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The UKIDSS Galactic Plane Survey (GPS) is one of the five near-infrared Public Legacy Surveys that are being undertaken by the UKIDSS consortium, using the Wide Field Camera on the United Kingdom Infrared Telescope. It is surveying 1868 deg2 of the northern and equatorial Galactic plane at Galactic latitudes -5° < b < 5° in the J, H and K filters and a ?200-deg2 area of the Taurus-Auriga-Perseus molecular cloud complex in these three filters and the 2.12 ?m (1-0) H2 filter. It will provide data on ?2 × 109 sources. Here we describe the properties of the data set and provide a user's guide for its exploitation. We also present brief Demonstration Science results from DR2 and from the Science Verification programme. These results illustrate how GPS data will frequently be combined with data taken in other wavebands to produce scientific results. The Demonstration Science comprises six studies. (1) A GPS-Spitzer-GLIMPSE cross-match for the star formation region G28.983-0.603 to identify YSOs. This increases the number of YSOs identified by a factor of 10 compared to GLIMPSE alone. (2) A wide-field study of the M17 nebula, in which an extinction map of the field is presented and the effect of source confusion on luminosity functions in different subregions is noted. (3) H2 emission in the ? Ophiuchi dark cloud. All the molecular jets are traced back to a single active clump containing only a few protostars, which suggests that the duration of strong jet activity and associated rapid accretion in low-mass protostars is brief. (4) X-ray sources in the nuclear bulge. The GPS data distinguishes local main-sequence counterparts with soft X-ray spectra from nuclear bulge giant counterparts with hard X-ray spectra. (5) External galaxies in the zone of avoidance. The galaxies are clearly distinguished from stars in fields at longitudes l > 90°. (6) IPHAS-GPS optical-infrared spectrophotometric typing. The (i?- J) versus (J - H) diagram is used to distinguish A-F type dwarfs, G

We present wide-field near-infrared (IR) images of the DR21/W75 high-mass star-forming region, obtained with the Wide Field Camera (WFCAM) on the United Kingdom Infrared Telescope. Broad-band JHK and narrow-band H-2 1-0S(1) images are compared to archival mid-IR images from the Spitzer Space Telescope, and 850-mu m dust-continuum maps obtained with the Submillimeter Common User Bolometer Array (SCUBA). Together these data give a complete picture of dynamic star formation across this extensive region, which includes at least four separate star-forming sites in various stages of evolution. The H-2 data reveal knots and bow shocks associated with more than 50 individual flows. Most are well collimated, and at least five qualify as parsec-scale flows. Most appear to be driven by embedded, low-mass protostars. The orientations of the outflows, particularly from the few higher mass sources in the region (DR21, DR21(OH), W75N and ERO 1), show some degree of order, being preferentially orientated roughly orthogonal to the chain of dusty cores that runs north-south through DR21. Clustering may inhibit disc accretion and therefore the production of outflows; we certainly do not see enhanced outflow activity from clusters of protostars. Finally, although the low-mass protostellar outflows are abundant and widely distributed, the current generation does not provide sufficient momentum and kinetic energy to account for the observed turbulent motions in the DR21/W75 giant molecular clouds. Rather, multiple epochs of outflow activity are required over the million-year time-scale for turbulent decay.

We present an in-depth analysis of molecular excitation in 11 H 2-bright planetary and protoplanetary nebulae (PN and PPN). From newly acquired K-band observations, we extract a number of spectra at positions across each source. H2 line intensities are plotted on 'column density ratio' diagrams so that we may examine the excitation in and across each region. To achieve this, we combine the shock models of Smith, Khanzadyan & Davis with the photodissociation region (PDR) models of Black & van Dishoeck to yield a shock-plus-fluorescence fit to each data set. Although the combined shock + fluorescence model is needed to explain the low- and high-energy H2 lines in most of the sources observed (fluorescence accounts for much of the emission from the higher-energy H2 lines), the relative importance of shocks over fluorescence does seem to change with evolutionary status. We find that shock excitation may well be the dominant excitation mechanism in the least evolved PPN (CRL 2688 - in both the bipolar lobes and in the equatorial plane) and in the most evolved PN considered (NGC 7048). Fluorescence, on the other hand, becomes more important at intermediate evolutionary stages (i.e. in 'young' PN), particularly in the inner core regions and along the inner edges of the expanding post- asymptotic giant branch (AGB) envelope. Since H2 line emission seems to be produced in almost all stages of post-AGB evolution, H2 excitation may prove to be a useful probe of the evolutionary status of PPN and PN alike. Moreover, shocks may play an important role in the molecular gas excitation in (P)PN, in addition to the low- and/or high-density fluorescence usually attributed to the excitation in these sources.

We present near-infrared observations of the protostellar outflow HH 7-11 in the molecular hydrogen 1-0 S(1) and [Fe II] 1.644-μm emission lines. Images at four epochs spread over 11 yr constrain the proper motions of the H2 emission knots to be less than 0.045 arcsec yr-1, corresponding to tangential speeds of under 47 km s-1 at 220-pc distance, consistent with the proper motions derived in the optical range for the atomic component. Reanalysis of previous observations shows that variability and distortion of the SVS 13 reference star led to erroneous high H2 proper motions. The SVS 13 K-band flux is a factor of 3 higher in 1994-2000 than in 1989. Numerous H2 subknots are identified on images taken in 1998 and 2000. No significant relative variability is found in these subknots, consistent with slow outflow evolution. Compact [Fe II] emission peaks are found at the locations of atomic emission-line features observed in the optical range. We also identify in this line an elongated feature corresponding to the proposed jet Mach disc.

We perform a detailed shock diagnosis of the Herbig-Haro object HH7, a well-defined bow shock from a protostellar outflow. We first present molecular hydrogen images in the 2-1 S(1) and 1-0 S(1) A-band emission lines. We then introduce revised models for magneto-hydrodynamic bow shocks that incorporate a limited C and O chemistry and account for the shock thickness. We employ these models to interpret the new images as well as ISO data, the line profile, H 2 position-velocity diagram, optical images and the proper motion. This yields a C-shock model that satisfies the constraints, confirming that ambipolar diffusion is the linchpin in the shock physics. The best model is a slow-moving paraboloidal bow of speed 55 km s-1, with a pre-shock density of 8 × 103 cm-3 and an H2/H number ratio of just 0.25. The bow moves at an angle of ?30° to the line of sight and at a position angle of ?95° in the plane of the sky rather than along the outflow axis of ?123°. The bow model also predicts the observed low line emission from H2O, without the need for gas-phase depletion. Predictions for imaging and spectroscopy at far-infrared wavelengths, employing the 63-?m [OI] line, are presented.

We demonstrate that a wide range of molecular hydrogen excitation can be observed in protostellar outflows at wavelengths in excess of 5 ?m. Cold H2 in DR 21 is detected through the pure rotational transitions in the ground vibrational level (0-0). Hot H2 is detected in pure rotational transitions within higher vibrational levels (1-1, 1-2, etc.). Although this emission is relatively weak, we have detected two 1-1 lines in the DR 21 outflow with the ISO SWS instrument. We thus investigate molecular excitation over energy levels corresponding to the temperature range 1015-15 722 K, without the uncertainty introduced by differential extinction when employing near-infrared data. This gas is thermally excited. We uncover a rather low H2 excitation in the DR 21 West Peak. The line emission cannot be produced from single C-shocks or J-shocks; a range of shock strengths is required. This suggests that bow shocks and/or bow-generated supersonic turbulence is responsible. We are able to distinguish this shock-excited gas from the fluoresced gas detected in the K band, providing support for the dual-excitation model of Fernandes, Brand & Burton.

Near-infrared images in H2 line emission and submillimetre maps in CO J = 3-2 emission illustrate the remarkable association between a molecular bow shock and the redshifted molecular outflow lobe in W75N. The flow lobe fits perfectly into the wake of the bow, as one would expect if the lobe represented swept-up gas. Indeed, these observations strongly support the 'bow shock' entrainment scenario for molecular outflows driven by young stars. The characteristics of the bow shock and CO outflow lobe are compared with those of numerical simulations of jet-driven flows. These models successfully reproduce the bulge and limb-brightening in the CO outflow, although the model H2 bow exhibits more structure extending back along the flow axis. We also find that the size of the flow, the high mass fraction in the flow at low outflow velocities (low ? values) and the high CO/H2 luminosity ratio indicate that the system is evolved. We also predict a correlation, in evolved systems, between outflow age and the CO/H2 luminosity ratio.