The infrared companions (IRCs) associated with several normal low-mass preÈmain-sequence (T Tauri) stars pose an interesting problem for theories of binary star formation. The IRCs have very low infrared color temperatures and large infrared excesses, which have led observers to suggest that they may be less evolved objects such as protostars. This paper presents an attempt to understand the IRCs as a class by examining a broad range of observations and applying simple arguments and models. We propose that the IRCs may represent relatively normal young low-mass stars experiencing episodes of enhanced circumstellar extinction, possibly due to rapid accretion of disk material perturbed by their gravitational inÑuence at aphelion or perihelion.

Abstract. The core of the nebula surrounding Eta Carinae has been observed with the VLT Adaptive Optics system NACO and with the interferometer VLTI/MIDI to constrain spatially and spectrally the warm dusty environment and the central object. In particular, narrow-band images at 3.74 µm and 4.05 µm reveal the butterfly shaped dusty environment close to the central star with unprecedented spatial resolution. A void whose radius corresponds to the expected sublimation radius has been discovered around the central source. Fringes have been obtained in the Mid-IR which reveal a correlated flux of about 100 Jy situated 0. 3 south-east of the photocenter of the nebula at 8.7 µm, which corresponds with the location of the star as seen in other wavelengths. This correlated flux is partly attributed to the central object, and these observations provide an upper limit for the SED of the central source from 2.2 µm to 13.5 µm. Moreover, we have been able to spectrally disperse the signal from the nebula itself at PA = 318 degree, i.e. in the direction of the bipolar nebula (∼310• ) within the MIDI field of view of 3 . A large amount of corundum (Al 2 O 3 ) is discovered, peaking at 0. 6-1. 2 south-east from the star, whereas the dust content of the Weigelt blobs is dominated by silicates. We discuss the mechanisms of dust formation which are closely related to the geometry of this Butterfly nebulae.

We present two wide-field (%5 0 ; 3A5), diffraction-limited (k=D ' 0B5 at 10 m), broadband 10 and 20 m images of the Orion Nebula, plus six 7-13 m narrowband (k=Ák ' 1) images of the BN/ KL complex taken at the 3.8 m UKIRT telescope with the MPIA MAX camera. The wide-field images, centered on the Trapezium and BN/ KL regions, are mosaics of 35 00 ; 35 00 frames obtained with standard chopping and nodding techniques and reconstructed using a new restoration method developed for this project. They show the filamentary structure of the dust emission from the walls of the H ii region and reveal a new remarkable group of arclike structures %1 0 to the south of the Trapezium. The morphology of the Ney-Allen Nebula, produced by wind-wind interaction in the vicinity of the Trapezium stars, suggests a complex kinematical structure at the center of the cluster. We find indications that one of the most massive members of the cluster, the B0.5 V star 1 Ori D, is surrounded by a photoevaporated circumstellar disk. Among the four historic Trapezium OB stars, this is the only one without a binary companion, suggesting that stellar multiplicity and the presence of massive circumstellar disks may be mutually exclusive. In what concerns the BN / KL complex, we find evidence for extended optically thin silicate emission on top of the deep 10 m absorption feature. Assuming a simple two-component model, we map with '0B5 spatial resolution the foreground optical depth, color temperature, and mid-IR luminosity of the embedded sources. We resolve a conspicuous point source at the location of the IRc2-A knot, approximately 0B5 north of the deeply embedded H ii region ''I.'' We analyze the spectral profile of the 10 m silicate absorption feature and find indication for grain crystallization in the harsh nebular environment. In the OMC-1 South region, we detect several point sources and discuss their association with the mass-loss phenomenology observed at optical and millimeter wavelengths. Finally, we list the position and photometry of 177 point sources, the large majority of which are detected for the first time in the mid-IR. Twenty-two of them lack a counterpart at shorter wavelengths and are therefore candidates for deeply embedded protostars. The comparison of photometric data obtained at two different epochs reveals that source variability at 10 m is present up to a level of %1 mag on a timescale of $2 yr. With the possible exception of a pair of OB stars, all point sources detected at shorter wavelengths display 10 m emission well above the photospheric level, which we attribute to disk circumstellar emission. The recent model of Robberto et al. provides the simplest explanation for the observed mid-IR excess.

Abstract. We present new high angular resolution observations at near-IR wavelengths of the core of the Luminous Blue Variable η Carinae, using NAOS-CONICA at the VLT and VINCI at the VLT Interferometer (VLTI). The latter observations provide spatial information on a scale of 5 milli-arcsec or ∼11 AU at the distance of η Carinae. The present-day stellar wind of η Carinae is resolved on a scale of several stellar radii. Assuming spherical symmetry, we find a mass loss rate of 1.6×10−3 M /yr and a wind clumping factor of 0.26. The VLTI data taken at a baseline of 24 m show that the object is elongated with a deprojected axis ratio of approximately 1.5; the major axis is aligned with that of the large bi-polar nebula that was ejected in the 19th century. The most likely explanation for this observation is a counter-intuitive model in which stellar rotation near the critical velocity causes enhanced mass loss along the rotation axis. This results from the large temperature difference between pole and equator in rapidly rotating stars. η Carinae must rotate in excess of 90 percent of its critical velocity to account for the observed shape. The large outburst may have been shaped in a similar way. Our observations provide strong support for the existence of a theoretically predicted rotational instability, known as the Ω limit.