The DESI Legacy Imaging Surveys (http://legacysurvey.org/) are a combination of three public projects (the Dark Energy Camera Legacy Survey, the Beijing-Arizona Sky Survey, and the Mayall z-band Legacy Survey) that will jointly image ≈14,000 deg 2 of the extragalactic sky visible from the northern hemisphere in three optical bands (g, r, and z) using telescopes at the Kitt Peak National Observatory and the Cerro Tololo Inter-American Observatory. The combined survey footprint is split into two contiguous areas by the Galactic plane. The optical imaging is conducted using a unique strategy of dynamically adjusting the exposure times and pointing selection during observing that results in a survey of nearly uniform depth. In addition to calibrated images, the project is delivering a catalog, constructed by using a probabilistic inference-based approach to estimate source shapes and brightnesses. The catalog includes photometry from the grz optical bands and from four mid-infrared bands (at 3.4, 4.6, 12, and 22 μm) observed by the Wide-field Infrared Survey Explorer satellite during its full operational lifetime. The project plans two public data releases each year. All the software used to generate the catalogs is also released with the data. This paper provides an overview of the Legacy Surveys project.

The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.

We present a new calibration of the Stroemgren metallicity index m 1 using red giant (RG) stars in a sample of Galactic globular clusters (GGCs: M92, M13, NGC 1851, 47 Tuc) that cover a broad range in metallicity (−2.2 ≤ [Fe/H] ≤ −0.7), are marginally affected by reddening uncertainties (E(B-V ) ≤ 0.04) and for which accurate u, v, b, y Stroemgren photometry is available to well below 1 Based in part on observations collected with the 1.54m Danish telescope operated at ESO (La Silla) and with the Nordic Optical Telescope (NOT) operated at La Palma (Spain).-2the turnoff region. The main difference between the new empirical metallicityindex-color (MIC) relations and similar relations available in the literature is that we have adopted the u-y and v-y colors instead of the b-y color. These colors present a stronger sensitivity to effective temperature, and the MIC relations show a linear and well-defined slope. The net difference between photometric estimates and spectroscopic measurements, for RG stars in five GGCs: M71, NGC 288, NGC 362, NGC 6397, NGC 6752, is 0.04 ± 0.03 dex with σ = 0.11 dex. We also apply the new MIC relations to a sample of field stars for which spectroscopic metallicity (−2.4 ≤ [Fe/H] ≤ −0.5), accurate Strömgren photometry, and reddening estimates (Anthony-Twarog & Twarog 1994 are all available. We find that the difference between photometric estimates and spectroscopic measurements is on average −0.14 ± 0.01 dex, with σ = 0.17 dex. We also provide two independent sets of MIC relations based on evolutionary models that have been transformed into the observational plane by adopting either semi-empirical or theoretical color-temperature relations (CTRs). We apply the semi-empirical α−enhanced MIC relations to the nine GCs and find that the difference between photometric estimates and spectroscopic measurements is 0.04 ± 0.03 dex, with σ = 0.10 dex. A similar agreement is also found for the sample of field stars, and indeed the difference is −0.09 ± 0.03 dex, with σ = 0.19 dex. The difference between metallicity estimates based on theoretical scaled-solar and spectroscopic measurements −0.11±0.03 dex, with σ = 0.14 dex for the nine GGCs and −0.24 ± 0.03 dex, with σ = 0.15 dex for the field stars. On the whole, current findings support the evidence that new Strömgren MIC relations provide metallicity estimates with an intrinsic accuracy better than 0.2 dex.

Context. UV observations of some massive globular clusters have revealed a significant population of stars hotter and fainter than the hot end of the horizontal branch (HB), the so-called blue hook stars. This feature might be explained either by the late hot flasher scenario where stars experience the helium flash while on the white dwarf cooling curve or by the progeny of the helium-enriched sub-population postulated to exist in some clusters. Previous spectroscopic analyses of blue hook stars in ω Cen and NGC 2808 support the late hot flasher scenario, but the stars contain much less helium than expected and the predicted C and N enrichment cannot be verified. Aims. We compare the observed effective temperatures, surface gravities, helium abundances, and carbon line strengths (where detectable) of our targets stars with the predictions of the two aforementioned scenarios. Methods. Moderately high resolution spectra of hot HB stars in the globular cluster ω Cen were analysed for radial velocity variations, atmospheric parameters, and abundances using LTE and non-LTE model atmospheres. Results. We find no evidence of close binaries among our target stars. All stars below 30 000 K are helium-poor and very similar to HB stars observed in that temperature range in other globular clusters. In the temperature range 30 000 K to 50 000 K, we find that 28% of our stars are helium-poor (log n He n H < −1.6), while 72% have roughly solar or super-solar helium abundance (log n He n H ≥ −1.5). We also find that carbon enrichment is strongly correlated with helium enrichment, with a maximum carbon enrichment of 3% by mass. Conclusions. A strong carbon enrichment in tandem with helium enrichment is predicted by the late hot flasher scenario, but not by the helium-enrichment scenario. We conclude that the helium-rich HB stars in ω Cen cannot be explained solely by the heliumenrichment scenario invoked to explain the blue main sequence.

We report the first unambiguous detection and mass measurement of an isolated stellar-mass black hole (BH). We used the Hubble Space Telescope (HST) to carry out precise astrometry of the source star of the long-duration (t E ≃ 270 days), high-magnification microlensing event MOA-2011-BLG-191/OGLE-2011-BLG-0462 (hereafter designated as MOA-11-191/OGLE-11-462), in the direction of the Galactic bulge. HST imaging, conducted at eight epochs over an interval of 6 yr, reveals a clear relativistic astrometric deflection of the background star’s apparent position. Ground-based photometry of MOA-11-191/OGLE-11-462 shows a parallactic signature of the effect of Earth’s motion on the microlensing light curve. Combining the HST astrometry with the ground-based light curve and the derived parallax, we obtain a lens mass of 7.1 ± 1.3 M ⊙ and a distance of 1.58 ± 0.18 kpc. We show that the lens emits no detectable light, which, along with having a mass higher than is possible for a white dwarf or neutron star, confirms its BH nature. Our analysis also provides an absolute proper motion for the BH. The proper motion is offset from the mean motion of Galactic disk stars at similar distances by an amount corresponding to a transverse space velocity of ∼45 km s−1, suggesting that the BH received a “natal kick” from its supernova explosion. Previous mass determinations for stellar-mass BHs have come from radial velocity measurements of Galactic X-ray binaries and from gravitational radiation emitted by merging BHs in binary systems in external galaxies. Our mass measurement is the first for an isolated stellar-mass BH using any technique.

New accurate and homogeneous optical UBVRI photometry has been obtained for variable stars in the Galactic globular cluster ω Cen (NGC 5139). We secured 8202 CCD images covering a time interval of 24 years and a sky area of 84×48 arcmin. The current data were complemented with data available in the literature and provided new, homogeneous pulsation parameters (mean magnitudes, luminosity amplitudes, periods) for 187 candidate ω Cen RR Lyrae (RRLs). Among them we have 101RRc (first overtone) and 85RRab (fundamental) variables, and a single candidate RRd (double-mode) variable. Candidate Blazhko RRLs show periods and colors that are intermediate between the RRc and RRab variables, suggesting that they are transitional objects. A comparison of the period distribution and the Bailey diagram indicates that RRLs in ω Cen show a long-period tail not present in typical Oosterhoff II (OoII) globulars. The RRLs in dwarf spheroidals and in ultra-faint dwarfs have properties between Oosterhoff intermediate and OoII clusters. Metallicity plays a key role in shaping the above evidence. These findings do not support the hypothesis that ω Cen is the core remnant of a spoiled dwarf galaxy. Using optical period-Wesenheit relations that are reddening-free and minimally dependent on metallicity we find a mean distance to ω Cen of 13.71±0.08±0.01 mag (semi-empirical and theoretical calibrations). Finally, we invert the I-band period-luminosity-metallicity relation to estimate individual RRLs' metal abundances. The metallicity distribution agrees quite well with spectroscopic and photometric metallicity estimates available in the literature.

Gravitational deflection of starlight around the Sun during the 1919 total solar eclipse provided measurements that confirmed Einstein's general theory of relativity. We have used the Hubble Space Telescope to measure the analogous process of astrometric microlensing caused by a nearby star, the white dwarf Stein 2051 B. As Stein 2051 B passed closely in front of a background star, the background star's position was deflected. Measurement of this deflection at multiple epochs allowed us to determine the mass of Stein 2051 B-the sixth-nearest white dwarf to the Sun-as 0.675 ± 0.051 solar masses. This mass determination provides confirmation of the physics of degenerate matter and lends support to white dwarf evolutionary theory.

Despite the huge amount of photometric and spectroscopic efforts targetting the Galactic bulge over the past few years, its age distribution remains controversial owing to both the complexity of determining the age of individual stars and the difficult observing conditions. Taking advantage of the recent release of very deep, propermotion-cleaned colour-magnitude diagrams (CMDs) of four low reddening windows obtained with the Hubble Space Telescope (HST), we used the CMD-fitting technique to calculate the star formation history (SFH) of the bulge at −2 • > b > −4 • along the minor axis. We find that over 80 percent of the stars formed before 8 Gyr ago, but that a significant fraction of the super-solar metallicity stars are younger than this age. Considering only the stars that are within reach of the current generation of spectrographs (i.e. V < ∼ 21), we find that 10 percent of the bulge stars are younger than 5 Gyr, while this fraction rises to 20-25 percent in the metal-rich peak. The agemetallicity relation is well parametrized by a linear fit implying an enrichment rate of dZ/dt ∼ 0.005 Gyr −1 . Our metallicity distribution function accurately reproduces that observed by several spectroscopic surveys of Baade's window, with the bulk of stars having metal-content in the range [Fe/H]∼ −0.7 to ∼0.6, along with a sparse tail to much lower metallicities.