Search

Context. Over the past years, the $\lambda-$Orionis cluster has been a prime location for the study of young very low mass stars, substellar and isolated planetary mass objects and the determination of the initial mass function and other properties of low mass cluster members.

Context. The triple stellar system δVel system presents a significant infrared excess, whose origin is still being debated. A large infrared bow shock has been discovered using Spitzer/MIPS observations. Although it appears as a significant contributor to the measured IR excess, the possibility exists that a circumstellar IR excess is present around the stars of the system.

Context.We have started a program of infrared (IR) studies of the stellar clusters associated with HII regions in order to understand the structure of the spiral arms and their interaction with the central bar of the Galaxy better.

Aims.The recent outburst of the young eruptive star V1647 Orionishas produced a spectacular appearance of a new reflection nebula in Orion (McNeil's nebula). We present an optical/near infrared investigation of McNeil's nebula. This analysis is aimed at determining the morphology, temporal evolution and nature of the nebula and its connection to the outburst.

Aims.The occurrence of new FU Orionis-like objects is fundamental to understand the outburst mechanism in young stars and their role in star formation and disk evolution. Our work is aimed at investigating the properties of the recent outburst of V1647 Ori.

Context.The nearby (~24 pc) triple stellar system δVelorum contains a close, eclipsing binary (Aa, Ab) discovered in 2000. Multiple systems provide an opportunity to determine the set of fundamental parameters (mass, luminosity, size, chemical composition) of coeval stars.

We report on a high-spatial-resolution survey for binary stars in the periphery of the Orion Nebula Cluster, at 5–15 arcmin (0.65–2 pc) from the cluster center. We observed 228 stars with adaptive optics systems, in order to find companions at separations of $0\farcs13$–$1\farcs12$(60–500 AU), and detected 13 new binaries. Combined with the results of Petr (1998), we have a sample of 275 objects, about half of which have masses from the literature and high probabilities to be cluster members. We used an improved method to derive the completeness limits of the observations, which takes into account the elongated point spread function of stars at relatively large distances from the adaptive optics guide star. The multiplicity of stars with masses ${>}2\,M_{\odot}$is found to be significantly larger than that of low-mass stars. The companion star frequency of low-mass stars is comparable to that of main-sequence M-dwarfs, less than half that of solar-type main-sequence stars, and 3.5 to 5 times lower than in the Taurus-Auriga and Scorpius-Centaurus star-forming regions. We find the binary frequency of low-mass stars in the periphery of the cluster to be the same or only slightly higher than for stars in the cluster core (<3 arcmin from $\theta^1$C Ori). This is in contrast to the prediction of the theory that the low binary frequency in the cluster is caused by the disruption of binaries due to dynamical interactions. There are two ways out of this dilemma: Either the initial binary frequency in the Orion Nebula Cluster was lower than in Taurus-Auriga, or the Orion Nebula Cluster was originally much denser and dynamically more active.

We have performed deep Ks-band observations of the starburst region NGC 3603 with the infrared camera ISAAC mounted on the VLT Antu. The total area covered by our data stretches from the NGC 3603 starburst cluster towards the south up to a maximal distance of about 370$\hbox{$^{\prime\prime}$}$(~12 pc). This enables us to reconsider and redetermine the radial extent of the cluster which is found to be about 150$\hbox{$^{\prime\prime}$}$±15$\hbox{$^{\prime\prime}$}$(~5 pc), exceeding previous estimates by a factor of 2.5. King model fits are used to disentangle the count statistics of cluster stars from those of field stars. With knowledge of the cluster radius we then construct and analyze the Ks-band luminosity function (KLF) of the NGC 3603 starburst cluster. The KLF for cluster radii >30$\hbox{$^{\prime\prime}$}$is rising down to the completeness limit of our study at Ks~ 17$\hbox{m}$–17$\hbox{m}\!\!\!.\,$5, corresponding to 1 Myr old cluster members of $\cal M$~ 0.5 $\cal M\hbox{$_{\odot}$}$. For the range of intermediate luminosity stars we obtain a KLF slope of α= 0.35 ±0.02 which is consistent with a Miller-Scalo type initial mass function (IMF) of a ~1 Myr old stellar cluster if one assumes a power-law index of β~ 2 for the mass-luminosity relation of both intermediate mass main sequence stars and low mass pre-main sequence stars. At the high luminosity end the observed KLF likely flattens (α~ 0.2), being consistent with a Salpeter type IMF for NGC 3603's massive main sequence stars.

Context.The triple stellar system δVel (composed of two A-type and one F-type main-sequence stars) is particularly interesting because it contains one of the nearest and brightest eclipsing binaries. It therefore presents a unique opportunity to determine independently the physical properties of the three components of the system, as well as its distance.

The variation in the period distribution function of late-type binaries is studied. It is shown that the Taurus-Auriga pre-main-sequence population and the main-sequence G dwarf sample do not stem from the same parent period distribution with better than 95 per cent confidence probability. The Lupus, Upper Scorpius A, and Taurus-Auriga populations are shown to be compatible with being drawn from the same initial period function (IPF), which is inconsistent with the main-sequence data. Two possible IPF forms are used to find parent distributions to various permutations of the available data, which include Upper Scorpius B (UScB), Chameleon, and Orion Nebula Cluster pre-main-sequence samples. All the pre-main-sequence samples studied here are consistent with the hypothesis that there exists a universal IPF that is modified by binary-star disruption if it forms in an embedded star cluster leading to a general decline of the observed period function with increasing period. The pre-main-sequence data admit a log-normal IPF similar to that arrived at by Duquennoy & Mayor (1991, A&A, 248, 485) for main-sequence stars, provided the binary fraction among pre-main-sequence stars is significantly higher. However, for consistency with proto-stellar data, the possibly universal IPF ought to be flat along the log-P or log-semi-major axis and must be similar to the K1 IPF form derived by means of inverse dynamical population synthesis, which has been shown to lead to the main-sequence period function if most stars form in typical embedded clusters.

Context.TMR-1 (IRAS 04361+2547) is a class I proto-stellar source located in the nearby Taurus star-forming region. Its circumstellar environment is characterized by extended dust emission with complex structures and conspicuous filaments. A faint companion, called TMR-1C, located near the proto-star was detected in previous studies, but its nature as a very young substellar object remains inconclusive.