We present the second realization of the International Celestial Reference Frame (ICRF2) at radio wavelengths using nearly 30 years of Very Long Baseline Interferometry observations. ICRF2 contains precise positions of 3414 compact radio astronomical objects and has a positional noise floor of ∼40 μas and a directional stability of the frame axes of ∼10 μas. A set of 295 new "defining" sources was selected on the basis of positional stability and the lack of extensive intrinsic source structure. The positional stability of these 295 defining sources and their more uniform sky distribution eliminates the two greatest weaknesses of the first realization of the International Celestial Reference Frame (ICRF1). Alignment of ICRF2 with the International Celestial Reference System was made using 138 positionally stable sources common to both ICRF2 and ICRF1. The resulting ICRF2 was adopted by the International Astronomical Union as the new fundamental celestial reference frame, replacing ICRF1 as of 2010 January 1.

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Aims. We present high spatial resolution observations of the dusty core of the Planetary Nebula with Wolf-Rayet central star CPD-56• 8032, for which indications of a compact disk have been found by HST/SITS observations. Methods. These observations were taken with the mid-infrared interferometer VLTI/MIDI in imaging mode providing a typical 300 mas resolution and in interferometric mode using UT2-UT3 47m baseline providing a typical spatial resolution of 20 mas. We also made use of unpublished HST/ACS images in the F435W and F606W filters. Results. The visible HST images exhibit a complex multilobal geometry dominated by faint lobes. The farthest structures are located at 7 from the star. The mid-IR environment of CPD-56• 8032 is dominated by a compact source, barely resolved by a single UT telescope in a 8.7 µm filter (∆λ = 1.6 µm, contaminated by PAH emission). The infrared core is almost fully resolved with the three 40-45 m projected baselines ranging from −5• to 51• but smooth oscillating fringes at low level have been detected in spectrally dispersed visibilities. This clear signal is interpreted in terms of a ring structure which would define the bright inner rim of the equatorial disk. Geometric models allowed us to derive the main geometrical parameters of the disk. For instance, a reasonably good fit is reached with an achromatic and elliptical truncated Gaussian with a radius of 97 ± 11 AU, an inclination of 28 ± 7• and a PA for the major axis at 345• ± 7• . Furthermore, we performed some radiative transfer modeling aimed at further constraining the geometry and mass content of the disk, by taking into account the MIDI dispersed visibilities, spectra, and the large aperture SED of the source. These models show that the disk is mostly optically thin in the N band and highly flared. As a consequence of the complex flux distribution, an edge-on inclination is not excluded by the data.

Context. The European space astrometry mission Gaia, to be launched by 2012, will construct a dense optical QSO-based celestial reference frame which will need to be linked to the International Celestial Reference Frame (ICRF; the IAU fundamental frame), with the highest accuracy. However, it has been found that only 10% of the ICRF sources (70 sources) are suitable to establish this link. The remaining sources are not useful either because they are not bright enough at optical wavelengths or because they have significant extended radio emission which precludes reaching the highest astrometric accuracy. Aims. In order to improve the accuracy of this alignment, we have developed a program of VLBI observations based on three steps to detect, image and measure astrometric positions of weak extragalactic radio sources, with bright optical counterparts, from a sample of 447 candidate sources. Methods. The experiments devoted to VLBI detection, carried out with the European VLBI Network (EVN) in June and October 2007, were very successful, with 398 sources detected at both S-and X-bands. From these, 105 sources were observed in March 2008 with a global VLBI array (EVN and VLBA; Very Long Baseline Array) for imaging their VLBI structures. Results. All sources were successfully imaged in both bands and about 50% (47 sources) were found to be point-like on VLBI scales. These images are available at http://www.obs.u-bordeaux1.fr/BVID/GC030/. VLBI positions of these sources will be measured accurately in future astrometric experiments.