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.

Spitzer Space Observatory IRAC and MIPS photometric observations are presented for 20 white dwarfs with T eff 20, 000 K and metal-contaminated photospheres. A warm circumstellar disk is detected at GD 16 and likely at PG 1457−086, while the remaining targets fail to reveal mid-infrared excess typical of dust disks, including a number of heavily polluted stars. Extending previous studies, over 50% of all single white dwarfs with implied metal accretion rates dM/dt 3 × 10 8 g s −1 display a warm infrared excess from orbiting dust; the likely result of a tidally-destroyed minor planet. This benchmark accretion rate lies between the dust production rates of 10 6 g s −1 in the solar system zodiacal cloud and 10 10 g s −1 often inferred for debris disks at main sequence A-type stars. It is estimated that between 1% and 3% of all single white dwarfs with cooling ages less than around 0.5 Gyr possess circumstellar dust, signifying an underlying population of minor planets.

Context. Heavy metals in the atmospheres of white dwarfs are thought in many cases to be accreted from a circumstellar debris disk, which was formed by the tidal disruption of a rocky planetary body within the Roche radius of the star. The abundance analysis of photospheric elements and conclusions about the chemical composition of the accreted matter are a new and promising method of studying the composition of extrasolar planetary systems. However, ground-based searches for metal-polluted white dwarfs that rely primarily on the detection of the Ca ii K line become insensitive at T eff > 15 000 K because this ionization state depopulates.Aims. We present the results of the first unbiased survey for metal pollution among hydrogen-atmosphere (DA type) white dwarfs with cooling ages in the range 20-200 Myr and 17 000 K < T eff < 27 000 K. Methods. The sample was observed with the Cosmic Origins Spectrograph on board the Hubble Space Telescope in the far ultraviolet range between 1130 and 1435 Å. The atmospheric parameters were obtained using these spectra and optical observations from the literature. Element abundances were determined using theoretical models, which include the effects of element stratification due to gravitational settling and radiative levitation. Results. We find 48 of the 85 DA white dwarfs studied, or 56% show traces of heavy elements. In 25 stars (showing only Si and occasionally C), the elements can be explained by radiative levitation alone, although we argue that accretion has very likely occurred recently. The remaining 23 white dwarfs (27%), however, must be currently accreting. Together with previous studies from the ground and adopting bulk Earth abundances for the debris, accretion rates range from a few 10 5 g s −1 to a few 10 8 g s −1 , with no evident trend in cooling age from ≈40 Myr to ≈2 Gyr. Only a single, modest case of metal pollution (Ṁ < 10 6 g s −1 ) is found among ten white dwarfs with T eff > 23 000 K, in excellent agreement with the absence of infrared excess from dust around these warmer stars. The median, main sequence progenitor of our sample corresponds to an A-type star of ≈2 M , and we find 13 of 23 white dwarfs descending from main sequence 2-3 M , late B-and A-type stars to be currently accreting. Only one of 14 targets with M wd > 0.8 M is found to be currently accreting, which suggests a large fraction of these stars result from double-degenerate mergers, and the merger disks do not commonly reform large planetesimals or otherwise pollute the remnant. We reconfirm our previous finding that two 625 Myr Hyades white dwarfs are currently accreting rocky planetary debris. Conclusions. At least 27% of all white dwarfs with cooling ages 20-200 Myr are accreting planetary debris, but that fraction could be as high as ≈50%. At T eff > 23 000 K, the luminosity of white dwarfs is probably sufficient to vaporize circumstellar dust grains, so no stars with strong metal-pollution are found. Planetesimal disruption events should occur in this cooling age and temper...