independent of the potential adopted. Our theoretical eccentricity distribution would be a vital tool of examining how far out in the halo the dynamical equilibrium has been achieved, through comparison with kinematics of halo stars sampled at greater distances. Given that large surveys of the SEGUE and Gaia projects would be in progress, we discuss how our results would serve as a new guide in exploring the formation and evolution of the Milky Way halo., Comment: 13 pages, 7 figures