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.

Aims. This study aims to determine the impact of stellar binary companions on the lifetime and evolution of circumstellar disks in the Chamaeleon I (Cha I) star-forming region by measuring the frequency and strength of accretion and circumstellar dust signatures around the individual components of T Tauri binary stars. Methods. We used high-angular resolution adaptive optics JHK s L -band photometry and 1.5-2.5 μm spectroscopy of 19 visual binary and 7 triple stars in Cha I -including one newly discovered tertiary component -with separations between ∼25 and ∼1000 AU. The data allowed us to infer stellar component masses and ages and, from the detection of near-infrared excess emission and the strength of Brackett-γ emission, the presence of ongoing accretion and hot circumstellar dust of the individual stellar components of each binary. Results. Of all the stellar components in close binaries with separations of 25-100 AU, 10 +15 −5 % show signs of accretion. This is less than half of the accretor fraction found in wider binaries, which itself appears significantly reduced (∼44%) compared with previous measurements of single stars in Cha I. Hot dust was found around 50 +30 −15 % of the target components, a value that is indistinguishable from that of Cha I single stars. Only the closest binaries (<25 AU) were inferred to have a significantly reduced fraction ( 25%) of components that harbor hot dust. Accretors were exclusively found in binary systems with unequal component masses M secondary /M primary < 0.8, implying that the detected accelerated disk dispersal is a function of mass-ratio. This agrees with the finding that only one accreting secondary star was found, which is also the weakest accretor in the sample. Conclusions. The results imply that disk dispersal is more accelerated the stronger the dynamical disk truncation, i.e., the smaller the inferred radius of the disk. Nonetheless, the overall measured mass accretion rates appear to be independent of the cluster environment or the existence of stellar companions at any separation 25 AU, because they agree well with observations from our previous binary study in the Orion Nebula cluster and with studies of single stars in these and other star-forming regions.

Aims. We present a study of protoplanetary disks in spatially resolved low-mass binary stars in the well-known Orion nebula cluster (ONC) to assess the impact of binarity on the properties of circumstellar disks. This is currently the largest such study in a clustered high-stellar-density star-forming environment, as opposed to previous studies, which have mostly been focussed on the young, lowstellar-density Taurus association. We particularly aim to determine the presence of magnetospheric accretion and dust disks for each binary component, and measure the overall disk frequency. Methods. We carried out spatially resolved adaptive-optics-assisted observations to acquire near-infrared photometry and spectroscopy of 26 binaries in the ONC, and determine stellar parameters such as effective temperatures, spectral types, luminosities, and masses, as well as accretion properties and near-infrared excesses for the individual binary components. On the basis of our medium resolution K-band spectroscopy, we infer the presence of magnetospheric accretion around each binary component by measuring the strength of the Brackett-γ emission. The accretion disk frequency among the ONC binaries is then estimated from Bayesian statistics. The observed disk signatures, measured accretion luminosities, and mass accretion rates are investigated with respect to the binary separation, mass ratios, and distance to the center of the ONC. Results. A fraction of 40 +10−9 % of the binary components in the sample can be inferred to be T Tauri stars possessing an accretion disk. This is only marginally smaller than the disk fraction of single stars of ∼50% in the ONC. We find that disks in wide binaries of >200 AU separation are consistent with random pairing, while the evolution of circumprimary and circumsecondary disks is observed to be synchronized in close binaries (separations <200 AU). Circumbinary disks appear to be unsuitable to explain this difference. Furthermore, we identify several mixed pairs of accreting and non-accreting components, suggesting that these systems are common and that there is no preference for either the more or less massive component to evolve faster. The derived accretion luminosities and mass accretion rates of the ONC binary components are of similar magnitude as those for both ONC single stars and binaries in the Taurus star-forming region. The paper concludes with a discussion of the (presumably weak) connection between the presence of inner accretion disks in young binary systems and the existence of planets in stellar multiples.

Context. Observations at various wavelengths of late B-type stars exhibiting strong atmospheric overabundances of the chemical elements Hg and Mn indicate that these stars are frequently found in binary and multiple systems. Aims. We intend to study the multiplicity of this type of chemically peculiar stars, looking for visual companions in the range of angular separation between 0. 05 and 8 . Methods. We carried out a survey of 56 stars using diffraction-limited near-infrared imaging with NAOS-CONICA at the VLT. Results. Thirty-three companion candidates in 24 binaries, three triples, and one quadruple system were detected. Nine companion candidates were found for the first time in this study. Five objects are likely chance projections. The detected companion candidates have K magnitudes between 5. m 95 and 18. m 07 and angular separations ranging from <0. 05 to 7. 8, corresponding to linear projected separations of 13.5-1700 AU. Conclusions. Our study clearly confirms that HgMn stars are frequently members of binary and multiple systems. Taking into account companions found by other techniques, the multiplicity fraction in our sample may be as high as 91%. The membership in binary and multiple systems seems to be a key point to understanding the abundance patterns in these stars.

The Very Large Telescope (VLT) Observatory on Cerro Paranal (2635 m) in Northern Chile is approaching completion. After the four 8-m Unit Telescopes (UT) individually saw first light in the last years, two of them were combined for the first time on October 30, 2001 to form a stellar interferometer, the VLT Interferometer. The remaining two UTs will be integrated into the interferometric array later this year. In this article, we will describe the subsystems of the VLTI and the planning for the following years.