Abstract. We present the first long baseline mid-infrared interferometric observations of the circumstellar disks surrounding Herbig Ae/Be stars. The observations were obtained using the mid-infrared interferometric instrument MIDI at the European Southern Observatory (ESO) Very Large Telescope Interferometer VLTI on Cerro Paranal. The 102 m baseline given by the telescopes UT1 and UT3 was employed, which provides a maximum full spatial resolution of 20 milli-arcsec (mas) at a wavelength of 10 µm. The interferometric signal was spectrally dispersed at a resolution of 30, giving spectrally resolved visibility information from 8 µm to 13.5 µm. We observed seven nearby Herbig Ae/Be stars and resolved all objects. The warm dust disk of HD 100546 could even be resolved in single-telescope imaging. Characteristic dimensions of the emitting regions at 10 µm are found to be from 1 AU to 10 AU. The 10 µm sizes of our sample stars correlate with the slope of the 10-25 µm infrared spectrum in the sense that the reddest objects are the largest ones. Such a correlation would be consistent with a different geometry in terms of flaring or flat (self-shadowed) disks for sources with strong or moderate mid-infrared excess, respectively. We compare the observed spectrally resolved visibilities with predictions based on existing models of passive centrally irradiated hydrostatic disks made to fit the SEDs of the observed stars. We find broad qualitative agreement of the spectral shape of visibilities corresponding to these models with our observations. Quantitatively, there are discrepancies that show the need for a next step in modelling of circumstellar disks, satisfying both the spatial constraints such as are now available from the MIDI observations and the flux constraints from the SEDs in a consistent way.Key words. stars: circumstellar matter -techniques: interferometric -stars: formation -stars: pre-main-sequenceinfrared: stars Based on observations made with the Very Large Telescope Interferometer at Paranal Observatory.

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Abstract. We present new total intensity and linear polarization VLBA observations of the ν = 2 and ν = 1 J = 1−0 maser transitions of SiO at 42.8 and 43.1 GHz in a number of Mira variable stars over a substantial fraction of their pulsation periods. These observations were part of an observing program that also includes interferometric measurements at 2.2 and 3.6 micron (Mennesson et al. 2002); comparison of the results from different wavelengths allows studying the envelope independently of the poorly known distances to these stars. Nine stars were observed at from one to four epochs during 2001. The SiO emission is largely confined to rings which are smaller than the inner radius of the dust shells reported by Danchi et al. (1994). Two stars (U Orionis, R Aquarii) have maser rings with diameters corresponding to the size of the hot molecular layer as measured at 3.6 micron; in the other cases, the SiO rings are substantially larger. Variations of ring diameter for most, but not all stars, had an rms amplitude in agreement with the models of Humphreys et al. (2002) although the expected relationship between the diameter and pulsation phase was not seen. The ring diameter in U Orionis shows remarkably small variation. A correlation between the 2.2/3.6 µm diameter ratio with that of the SiO/3.6 µm diameter ratio is likely due to differences in the opacities at 2.2 and 3.6 µm in a molecular layer. A further correlation with the inner size of the dust shell reported by Danchi et al. (1994) suggest some differences in the temperature structure. Clear evidence is seen in R Aquarii for an equatorial disk similar to that reported by Hollis et al. (2001); rotation is possibly also detected in S Coronae Boralis.

Abstract. Long baseline stellar interferometry shares with other techniques the need for calibrator stars in order to correct for instrumental and atmospheric effects. We present a catalogue of 374 stars carefully selected to be used for that purpose in the near infrared. Owing to several convergent criteria with the work of Cohen et al. (1999), this catalogue is in essence a subset of their self-consistent all-sky network of spectro-photometric calibrator stars. For every star, we provide the angular limb-darkened diameter, uniform disc angular diameters in the J, H and K bands, the Johnson photometry and other useful parameters. Most stars are type III giants with spectral types K or M0, magnitudes V = 3-7 and K = 0-3. Their angular limb-darkened diameters range from 1 to 3 mas with a median uncertainty as low as 1.2%. The median distance from a given point on the sky to the closest reference is 5.2• , whereas this distance never exceeds 16.4• for any celestial location.Key words. catalogs -stars: fundamental parameters -instrumentation: interferometers -techniques: interferometric Interferometry and calibrationA stellar interferometer collects and recombines light from separate apertures. The interferometric observable, called the visibility, is a measure of the spatio-temporal coherence of the target. It is a complex quantity whose modulus and phase are respectively derived from the contrast and the position of the fringe packet resulting from the recombination process. In order to take into account instrumental and atmospheric effects, observations of the target should be bracketed by observations of calibrator stars. Thus, the object visibility V can be deduced from the measured object and reference fringe contrasts µ and µ ref by A single star appears as a straightforward reference source whose simplest model is a uniform disc (UD) fully described by its angular diameter (single-parameter model). In the following, we will call calibrator star such a reference source. In this case, the visibility V ref is given bywhere B is the interferometric baseline projected on the sky, θ UD the stellar angular diameter and λ the wavelength. A more elaborate model would be a limb-darkened disc (LDD) with two parameters, the stellar angular diameter and either the coefficient of a linear law or the exponent of a power law for limbdarkening (e.g. Hestroffer 1997). However, for the purpose of calibration, the UD model is in general fully satisfactory provided the visibility is large enough. For instance, if V ≥ 50%, UD and LD models do not differ by more than 1% in the K band (Perrin et al. 1998).To improve calibration, µ and µ ref should be measured with the same instrumental and preferably atmospheric conditions, hence the need for a catalogue of calibrator stars with a sky coverage as complete as possible. Therefore, in the framework of the simplest single-parameter UD model, a dense grid of reference stars along with their angular diameters has to be worked out. Since only few stellar angular diameters have been direc...