Final published version including significant revisions. Twenty four pages, fourteen figures. Original version April 2006; final version published in MNRAS August 2007We describe the goals, design, implementation, and initial progress of the UKIRT Infrared Deep Sky Survey (UKIDSS), a seven year sky survey which began in May 2005, using the UKIRT Wide Field Camera. It is a portfolio of five survey components covering various combinations of the filter set ZYJHK and H_2. The Large Area Survey, the Galactic Clusters Survey, and the Galactic Plane Survey cover approximately 7000 square degrees to a depth of K~18; the Deep Extragalactic Survey covers 35 square degrees to K~21, and the Ultra Deep Survey covers 0.77 square degrees to K~23. Summed together UKIDSS is 12 times larger in effective volume than the 2MASS survey. The prime aim of UKIDSS is to provide a long term astronomical legacy database; the design is however driven by a series of specific goals -- for example to find the nearest and faintest sub-stellar objects; to discover Population II brown dwarfs, if they exist; to determine the substellar mass function; to break the z=7 quasar barrier; to determine the epoch of re-ionisation; to measure the growth of structure from z=3 to the present day; to determine the epoch of spheroid formation; and to map the Milky Way through the dust, to several kpc. The survey data are being uniformly processed, and released in stages through the WFCAM Science Archive (WSA : http://surveys.roe.ac.uk/wsa). Before the formal survey began, UKIRT and the UKIDSS consortium collaborated in obtaining and analysing a series of small science verification (SV) projects to complete the commissioning of the camera. We show some results from these SV projects in order to demonstrate the likely power of the eventual complete survey. Finally, using the data from the First Data Release we assess how well UKIDSS is meeting its design targets so far

We surveyed all stars in Taurus (3h 45m < α < 4h 15m, 15° < δ < 35°) for multiplicity which are contained in the Herbig-Bell catalogue of young stars and have a 2 micron brightness of K ≤ 9.5 mag. This sample consists of 106 stellar systems (single or multiple), of which 43 are double or multiple according to the criteria of our survey, i.e. with separations of ≈0″.2 ≤ d ≤ 10″. Of these, 23 binaries are new detections found in this survey. The resulting degree of multiplicity, 43/106 = 41±6%, is higher than found for main-sequence stars. Provided that the period distribution is the same for young stars as on the main sequence, our result implies that the vast majority of stars are born in binary or multiple systems.

We present an IR lunar occultation and direct imaging search for companions in the Ophiuchus starforming region and update a similar search of the Taurus region. The search is sensitive to companions in the angular separation range 0.005-10". In Ophiuchus we surveyed 35 young star targets; this sample contains at least 10 binaries, two triples, and one quadruple. Ten of the companion stars are newly discovered. In Taurus, the survey now includes 47 systems among which there are at least 22 binaries and four triples. Only two companion stars are newly identified because there is strong overlap with prior work. All the triples and quadruple are hierarchical. The observed binary frequency in Ophiuchus, in the 3-1400 AU range of separations, is at least 1.1 ± 0.3 that of the nearby solar-like stars. This value is a lower bound because we make no corrections for incompleteness. In Taurus, in the same range of separations, the observed binary frequency is at least 1.6 ± 0.3 that of the nearby solar-like stars. This value extends Ghez et al.'s (1993) and Leinert's et al.'s (1993) determination of an excess binary frequency to 3 AU separation. We used the WT/TT type and the K-L color index to distinguish between systems with and without inner disks. We find no convincing difference in the binary frequency or distribution of separations of the systems with and without inner disks. The 1.3 mm continuum emission of the single systems exceeds that of the multiples suggesting that their extensive outer disks are more massive. The specific angular momenta of the binaries overlap those of molecular cloud cores measured by Goodman et al. (1993). Subject headings: binaries: visual-occultations-open clusters and associations: generalstars: pre-main-sequence a coauthor. David Allen died on 1994 July 26. We dedicate the paper to his memory.

We present near‐infrared (1.15–2.50 μm) medium‐resolution (R= 1700) spectroscopy of a sample of 23 brown dwarf candidates in the young Upper Sco association. We confirm membership of 21 brown dwarfs based on their spectral shape, comparison with field dwarfs and presence of weak gravity‐sensitive features. Their spectral types range from M8 to L2 with an uncertainty of a subclass, suggesting effective temperatures between 2700 and 1800 K with an uncertainty up to 300 K and masses in the 30–8 MJup range. Among the non‐members, we have uncovered a field L2 dwarf at a distance of 120–140 pc, assuming that it is single. The success rate of our photometric selection based on five photometric passbands and complemented partly by proper motion is over 90 per cent, a very promising result for future studies of the low‐mass star and brown dwarf populations in young open clusters by the UKIDSS [UKIRT (United Kingdom Infrared Telescope) Infrared Deep Sky Survey] Galactic Cluster Survey. We observe a large dispersion in the magnitude versus spectral‐type relation which is likely the result of the combination of several effects including age dispersion, extent and depth of the association, a high degree of multiplicity and the occurrence of discs.

Accepted by MNRAS. 30 pages, 15 figures (some at reduced resolution due to upload restrictions - full res version at http://surveys.roe.ac.uk/wsa/pubs.html)This paper defines the UKIRT Infrared Deep Sky Survey (UKIDSS) Early Data Release (EDR). UKIDSS is a set of five large near-infra-red surveys defined by Lawrence et al. (2006), being undertaken with the UK Infra-red Telescope (UKIRT) Wide Field Camera (WFCAM). The programme began in May 2005 and has an expected duration of seven years. Each survey uses some or all of the broadband filter complement ZYJHK. The EDR is the first public release of data to the European Southern Observatory (ESO) community. All worldwide releases occur after a delay of 18 months from the ESO release. The EDR provides a small sample dataset, ~50 sq.deg (about 1% of the whole of UKIDSS), that is a lower limit to the expected quality of future survey data releases. In addition, an EDR+ dataset contains all EDR data plus extra data of similar quality, but for areas not observed in all of the required filters (amounting to ~220 sq.deg). The first large data release, DR1, will occur in mid-2006. We provide details of the observational implementation, the data reduction, the astrometric and photometric calibration, and the quality control procedures. We summarise the data coverage and quality (seeing, ellipticity, photometricity, depth) for each survey and give a brief guide to accessing the images and catalogues from the WFCAM Science Archive