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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)., Comment: Submitted to MNRAS

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, Comment: 11 pages, 10 figures. Accepted for publication in A&A

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., Comment: 20 pages, 11 figures. Accepted for publication in MNRAS

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., Comment: 21 pages, 18 figures, accepted for publication in Astronomy & Astrophysics

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., Comment: Submitted to the call for white papers for the Roman Core Community Survey (June 16th, 2023), and to the Bulletin of the AAS

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., Comment: A&A, in press

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., Comment: 11 pages, 8 figures, Accepted for publication in A&A

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., Comment: Accepted for publication in Astronomy & Astrophysics on 19 January 2023

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: $\texttt{https://github.com/gaia-unlimited/gaiaunlimited}$, Comment: submitted to A&A

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., Comment: Accepted for publication in A&A (31 pages, 20 figures, 12 tables)

A pioneering study showed that the fine structure in the luminosity function (LF) of young star clusters contains information about the evolutionary stage (age) and composition of the stellar population. The notable features include the H-peak, which is the result of the onset of hydrogen burning turning pre-main sequence stars into main sequence stars. The feature moves toward the faint end of the LF, and eventually disappears as the population evolves. Another detectable feature is the Wielen dip, a dip at M_V ~ 7 mag in the LF first identified in 1974 for stars in the solar environment. Later studies also identified this feature in the LF of star clusters. The Wielen dip is caused by the increased importance of H- opacity in a certain range of low-mass stars. We studied the detailed structure in the luminosity function using the data from Gaia DR2 and PARSEC stellar evolution models with the aim to further our understanding of young stellar populations. We analyzed the astrometric properties of stars in the solar neighborhood (< 20 pc) and in various relatively nearby (< 400 pc) young (< 50 Myr) open clusters and OB associations, and compare the features in the luminosity function with those generated by PARSEC models. The Wielen dip is confirmed in the LF of all the populations, including the solar neighborhood, at M_G ~7 mag. The H-peak is present in the LF of the field stars in the solar neighborhood. It likely signals that the population is mixed with a significant number of stars younger than 100 Myr. The H-peak is found in the LF of young open clusters and OB associations, and its location varies with age. Our observations with Gaia DR2 confirm the evolution of the H-peak from 5 Myr up to 47 Myr. The fine structure in the luminosity function in young stellar populations can be used to estimate their age., Comment: 16 pages, A&A in press

Statistical studies of astronomical data sets, in particular of cataloged properties for discrete objects, are central to astrophysics. One cannot model those objects' population properties or incidences without a quantitative understanding of the conditions under which these objects ended up in a catalog or sample, the sample's selection function. As systematic and didactic introductions to this topic are scarce in the astrophysical literature, we aim to provide one, addressing generically the following questions: What is a selection function? What arguments $\vec{q}$ should a selection function depend on? Over what domain must a selection function be defined? What approximations and simplifications can be made? And, how is a selection function used in `modelling'? We argue that volume-complete samples, with the volume drastically curtailed by the faintest objects, reflect a highly sub-optimal selection function that needlessly reduces the number of bright and usually rare objects in the sample. We illustrate these points by a worked example, deriving the space density of white dwarfs (WD) in the Galactic neighbourhood as a function of their luminosity and Gaia color, $\Phi_0(M_G,B-R)$ in [mag$^{-2}$pc$^{-3}$]. We construct a sample of $10^5$ presumed WDs through straightforward selection cuts on the Gaia EDR3 catalog, in magnitude, color, parallax, and astrometric fidelity $\vec{q}=(m_G,B-R,\varpi,p_{af})$. We then combine a simple model for $\Phi_0$ with the effective survey volume derived from this selection function $S_C(\vec{q})$ to derive a detailed and robust estimate of $\Phi_0(M_G,B-R)$. This resulting white dwarf luminosity-color function $\Phi_0(M_G,B-R)$ differs dramatically from the initial number density distribution in the luminosity-color plane: by orders of magnitude in density and by four magnitudes in density peak location.

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., Comment: 61 pages, 10 figures, author's version (before copy-editing) of invited review published in Annual Reviews of Astronomy and Astrophysics (2021)

Quasars behind the Galactic plane (GPQs) are important astrometric references and useful probes of Milky Way gas. However, the search for GPQs is difficult due to large extinctions and high source densities in the Galactic plane. Existing selection methods for quasars developed using high Galactic latitude (high-$b$) data cannot be applied to the Galactic plane directly because the photometric data obtained from high-$b$ regions and the Galactic plane follow different probability distributions. To alleviate this dataset shift problem for quasar candidate selection, we adopt a Transfer Learning Framework at both data and algorithm levels. At the data level, to make a training set in which dataset shift is modeled, we synthesize quasars and galaxies behind the Galactic plane based on SDSS sources and Galactic dust map. At the algorithm level, to reduce the effect of class imbalance, we transform the three-class classification problem for stars, galaxies, and quasars to two binary classification tasks. We apply XGBoost algorithm on Pan-STARRS1 (PS1) and AllWISE photometry for classification, and additional cut on Gaia proper motion to remove stellar contaminants. We obtain a reliable GPQ candidate catalog with 160,946 sources located at $|b|\leq 20^{\circ}$ in PS1-AllWISE footprint. Photometric redshifts of GPQ candidates achieved with XGBoost regression algorithm show that our selection method can identify quasars in a wide redshift range ($0

Radio campaigns using maser stellar beacons have provided crucial information to characterize Galactic stellar populations. Currently, the Bulge Asymmetries and Dynamical Evolution (BAaDE) project is surveying infrared (IR) color-selected targets for SiO masers. This provides a sample of evolved stars that can be used to study the inner, optically obscured Galaxy using line of sight velocities and possibly VLBI proper motions. In order to use the BAaDE sample for kinematic studies, the stellar population should be characterized. In this study, the BAaDE targets have been cross-matched with IR (2MASS) and optical Gaia samples. By exploring the synergies of this cross-match together with Gaia parallaxes and extinction maps, the local ($d < 2$ kpc) AGB stars can be characterized. We have defined a \textit{BAaDE-Gaia} sample of 20,111 sources resulting from cross-matching BAaDE targets with IR and optical surveys. From this sample, a~{\local} sample of 1,812 evolved stars with accurate parallax measurements, confirmed evolved stellar evolution stage, and within 2 kpc distance around the Sun was selected, for which absolute (bolometric) magnitudes are estimated. The evolved stellar population with Gaia counterparts that are variable seems to be predominantly associated with AGB stars with moderate luminosity ($1,500^{+3,000}_{-500} \ L_\odot$) and periods between 250 and 1,250 days., Comment: Accepted for publication in ApJ

For stars with unresolved companions, motions of the centre of light and that of mass decouple, causing a single-source astrometric model to perform poorly. We show that such stars can be easily detected with the reduced chi2 statistic, or RUWE, provided as part of Gaia DR2. We convert RUWE into the amplitude of the image centroid wobble, which, if scaled by the source distance, is proportional to the physical separation between companions (for periods up to several years). We test this idea on a sample of known spectroscopic binaries and demonstrate that the amplitude of the centroid perturbation scales with the binary period and the mass ratio as expected. We apply this technique to the Gaia DR2 data and show how the binary fraction evolves across the Hertzsprung--Russell diagram. The observed incidence of unresolved companions is high for massive young stars and drops steadily with stellar mass, reaching its lowest levels for white dwarfs. We highlight the elevated binary fraction for the nearby Blue Stragglers and Blue Horizontal Branch stars. We also illustrate how unresolved hierarchical triples inflate the relative velocity signal in wide binaries. Finally, we point out a hint of evidence for the existence of additional companions to the hosts of extrasolar hot jupiters., Comment: submitted to MNRAS, comments welcome

We investigate individual distances and luminosities of a sample of 889 nearby candidate red supergiants with reliable parallaxes (plx/plxerr > 4 and RUWE < 2.7) from Gaia DR2. The sample was extracted from the historical compilation of spectroscopically derived spectral types by Skiff (2014), and consists of K-M stars that are listed with class I at least once. The sample includes well-known red supergiants from Humphreys (1978), Elias et al. (1985), Jura and Kleinmann (1990), and Levesque et al. (2005). Infrared and optical measurements from the 2MASS, CIO, MSX, WISE, MIPSGAL, GLIMPSE, and NOMAD catalogs allow us to estimate the stellar bolometric magnitudes. We analyze the stars in the luminosity versus effective temperature plane and confirm that 43 sources are highly-probably red supergiants with Mbol<-7.1 mag. 43% of the sample is made of stars with masses > 7 Msun. Another 30% of the sample consists of giant stars., Comment: 14 pages, 7 Figures, accepted by ApJ. See https://somethingaboutrsgstars.wordpress.com/ or http://staff.ustc.edu.cn/~messineo

Asteroseismology provides fundamental stellar parameters independent of distance, but subject to systematics under calibration. Gaia DR2 has provided parallaxes for a billion stars, which are offset by a parallax zero-point. Red Clump (RC) stars have a narrow spread in luminosity, thus functioning as standard candles to calibrate these systematics. This work measures how the magnitude and spread of the RC in the Kepler field are affected by changes to temperature and scaling relations for seismology, and changes to the parallax zero-point for Gaia. We use a sample of 5576 RC stars classified through asteroseismology. We apply hierarchical Bayesian latent variable models, finding the population level properties of the RC with seismology, and use those as priors on Gaia parallaxes to find the parallax zero-point offset. We then find the position of the RC using published values for the zero-point. We find a seismic temperature insensitive spread of the RC of ~0.03 mag in the 2MASS K band and a larger and slightly temperature-dependent spread of ~0.13 mag in the Gaia G band. This intrinsic dispersion in the K band provides a distance precision of ~1% for RC stars. Using Gaia data alone, we find a mean zero-point of -41 $\pm$ 10 $\mu$as. This offset yields RC absolute magnitudes of -1.634 $\pm$ 0.018 in K and 0.546 $\pm$ 0.016 in G. Obtaining these same values through seismology would require a global temperature shift of ~-70 K, which is compatible with known systematics in spectroscopy., Comment: Accepted for publication in MNRAS

The importance of studying the Gaia DR2 parallax zero-point by external means was underlined by Lindegren et al. (2018), and initiated by several works making use of Cepheids, eclipsing binaries, and asteroseismology. Despite a very efficient elimination of basic-angle variations, a small fluctuation remains and shows up as a small offset in the Gaia DR2 parallaxes. By combining astrometric, asteroseismic, spectroscopic, and photometric constraints, we undertake a new analysis of the Gaia parallax offset for nearly 3000 red-giant branch (RGB) and 2200 red clump (RC) stars observed by Kepler, as well as about 500 and 700 red giants (both RGB and RC) selected by the K2 Galactic Archaeology Program in campaigns 3 and 6. Engaging into a thorough comparison of the astrometric and asteroseismic parallaxes, we are able to highlight the influence of the asteroseismic method, and measure parallax offsets in the Kepler field that are compatible with independent estimates from literature and open clusters. Moreover, adding the K2 fields to our investigation allows us to retrieve a clear illustration of the positional dependence of the zero-point, in general agreement with the information provided by quasars. Lastly, we initiate a two-step methodology to make progress in the simultaneous calibration of the asteroseismic scaling relations and of the Gaia DR2 parallax offset, which will greatly benefit from the gain in precision with the third Data Release of Gaia., Comment: 15 pages, 17 figures, Accepted for publication in A&A

It is challenging to reliably identify stars that were born together outside of actively star-forming regions and bound stellar systems. However, co-natal stars should be present throughout the Galaxy, and their demographics can shed light on the clustered nature of star formation and the dynamical state of the disk. In previous work we presented a set of simulations of the Galactic disk that followed the clustered formation and dynamical evolution of 4 billion individual stars over the last 5 Gyr. The simulations predict that a high fraction of co-moving stars with physical and 3D velocity separation of $\Delta r < 20$ pc and $\Delta v < 1.5$ km s$^{-1}$ are co-natal. In this \textit{Letter}, we use \textit{Gaia} DR2 and LAMOST DR4 data to identify and study co-moving pairs. We find that the distribution of relative velocities and separations of pairs in the data is in good agreement with the predictions from the simulation. We identify 111 co-moving pairs in the Solar neighborhood with reliable astrometric and spectroscopic measurements. These pairs show a strong preference for having similar metallicities when compared to random field pairs. We therefore conclude that these pairs were very likely born together. The simulations predict that co-natal pairs originate preferentially from high-mass and relatively young ($< 1$ Gyr) star clusters. \textit{Gaia} will eventually deliver well-determined metallicities for the brightest stars, enabling the identification of thousands of co-natal pairs due to disrupting star clusters in the solar neighborhood., Comment: 6 pages, 4 figures, 1 table. Submitted to ApJL. Catalog here: http://harshilkamdar.github.io/2019/04/03/pairs.html

We use Gaia DR2 data to isolate the pre-main sequence population corresponding to the Scorpius-Lupus-Centaurus-Crux area on the sky making use the Hertzsprung-Russell diagram. A sample of $120\,911$ sources was selected with galactic coordinates $285^\circ\leq\ell\leq360^\circ$ and $-10^\circ\leq b\leq+32^\circ$, and parallaxes between $5$ and $12$~mas where the relative uncertainty in parallax was restricted to be $<10\%$. The pre-main sequence sources were isolated due to a clear separation between the pre-main and main sequences, well above the expected $0.75$ magnitude from a possible observed population of equal mass binaries. The final sample contains $14\,459$ young stellar objects. The traditional boundaries of the Sco OB2 association are well traced by clear concentrations of young stars where the Upper Scorpius region stands out as the densest concentration. The IC 2602 cluster is visible naturally near $(\ell,b)=(290^\circ,-5^\circ)$. An additional population located at ($b\sim5^\circ$ and $\ell\sim345^\circ$) with a mean distance of $\sim180\mathrm{pc}$ ($5$-$6$~mas) is observed. This is consistent with previous studies reported in the literature., Comment: 3 pages, 1 figure, data available online, Published by Research Notes of the AAS

The assembly process of our Galaxy can be retrieved using the motions and chemistry of individual stars. Chemo-dynamical studies of the nearby halo have long hinted at the presence of multiple components such as streams, clumps, duality and correlations between the stars' chemical abundances and orbital parameters. More recently, the analysis of two large stellar surveys have revealed the presence of a well-populated chemical elemental abundance sequence, of two distinct sequences in the colour-magnitude diagram, and of a prominent slightly retrograde kinematic structure all in the nearby halo, which may trace an important accretion event experienced by the Galaxy. Here report an analysis of the kinematics, chemistry, age and spatial distribution of stars in a relatively large volume around the Sun that are mainly linked to two major Galactic components, the thick disk and the stellar halo. We demonstrate that the inner halo is dominated by debris from an object which at infall was slightly more massive than the Small Magellanic Cloud, and which we refer to as Gaia-Enceladus. The stars originating in Gaia-Enceladus cover nearly the full sky, their motions reveal the presence of streams and slightly retrograde and elongated trajectories. Hundreds of RR Lyrae stars and thirteen globular clusters following a consistent age-metallicity relation can be associated to Gaia-Enceladus on the basis of their orbits. With an estimated 4:1 mass-ratio, the merger with Gaia-Enceladus must have led to the dynamical heating of the precursor of the Galactic thick disk and therefore contributed to the formation of this component approximately 10 Gyr ago. These findings are in line with simulations of galaxy formation, which predict that the inner stellar halo should be dominated by debris from just a few massive progenitors., Comment: 19 pages, 8 figures. Published in Nature in the issue of Nov. 1st, 2018. This is the authors' version before final edits

In this contribution I provide a brief summary of the contents of Gaia DR1. This is followed by a discussion of studies in the literature that attempt to characterize the quality of the Tycho-Gaia Astrometric Solution parallaxes in Gaia DR1, and I point out a misconception about the handling of the known systematic errors in the Gaia DR1 parallaxes. I highlight some of the more unexpected uses of the Gaia DR1 data and close with a look ahead at the next Gaia data releases, with Gaia DR2 coming up in April 2018., Comment: 10 pages, 1 figure, to appear in Proceedings of IAU Symposium 330 "Astrometry and Astrophysics in the Gaia Sky", slightly edited version, updated references

Double white dwarf (DWD) binaries are expected to be very common in the Milky Way, but their intrinsic faintness challenges the detection of these systems. Currently, only a few tens of detached DWDs are know. Such systems offer the best chance of extracting the physical properties that would allow us to address a wealth of outstanding questions ranging from the nature of white dwarfs, over stellar and binary evolution to mapping the Galaxy. In this paper we explore the prospects for detections of ultra-compact (with binary separations of a few solar radii or less) detached DWDs in: 1) optical radiation with Gaia and the LSST and 2) gravitational wave radiation with LISA. We show that Gaia, LSST and LISA have the potential to detect respectively around a few hundreds, a thousand, and 25 thousand DWD systems. Moreover, Gaia and LSST data will extend by respectively a factor of two and seven the guaranteed sample of binaries detected in electromagnetic and gravitational wave radiation, opening the era of multi-messenger astronomy for these sources., Comment: submitted to MNRAS

The Gaia archive is being designed and implemented by the DPAC Consortium. The purpose of the archive is to maximize the scientific exploitation of the Gaia data by the astronomical community. Thus, it is crucial to gather and discuss with the community the features of the Gaia archive as much as possible. It is especially important from the point of view of the GENIUS project to gather the feedback and potential use cases for the archive. This paper presents very briefly the general ideas behind the Gaia archive and presents which tools are already provided to the community., Comment: Proceedings of the XXXVII Meeting of the Polish Astronomical Society

Aims: We study single and binary white dwarfs in the inner halo of the Milky Way in order to learn more about the conditions under which the population of halo stars was born, such as the initial mass function (IMF), the star formation history, or the binary fraction. Methods: We simulate the evolution of low-metallicity halo stars at distances up to ~ 3 kpc using the binary population synthesis code SeBa. We use two different white dwarf cooling models to predict the present-day luminosities of halo white dwarfs. We determine the white dwarf luminosity functions (WDLFs) for eight different halo models and compare these with the observed halo WDLF of white dwarfs in the SuperCOSMOS Sky Survey. Furthermore, we predict the properties of binary white dwarfs in the halo and determine the number of halo white dwarfs that is expected to be observed with the Gaia satellite. Results: By comparing the WDLFs, we find that a standard IMF matches the observations more accurately than a top-heavy one, but the difference with a bottom-heavy IMF is small. A burst of star formation 13 Gyr ago fits slightly better than a star formation burst 10 Gyr ago and also slightly better than continuous star formation $10-13$ Gyrs ago. Gaia will be the first instrument to constrain the bright end of the field halo WDLF, where contributions from binary WDs are considerable. Many of these will have He cores, of which a handful have atypical surface gravities ($\log g < 6$) and reach luminosities $\log(L/L_\odot) > 0$ in our standard model for WD cooling. These so called pre-WDs, if observed, can help us to constrain white dwarf cooling models and might teach us something about the fraction of halo stars that reside in binaries., Comment: 16 pages, 9 figures. Accepted for publication in Astronomy & Astrophysics

In this contribution I provide an overview of the the European Space Agency's Gaia mission just ahead of its launch scheduled for November 2013., Comment: 6 pages, 3 figures, to appear in the proceedings of the Societe Francaise d'Astronomie et d'Astrophysique (SF2A) 2013 (eds. L. Cambresy, F. Martins, E. Nuss and A. Palacios)

The Gaia mission has been designed to perform absolute astrometric measurements with unprecedented accuracy; the end-of-mission parallax standard error is required to be 30 micro-arcseconds for a G2V type star of magnitude 15. These requirements set a stringent constraint on the accuracy of the estimation of the location of the stellar image on the CCD for each observation: e.g., 0.3 milli-arseconds (mas) or 0.005 pixels for the same V=15 G2V star. However the Gaia CCDs will suffer from charge transfer inefficiency (CTI) caused by radiation damage that will degrade the stellar image quality and may degrade the astrometric performance of Gaia if not properly addressed. For the first time at this level of detail, the potential impact of radiation damage on the performance of Gaia is investigated. In this first paper we focus on the evaluation of the CTI impact on the image location accuracy. We show that CTI decreases the stellar image signal-to-noise ratio and irreversibly degrades the image location estimation precision. As a consequence the location estimation standard errors increase by up to 6% for a radiation damage level equivalent to the end-of-mission. In addition the CTI-induced image distortion introduces a systematic bias in the image location estimation (up to 0.05 pixels or 3 mas in the Gaia operating conditions). We present a novel approach to CTI mitigation that enables, without correction of the raw data, the unbiased estimation of the image location and flux from damaged observations. Its implementation reduces the maximum measured location bias for the faintest magnitude to 0.005 pixels (~4e-4 pixels at magnitude 15). In a second paper we will investigate how the CTI effects affect the final astrometric accuracy of Gaia by propagating residual errors through the astrometric solution., Comment: 23 figures, 6 tables, accepted for publication in the Monthly Notices of the Royal Astronomical Society the 4th of October 2011

(abridged) A complete census of planetary systems around a volume-limited sample of solar-type stars (FGK dwarfs) in the Solar neighborhood with uniform sensitivity down to Earth-mass planets within their Habitable Zones out to several AUs would be a major milestone in extrasolar planets astrophysics. This fundamental goal can be achieved with a mission concept such as NEAT - the Nearby Earth Astrometric Telescope. NEAT is designed to carry out space-borne extremely-high-precision astrometric measurements sufficient to detect dynamical effects due to orbiting planets of mass even lower than Earth's around the nearest stars. Such a survey mission would provide the actual planetary masses and the full orbital geometry for all the components of the detected planetary systems down to the Earth-mass limit. The NEAT performance limits can be achieved by carrying out differential astrometry between the targets and a set of suitable reference stars in the field. The NEAT instrument design consists of an off-axis parabola single-mirror telescope, a detector with a large field of view made of small movable CCDs located around a fixed central CCD, and an interferometric calibration system originating from metrology fibers located at the primary mirror. The proposed mission architecture relies on the use of two satellites operating at L2 for 5 years, flying in formation and offering a capability of more than 20,000 reconfigurations (alternative option uses deployable boom). The NEAT primary science program will encompass an astrometric survey of our 200 closest F-, G- and K-type stellar neighbors, with an average of 50 visits. The remaining time might be allocated to improve the characterization of the architecture of selected planetary systems around nearby targets of specific interest (low-mass stars, young stars, etc.) discovered by Gaia, ground-based high-precision radial-velocity surveys., Comment: Accepted for publication in Experimental Astronomy. The full member list of the NEAT proposal and the news about the project are available at http://neat.obs.ujf-grenoble.fr. The final publication is available at http://www.springerlink.com

ELSA stands for the ambitious goal of `European Leadership in Space Astrometry'. In this closing contribution I will examine how the ELSA network has advanced this goal. I also look ahead to the time when the Gaia data will be published and consider what needs to be done to maintain European leadership., Comment: Closing remarks at the ELSA conference "Gaia: at the frontiers of astrometry". Proceedings editors: C. Turon, F. Arenou & F. Meynadier

Wide-field multi-object spectroscopy is a high priority for European astronomy over the next decade. Most 8-10m telescopes have a small field of view, making 4-m class telescopes a particularly attractive option for wide-field instruments. We present a science case and design drivers for a wide-field multi-object spectrograph (MOS) with integral field units for the 4.2-m William Herschel Telescope (WHT) on La Palma. The instrument intends to take advantage of a future prime-focus corrector and atmospheric-dispersion corrector that will deliver a field of view 2 deg in diameter, with good throughput from 370 to 1,000 nm. The science programs cluster into three groups needing three different resolving powers R: (1) high-precision radial-velocities for Gaia-related Milky Way dynamics, cosmological redshift surveys, and galaxy evolution studies (R = 5,000), (2) galaxy disk velocity dispersions (R = 10,000) and (3) high-precision stellar element abundances for Milky Way archaeology (R = 20,000). The multiplex requirements of the different science cases range from a few hundred to a few thousand, and a range of fibre-positioner technologies are considered. Several options for the spectrograph are discussed, building in part on published design studies for E-ELT spectrographs. Indeed, a WHT MOS will not only efficiently deliver data for exploitation of important imaging surveys planned for the coming decade, but will also serve as a test-bed to optimize the design of MOS instruments for the future E-ELT., Comment: 15 pages, 4 figures, submitted to SPIE Astronomical Telescopes and Instrumentation 2010

We model the formation of the Galactic stellar halo via the accretion of satellite galaxies onto a time-dependent semi-cosmological galactic potential. Our goal is to characterize the substructure left by these accretion events in a close manner to what may be possible with the {\it Gaia} mission. We have created a synthetic {\it Gaia} Solar Neighbourhood catalogue by convolving the 6D phase-space coordinates of stellar particles from our disrupted satellites with the latest estimates of the {\it Gaia} measurement errors, and included realistic background contamination due to the Galactic disc(s) and bulge. We find that, even after accounting for the expected observational errors, the resulting phase-space is full of substructure. We are able to successfully isolate roughly 50% of the different satellites contributing to the `Solar Neighbourhood' by applying the Mean-Shift clustering algorithm in energy-angular momentum space. Furthermore, a Fourier analysis of the space of orbital frequencies allows us to obtain accurate estimates of time since accretion for approximately 30% of the recovered satellites., Comment: 13 pages, 11 figures, submitted to MNRAS

We describe the results of a search for the remnants of the Sun's birth cluster among stars in the Hipparcos Catalogue. This search is based on the predicted phase space distribution of the Sun's siblings from simple simulations of the orbits of the cluster stars in a smooth Galactic potential. For stars within 100 pc the simulations show that it is interesting to examine those that have small space motions relative to the Sun. From amongst the candidate siblings thus selected there are six stars with ages consistent with that of the Sun. Considering their radial velocities and abundances only one potential candidate, HIP 21158, remains but essentially the result of the search is negative. This is consistent with predictions by Portegies Zwart (2009) on the number of siblings near the Sun. We discuss the steps that should be taken in anticipation of the data from the Gaia mission in order to conduct fruitful searches for the Sun's siblings in the future., Comment: 7 pages, 9 figures, accepted for publication in MNRAS

The Gaia mission is reviewed together with the expected contents of the final catalogue. It is then argued that the ultimate goal of Galactic structure studies with Gaia astrometry should be to build a dynamical model of our galaxy which is capable of explaining the contents of the Gaia catalogue. This will be possible only by comparing predicted catalogue data to Gaia's actual measurements. To complement this approach the Gaia catalogue should be used to recalibrate photometric distance and abundance indicators across the HR-diagram in order to overcome the lack of precise parallax data at the faint end of the astrometric survey. Using complementary photometric and spectroscopic data from other surveys will be essential in this respect., Comment: Presented at the "Classification and Discovery in Large Astronomical Surveys" meeting, Ringberg Castle, 14-17 October, 2008

As the title of this symposium implies, one of the aims is to examine the future of astrometry as we move from an era in which thanks to the Hipparcos Catalogue everyone has become familiar with milliarcsecond astrometry to an era in which microarcsecond astrometry will become the norm. I will take this look into the future by first providing an overview of present astrometric programmes and how they fit together and then I will attempt to identify the most promising future directions. In addition I discuss the important conditions for the maximization of the scientific return of future large and highly accurate astrometric catalogues; catalogue access and analysis tools, the availability of sufficient auxiliary data and theoretical knowledge, and the education of the future generation of astrometrists., Comment: To appear in proceedings of IAU Symposium 248: A Giant Step: from Milli- to Micro-arcsecond Astrometry; 10 pages, 4 figures

The SINFONI instrument for ESO's VLT combines integral field spectroscopy and adaptive optics (AO). We discuss detailed simulations of the adaptive optics module. These simulations are aimed at assessing the AO module performance, specifically for operations with extended sources and a laser guide star. Simulated point spread function (PSF) images will be used to support scientific preparations and the development of an exposure time calculator, while simulated wavefront sensor measurements will be used to study PSF reconstruction methods. We explain how the adaptive optics simulations work, focusing on the realistic modelling of the laser guide star for a curvature wavefront sensor. The predicted performance of the AO module is discussed, resulting in recommendations for the operation of the SINFONI AO module at the telescope., Comment: 12 pages, 6 figures, to appear in SPIE conference proceedings vol 5490, "Advancements in Adaptive Optics", eds. D. Bonaccini, B.L. Ellerbroek, R. Ragazonni, Glasgow UK, 21-25 June 2004

If we want to understand the evolution from star clusters to the Galactic field star population, the Solar neighbourhood is an ideal place to start. It contains objects from dense, very young clusters to old and almost dissolved moving groups. I describe the observational evidence for the ongoing dissolution of OB associations and open clusters. Subsequently, the interpretation of the local phase space distribution of stars in terms of old moving groups is discussed, emphasising the present limitations. Finally, I discuss future observational requirements and the possibilities offered by upcoming astrometric space missions., Comment: 10 pages, 1 figure, for publication in ASP Conf. Ser., Modes of Star Formation and the Origin of Field Populations, eds. Grebel & Brandner

Since the previous (1990) edition of this meeting enormous progress in the field of OB associations has been made. Data from X-ray satellites have greatly advanced the study of the low-mass stellar content of associations, while astrometric data from the Hipparcos satellite allow for a characterization of the higher-mass content of associations with unprecedented accuracy. We review recent work on the OB associations located within 1.5 kpc from the Sun, discuss the Hipparcos results at length, and point out directions for future research., Comment: To appear in The Physics of Star Formation and Early Stellar Evolution II, eds C.J. Lada & N. Kylafis (Kluwer Academic), 30 pages, 9 EPS-figures, LaTeX using crckapb.sty, epsfig.sty, amssymb.sty

We review the current knowledge of the population of `older' stars in the Orion OB1 association, specifically those in subgroups 1a and 1b. We briefly outline the history of the subject and then continue with a summary of the present state of knowledge of the early-type stars in Orion OB1. New results from the Hipparcos parallaxes and proper motions will be presented. The main result is that subgroup 1a is located at about 330 pc from the Sun, much closer than the previously determined distance, and about 100 pc distant from the other subgroups of the association and the Orion molecular clouds. Unfortunately, due to the unfavorable kinematics of the association with respect to the Galactic background, Hipparcos proper motions do not allow a clear kinematic separation of the association from the field. For this purpose accurate and homogeneous radial velocities are needed. Traditionally, the massive O and B stars have received most of the attention in the studies of OB associations. However, we will present results showing that significant numbers of low-mass stars are associated with Orion OB1. Unbiased, optically complete, spectroscopic and photometric surveys of areas within subgroups 1a and 1b have the potential to determine the complete low-mass stellar population, down to the brown dwarf limit. This will provide much insight into the overall initial mass function and studies of the kinematics of the low-mass stars will yield insights into the dispersal of the association., Comment: To appear in The Orion Complex Revisited, eds. M. J. McCaughrean & A. Burkert (San Francisco, ASP), gzipped tar-file, 22 pages 7 EPS-figures, LaTeX using paspconf.sty and psfig.tex. Wrongly quoted errors on the average parallaxes of the Orion OB1 subgroups were corrected (Section 4)