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.

A B S T R A C TWe present contemporary optical and infrared spectroscopic observations of the type IIn SN 1998S covering the period between 3 and 127 days after discovery. During the first week the spectra are characterized by prominent broad H, He and C iiiaN iii emission lines with narrow peaks, superimposed on a very blue continuum T , 24 000 KX In the following two weeks the C iiiaN iii emission vanished, together with the broad emission components of the H and He lines. Broad, blueshifted absorption components appeared in the spectra. The temperature of the continuum also dropped to ,14 000 K. By the end of the first month the spectrum comprised broad, blueshifted absorptions in H, He, Si ii, Fe ii and Sc ii. By day 44, broad emission components in H and He reappeared in the spectra. These persisted to as late as days ,100±130Y becoming increasingly asymmetric. We agree with Leonard et al. that the broad emission lines indicate interaction between the ejecta and circumstellar material (CSM) emitted by the progenitor. We also agree that the progenitor of SN 1998S appears to have gone through at least two phases of mass-loss, giving rise to two CSM zones. Examination of the spectra indicates that the inner zone extended to #90 au, while the outer CSM extended from 185 au to over 1800 au.We also present high-resolution spectra obtained at days 17 and 36. These spectra exhibit narrow P Cygni H i and He i lines superimposed on shallower, broader absorption components. Narrow lines of [N ii], [O iii], [Ne iii] and [Fe iii] are also seen. We attribute the narrow lines to recombination and heating following ionization of the outer CSM shell by the UV/X-ray flash at shock breakout. Using these lines, we show that the outer CSM had a velocity of 40±50 km s 21 X Assuming a constant velocity, we can infer that the outer CSM wind commenced more than 170 years ago, and ceased about 20 years ago, while the inner CSM wind may have commenced less than 9 years ago. During the era of the outer CSM wind the outflow from the progenitor was high ± at least ,2 Â 10 25 M ( yr 21 X This corresponds to a mass-loss of at least ,0.003 M ( , suggesting a massive progenitor. The shallower, broader absorption is of width ,350 km s 21 , and may have arisen from a component of the outer CSM shell produced when the progenitor was going through a later q 2001 RAS

Optical photometry is presented for the quadruple gravitational lens PG1115+080. A preliminary reduction of data taken from November 1995 to June 1996 gives component "C" leading component "B" by 23.7 ± 3.4 days and -2components "A1" and "A2" by 9.4 days. A range of models has been fit to the image positions, none of which gives an adequate fit. The best fitting and most physically plausible of these, taking the lensing galaxy and the associated group of galaxies to be singular isothermal spheres, gives a Hubble constant of 42 km/s/Mpc for Ω = 1, with an observational uncertainty of 14%, as computed from the B − C time delay measurement. Taking the lensing galaxy to have an approximately E5 isothermal mass distribution yields H 0 = 64 km/sec/Mpc while taking the galaxy to be a point mass gives H 0 = 84 km/sec/Mpc. The former gives a particularly bad fit to the position of the lensing galaxy, while the latter is inconsistent with measurements of nearby galaxy rotation curves. Constraints on these and other possible models are expected to improve with planned HST observations.

We present extensive optical spectroscopy of the early-type magnetic star HD 191612 (O6.5f?pe-O8fp). The Balmer and He I lines show strongly variable emission which is highly reproducible on a well-determined 538-d period. He II absorptions and metal lines (including many selective emission lines but excluding He II λ4686 Å emission) are essentially constant in line strength, but are variable in velocity, establishing a double-lined binary orbit with P orb = 1542 d, e = 0.45. We conduct a model-atmosphere analysis of the spectrum, and find that the system is consistent with a ∼O8 giant with a ∼B1 main-sequence secondary. Since the periodic 538-d changes are unrelated to orbital motion, rotational modulation of a magnetically constrained plasma is strongly favoured as the most likely underlying 'clock'. An upper limit on the equatorial rotation is consistent with this hypothesis, but is too weak to provide a strong constraint.

HD 140283 is an extremely metal-deficient and high-velocity subgiant in the solar neighborhood, having a location in the Hertzsprung-Russell diagram where absolute magnitude is most sensitive to stellar age. Because it is bright, nearby, unreddened, and has a well-determined chemical composition, this star avoids most of the issues involved in age determinations for globular clusters. Using the Fine Guidance Sensors on the Hubble Space Telescope, we have measured a trigonometric parallax of 17.15 ± 0.14 mas for HD 140283, with an error one-fifth of that determined by the Hipparcos mission. Employing modern theoretical isochrones, which include effects of helium diffusion, revised nuclear reaction rates, and enhanced oxygen abundance, we use the precise distance to infer an age of 14.46 ± 0.31 Gyr. The quoted error includes only the uncertainty in the parallax, and is for adopted surface oxygen and iron abundances of [O/H] = −1.67 and [Fe/H] = −2.40. Uncertainties in the stellar parameters and chemical composition, especially the oxygen content, now contribute more to the error budget for the age of HD 140283 than does its distance, increasing the total uncertainty to about ±0.8 Gyr. Within the errors, the age of HD 140283 does not conflict with the age of the Universe, 13.77 ± 0.06 Gyr, based on the microwave background and Hubble constant, but it must have formed soon after the big bang.