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Full author list: A Pieres, L Girardi, E Balbinot, B Santiago, L N da Costa, A Carnero Rosell, A B Pace, K Bechtol, M A T Groenewegen, A Drlica-Wagner, T S Li, M A G Maia, R L C Ogando, M dal Ponte, H T Diehl, A Amara, S Avila, E Bertin, D Brooks, D L Burke, M Carrasco Kind, J Carretero, J De Vicente, S Desai, T F Eifler, B Flaugher, P Fosalba, J Frieman, J García-Bellido, E Gaztanaga, D W Gerdes, D Gruen, R A Gruendl, J Gschwend, G Gutierrez, D L Hollowood, K Honscheid, D J James, K Kuehn, N Kuropatkin, J L Marshall, R Miquel, A A Plazas, E Sanchez, S Serrano, I Sevilla-Noarbe, E Sheldon, M Smith, M Soares-Santos, F Sobreira, E Suchyta, M E C Swanson, G Tarle, D Thomas, V Vikram, A R Walker, We present a technique to fit the stellar components of the Galaxy by comparing Hess Diagrams (HDs) generated from TRILEGAL models to real data. We apply this technique, which we call MWFITTING, to photometric data from the first 3 yr of the Dark Energy Survey (DES). After removing regions containing known resolved stellar systems such as globular clusters, dwarf galaxies, nearby galaxies, the Large Magellanic Cloud, and the Sagittarius Stream, our main sample spans a total area of ~2300 deg2. We further explore a smaller subset (~1300 deg2) that excludes all regions with known stellar streams and stellar overdensities. Validation tests on synthetic data possessing similar properties to the DES data show that the method is able to recover input parameters with a precision better than 3 per cent. We fit the DES data with an exponential thick disc model and an oblate double power-law halo model. We find that the best-fitting thick disc model has radial and vertical scale heights of 2.67 ± 0.09 kpc and 925 ± 40 pc, respectively. The stellar halo is fit with a broken power-law density profile with an oblateness of 0.75 ± 0.01, an inner index of 1.82 ± 0.08, an outer index of 4.14 ± 0.05, and a break at 18.52 ± 0.27 kpc from the Galactic centre. Several previously discovered stellar overdensities are recovered in the residual stellar density map, showing the reliability of MWFITTING in determining the Galactic components. Simulations made with the best-fitting parameters are a promising way to predict Milky Way star counts for surveys such as the LSST and Euclid., The DESDM is supported by the National Science Foundation under grants AST-1138766 and AST-1536171. The DES participants from Spanish institutions are partially supported by MINECO under grants AYA2015-71825, ESP2015-66861, FPA2015-68048, SEV-2016-0588, SEV-2016-0597, and MDM-2015-0509, some of which include ERDF funds from the European Union. IFAE is partially funded by the CERCA program of the Generalitat de Catalunya. Research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Program (FP7/2007-2013) including ERC grant agreements 240672, 291329, and 306478. We acknowledge support from the Australian Research Council Centre of Excellence for All-sky Astrophysics (CAASTRO), through project number CE110001020, and the Brazilian Instituto Nacional de Ci?ncia e Tecnologia (INCT) do e-Universe (CNPq grant 465376/2014-2). This manuscript has been authored by Fermi Research Alliance, LLC under Contract No. DE-AC02-07CH11359 with the U.S. Department of Energy, Office of Science, Office of High Energy Physics. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. Funding for the DES Projects has been provided by the U.S. Department of Energy, the U.S. National Science Foundation, the Ministry of Science and Education of Spain, the Science and Technology Facilities Council of the United Kingdom, the Higher Education Funding Council for England, the National Centre for Supercomputing Applications at the University of Illinois at Urbana-Champaign, the Kavli Institute of Cosmological Physics at the University of Chicago, the Centre for Cosmology and AstroParticle Physics at the Ohio State University, the Mitchell Institute for Fundamental Physics and Astronomy at Texas A&M University, Financiadora de Estudos e Projetos, Fundac¸ão Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro, Conselho Nacional de Desenvolvimento Científico e Tecnológico and the Ministério da Ciência, Tecnologia e Inovac¸ão, the Deutsche Forschungsgemeinschaft, and the Collaborating Institutions in the DES.

The extended stellar halos of galaxies contain important clues for investigating their assembly history and evolution. We investigate the resolved stellar content and the extended halo of NGC 5128 as a function of galactocentric distance. We used HST images to resolve individual red giant branch (RGB) stars in 28 independent pointings. Star counts from deep VI color-magnitude diagrams reaching at least 1.5 mag below the tip of the RGB are used to derive the surface density distribution of the halo. The contamination by Milky Way stars is assessed with a new control field, with models, and by combining optical and near-IR photometry. We present a new calibration of the WFC3 F606W+F814W photometry to the ground-based VI photometric system. The photometry shows that the stellar halo of NGC 5128 is dominated by old RGB stars that are present in all fields. The V-band surface brightness changes from 23 to 32 mag/arcsec$^2$ between 8.3 kpc from the galaxy center to our outermost halo fields located 140 kpc away from the center along the major axis and 92 kpc along the minor axis. Within ~30 kpc, we also find evidence for a 2-3 Gyr old population traced by bright asymptotic giant branch stars. This population contributes only up to 10% in total stellar mass if it is 2 Gyr old, but a larger fraction of 30-40% is required if its age is 3 Gyr. The stellar surface density profile is well fit by a r$^{1/4}$ curve or a power-law $\sim r^{-3.1}$ over the full radial range, with no obvious break in the slope, but with large field-to-field scatter. The ellipticity measured from integrated-light photometry in the inner parts, $e=(b/a)=0.77$, flattens to $e=0.54 \pm 0.02$ beyond 30 kpc. Considering the flattening of the outer halo, the projection of the elliptical isophote on the semimajor axis for our most distant field reaches nearly 30 effective radii. [abridged], 23 pages, 24 figures, accepted for publication in A&A

Despite the impressive advances in Galactic structure studies, thanks to the large astronomical surveys, there remain several open questions. Although at low distances, optical surveys can bring us important information, the potential of NIR surveys, combined with the optical data, should be considered. In the present work, we explore the stellar distribution through the most recent NIR surveys toward low latitudes (|b| < 2° for 20° ≤ ℓ ≤ 346°) in the Galactic disk, such as 2MASS (entire plane), UKIDSS (20° ≤ ℓ ≤ 231°), and VVV-PSF data (295° ≤ ℓ ≤ 346°), avoiding directions toward the Galactic bar and bulge. Our final compilation contains nearly 140 million stars. We used this sample to perform total star counts at different longitudes, obtaining longitudinal profiles that are compared with those of other authors. For some directions, we obtained the stellar density as a function of distance to investigate the stellar distribution in the Galactic disk. As an example, the variation of the counts toward the Scutum arm tangential direction reveals the stellar content of two spiral arms, e.g., Sagittarius and Scutum. These are the preliminary results of a study that will cover a large extension of the Galactic disk.

The peripheral regions of globular clusters (GCs) are extremely challenging to study due to their low surface brightness nature and the dominance of Milky Way contaminant populations along their sightlines. We have developed a probabilistic approach to this problem through utilising a mixture model in spatial and proper motion space which separately models the cluster, extra-tidal and contaminant stellar populations. We demonstrate the efficacy of our method through application to Gaia EDR3 photometry and astrometry in the direction of NGC 5139 ($\omega$ Cen), a highly challenging target on account of its Galactic latitude ($b\approx 15^{\circ}$) and low proper motion contrast with the surrounding field. We recover the spectacular tidal extensions, spanning the $10^{\circ}$ on the sky explored here, seen in earlier work and quantify the star count profile and ellipticity of the system out to a cluster-centric radius of $4^{\circ}$. We show that both RR Lyrae and blue horizontal branch stars consistent with belonging to $\omega$ Cen are found in the tidal tails, and calculate that these extensions contain at least $\approx 0.1$ per cent of the total stellar mass in the system. Our high probability members provide prime targets for future spectroscopic studies of $\omega$ Cen out to unprecedented radii., Comment: 11 pages, 7 figures, accepted for publication in MNRAS

We present a new identity card for the cluster NGC 6440 in the Galactic Bulge. We have used a combination of high-resolution Hubble Space Telescope images, wide-field ground-based observations performed with the ESO-FORS2, and the public survey catalog Pan-STARRS, to determine the gravitational center, projected density profile and structural parameters of this globular from resolved star counts. The new determination of the cluster center differs by ~ 2" (corresponding to 0.08 pc) from the previous estimate, which was based on the surface brightness peak. The star density profile, extending out to 700" from the center and suitably decontaminated from the Galactic field contribution, is best-fitted by a King model with significantly larger concentration ($c=1.86\pm0.06$) and smaller core radius ($r_c=6.4"\pm0.3"$) with respect to the literature values. By taking advantage of high-quality optical and near-infrared color-magnitude diagrams, we also estimated the cluster age, distance and reddening. The luminosity of the RGB-bump was also determined. This study indicates that the extinction coefficient in the bulge, in the direction of the cluster has a value ($R_V=2.7$) that is significantly smaller than that traditionally used for the Galaxy ($R_V=3.1$). The corresponding best-fit values of the age, distance and color excess of NGC 6440 are 13 Gyr, 8.3 kpc and $E(B-V)\sim 1.27$, respectively. These new determinations also allowed us to update the values of the central ($t_{rc}=2.5\ 10^7$ yr) and half-mass ($t_{rh}=10^9$ yr) relaxation times, suggesting that NGC 6440 is in a dynamically evolved stage., Accepted for publication in The Astrophysical Journal; 19 pages, 9 figures and 2 tables

We present an updated version of the semi-analytic Just-Jahrei{\ss} (JJ) model of the Galactic disk and constrain its parameters in the Solar neighbourhood. The new features of the JJ model include a simple two-component gaseous disk, a star-formation rate (SFR) function of the thick disk that has been extended in time, and a correlation between the kinematics of molecular gas and thin-disk populations. Here, we study the vertical number density profiles and W-velocity distributions determined from ~2 million local stars of the Gaia DR2. We also investigate an apparent Hess diagram of the Gaia stars selected in a conic volume towards the Galactic poles. Using a stellar evolution library, we synthesise stellar populations with a four-slope broken power-law initial mass function (IMF), the SFR, and an age-metallicity relation (AMR). The latter is consistently derived with the observed metallicity distribution of the local Red Clump (RC) giants from the APOGEE. Working within a Bayesian approach, we sample the posterior probability distribution in a multidimensional parameter space using the MCMC method. We find that the spatial distribution and motion of the Gaia stars imply two recent SF bursts centered at ages of ~0.5 Gyr and ~3 Gyr and characterised by a ~30% and ~55% SF enhancement, respectively, relative to a monotonously declining SFR continuum. The stellar populations associated with this SF excess are found to be dynamically hot for their age: they have W-velocity dispersions of ~12.5 km/s and ~26 km/s. The new JJ model is able to reproduce the local star counts with an accuracy of ~5 %. Using Gaia DR2 data, we self-consistently constrained 22 parameters of the updated JJ model. Our optimised model predicts two SF bursts within the last ~4 Gyr, which may point to recent episodes of gas infall., Comment: 29 pages, 16 figures, 4 tables. Accepted for publication in Astronomy&Astrophysics on December 1, 2020

We developed a parametric Galactic model toward the Galactic bulge by fitting to spatial distributions of the Gaia DR2 disk velocity, VVV proper motion, BRAVA radial velocity, OGLE-III red clump star count, and OGLE-IV star count and microlens rate, optimized for use in microlensing studies. We include the asymmetric drift of Galactic disk stars and the dependence of velocity dispersion on Galactic location in the kinematic model, which has been ignored in most previous models used for microlensing studies. We show that our model predicts a microlensing parameter distribution significantly different from those typically used in previous studies. We estimate various fundamental model parameters for our Galaxy through our modeling, including the initial mass function (IMF) in the inner Galaxy. Combined constraints from star counts and the microlensing event timescale distribution from the OGLE-IV survey, in addition to a prior on the bulge stellar mass, enable us to successfully measure IMF slopes using a broken power-law form over a broad mass range, $\alpha_{\rm bd}=0.22^{+0.20}_{-0.55}$ for $M, Comment: 47 pages, 13 figures, 7 tables, accepted to ApJ. A code for microlensing simulation using the developed Galactic model is available via https://github.com/nkoshimoto/genulens

The structure of our Galaxy has been studied from ultraviolet (UV) star counts obtained with the Ultra-Violet Imaging Telescope (UVIT) on board the {\em AstroSat} satellite, in Far-UV (FUV) and Near-UV (NUV) bands. The F154W (BaF2) and N263M (NUVB4) filters were used in the FUV and NUV bands, respectively. The point sources are separated from the extra-galactic sources of UVIT observations using infrared (IR) color cut method. The observed UVIT star counts match well with the simulations obtained from the Besan\c{c}on model of stellar population synthesis towards several Galactic directions. We also estimated the scale length and scale height of the thick disc and the scale height of the thin disc using the space density function and the exponential density law for the stars of intermediate Galactic latitudes. The scale length of the thick disc ranges from 3.11 to 5.40 kpc whereas the scale height ranges from 530$\pm$32 pc to 630$\pm$29 pc. The scale height of the thin disc comes out to be in the range of 230$\pm$20 pc to 330$\pm$11 pc., Comment: Accepted for publication in Journal of Astrophysics and Astronomy

We discuss the Maunakea Spectroscopic Explorer (MSE) Acquisition and Guide (A and G) System conceptual focal plane hardware and operational requirements and pay detailed attention to the A and G system’s three CMOS cameras’ areas and sensitivities needed to assure a high success rate in acquiring suitable guide stars. Ways to provide auxiliary functions, including the measurement of defocus and misalignment of the telescope optics, are also discussed.

We present results of a high angular resolution survey of massive OB stars in the Cygnus OB2 association that we conducted with the NIRI camera and ALTAIR adaptive optics system of the Gemini North telescope. We observed 74 O- and early B-type stars in Cyg OB2 in the $JHK$ infrared bands in order to detect binary and multiple companions. The observations are sensitive to equal-brightness pairs at separations as small as 0.08 \arcsec, and progressively fainter companions are detectable out to $\Delta$ K = 9 mag at a separation of 2 arcsec. This faint contrast limit due to readnoise continues out to 10 arcsec near the edge of the detector. We assigned a simple probability of chance alignment to each companion based upon its separation and magnitude difference from the central target star and upon areal star counts for the general star field of Cyg OB2. Companion stars with a field membership probability of less than 1% are assumed to be physical companions. This assessment indicates that 47% of the targets have at least one resolved companion that is probably gravitationally bound. Including known spectroscopic binaries, our sample includes 27 binary, 12 triple, and 9 systems with four or more components. These results confirm studies of high mass stars in other environments that find that massive stars are born with a high multiplicity fraction. The results are important for the placement of the stars in the H-R diagram, the interpretation of their spectroscopic analyses, and for future mass determinations through measurement of orbital motion., Comment: 53 pages, 5 figures, Accepted to AJ

We have matched the ultraviolet (UV) sources in GUVcat_AIS (Bianchi et al.2017) with optical databases having similar depth and wide sky coverage. GUVcat_AIS has GALEX far-UV (FUV, lambda-eff ~1528\AA ) and near-UV (NUV, lambda-eff ~2310\AA) photometry of approx 83~million sources, covering 24,788 square degrees of the sky, with typical depth of FUV=19.9, NUV=20.8~ABmag. Matches with Gaia and SDSS databases are presented here. Gaia data release 2 (DR2), covering the entire GUVcat footprint (Bianchi et al. 2019), detected about one third of the $GUVcat\_AIS$ sources. We found 31,925,294 Gaia~DR2 counterparts to 30,024,791 GUVcat_AIS unique sources, with photometry in $Gaia$~$G$ band, and often also in $Gaia$~$BP$ and $RP$ bands; 26,275,572 matches have a parallax measurement, 21,084,628/18,588,140/16,357,505 with parallax error less than 50%/30%/20%. The match with SDSS data release 14 (DR14) yields 23,310,532 counterparts to 22,207,563 unique GUVcat_AIS sources, 10,167,460 of which are point-like, over a total overlap area of $\approx$11,100~square~degrees (Bianchi et al. 2019)}. SDSS adds to the UV photometry five optical magnitudes: $u,g,r,i,z$, and optical spectra of 860,224 matched sources. We used a match radius of 3arcsec, consistent with previous works (e.g., Bianchi et al. 2011a), although the positions agree to $\lesssim$1.5arcsec for the majority of [point-like] matched-sources, in order to identify possible multiple matches whose UV flux could be unresolved in GALEX imaging. The catalogs can be trimmed to a tighter match radius using the provided separation. The multi-band photometry is used to identify classes of astrophysical objects that are prominent in UV, to characterise the content of the $GUVmatch$ catalogs, where stars in different evolutionary stages, QSOs, and galaxies can be separated., Comment: accepted for ApJS

The abundance and narrow magnitude dispersion of Red Clump (RC) stars make them a popular candidate for mapping the morphology of the bulge region of the Milky Way. Using an estimate of the RC's intrinsic luminosity function, we extracted the three-dimensional density distribution of the RC from deep photometric catalogues of the VISTA Variables in the Via Lactea (VVV) survey. We used maximum entropy based deconvolution to extract the spatial distribution of the bulge from Ks-band star counts. We obtained our extrapolated non-parametric model of the bulge over the inner 40 by 40 degrees squared region of the Galactic centre. Our reconstruction also naturally matches onto a parametric fit to the bulge outside the VVV region and inpaints overcrowded and high extinction regions. We found a range of bulge properties consistent with other recent investigations based on the VVV data. In particular, we estimated the bulge mass to be in the range 13 to 17 billion solar masses, the X-component to be between 18% and 25% of the bulge mass, and the bulge angle with respect to the Sun-Galactic centre line to be between 18 and 32 degrees. Studies of the Fermi Large Area Telescope (LAT) gamma-ray Galactic centre excess suggests that the excess may be traced by Galactic bulge distributed sources. We applied our deconvolved density in a template fitting analysis of this Fermi-LAT GeV excess and found an improvement in the fit compared to previous parametric based templates., 25 pages, 27 figures, minor typo corrected

AIMS. We calculate the stellar density using star counts obtained from Gaia DR2 up to a Galactocentric distance R=20 kpc with a deconvolution technique for the parallax errors. Then we analyse the density in order to study the structure of the outer Galactic disc, mainly the warp. METHODS. In order to carry out the deconvolution, we used the Lucy inversion technique for recovering the corrected star counts. We also used the Gaia luminosity function of stars with $M_G, Comment: 19 pages, 21 figures, accepted to be published in A&A

Context. The environment of Sagittarius A* (Sgr A*), the central black hole of the Milky Way, is the only place in the Universe where we can currently study the interaction between a nuclear star cluster and a massive black hole and infer the properties of a nuclear cluster from observations of individual stars.Aims. This work aims to explore the star formation history of the nuclear cluster and the structure of the innermost stellar cusp around Sgr A*.Methods. We combined and analysed multi epoch high quality AO observations. For the region close to Sgr A* we apply the speckle holography technique to the AO data and obtain images that are >= 50% complete down to K-s approximate to 19 within a projected radius of 5 '' around Sgr A*. We used H-band images to derive extinction maps.Results. We provide K-s photometry for roughly 39 000 stars and H-band photometry for similar to 11 000 stars within a field of about 40 '' x40 '', centred on Sgr A*. In addition, we provide K-s photometry of similar to 3000 stars in a very deep central field of 10 '' x10 '', centred on Sgr A*. We find that the K-s luminosity function (KLF) is rather homogeneous within the studied field and does not show any significant changes as a function of distance from the central black hole on scales of a few 0.1 pc. By fitting theoretical luminosity functions to the KLF, we derive the star formation history of the nuclear star cluster. We find that about 80% of the original star formation took place 10 Gyr ago or longer, followed by a largely quiescent phase that lasted for more than 5 Gyr. We clearly detect the presence of intermediate-age stars of about 3 Gyr in age. This event makes up about 15% of the originally formed stellar mass of the cluster. A few percent of the stellar mass formed in the past few 100 Myr. Our results appear to be inconsistent with a quasi-continuous star formation history. The mean metallicity of the stars is consistent with being slightly super solar. The stellar density increases exponentially towards Sgr A* at all magnitudes between K-s=15-19. We also show that the precise properties of the stellar cusp around Sgr A* are hard to determine because the star formation history suggests that the star counts can be significantly contaminated, at all magnitudes, by stars that are too young to be dynamically relaxed. We find that the probability of observing any young (non-millisecond) pulsar in a tight orbit around Sgr A* and beamed towards Earth is very low. We argue that typical globular clusters, such as they are observed in and around the Milky Way today, have probably not contributed to the nuclear cluster's mass in any significant way. The nuclear cluster may have formed following major merger events in the early history of the Milky Way. © ESO 2020, The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013) / ERC grant agreement no: [614922]. RS, FNL, EGC, ATGC, and BS acknowledge financial support from the State Agency for Research of the Spanish MCIU through the "Center of Excellence Severo Ochoa" award for the Instituto de Astrofisica de Andalucia (SEV-2017-0709). ATGC, BS, and RS acknowledge financial support from national project PGC2018-095049-B-C21 (MCIU/AEI/FEDER, UE). F. N.-L. gratefully acknowledges funding by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) - Project-ID 138713538 - SFB 881 ("The Milky Way System", subproject B8). This work is based on observations made with ESO Telescopes at the La Silla Paranal Observatory under programmes IDs 083.B-0390, 183.B-0100 and 089.B-0162. We thank the staff of ESO for their great efforts and helpfulness.

Windows of low extinction in the Milky Way (MW) have been used along the past decades for the study of the Galactic structure and the stellar population across the inner bulge and disk. Here we report the analysis of another low extinction near-IR window discovered by the VISTA Variables in the V\'ia L\'actea Survey. VVV WIN 1733$-$3349 is about half a degree in size and is conveniently located right in the MW plane, at Galactic coordinates $(l, b) = (-5.2, -0.3)$. The mean extinction of VVV WIN 1733$-$3349 is $A_{Ks} = 0.61\pm0.08$ mag, which is much smaller than the extinction in the surrounding area. The excess in the star counts is consistent with the reduced extinction, and complemented by studying the distribution of red clump (RC) stars. Thanks to the strategic low-latitude location of VVV WIN 1733$-$3349, we are able to interpret their RC density fluctuations with the expected overdensities due to the presence of the spiral arms beyond the bulge. In addition, we find a clear excess in the number of microlensing events within the window, which corroborates our interpretation that VVV WIN 1733$-$3349 is revealing the far side of the MW bulge., Comment: 5 pages, 4 figures. Accepted for publication in MNRAS Letters

RV variable stars are important in astrophysics. The Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) spectroscopic survey has provided ~ 6.5 million stellar spectra in its Data Release 4 (DR4). During the survey, ~ 4.7 million unique sources were targeted and ~ 1 million stars observed repeatedly. The probabilities of stars being RV variables are estimated by comparing the observed radial velocity variations with the simulated ones. We build a catalog of 80,702 RV variable candidates with probability greater than 0.60 by analyzing the duplicate-observed multi-epoch sources covered by the LAMOST DR4. Simulations and cross-identifications show that the purity of the catalog is higher than 80%. The catalog consists of 77% binary systems and 7% pulsating stars as well as 16% pollution by single stars. 3,138 RV variables are classified through cross-identifications with published results in literatures. By using the 3,138 sources common to both LAMOST and a collection of published RV variable catalogs we are able to analyze LAMOST's RV variable detection rate. The efficiency of the method adopted in this work relies not only on the sampling frequency of observations but also periods and amplitudes of RV variables. With the progress of LAMOST, Gaia and other surveys, more and more RV variables would will be confirmed and classified. This catalog is valuable for other large-scale surveys, especially for RV variable searches. The catalog will be released according to the LAMOST Data Policy via http://dr4.lamost.org., Comment: 16 pages, 23 figures, Accepted for publication in ApJS

With the imminent start of the Legacy Survey for Space and Time (LSST) on the Vera C. Rubin Observatory, and several new space telescopes expected to begin operations later in this decade, both time domain and wide-field astronomy are on the threshold of a new era. In this paper, we use a new, multi-component model for the distribution of white dwarfs (WDs) in our Galaxy to simulate the WD populations in four upcoming wide-field surveys (i.e., LSST, Euclid, the Roman Space Telescope and CASTOR) and use the resulting samples to explore some representative WD science cases. Our results confirm that LSST will provide a wealth of information for Galactic WDs, detecting more than 150 million WDs at the final depth of its stacked, 10-year survey. Within this sample, nearly 300,000 objects will have 5$\sigma$ parallax measurements and nearly 7 million will have 5$\sigma$ proper motion measurements, allowing the detection of the turn-off in the halo WD luminosity function and the discovery of more than 200,000 ZZ Ceti stars. The wide wavelength coverage that will be possible by combining LSST data with observations from Euclid, and/or the Roman Space Telescope, will also discover more than 3,500 WDs with debris disks, highlighting the advantages of combining data between the ground- and space-based missions., Comment: 23 pages, 12 figures

We study a set of solar neighborhood ($d < 3$ kpc) stars from Gaia Data Release 2 to determine azimuthal star count differences, i.e., left and right of the line from the Galactic center through the sun - and compare these differences north and south. In this companion paper to Gardner et al. (2020), we delineate our procedures to remove false asymmetries from sampling effects, incompleteness, and/or interloper populations, as this is crucial to tests of axisymmetry. Particularly, we have taken care to make appropriate selections of magnitude, color, in-plane Galactocentric radius and Galactic $|b|$ and $|z|$. We find that requiring parallax determinations of high precision induces sampling biases, so that we eschew such requirements and exclude, e.g., regions around the lines of sight to the Magellanic clouds, along with their mirror-image lines of sight, to ensure well-matched data sets. After making conservative cuts, we demonstrate the existence of azimuthal asymmetries, and find differences in those, north and south. These asymmetries give key insights into the nature and origins of the perturbations on Galactic matter, allowing us to assess the relative influence of the Magellanic Clouds (LMC & SMC), the Galactic bar, and other masses on the Galactic mass distribution, as described in Gardner et al. (2020). The asymmetry's radial dependence reveals variations that we attribute to the Galactic bar, and it changes sign at a radius of $(0.95 \pm 0.03) R_0$, with $R_0$ the Sun-Galactic-Center (GC) distance, to give us the first direct assessment of the outer Lindblad resonant radius., 22 pages, 14 figures, accepted for publication in ApJ

Context. Although a variety of techniques have been employed for determining the Milky Way dark matter halo mass distribution, the range of allowed masses spans both light and heavy values. Knowing the precise mass of our Galaxy is important for placing the Milky Way in a cosmological $\Lambda$CDM context. Aims. We show that hypervelocity stars (HVSs) ejected from the center of the Milky Way galaxy can be used to constrain the mass of its dark matter halo. Methods. We use the asymmetry in the radial velocity distribution of halo stars due to escaping HVSs, which depends on the halo potential (escape speed) as long as the round trip orbital time is shorter than the stellar lifetime, to discriminate between different models for the Milky Way gravitational potential. Results. Adopting a characteristic HVS travel time of $330$ Myr, which corresponds to the average mass of main sequence HVSs, we find that current data favors a mass for the Milky Way in the range $(1.2$-$1.9)\times 10^{12} \mathrm{M}_\odot$., Comment: 6 pages, 4 figures, 3 tables

Digital zenith camera system is a highly accurate positioning instrument in geodetic astronomy. It can capture 16 star images in one experiment unit in different azimuthal camera orientations and the stars in each star image should be identified using star catalogue. But the amount of calculation is very large, the efficiency of star identification is low. Now, the improvement of star identification is mainly through updating algorithm. Considering that there exist identical stars in these star images, this paper provides a new method to improve star identification. The star trace is deduced taking the earth rotation and instrument rotation into consideration. The errors of star trace are also analyzed. Then select the same stars that appear in the star images taken in the initial position and after rotating one angle. Using the identical stars instead of star catalogue and the star trace to identify the rest stars, the efficiency will get improved. The analysis of experimental data shows that there are many identical stars in these star images. The appearance of one star can be predicted according to the star trace. The star identification get improved by using the same stars in these star images. And it costs less time to identify the stars in star images, but the precision is approximately equal.

Careful analyses of photometric and star count data available for the nine putative young clusters identified by Camargo et al. (2015, 2016) at high Galactic latitudes reveal that none of the groups contain early-type stars, and most are not significant density enhancements above field level. 2MASS colours for stars in the groups match those of unreddened late-type dwarfs and giants, as expected for contamination by (mostly) thin disk objects. A simulation of one such field using only typical high latitude foreground stars yields a colour-magnitude diagram that is very similar to those constructed by Camargo et al. (2015, 2016) as evidence for their young groups as well as the means of deriving their reddenings and distances. Although some of the fields are coincident with clusters of galaxies, one must conclude that there is no evidence that the putative clusters are extremely young stellar groups., Accepted for publication (MNRAS)

We use Monte Carlo simulations to explore the statistical challenges of constraining the characteristic mass ($m_c$) and width ($\sigma$) of a lognormal sub-solar initial mass function (IMF) in Local Group dwarf galaxies using direct star counts. For a typical Milky Way (MW) satellite ($M_{V} = -8$), jointly constraining $m_c$ and $\sigma$ to a precision of $\lesssim 20\%$ requires that observations be complete to $\lesssim 0.2 M_{\odot}$, if the IMF is similar to the MW IMF. A similar statistical precision can be obtained if observations are only complete down to $0.4M_{\odot}$, but this requires measurement of nearly 100$\times$ more stars, and thus, a significantly more massive satellite ($M_{V} \sim -12$). In the absence of sufficiently deep data to constrain the low-mass turnover, it is common practice to fit a single-sloped power law to the low-mass IMF, or to fit $m_c$ for a lognormal while holding $\sigma$ fixed. We show that the former approximation leads to best-fit power law slopes that vary with the mass range observed and can largely explain existing claims of low-mass IMF variations in MW satellites, even if satellite galaxies have the same IMF as the MW. In addition, fixing $\sigma$ during fitting leads to substantially underestimated uncertainties in the recovered value of $m_c$ (by a factor of $\sim 4$ for typical observations). If the IMFs of nearby dwarf galaxies are lognormal and do vary, observations must reach down to $\sim m_c$ in order to robustly detect these variations. The high-sensitivity, near-infrared capabilities of JWST and WFIRST have the potential to dramatically improve constraints on the low-mass IMF. We present an efficient observational strategy for using these facilities to measure the IMFs of Local Group dwarf galaxies., Comment: Accepted to MNRAS with minor revisions since v1. 12 pages, 9 figures, plus appendices

We used resolved star counts from Hubble Space Telescope images to determine the center of gravity and the projected density profiles of 6 old globular clusters in the Large Magellanic Cloud (LMC), namely NGC 1466, NGC 1841, NGC 1898, NGC 2210, NGC 2257 and Hodge 11. For each system, the LMC field contribution was properly taken into account by making use, when needed, of parallel HST observations. The derived values of the center of gravity may differ by several arcseconds (corresponding to more than 1 pc at the distance of the LMC) from previous determinations. The cluster density profiles are all well fit by King models, with structural parameters that may differ from the literature ones by even factors of two. Similarly to what observed for Galactic globular clusters, the ratio between the effective and the core radii has been found to anti-correlate with the cluster dynamical age., 15 pages, 12 figures, in press on the ApJ

We apply a flexible parametric model, a combination of generalized Plummer profiles, to infer the shapes of the stellar density profiles of the Milky Way's satellite dwarf spheroidal galaxies (dSphs). We apply this model to 40 dSphs using star counts from the Sloan Digital Sky Survey, PanStarrs-1 Survey, Dark Energy Survey, and Dark Energy Camera Legacy Survey. Using mock data, we examine systematic errors associated with modelling assumptions and identify conditions under which our model can identify `non-standard' stellar density profiles that have central cusps and/or steepened outer slopes. Applying our model to real dwarf spheroidals, we do not find evidence for centrally cusped density profiles among the fifteen Milky Way satellites for which our tests with mock data indicate there would be sufficient detectability. We do detect steepened (with respect to a standard Plummer model) outer profiles in several dSphs--Fornax, Leo I, Leo II, and Reticulum II--which may point to distinct evolutionary pathways for these objects. However, the outer slope of the stellar density profile does not yet obviously correlate with other observed galaxy properties., 25 pages, 17 figures. Published in ApJ

Constraining parameters such as the initial mass function high-mass slope and the frequency of type Ia supernovae is of critical importance in the ongoing quest to understand galactic physics and create realistic hydrodynamical simulations. In this paper, we demonstrate a method to precisely determine these using individual chemical abundances from a large set of stars, coupled with some estimate of their ages. Inference is performed via the simple chemical evolution model Chempy in a Bayesian framework, marginalizing over each star's specific interstellar medium parameters, including an element-specific `model error' parameter to account for inadequacies in our model. Hamiltonian Monte Carlo (HMC) methods are used to sample the posterior function, made possible by replacing Chempy with a trained neural network at negligible error. The approach is tested using data from both Chempy and the IllustrisTNG simulation, showing sub-percent agreement between inferred and true parameters using data from up to 1600 individual stellar abundances. For IllustrisTNG, strongest constraints are obtained from metal ratios, competitive with those from other methods including star counts. Analysis using a different set of nucleosynthetic yields shows that incorrectly assumed yield models can give non-negligible bias in the derived parameters; this is reduced by our model errors, which further show how well the yield tables match data. We also find a significant bias from analyzing only a small set of stars, as is often done in current analyses. The method can be easily applied to observational data, giving tight bounds on key galactic parameters from chemical abundances alone., 25 pages, 9 figures, accepted by ApJ. Code has been made publicly available at https://github.com/oliverphilcox/ChempyMulti

Chemical abundances and abundance ratios measured in galaxies provide precious information about the mechanisms, modes and time scales of the assembly of cosmic structures. Yet, the nucleogenesis and chemical evolution of elements heavier than helium are dictated mostly by the physics of the stars and the shape of the stellar mass spectrum. In particular, estimates of CNO isotopic abundances in the hot, dusty media of high-redshift starburst galaxies offer a unique glimpse into the shape of the stellar initial mass function (IMF) in extreme environments that can not be accessed with direct observations (star counts). Underlying uncertainties in stellar evolution and nucleosynthesis theory, however, may hurt our chances of getting a firm grasp of the IMF in these galaxies. In this work, we adopt new yields for massive stars, covering different initial rotational velocities. First, we implement the new yield set in a well-tested chemical evolution model for the Milky Way. The calibrated model is then adapted to the specific case of a prototype submillimeter galaxy (SMG). We show that, if the formation of fast-rotating stars is favoured in the turbulent medium of violently star-forming galaxies irrespective of metallicity, the IMF needs to be skewed towards high-mass stars in order to explain the CNO isotope ratios observed in SMGs. If, instead, stellar rotation becomes negligible beyond a given metallicity threshold, as is the case for our own Galaxy, there is no need to invoke a top-heavy IMF in starbursts., 17 pages, 10 figures, 2 tables; submitted to MNRAS; comments welcome

The bulge represents the best compromise between old and massive Galactic component, and as such its study is a valuable opportunity to understand how the bulk of the Milky Way formed and evolved. In addition, being the only bulge in which we can individually resolve stars in all evolutionary sequences, the properties of its stellar content provide crucial insights on the formation of bulges at large. We aim at providing a detailed and comprehensive census of the Milky Way bulge stellar populations by producing deep and accurate photometric catalogs of the inner ~300 sqdeg of the Galaxy. We perform DAOPHOT/ALLFRAME PSF-fitting photometry of multi-epochs J and Ks images provided by the VVV survey to obtain deep photometric catalogs. Artificial star experiments have been conducted on all images to properly assess the completeness and the accuracy of the photometric measurements. We present a photometric database containing nearly 600 million stars across the bulge area surveyed by the VVV. Through the comparison of derived color-magnitude diagrams of selected fields representative of different levels of extinction and crowding, we show the quality, completeness and depth of the new catalogs. With the exception of the fields located along the plane, this new photometry samples stars down to ~1-2 mag below the MS-TO with unprecedented accuracy. To demonstrate the tremendous potential inherent to this new dataset, we give few examples of possible applications such as: i) star counts studies through the dataset completeness map; ii) surface brightness map; and iii) cross-correlation with Gaia DR2. The database presented here represents an invaluable collection for the whole community, and we encourage its exploitation. The photometric catalogs including completeness information are publicly available through the ESO Science Archive., Accepted for publication on A&A. The data will shortly be available from the ESO science archive. Until then, anyone interested to access the data should contact E. Valenti

The dense clusters within the Serpens Molecular Cloud are among the most active regions of nearby star formation. In this paper, we use Gaia DR2 parallaxes and proper motions to statistically measure 1167 kinematic members of Serpens, few of which were previously identified, to evaluate the star formation history of the complex. The optical members of Serpens are concentrated in three distinct groups located at 380 to 480 pc; the densest clusters are still highly obscured by optically-thick dust and have few optical members. The total population of young stars and protostars in Serpens is at least 2000 stars, including past surveys that were most sensitive to protostars and disks, and may be far higher. Distances to dark clouds measured from deficits in star counts are consistent with the distances to the optical star clusters. The Serpens Molecular Cloud is seen in the foreground of the Aquila Rift, dark clouds located at 600 to 700 pc, and behind patchy extinction, here called the Serpens Cirrus, located at ~250 pc. Based on the lack of a distributed population of older stars, the star formation rate throughout the Serpens Molecular Cloud increased by at least a factor of 20 within the past ~5 Myr. The optically bright stars in Serpens Northeast are visible because their natal molecular cloud has been eroded and not because they were flung outwards from a central factory of star formation. The separation between subclusters of 20 to 100 pc and the absence of an older population leads to speculation that an external forcing was needed to trigger the active star formation., 25 pages, accepted by ApJ

In a dynamically relaxed cluster around a massive black hole a dense stellar cusp of old stars is expected to form. Previous observations showed a relative paucity of red giant stars within the central 0.5 pc in the Galactic Center. By co-adding spectroscopic observations taken over a decade, we identify new late-type stars, including the first five warm giants (G2-G8III), within the central 1 arcsec 2 (0.04 {\times} 0.04 pc^2) of the Galaxy. Our findings increase the number of late-type stars to 21, of which we present deep spectra for 16. The updated star count, based on individual spectral classification, is used to reconstruct the surface density profile of giant stars. Our study, for the first time, finds a cusp in the surface number density of the spectroscopically identified old (>3 Gyr) giants population (m K, ApJ Letters, in press

Binary stars are present in all stellar systems, yet their role is far from being fully understood. We investigate the effect of unresolved binaries in the derivation of open clusters' mass by star counts. We start from the luminosity functions of five open clusters: IC 2714, NGC 1912, NGC 2099, NGC 6834 and NGC 7142. Luminosity functions are obtained via star counts extracted from the 2MASS database. The fraction of binaries is considered to be independent on stellar magnitude. We take into account different assumptions for the binary mass ratio distribution and assign binary masses using the so-called {\it luminosity-limited pairing} method and Monte-Carlo simulations. We show that cluster masses increase when binary stars are appropriately taken into account., 9 pages, 2 figures, 2 tables

We employed recent Gaia/DR2 data to investigate the dynamical status of the nearby (300 pc), old (2.5 Gyr) open cluster Ruprecht~147. We found prominent leading and trailing tails of stars along the cluster orbit, which demonstrates that Ruprecht~147 is losing stars at fast pace. Star counts indicate the cluster has a core radius of 33.3 arcmin, and a tidal radius of 137.5 arcmin. The cluster also possesses an extended corona, which cannot be reproduced by a simple King model. We computed the present-day cluster mass using its luminosity and mass function, and derived an estimate of 234$\pm$52 $M_{\odot}$. We also estimated the cluster original mass using available recipes extracted from N-body simulations obtaining a mass at birth of 50000$\pm$6500 $M_{\odot}$. Therefore dynamical mass loss, mostly caused by tidal interaction with the Milky Way, reduced the cluster mass by about 99\%. We then conclude that Ruprecht~147 is rapidly dissolving into the general Galactic disc., 13 pages, 10 figure, in press in the Astronomical Journal

The thermally-pulsing asymptotic giant branch (TP-AGB) experienced by low- and intermediate-mass stars is one of the most uncertain phases of stellar evolution and the models need to be calibrated with the aid of observations. To this purpose, we couple high-quality observations of resolved stars in the Small Magellanic Cloud (SMC) with detailed stellar population synthesis simulations computed with the TRILEGAL code. The strength of our approach relies on the detailed spatially-resolved star formation history of the SMC, derived from the deep near-infrared photometry of the VISTA survey of the Magellanic Clouds, as well as on the capability to quickly and accurately explore a wide variety of parameters and effects with the COLIBRI code for the TP-AGB evolution. Adopting a well-characterized set of observations -- star counts and luminosity functions -- we set up a calibration cycle along which we iteratively change a few key parameters of the TP-AGB models until we eventually reach a good fit to the observations. Our work leads to identify two best-fitting models that mainly differ in the efficiencies of the third dredge-up and mass loss in TP-AGB stars with initial masses larger than about 3 M$_{\odot}$. On the basis of these calibrated models we provide a full characterization of the TP-AGB stellar population in the SMC in terms of stellar parameters (initial masses, C/O ratios, carbon excess, mass-loss rates). Extensive tables of isochrones including these improved models are publicly available., accepted for publication in MNRAS. 29 pages, 24 figures, 2 appendices. One online appendix available as ancillary file on this page

Recent studies of warp kinematics using Gaia DR2 data have produced detections of warp precession for the first time, which greatly exceeds theoretical predictions of models. However, this detection assumes a warp model derived for a young population (few tens of megayears) to fit velocities of an average older stellar population of the thin disk (several gigayears) in Gaia-DR2 observations, which may lead to unaccounted systematic errors. Here, we recalculate the warp precession with the same approach and Gaia DR2 kinematic data, but using different warp parameters based on the fit of star counts of the Gaia DR2 sample, which has a much lower warp amplitude than the young population. When we take into account this variation of the warp amplitude with the age of the population, we find that there is no need for precession. We find the value of warp precession $\beta = 4^{+6}_{-4}$ km s$^{-1}$ kpc$^{-1}$, which does not exclude nonprecessing warp., Comment: 12 pages, 8 figures, accepted to be published in ApJ

Copyright © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimAge-dating the Milky Way stellar populations plays a major role in studies of galaxy formation and evolution. Ages of old stars that represent the fossil record of the early stages of galaxy formation are particular important. Current spectroscopic surveys provide effective temperature, surface gravity and photospheric composition of large samples of Milky Way stars, and these data can be used to determine stellar ages by employing stellar evolution models. Here it is shown how current uncertainties on the efficiency of atomic diffusion in the interiors of low-mass metal poor stars can affect the derived ages at the level of several 10 %. Asteroseismic constraints on the stellar masses can reduce these errors and in the limit of high accuracy can indirectly constrain the efficiency of diffusion in the interiors of these stars.

We show how present-day 10 meter class telescopes can provide broadband imaging 1.5-2 mag deeper than most previous results within a reasonable amount of time ( ~ 8h on source integration). We illustrate the ability of the 10.4 m Gran Telescopio de Canarias (GTC) telescope to produce imaging with a limiting surface brightness of 31.5 mag/arcsec2 (3σ in 10 × 10 arcsec boxes). We explore the stellar halos of nearby galaxies obtaining surface brightness radial profiles down to μr ~ 33 mag/arcsec2. This depth is similar to that obtained using star counts techniques of Local Group galaxies, but is achieved at a distance where this technique is unfeasible.

Our recent studies of axial-symmetry breaking in the nearby ($d, Comment: 12 pages, 2 figures (3 panels), accepted for publication in ApJ Letters

Both rotation and interactions with binary companions can significantly affect massive star evolution, altering interior and surface abundances, mass loss rates and mechanisms, observed temperatures and luminosities, and their ultimate core-collapse fates. The Geneva and BPASS stellar evolution codes include detailed treatments of rotation and binary evolutionary effects, respectively, and can illustrate the impact of these phenomena on massive stars and stellar populations. However, a direct comparison of these two widely-used codes is vital if we hope to use their predictions for interpreting observations. In particular, rotating and binary models will predict different young stellar populations, impacting the outputs of stellar population synthesis (SPS) and the resulting interpretation of large massive star samples based on commonly-used tools such as star count ratios. Here we compare the Geneva and BPASS evolutionary models, using an interpolated SPS scheme introduced in our previous work and a novel Bayesian framework to present the first in-depth direct comparison of massive stellar populations produced from single, rotating, and binary non-rotating evolution models. We calculate both models' predicted values of star count ratios and compare the results to observations of massive stars in Westerlund 1, $h + \chi$ Persei, and both Magellanic Clouds. We also consider the limitations of both the observations and the models, and how to quantitatively include observational completeness limits in SPS models. We demonstrate that the methods presented here, when combined with robust stellar evolutionary models, offer a potential means of estimating the physical properties of massive stars in large stellar populations., Comment: 17 pages, 7 figures, 2 tables. Accepted for publication in ApJ

Context. The Milky Way nuclear star cluster (MWNSC) is a crucial laboratory for studying the galactic nuclei of other galaxies, but its properties have not been determined unambiguously until now.Aims. We aim to study the size and spatial structure of the MWNSC.Methods. This study uses data and methods that address potential shortcomings of previous studies on the topic. We use 0.2 '' angular resolution K-s data to create a stellar density map in the central 86.4 pc x 21 pc at the Galactic center. We include data from selected adaptive-optics-assisted images obtained for the inner parsecs. In addition, we use Spitzer/IRAC mid-infrared (MIR) images. We model the Galactic bulge and the nuclear stellar disk in order to subtract them from the MWNSC. Finally, we fit a Sersic model to the MWNSC and investigate its symmetry.Results. Our results are consistent with previous work. The MWNSC is flattened with an axis ratio of q=0.710.10, an effective radius of R-e=(5.1 +/- 1.0) pc, and a Sersic index of n=2.2 +/- 0.7. Its major axis may be tilted out of the Galactic plane by up to -10 degrees. The distribution of the giants brighter than the Red Clump (RC) is found to be significantly flatter than the distribution of the faint stars. We investigate the 3D structure of the central stellar cusp using our results on the MWNSC structure on large scales to constrain the deprojection of the measured stellar surface number density, obtaining a value of the 3D inner power law of gamma=1.38 +/- 0.06(sys)+/- 0.01(stat).Conclusions. The MWNSC shares its main properties with other extragalactic NSCs found in spiral galaxies. The differences in the structure between bright giants and RC stars might be related to the existence of not completely mixed populations of different ages. This may hint at recent growth of the MWNSC through star formation or cluster accretion.© ESO 2020, The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement n degrees[614922]. This work is based on observations made with ESO Telescopes at the La Silla Paranal Observatory under programmes IDs 195.B-0283 and 091.B-0418. We thank the staff of ESO for their great efforts and helpfulness. N.N. acknowledges support by the Sonderforschungsbereich SFB 881 "The Milky Way System" of the German Research Foundation (DFG). F.N.-L. acknowledges financial support from a MECD pre-doctoral contract, code FPU14/01700. We acknowledge support by the State Agency for Research of the Spanish MCIU through the "Center of Excellence Severo Ochoa" award for the Instituto de Astrofisica de Andalucia (CSIC) (SEV-2017-0709).

We present an analysis of Galactic bulge stars from Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) observations of the Stanek window (l,b=[0.25,-2.15]) from two epochs approximately two years apart. This dataset is adjacent to the provisional Wide-field Infrared Survey Telescope (WFIRST) microlensing field. Proper motions are measured for approximately 115,000 stars down to 28th mag in V band and 25th mag in I band, with accuracies of 0.5 mas yr$^{-1}$ (20 km s$^{-1}$) at I $\approx$ 21. A cut on the longitudinal proper motion $\mu_l$ allows us to separate disk and bulge populations and produce bulge-only star counts that are corrected for photometric completeness and efficiency of the proper-motion cut. The kinematic dispersions and surface density in the field are compared to the nearby SWEEPS sight-line, finding a marginally larger than expected gradient in stellar density. The observed bulge star counts and kinematics are further compared to the Besan\c{c}on, Galaxia, and GalMod Galactic population synthesis models. We find that most of the models underpredict low-mass bulge stars by $\sim$33% below the main-sequence turnoff, and upwards of $\sim$70% at redder J and H wavebands. While considering inaccuracies in the Galactic models, we give implications for the exoplanet yield from the WFIRST microlensing mission., Comment: 18 pages, 10 figures. Published in ApJ

Context. Dynamically self-consistent galactic models are necessary for analysing and interpreting star counts, stellar density distributions, and stellar kinematics in order to understand the formation and the evolution of our Galaxy. Aims. We modify and improve the dynamical self-consistency of the Besan\c{c}on Galaxy model in the case of a stationary and axisymmetric gravitational potential. Methods. Each stellar orbit is modelled by determining a Staeckel approximate integral of motion. Generalised Shu distribution functions with three integrals of motion are used to model the stellar distribution functions. Results. This new version of the Besan\c{c}on model is compared with the previous axisymmetric BGM2014 version and we find that the two versions have similar densities for each stellar component. The dynamically self-consistency is improved and can be tested by recovering the forces and the potential through the Jeans equations applied to each stellar distribution function. Forces are recovered with an accuracy better than one per cent over most of the volume of the Galaxy., Comment: 8 pages, 6 figures Accepted for publication in Astronomy and Astrophysics

It is well established that roughly half of all nearby solar-type stars have at least one companion. Stellar companions can have significant implications for the detection and characterization of exoplanets, including triggering false positives and masking the true radii of planets. Determining the fraction of exoplanet host stars that are also binaries allows us to better determine planetary characteristics as well as establish the relationship between binarity and planet formation. Using high angular resolution speckle imaging, we detect stellar companions within ~1 arcsec of K2 planet candidate host stars. Comparing our detected companion rate to TRILEGAL star count simulations and known detection limits of speckle imaging we estimate the binary fraction of K2 planet host stars to be 40 - 50%, similar to that of Kepler exoplanet hosts and field stars., Accepted for publication in The Astronomical Journal. 18 pages, 9 figures, 3 tables

CONTEXT. The maximum size of the Galactic stellar disk is not yet known. Some studies have suggested an abrupt drop-off of the stellar density of the disk at Galactocentric distances $R\gtrsim 15$ kpc, which means that in practice no disk stars or only very few of them should be found beyond this limit. However, stars in the Milky Way plane are detected at larger distances. In addition to the halo component, star counts have placed the end of the disk beyond 20 kpc, although this has not been spectroscopically confirmed so far. AIMS. Here, we aim to spectroscopically confirm the presence of the disk stars up to much larger distances. METHODS. With data from the LAMOST and SDSS-APOGEE spectroscopic surveys, we statistically derived the maximum distance at which the metallicity distribution of stars in the Galactic plane is distinct from that of the halo populations. RESULTS. Our analysis reveals the presence of disk stars at R>26 kpc (99.7% C.L.) and even at R>31 kpc (95.4% C.L.)., 4 pages, accepted to be published in A&A-Letters

In the framework of the STREGA (STRucture and Evolution of the GAlaxy) survey, two fields around the globular cluster Pal 12 were observed with the aim of detecting the possible presence of streams and/or an extended halo. The adopted stellar tracers are the Main Sequence, Turn-off and Red Giant Branch stars. We discuss the lumi- nosity function and the star counts in the observed region covering about 2 tidal radii, confirming that Pal 12 appears to be embedded in the Sagittarius Stream. Adopting an original approach to separate cluster and field stars, we do not find any evidence of sig- nificant extra-tidal Pal 12 stellar populations. The presence of the Sagittarius stream seems to have mimicked a larger tidal radius in previous studies. Indeed, adopting a King model, a redetermination of this value gives r_T = 0.22 +- 0.1 deg., 13 pages, 17 figures, accepted for publication in MNRAS on 22 September 2017

Context. The distribution of stars around a massive black hole (MBH) has been addressed in stellar dynamics for the last four decades by a number of authors. Because of its proximity, the centre of the Milky Way is the only observational test case where the stellar distribution can be accurately tested. Past observational work indicated that the brightest giants in the Galactic centre (GC) may show a density deficit around the central black hole, not a cusp-like distribution, while we theoretically expect the presence of a stellar cusp. Aims. We here present a solution to this long-standing problem. Methods. We performed direct-summation N-body simulations of star clusters around massive black holes and compared the results of our simulations with new observational data of the GC's nuclear cluster. Results. We find that after a Hubble time, the distribution of bright stars as well as the diffuse light follow power-law distributions in projection with slopes of Γ ≈ 0.3 in our simulations. This is in excellent agreement with what is seen in star counts and in the distribution of the diffuse stellar light extracted from adaptive-optics (AO) assisted near-infrared observations of the GC. Conclusions. Our simulations also confirm that there exists a missing giant star population within a projected radius of a few arcsec around Sgr A∗. Such a depletion of giant stars in the innermost 0.1 pc could be explained by a previously present gaseous disc and collisions, which means that a stellar cusp would also be present at the innermost radii, but in the form of degenerate compact cores.© ESO, 2017., The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement no. [614922]. PAS acknowledges support from the Ramon y Cajal Programme of the Ministry of Economy, Industry and Competitiveness of Spain. This work has been partially supported by the CAS President's International Fellowship Initiative.

Context. The existence of dynamically relaxed stellar density cusps in dense clusters around massive black holes is a long-standing prediction of stellar dynamics, but it has so far escaped unambiguous observational confirmation. Aims. In this paper we aim to revisit the problem of inferring the innermost structure of the Milky Way's nuclear star cluster via star counts, to clarify whether it displays a core or a cusp around the central black hole. Methods. We used judiciously selected adaptive optics assisted high angular resolution images obtained with the NACO instrument at the ESO VLT. Through image stacking and improved point spread function fitting we pushed the completeness limit about one magnitude deeper than in previous, comparable work. Crowding and extinction corrections were derived and applied to the surface density estimates. Known young, and therefore dynamically not relaxed stars, are excluded from the analysis. Contrary to previous work, we analyse the stellar density in well-defined magnitude ranges in order to be able to constrain stellar masses and ages. Results. We focus on giant stars, with observed magnitudes K = 12.5-16, and on stars with observed magnitudes K ≈ 18, which may have similar mean ages and masses than the former. The giants display a core-like surface density profile within a projected radius R ≤ 0.3 pc of the central black hole, in agreement with previous studies, but their 3D density distribution is not inconsistent with a shallow cusp if we take into account the extent of the entire cluster, beyond the radius of influence of the central black hole. The surface density of the fainter stars can be described well by a single power-law at R < 2 pc. The cusp-like profile of the faint stars persists even if we take into account the possible contamination of stars in this brightness range by young pre-main sequence stars. The data are inconsistent with a core-profile for the faint stars. Finally, we show that a 3D Nuker law provides a good description of the cluster structure. Conclusions. We conclude that the observed density of the faintest stars detectable with reasonable completeness at the Galactic centre, is consistent with the existence of a stellar cusp around the Milky Way's central black hole, Sagittarius A∗. This cusp is well developed inside the influence radius of Sagittarius A∗and can be described by a single three-dimensional power-law with an exponent γ = 1.43 ± 0.02 ± 0.1. This corroborates existing conclusions from Nbody simulations performed in a companion paper. An important caveat is that the faint stars analysed here may be contaminated significantly by dynamically unrelaxed stars that formed about 100 Myr ago. The apparent lack of giants at projected distances of R ≲ 0.3 pc (R ≲ 8″) of the massive black hole may indicate that some mechanism may have altered their distribution or intrinsic luminosity. We roughly estimate the number of possibly missing giants to about 100.© ESO 2017., The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement No. [614922].PAS acknowledges support from the Ramon y Cajal Programme of the Ministry of Economy, Industry and Competitiveness of Spain. This work has been partially supported by the CAS President's International Fellowship Initiative.