arXiv subjects

Daisuke Kawata

Publications and source records attributed to Daisuke Kawata.

At least 19 recordsLinked to original sources

arXiv2026

Milky Way's warped disc traced by AGB stars

While the presence of the Galactic warp has long been established from observations of \HI\, gas, the \textit{Gaia} measurements of over 1 billion stars with parallaxes have enabled much more detailed studies using stellar populations. Here, we demonstrate that asymptotic giant branch (AGB) stars, an evolved phase of low- and intermediate-mass stars, can serve as an effective tracer of the Galactic warp. We use two distinct AGB populations: C-rich AGB stars, representing stars of about 1~Gyr in age with main-sequence masses of 2--2.5~\Msun, and intermediate-mass (3--5~\Msun) O-rich AGB stars, corresponding to ages of 100--300~Myr. The downward warp traced by O-rich AGB stars is consistent with that found from Cepheids, which is expected given their similar ages. The more numerous C-rich AGB stars clearly reveal the Galactic warp over a wide range of azimuthal angles. Their warp appears to reach larger amplitudes than that of Cepheids across azimuthal angles. Our results show that C-rich AGB stars, together with intermediate-mass O-rich AGB stars, provide new constraints on the Galactic warp at intermediate stellar ages, offering a new insight into the stellar age and warp amplitude relation.

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arXiv2026

A simulation-based inference of the Milky Way merger history

Accreted stars in the Milky Way (MW) preserve information about the progenitor galaxies where they formed in their chemical and kinematic properties. In this study, we use the chemo-dynamical signatures in the merger debris to approximate the posterior distribution of disrupted satellite properties at the time of infall. Adopting a simulation-based inference framework, we train an ensemble of normalizing flows using samples of merger debris from the Auriga suite of simulations of MW-like galaxies. Applying this methodology to a local sample of accreted stars in the MW, we infer the lookback times, stellar and halo masses, and halo mass merger ratios of several known accretion events in the Galaxy: Gaia Enceladus-Sausage (GES), Helmi streams, Heracles, I'itoi, LMS-1/Wukong, Sagittarius (Sgr), Sequoia and Thamnos. Our predictions align with the accretion time and mass estimates from the literature, and the expected relation between the progenitor stellar masses and debris metallicities across redshifts. The total stellar mass accreted from these events is predicted to be $2.2^{+1.1}_{-0.6}\times10^{9}~\rm{M_{\odot}}$, with GES and Sgr being the largest contributors. The predicted stellar mass accreted from fully disrupted progenitors in the stellar halo is $1.3^{+1.0}_{-0.5}\times10^{9}~\rm{M_{\odot}}$, which is consistent with previous mass measurements of this component. We provide a prediction for the evolution of the MW halo mass until the accretion of Sgr ($z\approx1$): specifically, we find that the mass growth of the Galaxy from the time of its first merger ($z\approx5$) to $z\approx2$ exceeds the total mass of the known progenitors accreted during that interval, suggesting the presence of unidentified substructures. Our estimate of the Galaxy halo mass after the Sgr merger, but prior to the accretion of the Magellanic Clouds, is $5.9^{+1.4}_{-1.1}\times10^{11}~\rm{M_{\odot}}$.

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arXiv2025

Transformational astrophysics and exoplanet science with Habitable Worlds Observatory's High Resolution Imager

Habitable Worlds Observatory (HWO) will be NASA's flagship space telescope of the 2040s, designed to search for life on other planets and to transform broad areas of astrophysics. NASA are seeking international partners, and the UK is well-placed to lead the design and construction of its imaging camera - which is likely to produce the mission's most visible public impact. Early participation in the mission would return investment to UK industry, and bring generational leadership for the UK in space science, space technology, and astrophysics.

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arXiv2025

Orbital migration and heating history of the Galactic disc: a transition between the bimodal discs

Stellar orbits in the Galactic disc evolve from their birth to the current shape through both radial migration and dynamical heating. The history of their secular evolution is imprinted in the current kinematics and age-metallicity distribution. We construct a chrono-chemo-dynamical model of the disc, incorporating inside-out growth, metallicity evolution, radial migration, and heating to fit the observed age-metallicity-kinematics distribution of LAMOST subgiant stars in both the low and high-$\alpha$ disc. By modelling all distribution parameters with spline fitting, we present the first non-parametric stellar migration and heating history of the Galaxy. We determine the heating-to-migration ratio, the ratio of the root-mean-square changes in radial/vertical and azimuthal actions, to be $\approx0.075$ for radial to azimuthal actions and $\approx0.015$ for vertical to azimuthal actions, implying a highly anisotropic diffusion in the action space. Furthermore, we identify a transition in radial migration efficiency coinciding with the transition moment of the bimodal disc, for which the radial migration was more efficient for the high-$\alpha$ disc than for the low-$\alpha$ disc. This transition may be attributed to two correlated scenarios: 1) a bar formation epoch accompanied by violent outward migration, and 2) a drop in the gas mass fraction in the disc when the low-$\alpha$ disc began to form. These findings offer further constraints on the formation mechanisms of bimodal discs, favouring the downsizing scenario. We also briefly discuss the connection between our results and recent high-redshift observations. In addition to the secular evolution history, our model maps the Milky Way ISM metallicity gradient at different lookback times, which we find has only varied a little (in the range of $-0.07~\rm to~-0.10~dex/kpc$) since disc formation.

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arXiv2025

Modelling the Milky Way's exoplanet population based on cosmological galaxy simulations

In this paper we aim to simulate realistic exoplanet populations across different regions of the MW by combining state-of-the-art cosmological simulations of our Galaxy with exoplanet formation models and observations. We model the exoplanet populations around single stars, using planet occurrence rates and multiplicity depending on stellar mass, metallicity, and planet type, and assign them physical parameters such as mass and orbital period. Focussing first on the solar vicinity, we find mostly metallicity-driven differences in the distributions of non-hosting and planet-hosting single stars. In our simulated solar neighbourhood, 52.5% of all planets are Earth-like (23% of them located in the Habitable Zone), 44% are super-Earths/Neptunes, and 3.5% are giant planets. A comparison with the census of Kepler exoplanets and candidates shows that, when taking into account the most relevant selection effects, we obtain a similar distribution of exoplanets compared to the observed population. However, we also detect significant differences in the exoplanet and host star distributions (e.g. more planets around F-type and red-giant stars compared to observations) that we attribute mostly to a too strong recent star formation and a too large disc scale height in the simulation, as well as to some caveats in our exoplanet population synthesis that will be addressed in future work. Extending our analysis to other regions of the simulated MW and to other simulated galaxies, we find that the relative percentages of planet types remain largely consistent as long as the simulated galaxy matches the morphology and mass of the MW. We have created a fast and flexible framework to produce exoplanet populations based on MW-like simulations that can easily be adapted to produce predictions for the yields of future exoplanet detection missions. (abridged)

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arXiv2025

Thick-to-thin disc transition and gas disc shrinking induced by the Gaia-Sausage-Enceladus merger

Understanding the Milky Way disc formation requires characterising its structural and kinematic properties as functions of stellar age. Using red giant stars from APOGEE DR17 and Gaia DR3, we model the age-dependent stellar kinematics with a quasi-isothermal distribution function and fit disc parameters as a function of age using non-parametric splines. We identify a transition from thick to thin disc populations around 10 Gyr ago. Stars older than this have short scale lengths ($\sim$1.7 kpc), typical of the thick disc, while younger stars exhibit increasing scale length with decreasing age, consistent with inside-out formation of the thin disc. This transition possibly coincides with the end of the starburst triggered by the Gaia-Sausage-Enceladus (GSE) merger. Stars formed around 10 Gyr ago exhibit a dip in scale length, even shorter than that of the thick disc. Comparison with an Auriga simulation suggests that this scale-length dip reflects gas disc shrinking caused by the transition from a cold to hot gas accretion mode. We propose the following disc formation scenario: (1) the thick disc formed under cold-mode accretion; (2) the GSE merger triggered a starburst and increased the total mass of the Galaxy, causing the transition to hot-mode accretion; (3) rapid gas consumption led to temporary shrinking of the star-forming gas disc; and then (4) thin disc grows in an inside-out fashion, as the size of the star-forming gas disc grows via hot-mode smooth gas accretion.

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arXiv2025

Investigating the eccentricity distribution of transiting, long-period giant planets

Eccentric giant planets are predicted to have acquired their eccentricity through two major mechanisms: the Kozai-Lidov effect or planet-planet scattering, but it is normally difficult to separate the two mechanisms and determine the true eccentricity origin for a given system. In this work, we focus on a sample of 92 transiting, long-period giant planets (TLGs) as part of an eccentricity distribution study for this planet population in order to understand their eccentricity origin. Using archival high-contrast imaging observations, public stellar catalogs, precise Gaia astrometry, and the NASA Exoplanet Archive database, we explored the eccentricity distribution correlation with different planet and host-star properties of our sample. We also homogeneously characterized the basic stellar properties for all 86 host-stars in our sample, including stellar age and metallicity. We found a correlation between eccentricity and stellar metallicity, where lower-metallicity stars ([Fe/H] <= 0.1) did not host any planets beyond e > 0.4, while higher-metallicity stars hosted planets across the entire eccentricity range. Interestingly, we found no correlation between the eccentricity distribution and the presence of stellar companions, indicating that planet-planet scattering is likely a more dominant mechanism than the Kozai-Lidov effect for TLGs. This is further supported by an anti-correlation trend found between planet multiplicity and eccentricity, as well as a lack of strong tidal dissipation effects for planets in our sample, which favor planet-planet scattering scenarios for the eccentricity origin.

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arXiv2025

Demonstration of Plate Analysis, an algorithm for precise relative astrometry

Astrometry plays a crucial role in understanding the structure, dynamics, and evolution of celestial objects by providing precise measurements of their positions and motions. We propose a new approach to wide-field, relative astrometry, Plate Analysis. Plate Analysis is an innovative algorithm that estimates stellar coordinates and corrects geometric distortion based on precise reference sources, without relying on dedicated calibration fields. It is implemented as a probabilistic framework using Stochastic Variational Inference to efficiently optimize the numerous parameters involved in the model. The methodology was tested through a simplified simulation designed after the Galactic center survey of the JASMINE mission. This simulation, called the JASMINE mini-mock survey, covered three years of observation with 100 satellite orbits, providing a comprehensive dataset for evaluating the performance of Plate Analysis. Although the observation model incorporated more than 30,000 parameters, the parameters were efficiently optimized through Plate Analysis. The results showed that the estimated coordinates closely match the expected stellar motions, with an average positional error of about $70\,\mathrm{\mu as}$. These findings validate the potential of Plate Analysis for precise wide-field astrometric applications, offering significant insights into improving the accuracy of stellar dynamics measurements.

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arXiv2025

GalactiKit: reconstructing mergers from $z=0$ debris using simulation-based inference in Auriga

We present GalactiKit, a data-driven methodology for estimating the lookback infall time, stellar mass, halo mass and mass ratio of the disrupted progenitors of Milky Way-like galaxies at the time of infall. GalactiKit uses simulation-based inference to extract the information on galaxy formation processes encoded in the Auriga cosmological MHD simulations of Milky Way-mass halos to create a model that relates the properties of mergers to those of the corresponding merger debris at $z=0$. We investigate how well GalactiKit can reconstruct the merger properties given the dynamical, chemical, and the combined chemo-dynamical information of debris. For this purpose, three models were implemented considering the following properties of merger debris: (a) total energy and angular momentum, (b) iron-to-hydrogen and alpha-to-iron abundance ratios, and (c) a combination of all of these. We find that the kinematics of the debris can be used to trace the lookback time at which the progenitor was first accreted into the main halo. However, chemical information is necessary for inferring the stellar and halo masses of the progenitors. In both models (b) and (c), the stellar masses are predicted more accurately than the halo masses, which could be related to the scatter in the stellar mass-halo mass relation. Model (c) provides the most accurate predictions for the merger parameters, which suggests that combining chemical and dynamical data of debris can significantly improve the reconstruction of the Milky Way's assembly history.

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arXiv2024

Milky Way Disk

Our understanding of the Milky Way disk is rapidly improving with the recent advent of the high quality and vast amount of observational data. We summarize our current view of the structure of the Milky Way disk, such as the masses and sizes of the gas and stellar disks, and the position and motion of the Sun in the disk. We also discuss the different definitions of the thick and thin disks of the Milky Way, the non-axisymmetric structures of the stellar disk, such as the bar and spiral arms, and the radial migration which can be triggered by these non-axisymmetric stellar structures. After the revolutionary data from the European Space Agency's Gaia mission, our view of the Milky Way disk has been transformed to a non-equilibrium system with many complicated structures in stellar kinematic distribution. We also summarize the recent findings of Galactoseismology research. These detailed observational data provide the archaeological information for us to unveil the formation and evolution history of the Milky Way disk, with the aid of the high-resolution numerical simulations of the Milky Way-like galaxy formation. We also discuss the current view of the formation history of the Milky Way disk.

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arXiv2024

Towards Galactic Archaeology with Inferred Ages of Giant Stars From Gaia Spectra

In the era of Gaia, the accurate determination of stellar ages is transforming Galactic archaeology. We demonstrate the feasibility of inferring stellar ages from Gaia's RVS spectra and the BP/RP (XP) spectrophotometric data, specifically for red giant branch and high-mass red clump stars. We successfully train two machine learning models, dubbed SIDRA: Stellar age Inference Derived from Gaia spectRA to predict the age. The SIDRA-RVS model uses the RVS spectra and SIDRA-XP the stellar parameters obtained from the XP spectra. Both models use BINGO, an APOGEE-derived stellar age as the training data. SIDRA-RVS estimates ages of stars whose age is around $\tau_\mathrm{BINGO}=10$~Gyr with a standard deviation of residuals of $\sim$ 0.12 dex in the unseen test dataset, while SIDRA-XP achieves higher precision with residuals $\sim$ 0.064 dex for stars around $\tau_\mathrm{BINGO}=10$~ Gyr. Since SIDRA-XP outperforms SIDRA-RVS, we apply SIDRA-XP to analyse the ages for 2,218,154 stars. This allowed us to map the chronological and chemical properties of Galactic disc stars, reproducing the known distinct features such as the Gaia-Sausage-Enceladus merger and a potential gas-rich interaction event linked to the first infall of the Sagittarius dwarf galaxy. This study demonstrates that machine learning techniques applied to Gaia's spectra can provide valuable individual age information, particularly for giant stars, thereby enhancing our understanding of the Milky Way's formation and evolution.

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arXiv2024

A link between rocky exoplanet composition and stellar age

Interior compositions are key for our understanding of Earth-like exoplanets. The composition of the core can influence the presence of a magnetic dynamo and the strength of gravity on the planetary surface, both of which heavily impact thermal and possible biological processes and thus the habitability for life and its evolution on the planet. However, detailed measurements of the planetary interiors are extremely challenging for small exoplanets, and existing data suggest a wide diversity in planet compositions. Hitherto, only certain photospheric chemical abundances of the host stars have been considered as tracers to explain the diversity of exoplanet compositions. Here we present a homogeneous analysis of stars hosting rocky exoplanets, with ages between 2 and 14 Gyr, revealing a correlation between rocky exoplanet compositions and the ages of the planetary systems. Denser rocky planets are found around younger stars. This suggests that the compositional diversity of rocky exoplanets can be linked to the ages of their host stars. We interpret this to be a result of chemical evolution of stars in the Milky Way, which modifies the material out of which stars and planets form. The results imply that rocky planets which form today, at similar galactocentric radii, may have different formation conditions, and thus different properties than planets which formed several billion years ago, such as the Earth.

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arXiv2024

JASMINE image simulator for high-precision astrometry and photometry

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.

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arXiv2024

Clues to growth and disruption of two neighbouring spiral arms of the Milky Way

Studying the nature of spiral arms is essential for understanding the formation of the intricate disc structure of the Milky Way. The European Space Agency's Gaia mission has provided revolutionary observational data that have uncovered detailed kinematical features of stars in the Milky Way. However, so far the nature of spiral arms continues to remain a mystery. Here we present that the stellar kinematics traced by the classical Cepheids around the Perseus and Outer spiral arms in the Milky Way shows strikingly different kinematical properties from each other: the radial and azimuthal velocities of Cepheids with respect to the Galactic centre show positive and negative correlations in the Perseus and Outer arms, respectively. We also found that the dynamic spiral arms commonly seen in an N-body/hydrodynamics simulation of a Milky Way-like galaxy can naturally explain the observed kinematic trends. Furthermore, a comparison with such a simulation suggests that the Perseus arm is being disrupted while the Outer arm is growing. Our findings suggest that two neighbouring spiral arms in distinct evolutionary phases - growing and disrupting phases - coexist in the Milky Way.

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arXiv2024

JASMINE: Near-Infrared Astrometry and Time Series Photometry Science

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 $μ$as) astrometry in the Near-Infrared (NIR) Hw-band (1.0-1.6 $μ$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.

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arXiv2023

Growing Local arm inferred by the breathing motion

Theoretical models of spiral arms suggest that the spiral arms provoke a vertical bulk motion in disc stars. By analysing the breathing motion, a coherent asymmetric vertical motion around the mid-plane of the Milky Way disc, with $\textit{Gaia}$ DR3, we found that a compressing breathing motion presents along the Local arm. On the other hand, with an $N$-body simulation of an isolated Milky Way-like disc galaxy, we found that the transient and dynamic spiral arms induce compressing breathing motions when the arms are in the growth phase, while the expanding breathing motion appears in the disruption phase. The observed clear alignment of the compressing breathing motion with the Local arm is similar to what is seen in the growth phase of the simulated spiral arms. Hence, we suggest that the Local arm's compressing breathing motion can be explained by the Local arm being in the growth phase of a transient and dynamic spiral arm. We also identified the tentative signatures of the expanding breathing motion associated with the Perseus arm and also the Outer arm coinciding with the compressing breathing motion. This may infer that the Perseus and Outer arms are in the disruption and growth phases, respectively.

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arXiv2023

The epoch of the Milky Way's bar formation: dynamical modelling of Mira variables in the nuclear stellar disc

A key event in the history of the Milky Way is the formation of the bar. This event affects the subsequent structural and dynamical evolution of the entire Galaxy. When the bar formed, gas was likely rapidly funnelled to the centre of the Galaxy settling in a star-forming nuclear disc. The Milky Way bar formation can then be dated by considering the oldest stars in the formed nuclear stellar disc. In this highly obscured and crowded region, reliable age tracers are limited, but bright, high-amplitude Mira variables make useful age indicators as they follow a period--age relation. We fit dynamical models to the proper motions of a sample of Mira variables in the Milky Way's nuclear stellar disc region. Weak evidence for inside-out growth and both radial and vertical dynamical heating with time of the nuclear stellar disc is presented suggesting the nuclear stellar disc is dynamically well-mixed. Furthermore, for Mira variables around a $\sim350$ day period, there is a clear transition from nuclear stellar disc-dominated kinematics to background bar-bulge-dominated kinematics. Using a Mira variable period-age relation calibrated in the solar neighbourhood, this suggests the nuclear stellar disc formed in a significant burst in star formation $(8\pm 1)\,\mathrm{Gyr}$ ago, although the data are also weakly consistent with a more gradual formation of the nuclear stellar disc at even earlier epochs. This implies a relatively early formation time for the Milky Way bar ($\gtrsim8\,\mathrm{Gyr}$), which has implications for the growth and state of the young Milky Way and its subsequent history.

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arXiv2023

A high fidelity Milky Way simulation with Kraken, Gaia-Enceladus, and Sequoia analogues: clues to their accretion histories

Within a simulated Milky Way-like galaxy, we identify and analyse analogues of the Gaia-Enceladus (GE), Kraken and Sequoia mergers that each matches remarkably well observational results, including in velocity and chemical abundance space, and their distributions in the $j_{z}$-Energy plane. The Kraken analogue is the earliest merger and has the highest total mass ratio. Consistent with previous studies, it is chemically indistinguishable from old in-situ stars at the time of its accretion. The GE and Sequoia analogue events accrete at similar times in our simulation, both along filaments but from opposite sides of the main galaxy. The mean stellar ages of the GE and Sequoia analogues are both similar and, from our simulation results, we see that they can be separate entities and still naturally reproduce the observed properties of their stellar remnants at the present day, including the significant retrograde velocities of the Sequoia analogue remnant stars and the difference in the tracks of the two galaxies through chemical abundance space. Our results provide supporting information about the properties of these three merger events, and show for the first time that they can all be reproduced with a fully cosmological simulation, providing a possible self consistent evolutionary pathway for the Milky Way's formation.

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