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We describe the design, development, and performance of HAWK-I, the new High-Acuity Wide-field K-band Imager for ESO's Very Large Telescope, which is equipped with a mosaic of four 2 k $\times$2 k arrays and operates from 0.9-2.4 μm over $7.5\arcmin \times 7.5\arcmin$with 0.1´´ pixels. A novel feature is the use of all reflective optics that, together with filters of excellent throughput and detectors of high quantum efficiency, has yielded an extremely high throughput. Commissioning and science verification observations have already delivered a variety of excellent and deep images that demonstrate its high scientific potential for addressing important astrophysical questions of current interest.

Context.The infrared wide-field camera (WFCAM) is now in operation on the 3.8 m UK Infrared Telescope on Mauna Kea. WFCAM currently has the fastest survey speed of any infrared camera in the world, and combined with generous allocations of telescope time, will produce deep maps of the sky from Zto Kband. The data from a set of public surveys, known as UKIDSS, will be initially available to astronomers in ESO member states, and later to the world.

The results of a deep mid-IR ISOCAM survey of the L1551dark molecular cloud are presented. The aim of this survey is a search for new YSO (Young Stellar Object) candidates, using two broad-band filters centred at 6.7 and 14.3 μm. Although two regions close to the centre of L1551had to be avoided due to saturation problems, 96 sources were detected in total (76 sources at 6.7 μm and 44 sources at 14.3 μm). Using the 24 sources detected in both filters, 14 were found to have intrinsic mid-IR excess at 14.3 μm and were therefore classified as YSO candidates. Using additional observations in B, V, I, J, Hand Kobtained from the ground, most candidates detected at these wavelengths were confirmed to have mid-IR excess at 6.7 μm as well, and three additional YSO candidates were found. Prior to this survey only three YSOs were known in the observed region (avoiding L1551IRS 5/NE and HL/XZ Tau). This survey reveals 15 new YSO candidates, although several of these are uncertain due to their extended nature either in the mid-IR or in the optical/near-IR observations. Two of the sources with mid-IR excess are previously known YSOs, one is a brown dwarf (MHO 5) and the other is the well known T Tauri star HH 30, consisting of an outflow and an optically thick disk seen edge on.

We present ISOCAM observations (5–18 μm) of the Horsehead nebula, together with observations of the ($J=1{-}0$) and ($J=2{-}1$) transitions of 12CO, 13CO and C18O taken at the IRAM 30-m telescope. The Horsehead nebula presents a typical photodissociation region illuminated by the O9.5 V system σOri. The ISOCAM emission is due to very small particles transiently heated to high temperature each time they absorb a UV photon. A very sharp filament (width: ${\sim}10^{\prime\prime}$or ${\sim}0.02$pc) is detected by ISOCAM at the illuminated edge of the nebula. This filament is due to a combined effect of steep increase of the column density and extinction of incident radiation, on typical sizes below ${\sim}0.01$pc. Both the three-dimensional shape and the local density of the illuminated interface are strongly constrained. The dense material forming the edge of the Horsehead nebula appears illuminated edge-on by σOri, and the particles located beyond the border should not be affected by the incident radiation field. This structure may be due to dense filaments in the parental cloud which have shielded the material located in their shadow from the photo-dissociating radiations. The measurement of the penetration depth of the incident radiation from the infrared data (${\sim}0.01$pc) gives a density of a few 104cm-3just behind the bright filament. This value is comparable to the estimate of the density beyond the edge and deduced from our molecular observations, and also to the density behind the ionization front calculated in the stationary case. The material behind the illuminated edge could also be non-homogeneous, with clump sizes significantly smaller than the observed penetration depth of ${\sim}0.01$pc. In that case no upper limit on the average density just behind the illuminated edge can be given.

We present the results of a mid-infrared (5–16.5 μm) imaging spectroscopy survey of Young Stellar Objects (YSOs) and their surrounding environment in four low-mass star formation regions: RCrA, ρ Ophiuchi, Serpens and Chamaeleon I. This survey was performed using ISOCAM and its Circular Variable Filters (CVF) and observed 42 YSO candidates: we were able to obtain complete 5–16.5 μm spectra for 40 of these with a spectral resolving power of $\lambda/\Delta\lambda\simeq40$. A number of spectral features were measured, most notably the 9.7 μm silicate feature, the bending modes of both water and CO2ices at 6.0 and 15.2 μm respectively and the well-known unidentified feature at 6.8 μm. The strength of the unidentified feature was observed to correlate very well with that of the water ice bending mode and far less strongly with the CO2ice bending mode. This suggests, in a manner consistent with previous observations, that the carrier of the unidentified feature is a strongly polar ice. Absorption profiles of the bending mode of CO2ice are observed to show a significant long wavelength wing, which suggests that a significant fraction of the CO2ice observed exists in a polar (H2O-rich) phase. The sources observed in RCrA, ρ Oph and Serpens show similar spectral characteristics, whilst the sources observed in Cha I are somewhat anomalous, predominantly showing silicate emission and little or no absorption due to volatile ices. However this is consistent with previous studies of this region of the Cha I cloud, which contains an unusual cluster of YSOs. From comparisons of the strengths of the water ice and silicate bands we detect an apparent under-abundance of water ice towards the sources in ρ Oph, relative to both RCrA and Serpens. This may be indicative of differences in chemical composition between the different clouds, or may be due to evaporation. Finally the CO2:H2O ice ratios observed towards the sources in ρ Oph show significantly greater scatter than in the other regions, possibly due to varying local conditions around the YSOs in ρ Oph.

We present ISOCAM observations (5-18 μm) of the southern part of Orion B, including the reflection nebula NGC 2023 and the Horsehead nebula illuminated by the B star HD 37903 and the O star σOrionis, respectively. Due to the limited radiation field, the emission in these wavelengths is due to very small particles which are heated each time they absorb a UV photon. A filamentary structure is detected at small angular scales (down to the angular resolution of 6″) on top of a smoother background. The particular case of the Horsehead nebula suggests that the filaments in general result from the illuminated surfaces of dense structures, while the smoother background comes from lower density matters probably ionised. Striking spatial variations of the infrared colour (5-8.5 μm/12–18 μm) are also detected. Spectroscopic observations show that they are due to variations of the intensity of the aromatic features (especially at $7.7~\mu$m) relative to a continuum emission increasing in intensity towards longer wavelengths. The contribution of the continuum relative to the aromatic features appears significantly higher at the illuminated surfaces of dense structures than in lower density matter. This effect could be the signature of the evolution of the very small particles from shielded molecular material to photo-dissociated and photo-ionised matter. We also show that size segregation due to grain dynamics in uni-directional radiation fields may play a major role.

We report the discovery of an extremely red object (ERO) in a medium-deep ISOCAM extragalactic survey. The object is also a radio source. Subsequent VLT NIR spectroscopy revealed a prominent Hαline giving a redshift of 1.5. We present the spectrum and photometric data points and discuss evidence that ISO J1324-2016 is a quasar harbouring a significant amount of very hot dust.

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Context. Variability is a common characteristic of pre-main-sequence stars (PMS). Near-IR?variability surveys of young stellar objects (YSOs) can probe stellar and circumstellar environments and provide information about the dynamics of the ongoing magnetic and accretion processes. Furthermore, variability can be used as a tool to uncover new cluster members in star formation regions.Aims. We hope to achieve the deepest near-IR?variability study of YSOs targeting the ??Ophiuchi cluster.Methods. Fourteen epochs of observations were obtained with the Wide Field Camera (WFCAM) at the UKIRT telescope scheduled in a manner that allowed the study of variability on timescales of days, months, and years. Statistical tools, such as the multi-band cross correlation index and the reduced chi-square, were used to disentangle signals of variability from noise. Variability characteristics are compared to existing models of YSOs in order to relate them to physical processes, and then used to select new candidate members of this star-forming region. Results. Variability in the near-IR?is found to be present in 41% of the known population of ??Ophiuchi recovered in our sample. The behaviours shown are several and can be associated with the existence of spots on the stellar surface, variations in circumstellar extinction, or changes in the geometry of an accretion disc. Using variability, a new population of objects has been uncovered that is believed to be part of the ??Ophiuchi cluster.

The characteristics required of a Dome-C widefield IR imaging facility in order to compete with ground and space-based telescopes are discussed. Although the K-dark filter offers an opportunity for a 2 m-class facility to compete with much larger survey telescopes, a diameter approaching 8 m together with an ambitious focal plane are required if such a facility is to be competitive with near-term space facilities in the thermal IR.

The characteristics required of a Dome-C widefield IR imaging facility in order to compete with ground and space-based telescopes are discussed. Although the K-dark filter offers an opportunity for a 2?m-class facility to compete with much larger survey telescopes, a diameter approaching 8?m together with an ambitious focal plane are required if such a facility is to be competitive with near-term space facilities in the thermal IR. ?

We present results from an ISOCAM survey in the two broad band filters LW2 (5-8.5 $\mu$m) and LW3 (12-18 $\mu$m) of a 0.13 square degree coverage of the Serpens Main Cloud Core. A total of 392 sources were detected in the 6.7 $\mu$m band and 139 in the 14.3 $\mu$m band to a limiting sensitivity of ~2 mJy. We identified 53 Young Stellar Objects (YSOs) with mid-IR excess from the single colour index $[14.3/6.7]$, and 8 additional YSOs from the $H-K/K-m_{6.7}$ diagram. Only 32 of these 61 sources were previously known to be YSO candidates. Only about 50% of the mid-IR excess sources show excesses in the near-IR $J-H/H-K$ diagram. In the 48 square arcmin field covering the central Cloud Core the Class I/Class II number ratio is 19/18, i.e. about 10 times larger than in other young embedded clusters such as $\rho$ Ophiuchi or Chamaeleon. The mid-IR fluxes of the Class I and flat-spectrum sources are found to be on the average larger than those of Class II sources. Stellar luminosities are estimated for the Class II sample, and its luminosity function is compatible with a coeval population of about 2 Myr which follows a three segment power-law IMF. For this age about 20% of the Class IIs are found to be young brown dwarf candidates. The YSOs are in general strongly clustered, the Class I sources more than the Class II sources, and there is an indication of sub-clustering. The sub-clustering of the protostar candidates has a spatial scale of 0.12 pc. These sub-clusters are found along the NW-SE oriented ridge and in very good agreement with the location of dense cores traced by millimeter data. The smallest clustering scale for the Class II sources is about 0.25 pc, similar to what was found for $\rho$ Ophiuchi. Our data show evidence that star formation in Serpens has proceeded in several phases, and that a ”microburst” of star formation has taken place very recently, probably within the last 10$^5$ yrs.

We report the detection of extended infrared emission 5.2 kpc above the plane of the edge‐on spiral galaxy NGC 5907 in the JKbands. This provides confirmation at two other wavelengths of the R‐band result of Sackett et al. The halo J − Kcolour is found to be 1.3 ± 0.3. If we make the assumption that the halo emission is of the form necessary to give a flat rotation curve at large radii, with a power‐law index in projection of −1.0, the Ksurface brightness, M/LKratio and R − Kcolour are found to be 23.0 mag arcsec−2, 100 and 3.5 respectively, at 95‐arcsec radius. Adopting a halo index of −2.5 in projection, typical of known stellar haloes but inconsistent with the halo measured by Sackett et al., the K surface brightness at 95 arcsec drops to 24.0 mag arcsec−2and the J − Kcolour to 2.5, while the M/LKratio is undetermined since such a halo would be unrelated to the dark matter component. If the emission is interpreted as arising from the massive halo, then it is broadly consistent with a population of stars at the very lowest masses, possibly extending into the brown dwarf regime.

The Serpens cloud core is populated by a young, low-mass stellar cluster with members in many different evolutionary stages. We revisit the near-IR properties of the cluster in this contribution. A total of 188 near-IR sources are identified, including new members suggested in this work. Near-IR colour-magnitude and colour-colour diagrams allow us to suggest 42 objects among the cluster members as brown dwarf candidates. A considerable fraction of them present near-IR excesses which indicates that many of these very low mass objects are still surrounded by prominent dust accretion disks. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Aims. We demonstrate the feasibility of determining parallaxes for nearby objects with the Wide Field Camera on the United Kingdom Infrared Telescope (UKIRT) using the UKIRT Infrared Deep Sky Survey as a first epoch. We determine physical parameters for ULAS J003402.77-005206.7, one of the coolest brown dwarfs currently known, using atmospheric and evolutionary models with the distance found here.Methods. Observations over the period 10/2005 to 07/2009 were pipeline processed at the Cambridge Astronomical Survey Unit and combined to produce a parallax and proper motion using standard procedures. Results. We determined $\pi = 79.6 \pm 3.8$mas, $\mu_{\alpha} = -20.0 \pm 3.7$mas/yr and $\mu_{\delta} = -363.8 \pm 4.3$mas/yr for ULAS J003402.77-005206.7. Conclusions. We have made a direct parallax determination for one of the coolest objects outside of the solar system. The distance is consistent with a relatively young, 1-2 Gyr, low mass, 13-20 $M_{\rm J}$, cool, 550-600 K, brown dwarf. We present a measurement of the radial velocity that is consistent with an age between 0.5 and 4.0 Gyr.

We present ISOCAM observations (5-18 μm) of illuminated surfaces of molecular clouds. The emission properties of the transiently heated small particles, which dominate the emission observed by ISOCAM, are analysed in relation with the spatial structure of the material. Striking spatial variations of the infrared colour (5-8.5 μm/12-18 μm) are detected. Spectroscopic observations show that they are due to variations of the intensity of the aromatic features (especially at 7.7 μm) relatively to the continuum emission increasing towards long wavelengths. Compared to the intensity of the continuum emission, the intensity of the aromatic features are significantly fainter at the illuminated surfaces of dense structures than in low density regions surrounding these dense structures. This effect could be the signature of photo-chemical evolution, size segregation due to grain dynamics in uni-directional radiation fields, or abundance variations of very small particles.