arXiv subjects

Anthony G. A. Brown

Publications and source records attributed to Anthony G. A. Brown.

At least 19 recordsLinked to original sources

arXiv2026

Radial velocity and atmospheric parameter calculations for the GaiaNIR spectrograph

Context. The upcoming GaiaNIR mission is currently planning to add a near-infrared spectrograph to its payload in order to enhance its scientific return, particularly for mapping the dust-obscured regions of the Milky Way. Aims. This study aims to identify the optimal wavelength region between 800 and 2300 nm for the proposed GaiaNIR spectrograph to maximize the precision of radial velocities and atmospheric parameters. Methods. To find its spectral range, we generated 10000 synthetic spectra from the BOSZ library across a wide range of stellar parameters, with resolutions varying from 5000 to 20000. By cross-correlating these mock observations with ideal templates, we assessed the statistical scatter of velocity residuals to isolate six candidate windows for further atmospheric parameter testing. Results. Our analysis finds that the 1926 - 1968 nm window at R = 16100 - 20100 in the K-band is the preferred strategic choice, because it has the potential to reach radial velocity precision for the brightest FGKM stars of about 160 - 260 m/s depending on resolution, while providing precision of the atmospheric parameters close to Gaia's Radial Velocity Spectrometer. We also identified a second region between 1158 and 1202 nm (R = 9300 - 11600), that has slightly lower radial velocity precision, but at a wider temperature range than the K-band. Both regions make it possible to derive abundances of ten species at these resolutions: O, Na, Mg, Si, Ca, Ti, V, Cr, Mn, Ni in the K-band, and Mg, Si, K, Ca, Ti, V, Cr, Mn, Co, Ni between 1158 - 1202 nm. Conclusions. The K-band delivers sufficiently precise measurements for the mission's primary cool star targets while taking advantage of significantly lower interstellar extinction, enabling the mapping of the dust-obscured regions of the Milky Way.

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arXiv2026

Phase spirals induced by the gas warp

The discovery of the phase space spirals in the Solar neighborhood in Gaia Data Release 2 has prompted various attempts to understand their origin. A source of bending waves, which has been neglected as a cause of the phase spiral, is irregular gas inflow along the warp. We aim to study whether perturbations by the gas warp could induce phase spirals. Accounting for this additional formation scenario for phase spirals could improve our current understanding of the perturbation history of the Milky Way disc. We use two N-body + SPH (Smooth Particle Hydrodynamics) simulations of an isolated galaxy to search for, and study, warp-induced phase spirals. We study the emergence and propagation of the detected phase spirals using Fourier decomposition. We detect strong one-armed phase spirals in the warped simulation. These phase spirals are prevalent and persist over ~10 Gyr. The morphology of these phase spirals varies with location and evolves with time. In particular, the emergence rate of the phase spiral evolves with the gas inflow at the outer disc and the bending wave amplitude, indicating that these phase spirals are a record of warp-induced bending waves. We find that these phase spirals can reach amplitudes comparable to those in the Gaia DR3. We only detect weak and stochastically distributed phase spirals in an unwarped control simulation. We conclude that phase spirals can be induced by the irregular gas accretion along the warp. These phase spirals occur globally and are long-lived.

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arXiv2026

A Path to an All-Sky Survey with Roman

A deep, space-based, all-sky near-infrared survey carried out with the Nancy Grace Roman Space Telescope would constitute a foundational astronomical infrastructure for decades to come. In this white paper, we present a concrete and feasible path to imaging the entire sky at $\sim0.1''$ resolution, beginning with high-impact fields in Cycle 1 and scaling to ultra-wide coverage within the nominal mission. This first-epoch survey will reach $\mathrm{H}\sim25.5$ AB mag (5$σ$) and maximize synergies with contemporaneous observatories, while preserving substantial time for other ambitious Roman programs. We outline representative scheduling scenarios and an example Cycle 1 program that triples early Roman-LSST overlap and delivers high-value community data products such as LSST forced photometry, joint \textit{Gaia}-Roman astrometry, and catalogs of Galactic substructure, stong lenses, and other rare systems. The Cycle 1 program will lay the foundation for an eventual all-sky survey, while also delivering high-impact early science. We invite broad community participation in shaping and carrying out both the initial program and the long-term vision of an all-sky Roman survey.

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arXiv2025

The completeness of the open cluster census towards the Galactic anticentre

Open clusters have long been used as tracers of Galactic structure. However, without a selection function to describe the completeness of the cluster census, it is difficult to quantitatively interpret their distribution. We create a method to empirically determine the selection function of a Galactic cluster catalogue. We test it by investigating the completeness of the cluster census in the outer Milky Way, where old and young clusters exhibit different spatial distributions. We develop a method to generate realistic mock clusters as a function of their parameters, in addition to accounting for Gaia's selection function and astrometric errors. We then inject mock clusters into Gaia DR3 data, and attempt to recover them in a blind search using HDBSCAN. We find that the main parameters influencing cluster detectability are mass, extinction, and distance. Age also plays an important role, making older clusters harder to detect due to their fainter luminosity function. High proper motions also improve detectability. After correcting for these selection effects, we find that old clusters are $2.97\pm0.11$ times more common at a Galactocentric radius of 13~kpc than in the solar neighbourhood -- despite positive detection biases in their favour, such as hotter orbits or a higher scale height. The larger fraction of older clusters in the outer Galaxy cannot be explained by an observational bias, and must be a physical property of the Milky Way: young outer-disc clusters are not forming in the outer Galaxy, or at least not with sufficient masses to be identified as clusters in Gaia DR3. We predict that in this region, more old clusters than young ones remain to be discovered. The current presence of old, massive outer-disc clusters could be explained by radial heating and migration, or alternatively by a lower cluster destruction rate in the anticentre.

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arXiv2025

A 3D view of dwarf galaxies with Gaia and VLT/FLAMES II. The Sextans dwarf spheroidal

The Sextans dwarf spheroidal galaxy has been challenging to study in a comprehensive way as it is highly extended on the sky, with an uncertain but large tidal radius of between 80-160 arcminutes (or 3-4kpc), and an extremely low central surface brightness of SigmaV = 26.2 mag/arcsec2. Here we present a new homogeneous survey of 41 VLT/FLAMES multi-fibre spectroscopic pointings that contain 2108 individual spectra, and combined with Gaia DR3 photometry and astrometry we present v-los measurements for 333 individual Red Giant Branch stars that are consistent with membership in the Sextans dwarf spheroidal galaxy. In addition, we provide the metallicity, [Fe/H], determined from the two strongest CaII triplet lines, for 312 of these stars. We look again at the global characteristics of Sextans, deriving a mean line-of-sight velocity of = +227.1km/s and a mean metallicity of <[Fe/H]> = -2.37. The metallicity distribution is clearly double peaked, with the highest peak at [Fe/H]= -2.81 and another broader peak at [Fe/H]= -2.09. Thus it appears that Sextans hosts two populations and the superposition leads to a radial variation in the mean metallicity, with the more metal rich population being centrally concentrated. In addition there is an intriguing group of 9 probable members in the outer region of Sextans at higher [Fe/H] than the mean in this region. If this group could be confirmed as members they would eliminate the metallicity gradient. We also look again at the Colour-Magnitude Diagram of the resolved stellar population in Sextans. We also look again at the relation between Sextans and the intriguingly nearby globular cluster, Pal3. The global properties of Sextans have not changed significantly compared to previous studies, but they are now more precise, and the sample of known members in the outer regions is now more complete.

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arXiv2025

The Gaia Astrometric Catalogue and Secular Aberration Drift in Proper Motions

A recent paper demonstrated the existence of a secular aberration drift term in stellar proper motions that arises when transforming an astrometric catalogue defined for an observer at rest with respect to the solar system barycentre to some other reference frame in which, for example, the observer is at rest with respect to the Galactic centre. Such a transformation requires an accurate and precise estimate of the velocity of the solar system barycentre. It was argued that the Gaia catalogue construction should account for this effect and also for the aberrational effect due to acceleration of the solar system barycentre. We argue that these two effects should not be accounted for in the construction of the Gaia astrometric catalogue. We briefly review the Gaia catalogue reference frame, the concepts of stellar aberration and secular aberration drift, and their observable consequences. The Gaia catalogue is (and should be) constructed in the Barycentric Celestial Reference System: the reference system with the origin at the solar system barycentre as defined by the underlying solar system ephemerides. We explain that the Gaia catalogue is consistent with the International Celestial Reference System despite the presence of proper motion terms due to the acceleration of the solar system barycentre. We also explain why transformation of the astrometry to a frame in which the observer is at rest with respect to the Galactic centre or distant universe is not needed for the interpretation of stellar kinematics, and that there are practical concerns with such a transformation. The estimation of the velocity and acceleration of the solar system barycentre, although important as a matter of scientific investigation, are not needed for the construction of the Gaia astrometric catalogue.

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arXiv2025

Gaia: Ten Years of Surveying the Milky Way and Beyond

On January 15 2025, the Gaia mission completed the collection of the astrometric, photometric, and spectroscopic data for about 2.5 billion celestial sources, from the solar system to the Milky Way to the distant universe. Work is ongoing to produce Gaia DR4 based on the first 5.5 years of data, with the release expected in 2026. The full 10.5 year survey will be turned into Giaa DR5 which will open up scientific possibilities beyond Gaia DR4}. In this contribution I give a brief overview of the Gaia mission, summarize results from the GaiaUnlimited project, provide a glimpse of what is to come in Gaia DR4, and summarize the new science opportunities that Gaia DR5 will bring. I close with a look ahead at the successor to Gaia, the GaiaNIR mission, which will survey the Milky Way in the infrared, thus probing the Galactic ecosystem in the regions hidden to the Gaia mission.

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arXiv2024

The realm of Aurora. Density distribution of metal-poor giants in the heart of the Galaxy

The innermost portions of the Milky Way's stellar halo have avoided scrutiny until recently. The lack of wide-area survey data, made it difficult to reconstruct an uninterrupted view of the density distribution of the metal-poor stars inside the Solar radius. In this study, we utilize red giant branch (RGB) stars from Gaia, with metallicities estimated using spectro-photometry from Gaia Data Release 3. Accounting for Gaia's selection function, we examine the spatial distribution of metal-poor ([M/H]<-1.3) RGB stars, from the Galactic centre (r~1 kpc) out to beyond the Solar radius (r~18 kpc). Our best-fitting single-component cored power-law model shows a vertical flattening of ~0.5 and a slope -3.4, consistent with previous studies. Motivated by the mounting evidence for two distinct stellar populations in the inner halo, we additionally test a range of two-component models. One of the components models the tidal debris from the Gaia Sausage/Enceladus merger, while the other captures the Aurora population -- stars that predate the Galactic disk formation. Our best-fit two-component model suggests that both populations contribute equally around the Solar radius, but Aurora dominates the inner halo with a steeper power-law index of -4.5, in agreement with the nitrogen-rich star distribution measured by Horta et al. (2021).

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arXiv2024

Gaia DR3 detectability of unresolved binary systems

Gaia can not individually resolve very close binary systems, however, the collected data can still be used to identify them. A powerful indicator of stellar multiplicity is the sources reported Renormalized Unit Weight Error (ruwe), which effectively captures the astrometric deviations from single-source solutions. We aim to characterise the imprints left on ruwe caused by binarity. By flagging potential binary systems based on ruwe, we aim to characterise which of their properties will contribute the most to their detectability. We develop a model to estimate ruwe values for observations of Gaia sources, based on the biases to the single-source astrometric track arising from the presence of an unseen companion. Then, using the recipes from previous GaiaUnlimited selection functions, we estimate the selection probability of sources with high ruwe, and discuss what binary properties contribute to increasing the sources ruwe. We compute the maximum ruwe value which is compatible with single-source solutions as a function of their location on-sky. We see that binary systems selected as sources with a ruwe higher than this sky-varying threshold have a strong detectability window in their orbital period distribution, which peaks at periods equal to the Gaia observation time baseline. We demonstrate how our sky-varying ruwe threshold provides a more complete sample of binary systems when compared to single sky-averaged values by studying the unresolved binary population in the Gaia Catalogue of Nearby Stars. We provide the code and tools used in this study, as well as the sky-varying ruwe threshold through the GaiaUnlimited Python package

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arXiv2024

The density of the Milky Way's corona at $z\approx 1.6$ through ram pressure stripping of the Draco dSph galaxy

Satellite galaxies within the Milky Way's (MW) virial radius $R_{\mathrm{vir}}$ are typically devoid of cold gas due to ram pressure stripping by the MW's corona. The density of this corona is poorly constrained today and essentially unconstrained in the past, but can be estimated using ram pressure stripping. In this paper, we probe the MW corona at $z\approx 1.6$ using the Draco dwarf spheroidal galaxy. We assume that i) Draco's orbit is determined by its interaction with the MW, whose dark matter halo we evolve in time following cosmologically-motivated prescriptions, ii) Draco's star formation was quenched by ram pressure stripping and iii) the MW's corona is approximately smooth, spherical and in hydrostatic equilibrium. We used GAIA proper motions to set the initial conditions and Draco's star formation history to estimate its past gas content. We found indications that Draco was stripped of its gas during the first pericentric passage. Using 3D hydrodynamical simulations at a resolution that enables us to resolve individual supernovae and assuming no tidal stripping, which we estimate to be a minor effect, we find a density of the MW corona $\geq 8\times 10^{-4}$ cm$^{-3}$ at a radius $\approx 0.72R_{\mathrm{vir}}$. This provides evidence that the MW's corona was already in place at $z\approx 1.6$ and with a higher density than today. If isothermal, this corona would have contained all the baryons expected by the cosmological baryon fraction. Extrapolating to today shows good agreement with literature constraints if feedback has removed $\lesssim 30$% of baryons accreted onto the halo.

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arXiv2024

Uniting Gaia and APOGEE to unveil the cosmic chemistry of the Milky Way disc

The spatial distribution of Galactic stars with different chemical abundances encodes information on the processes that drove the formation and evolution of the Milky Way. Survey selection functions are indispensable for analysing astronomical catalogues produced by large-scale surveys. The use of these selection functions in data modelling is more complex when data from different surveys are to be modelled simultaneously. We introduce a procedure for constructing the selection function of a sample of red clump stars that have parallaxes and elemental abundances from the Gaia mission. We separately constructed the selection function of the APOGEE DR17 red clump stars, which depends on very different observables and has a very different spatial coverage. We combined the two surveys and accounted for their joint selection function to provide strong constraints on the radial and vertical density distribution of mono-abundance populations, with Gaia offering a dense coverage of the solar neighbourhood, while APOGEE reaches larger distances near the Galactic plane. We confirm that the radial density profile steepens with increasing metallicity. The combined sample also indicates a metallicity-dependent flaring of the alpha-poor disc. We provide the code for constructing the Gaia selection function we used in this study through the GaiaUnlimited Python package.

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arXiv2023

NANCY: Next-generation All-sky Near-infrared Community surveY

The Nancy Grace Roman Space Telescope is capable of delivering an unprecedented all-sky, high-spatial resolution, multi-epoch infrared map to the astronomical community. This opportunity arises in the midst of numerous ground- and space-based surveys that will provide extensive spectroscopy and imaging together covering the entire sky (such as Rubin/LSST, Euclid, UNIONS, SPHEREx, DESI, SDSS-V, GALAH, 4MOST, WEAVE, MOONS, PFS, UVEX, NEO Surveyor, etc.). Roman can uniquely provide uniform high-spatial-resolution (~0.1 arcsec) imaging over the entire sky, vastly expanding the science reach and precision of all of these near-term and future surveys. This imaging will not only enhance other surveys, but also facilitate completely new science. By imaging the full sky over two epochs, Roman can measure the proper motions for stars across the entire Milky Way, probing 100 times fainter than Gaia out to the very edge of the Galaxy. Here, we propose NANCY: a completely public, all-sky survey that will create a high-value legacy dataset benefiting innumerable ongoing and forthcoming studies of the universe. NANCY is a pure expression of Roman's potential: it images the entire sky, at high spatial resolution, in a broad infrared bandpass that collects as many photons as possible. The majority of all ongoing astronomical surveys would benefit from incorporating observations of NANCY into their analyses, whether these surveys focus on nearby stars, the Milky Way, near-field cosmology, or the broader universe.

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arXiv2023

A 3D view of dwarf galaxies with Gaia and VLT/FLAMES I. The Sculptor dwarf spheroidal

We present a new homogeneous survey of VLT/FLAMES LR8 line-of-sight radial velocities (vlos) for 1604 resolved red giant branch stars in the Sculptor dwarf spheroidal galaxy. In addition, we provide reliable Ca II triplet metallicities, [Fe/H], for 1339 of these stars. From this combination of new observations (2257 individual spectra) with ESO archival data (2389 spectra), we obtain the largest and most complete sample of vlos and [Fe/H] measurements for individual stars in any dwarf galaxy. Our sample includes VLT/FLAMES LR8 spectra for 55% of the red giant branch stars at G $<20$ from Gaia DR3, and $>70$% of the brightest stars, G $<18.75$. Our spectroscopic velocities are combined with Gaia DR3 proper motions and parallax measurements for a new and more precise membership analysis. We look again at the global characteristics of Sculptor, deriving a mean metallicity of $\langle$[Fe/H]$\rangle = -1.82 \pm 0.45$ and a mean line-of-sight velocity of $\langle$vlos$\rangle = +111.2 \pm 0.25$km/s. There is a clear metallicity gradient in Sculptor, -0.7deg/dex, with the most metal-rich population being the most centrally concentrated. Furthermore, the most metal-poor population in Sculptor, [Fe/H]$<-2.5$, appears to show kinematic properties distinct from the rest of the stellar population. Finally, we combine our results with the exquisite Gaia DR3 multi-colour photometry to further investigate the colour-magnitude diagram of the resolved stellar population in Sculptor. Our detailed analysis shows a similar global picture as previous studies, but with much more precise detail, revealing that Sculptor has more complex properties than previously thought. This survey emphasises the role of the stellar spectroscopy technique and this galaxy as a benchmark system for modelling galaxy formation and evolution on small scales.

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arXiv2023

Investigating Gaia EDR3 parallax systematics using asteroseismology of Cool Giant Stars observed by Kepler, K2, and TESS I. Asteroseismic distances to 12,500 red-giant stars

Gaia EDR3 has provided unprecedented data that generate a lot of interest in the astrophysical community, despite the fact that systematics affect the reported parallaxes at the level of ~ 10 muas. Independent distance measurements are available from asteroseismology of red-giant stars with measurable parallaxes, whose magnitude and colour ranges more closely reflect those of other stars of interest. In this paper, we determine distances to nearly 12,500 red-giant branch and red clump stars observed by Kepler, K2, and TESS. This is done via a grid-based modelling method, where global asteroseismic observables, constraints on the photospheric chemical composition, and on the unreddened photometry are used as observational inputs. This large catalogue of asteroseismic distances allows us to provide a first comparison with Gaia EDR3 parallaxes. Offset values estimated with asteroseismology show no clear trend with ecliptic latitude or magnitude, and the trend whereby they increase (in absolute terms) as we move towards redder colours is dominated by the brightest stars. The correction model proposed by Lindegren et al. (2021) is not suitable for all the fields considered in this study. We find a good agreement between asteroseismic results and model predictions of the red clump magnitude. We discuss possible trends with the Gaia scan law statistics, and show that two magnitude regimes exist where either asteroseismology or Gaia provides the best precision in parallax.

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arXiv2023

VISIONS: The VISTA Star Formation Atlas -- I. Survey overview

VISIONS is an ESO public survey of five nearby (d < 500 pc) star-forming molecular cloud complexes that are canonically associated with the constellations of Chamaeleon, Corona Australis, Lupus, Ophiuchus, and Orion. The survey was carried out with VISTA, using VIRCAM, and collected data in the near-infrared passbands J, H, and Ks. With a total on-sky exposure time of 49.4 h VISIONS covers an area of 650 deg$^2$, and it was designed to build an infrared legacy archive similar to that of 2MASS. Taking place between April 2017 and March 2022, the observations yielded approximately 1.15 million images, which comprise 19 TB of raw data. The observations are grouped into three different subsurveys: The wide subsurvey comprises shallow, large-scale observations and has visited the star-forming complexes six times over the course of its execution. The deep subsurvey of dedicated high-sensitivity observations has collected data on the areas with the largest amounts of dust extinction. The control subsurvey includes observations of areas of low-to-negligible dust extinction. Using this strategy, the VISIONS survey offers multi-epoch position measurements, is able to access deeply embedded objects, and provides a baseline for statistical comparisons and sample completeness. In particular, VISIONS is designed to measure the proper motions of point sources with a precision of 1 mas/yr or better, when complemented with data from VHS. Hence, VISIONS can provide proper motions for sources inaccessible to Gaia. VISIONS will enable addressing a range of topics, including the 3D distribution and motion of embedded stars and the nearby interstellar medium, the identification and characterization of young stellar objects, the formation and evolution of embedded stellar clusters and their initial mass function, as well as the characteristics of interstellar dust and the reddening law.

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arXiv2022

An empirical model of the Gaia DR3 selection function

Interpreting and modelling astronomical catalogues requires an understanding of the catalogues' completeness or selection function: objects of what properties had a chance to end up in the catalogue. Here we set out to empirically quantify the completeness of the overall Gaia DR3 catalogue. This task is not straightforward because Gaia is the all-sky optical survey with the highest angular resolution to date and no consistent ``ground truth'' exists to allow direct comparisons. However, well-characterised deeper imaging enables an empirical assessment of Gaia's $G$-band completeness across parts of the sky. On this basis, we devised a simple analytical completeness model of Gaia as a function of the observed $G$ magnitude and position over the sky, which accounts for both the effects of crowding and the complex Gaia scanning law. Our model only depends on a single quantity: the median magnitude $M_{10}$ in a patch of the sky of catalogued sources with $\texttt{astrometric_matched_transits}$ $\leq 10$. $M_{10}$ reflects elementary completeness decisions in the Gaia pipeline and is computable from the Gaia DR3 catalogue itself and therefore applicable across the whole sky. We calibrate our model using the Dark Energy Camera Plane Survey (DECaPS) and test its predictions against Hubble Space Telescope observations of globular clusters. We find that our model predicts Gaia's completeness values to a few per cent across the sky. We make the model available as a part of the $\texttt{gaiasf}$ Python package built and maintained by the GaiaUnlimited project: $\texttt{https://github.com/gaia-unlimited/gaiaunlimited}$

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arXiv2021

Unveiling wide-orbit companions to K-type stars in Sco-Cen with Gaia EDR3

Abbreviated. We aim to identify new low-mass companions to young stars using the astrometric measurements provided by the Gaia space mission and complementary VLT/SPHERE data. We identify companion candidates from a sample of K-type, pre-main sequence stars in the Scorpius Centaurus association using the early version of the third data release of the Gaia space mission. Based on the provided positions, proper motions, and magnitudes, we identify all objects within a predefined radius whose differential proper motions are consistent with a gravitationally bound system. We derive companion masses through comparison with evolutionary tracks. For seven identified companion candidates we use additional data collected with VLT/SPHERE and VLT/NACO to assess the accuracy of the properties of the companions based on Gaia photometry alone. We identify 110 comoving companions that have a companionship likelihood of more than $95\,\%$. We identify ten especially intriguing companions that have masses in the brown dwarf regime down to $20\,M_\mathrm{Jup}$. Our high-contrast imaging data confirm both astrometry and photometric masses derived from Gaia alone. We discover a new brown dwarf companion, TYC 8252-533-1 B, with a projected separation of approximately $570\,\mathrm{au}$ from its Sun-like primary. SED modeling provides a companion mass of $52^{+17}_{-11}\,M_\mathrm{Jup}$. We show that the Gaia database can identify low-mass companions at wide separations from their host stars. For K-type Sco-Cen members Gaia can detect sub-stellar objects at projected separations larger than $300\,\mathrm{au}$ and is sensitivity limited beyond $1,000\,\mathrm{au}$ with a lower mass limit down to $20\,M_\mathrm{Jup}$. A similar analysis of other star-forming regions could significantly enlarge the sample size of such objects and test formation and evolution theories of planetary systems.

astro-ph.EP↗

arXiv2021

Microarcsecond Astrometry: Science Highlights from Gaia

Access to microarcsecond astrometry is now routine in the radio, infrared, and optical domains. In particular the publication of the second data release from the Gaia mission made it possible for every astronomer to work with easily accessible, high-precision astrometry for 1.7 billion sources to 21st magnitude over the full sky. * Gaia provides splendid astrometry but at the limits of the data small systematic errors are present. A good understanding of the Hipparcos/Gaia astrometry concept, and of the data collection and processing, provides insights into the origins of the systematic errors and how to mitigate their effects. * A selected set of results from Gaia highlight the breadth of exciting science and unexpected results, from the solar system to the distant universe, to creative uses of the data. * Gaia DR2 provides for the first time a dense sampling of Galactic phase space with high precision astrometry, photometry, and radial velocities, allowing to uncover subtle features in phase space and the observational HR diagram. * In the coming decade, we can look forward to more accurate and richer Gaia data releases, and new photometric and spectroscopic surveys coming online that will provide essential complementary data. * The longer term promises exciting new opportunities for microarcsecond astrometry and beyond, including the plans for an infrared version of Gaia which would offer the dense sampling of phase space deep into the Milky Way's nuclear regions.

astro-ph.IM↗