Abstract. We present a volume-limited multiplicity survey with magnitude cutoff (m K ≤ 10.5 mag) of 158 young stellar objects located within or in the vicinity of the ρ Ophiuchi Dark Cloud. With exception of eleven already well observed objects, all sources have been observed by us in the K-band with 3.5 m telescopes by using speckle techniques. The separation range covered by our survey is 0.13 ≤ θ ≤ 6.4 , where the lower limit is given by the diffraction limit of the telescopes and the upper limit by confusion with background stars. The multiplicity survey is complete for flux ratios ≥0.1 (∆m K ≤ 2.5) at the diffraction limit. After taking the background density into account the degree of multiplicity is 29.1% ± 4.3% and thus only marginally higher than the value 23.5% ± 4.8% derived for the given separation range for the main-sequence solar-like stars in the solar neighbourhood (Duquennoy & Mayor 1991). We discuss the implications of these findings.

Aims. We want to improve knowledge of the structure of the inner few AU of the circumstellar disk around the nearby T Tauri star TW Hya. Earlier studies have suggested the existence of a large inner hole, possibly caused by interactions with a growing protoplanet. Methods. We used interferometric observations in the N-band obtained with the MIDI instrument on the Very Large Telescope Interferometer, together with 10 µm spectra recorded by the infrared satellite Spitzer. The fact that we were able to determine N-band correlated fluxes and visibilities for this comparatively faint source shows that mid-infrared interferometry can be applied to a large number of low-mass young stellar objects. Results. The mid-infrared spectra obtained with Spitzer reveal emission lines from H I (6-5), H I (7-6), and [Ne II] and show that over 90% of the dust we see in this wavelength regime is amorphous. According to the correlated flux measured with MIDI, most of the crystalline material is in the inner, unresolved part of the disk, about 1 AU in radius. The visibilities exclude the existence of a very large (3−4 AU radius) inner hole in the circumstellar disk of TW Hya, which was required in earlier models. We propose instead a geometry of the inner disk where an inner hole still exists, but at a much reduced radius, with the transition from zero to full disk height between 0.5 and 0.8 AU, and with an optically thin distribution of dust inside. Such a model can comply with SED and mid-infrared visibilities, as well as with visibility and extended emission observed in the near-infrared at 2 µm. If a massive planet was the reason for this inner hole, as has been speculated, its orbit would have to be closer to the star than 0.3 AU. Alternatively, we may be witnessing the end of the accretion phase and an early phase of an inward-out dispersal of the circumstellar disk.

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.

Aims. This paper describes a systematic search for high-order multiplicity among wide visual Pre-Main Sequence (PMS) binaries. Methods. We conducted an Adaptive Optics survey of a sample of 58 PMS wide binaries from various star-forming regions, which include 52 T Tauri systems with mostly K-and M-type primaries, with the NIR instrument NACO at the VLT. Results. Of these 52 systems, 7 are found to be triple (2 new) and 7 quadruple (1 new). The new close companions are most likely physically bound based on their probability of chance projection and, for some of them, on their position on a color-color diagram. The corresponding degree of multiplicity among wide binaries (number of triples and quadruples divided by the number of systems) is 26.9 ± 7.2% in the projected separation range ∼0. 07-12 , with the largest contribution from the Taurus-Auriga cloud. We also found that this degree of multiplicity is twice in Taurus compared to Ophiuchus and Chamaeleon for which the same number of sources are present in our sample. Considering a restricted sample composed of systems at distance 140-190 pc, the degree of multiplicity is 26.8 ± 8.1%, in the separation range 10/14 AU-1700/2300 AU (30 binaries, 5 triples, 6 quadruples). The observed frequency agrees with results from previous multiplicity surveys within the uncertainties, although a significant overabundance of quadruple systems compared to triple systems is apparent. Tentatively including the spectroscopic pairs in our restricted sample and comparing the multiplicity fractions to those measured for solar-type main-sequence stars in the solar neighborhood leads to the conclusion that both the ratio of triples to binaries and the ratio of quadruples to triples seems to be in excess among young stars. Most of the current numerical simulations of multiple star formation, and especially smoothed particles hydrodynamics simulations, over-predict the fraction of high-order multiplicity when compared to our results. The circumstellar properties around the individual components of our high-order multiple systems tend to favor mixed systems (i.e. systems including components of wTTS and cTTS type), which is in general agreement with previous studies of disks in binaries, with the exception of Taurus, where we find a preponderance of similar type of components among the multiples studied.

We report on ALMA observations of continuum and molecular line emission with 0. 4 resolution towards the high-mass star-forming region G35.20-0.74 N. Two dense cores are detected in typical hot-core tracers (e.g., CH 3 CN) that reveal velocity gradients. In one of these cores, the velocity field can be fitted with an almost edge-on Keplerian disk rotating about a central mass of ∼18 M . This finding is consistent with the results of a recent study of the CO first overtone bandhead emission at 2.3 μm towards G35.20-0.74 N. The disk radius and mass are > ∼ 2500 au and ∼3 M . To reconcile the observed bolometric luminosity (∼3 × 10 4 L ) with the estimated stellar mass of 18 M , we propose that the latter is the total mass of a binary system.