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Aims: We wish to establish whether sequential star formation is taking place at the periphery of the Galactic HII region Sh2-212. Methods: We present CO millimetre observations of this region obtained at the IRAM 30-m telescope to investigate the distribution of associated molecular material. We also use deep JHK observations obtained at the CFHT to study the stellar content of the region, and radio observations obtained at the VLA to look for the presence of an ultra-compact (UC) HII region and for maser emission. Results: In the optical, Sh2-212 is spherically symmetric around its central exciting cluster. This HII region is located along a molecular filament. A thin, well-defined half ring of molecular material surrounds the brightest part of the HII region at the rear and is fragmented. The most massive fragment (~200 solar masses) contains a massive young stellar object displaying a near-IR excess; its spectral energy distribution indicates a high-mass (~14solar masses), high-temperature (~30000K), and high-luminosity (~17000 solar luminosities) source. This object ionizes a UC HII region. Conclusions: Sh2-212 is a good example of massive-star formation triggered via the collect and collapse process. The massive YSO observed at its periphery is a good candidate for a massive star formed in isolation., Comment: 12 pages, 14 figures. To be published in A&A more...

Context: L-type ultra-cool dwarfs and brown dwarfs have cloudy atmospheres that could host weather-like phenomena. The detection of photometric or spectral variability would provide insight into unresolved atmospheric heterogeneities, such as holes in a global cloud deck. Aims: It has been proposed that growth of heterogeneities in the global cloud deck may account for the L- to T-type transition as brown dwarf photospheres evolve from cloudy to clear conditions. Such a mechanism is compatible with variability. We searched for variability in the spectra of five L6 to T6 brown dwarfs in order to test this hypothesis. Methods: We obtained spectroscopic time series using VLT/ISAAC, over 0.99-1.13um, and IRTF/SpeX for two of our targets, in J, H and K bands. We search for statistically variable lines and correlation between those. Results: High spectral-frequency variations are seen in some objects, but these detections are marginal and need to be confirmed. We find no evidence for large amplitude variations in spectral morphology and we place firm upper limits of 2 to 3% on broad-band variability, on the time scale of a few hours. The T2 transition brown dwarf SDSS J1254-0122 shows numerous variable features, but a secure variability diagnosis would require further observations. Conclusions: Assuming that any variability arises from the rotation of patterns of large-scale clear and cloudy regions across the surface, we find that the typical physical scale of cloud cover disruption should be smaller than 5-8% of the disk area for four of our targets. The possible variations seen in SDSS J1254-0122 are not strong enough to allow us to confirm the cloud breaking hypothesis., Comment: 17 pages, 14 figures, accepted by A&A more...

Angular differential imaging is a high-contrast imaging technique that reduces quasi-static speckle noise and facilitates the detection of nearby companions. A sequence of images is acquired with an altitude/azimuth telescope while the instrument field derotator is switched off. This keeps the instrument and telescope optics aligned and allows the field of view to rotate with respect to the instrument. For each image, a reference PSF is constructed from other appropriately-selected images of the same sequence and subtracted to remove quasi-static PSF structure. All residual images are then rotated to align the field and are combined. Observed performances are reported for Gemini North data. It is shown that quasi-static PSF noise can be reduced by a factor \~5 for each image subtraction. The combination of all residuals then provides an additional gain of the order of the square root of the total number of acquired images. A total speckle noise attenuation of 20-50 is obtained for one-hour long observing sequences compared to a single 30s exposure. A PSF noise attenuation of 100 was achieved for two-hour long sequences of images of Vega, reaching a 5-sigma contrast of 20 magnitudes for separations greater than 8". For a 30-minute long sequence, ADI achieves 30 times better signal-to-noise than a classical observation technique. The ADI technique can be used with currently available instruments to search for ~1MJup exoplanets with orbits of radii between 50 and 300 AU around nearby young stars. The possibility of combining the technique with other high-contrast imaging methods is briefly discussed., Comment: 27 pages, 7 figures, accepted for publication in ApJ more...

A near-infrared camera in use at the Canada-France-Hawaii Telescope (CFHT) and at the 1.6-m telescope of the Observatoire du Mont-Megantic is described. The camera is based on a Hawaii-1 1024x1024 HgCdTe array detector. Its main feature is to acquire three simultaneous images at three wavelengths across the methane absorption bandhead at 1.6 microns, enabling, in theory, an accurate subtraction of the stellar point spread function (PSF) and the detection of faint close methanated companions. The instrument has no coronagraph and features fast data acquisition, yielding high observing efficiency on bright stars. The performance of the instrument is described, and it is illustrated by laboratory tests and CFHT observations of the nearby stars GL526, Ups And and Chi And. TRIDENT can detect (6 sigma) a methanated companion with delta H = 9.5 at 0.5" separation from the star in one hour of observing time. Non-common path aberrations and amplitude modulation differences between the three optical paths are likely to be the limiting factors preventing further PSF attenuation. Instrument rotation and reference star subtraction improve the detection limit by a factor of 2 and 4 respectively. A PSF noise attenuation model is presented to estimate the non-common path wavefront difference effect on PSF subtraction performance., Comment: 41 pages, 16 figures, accepted for publication in PASP more...

Simultaneous spectral differential imaging is a high contrast technique by which subtraction of simultaneous images reduces noise from atmospheric speckles and optical aberrations. Small non-common wave front errors between channels can seriously degrade its performance. We present a new concept, a multicolor detector assembly (MCDA), which can eliminate this problem. The device consists of an infrared detector and a microlens array onto the flat side of which a checkerboard pattern of narrow-band micro-filters is deposited, each micro-filter coinciding with a microlens. Practical considerations for successful implementation of the technique are mentioned. Numerical simulations predict a noise attenuation of 10^-3 at 0.5" for a 10^5 seconds integration on a mH=5 star of Strehl ratio 0.9 taken with an 8-m telescope. This reaches a contrast of 10^-7 at an angular distance of 0.5" from the center of the star image., Comment: 13 pages, 5 figures, accepted APJL more...

We present millimetre-line imaging of the Galactic HII region Sh 104. We show that it is surrounded by a ring of molecular gas and dust. Four large molecular condensations are regularly spaced around the ring. These condensations are themselves fragmented and contain several massive dense cores. A deeply embedded cluster is observed in the near IR towards the largest condensation. It contains at least one massive star ionizing an ultra-compact HII region. The Sh 104 region is a good illustration of the `collect and collapse' model for star formation triggered by the expansion of an HII region., Comment: 4 pages, 3 figures more...

We present the first results obtained at CFHT with the TRIDENT infrared camera, dedicated to the detection of faint companions close to bright nearby stars. The camera's main feature is the acquisition of three simultaneous images in three wavelengths (simultaneous differential imaging) across the methane absorption bandhead at 1.6 micron, that enables a precise subtraction of the primary star PSF while keeping the companion signal. The main limitation is non-common path aberrations between the three optical paths that slightly decorrelate the PSFs. Two types of PSF calibrations are combined with the differential simultaneous imaging technique to further attenuate the PSF: reference star subtraction and instrument rotation to smooth aberrations. It is shown that a faint companion with a DeltaH of 10 magnitudes would be detected at 0.5 arcsec from the primary., Comment: 12 pages, 10 figures, to appear in Astronomy with High Contrast Imaging, EAS Publications Series more...

A near-infrared (0.85-2.5 microns) camera in use at the Canada-France-Hawaii Telescope and at the 1.6m telescope of the Observatoire du Mont-Megantic is described. The camera is based on a Hawaii-1 1024x1024 HgCdTe array detector. Its main feature is to acquire three simultaneous images at three wavelengths (simultaneous differential imaging) across the methane absorption bandhead at 1.6 micron, enabling an accurate subtraction of the stellar point spread function (PSF) and the detection of faint close methanated companions. The instrument has no coronagraph and features a fast (1 MHz) data acquisition system without reset anomaly, yielding high observing efficiencies on bright stars. The performance of the instrument is described, and it is illustrated by CFHT images of the nearby star Ups And. TRIDENT can detect (3 sigma) a methanated companion with DeltaH=10 at 0.5 arcsec from the star in one hour of observing time. Non-common path aberrations between the three optical paths are the limiting factors preventing further PSF attenuation. Reference star subtraction and instrument rotation improve the detection limit by one order of magnitude., Comment: 8 pages, 6 figures, to appear in SPIE 4860 more...

We present the first results obtained at CFHT with the TRIDENT infrared camera, dedicated to the detection of faint companions close to bright nearby stars. Its main feature is the acquisition of three simultaneous images in three wavelengths (simultaneous differential imaging) across the methane absorption bandhead at 1.6 micron, that enables a precise subtraction of the primary star PSF while keeping the companion signal. Gl229 and 55Cnc observations are presented to demonstrate TRIDENT subtraction performances. It is shown that a faint companion with a DeltaH of 10 magnitudes would be detected at 0.5 arcsec from the primary., Comment: 4 pages, 3 figures, to appear in Scientific Frontiers in Research on Extrasolar Planets, ASP Conference Series, D. Deming and S. Seager, eds more...

We present the first results obtained at CFHT with the TRIDENT infrared camera, dedicated to the detection of faint companions close to bright nearby stars. Its main feature is the acquisition of three simultaneous images in three wavelengths (simultaneous differential imaging) across the methane absorption bandhead at 1.6 micron, that enables a precise subtraction of the primary star PSF while keeping the companion signal. Thirty-five stars have been observed in two observing missions, with no detection so far. It is shown that a faint companion with a DeltaH of 10 magnitudes would be detected at 0.5 arcsec from the primary., Comment: 4 pages, 1 figure, to appear in IAU Symposium, Vol. 211, E. L. Martin, ed more...

We present sub-arcsecond resolution ground-based near-infrared images of the central regions of a sample of twelve barred galaxies with circumnuclear star formation activity, which is organized in ring-like regions typically one kiloparsec in diameter. We also present Hubble Space Telescope near-infrared images of ten of our sample galaxies, and compare them with our ground-based data. Although our sample galaxies were selected for the presence of circumnuclear star formation activity, our broad-band near-infrared images are heterogeneous, showing a substantial amount of small-scale structure in some galaxies, and practically none in others. We argue that, where it exists, this structure is caused by young stars, which also cause the characteristic bumps or changes in slope in the radial profiles of ellipticity, major axis position angle, surface brightness and colour at the radius of the circumnuclear ring in most of our sample galaxies. In 7 out of 10 HST images, star formation in the nuclear ring is clearly visible as a large number of small emitting regions, organised into spiral arm fragments, which are accompanied by dust lanes. NIR colour index maps show much more clearly the location of dust lanes and, in certain cases, regions of star formation than single broad-band images. Circumnuclear spiral structure thus outlined appears to be common in barred spiral galaxies with circumnuclear star formation., Comment: Accepted for publication in MNRAS. 16 pages, includes 1 gif and 12 postscript figures. Complete version with high-quality postscript figures available on ftp://star.herts.ac.uk/pub/Knapen/perez/ more...

We present new high-resolution near-infrared observations in the J,H and K bands, obtained to study the properties of Seyfert host galaxies. The dataset consists of images in the three bands of practically the entire CfA sample of Seyfert galaxies, and K-band images of a control sample of non-active, `normal', galaxies, matched to the Seyfert sample in the distribution of type and inclination. The spatial resolution and sampling of the new images is a factor 2 better than previously published K-band data. In this paper, we present the data in the form of profiles of surface brightness and color, ellipticity and major axis position angle, as well as greyscale maps of surface brightness in H or K and both J-H and H-K colors. We compare our surface brightness and color profiles with the literature, and find good agreement. Our data are discussed in detail in three subsequent publications, where we analyze the morphologies of Seyfert and normal hosts, quantify the strength of nonaxisymmetric features in disks and their relationship to nuclear activity, address the question of bar fraction in Seyferts and normal galaxies, and analyze the color information in the framework of emission mechanisms in Seyfert 1s and 2s, and in non-active galaxies., Comment: 17 pages + 2 figures + data tables. Accepted for publication in the Astrophysical Journal Supplement. Figure 1, together with much more information about the Cfa sample of Seyfert galaxies, can be found at http://star-www.dur.ac.uk/~peletier/SEYFERT and http://www.pa.uky.edu/~shlosman/gals/SEYFERT more...

We report the discovery of trailing grand-design nuclear spiral structure in the spiral galaxy NGC 5248. Two tightly-wound red spiral arms, with widths less than 1 arcsec (~75 pc), emanate at 1 arcsec outside the quiescent nucleus of the galaxy, and can be followed for 3 arcsec, appearing to end before the radius of the circumnuclear starburst "ring", which is at 5--6 arcsec or around 400 parsec. Combining our near-infrared Canada-France-Hawaii Telescope adaptive optics results with traditional near-infrared and optical imaging, we show that spiral structure is present in this galaxy at spatial scales covering two orders of magnitude, from a hundred parsecs to 15 kpc. Comparison with a Hubble Space Telescope ultraviolet image shows how the location of the circumnuclear star formation relates to the nuclear spiral structure. We briefly discuss possible mechanisms which could be responsible for the observed nuclear grand-design spiral structure., Comment: 5 pages, Latex. 5 postscript files included. Better quality versions of the figures are available from ftp://star.herts.ac.uk/pub/Laine/ngc5248/ . MNRAS Letters, accepted more...

We have studied the core morphology of the ultraluminous infrared galaxy Mrk 273 by combining a high-resolution adaptive optics near-infrared image with an optical image from the Hubble Space Telescope and interferometric radio continuum data, all at spatial resolutions of 150 mas or better. The near-infrared image reveals that the nucleus has two main components, both of which have radio counterparts. The strongest component (N) shows very similar extended structure in the radio and near-infrared. It has a flat radio spectrum and is resolved into a double-lobed structure (Ne; Nw), with a separation of 90\pm5 mas (70 parsec). A similar structure is detected in the near-infrared. We identify this component as the location of the active nucleus. The second component (SW), strong in the near-infrared but relatively weak in the radio, is located $\sim1$ arcsecond to the southwest. We interpret this as an obscured starburst region associated with the merger. The radio continuum images show a third, strong, component (SE) which has previously been interpreted as a second nucleus. However, it shows no associated optical or near-infrared emission, suggesting that it is in fact a background source., Comment: 8 pages, Latex. 4 postscript files. Better quality version of figure 1 available from ftp://star.herts.ac.uk/pub/Knapen/mrk273 . Accepted, ApJ Letters more...