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We present a high-contrast imaging survey of intermediate-mass (1.75--4.5 $M_\odot$) stars to search for the most extreme stellar binaries, i.e., for the lowest mass stellar companions. Using adaptive optics at the Lick and Gemini…
Using the helium abundance measured by Galileo in the atmosphere of Jupiter and interior models reproducing the observed external gravitational field, we derive new constraints on the composition and structure of the planet. We conclude…
Mass, radius, and age are three of the most fundamental parameters for celestial objects, enabling studies of the evolution and internal physics of stars, brown dwarfs, and planets. Brown dwarfs are hydrogen-rich objects that are unable to…
Most stars form in a clustered environment. Both single and binary stars will sometimes encounter planetary systems in such crowded environments. Encounter rates for binaries may be larger than for single stars, even for binary fractions as…
This review summarizes the observational constraints on the mass spectrum of compact objects in X-ray binaries. We currently have 20 X-ray binaries with confirmed black holes, based on dynamical information (i.e. mass in excess of 3 Msun).…
The standard model for giant planet formation is based on the accretion of solids by a growing planetary embryo, followed by rapid gas accretion once the planet exceeds a so-called critical mass. The dominant size of the accreted solids…
It is unclear how very close binary stars form, given that during the pre-main-sequence phase the component stars would have been inside each other. One hypothesis is that they formed further apart, but were brought in closer after…
We present the first high-angular resolution survey for multiple systems among very low-mass stars and brown dwarfs in the Hyades open cluster. Using the Keck\,II adaptive optics system, we observed a complete sample of 16 objects with…
The discovery of planets orbiting at less than 1 au from their host star and less massive than Saturn in various exoplanetary systems revolutionized our theories of planetary formation. The fundamental question is whether these close-in…
It has now been known for over a decade that low-mass stars located in star-forming regions are very frequently members of binary and multiple systems, even more so than main sequence stars in the solar neighborhood. This high multiplicity…
We suggest that planets, brown dwarfs, and even low mass stars can be formed by fragmentation of protoplanetary disks around very massive stars M>~100 solar masses. We discuss how fragmentation conditions make the formation of very massive…
The region around the H$_2$O ice line, due to its higher surface density, seems to be the ideal location to form planets. The core of Jupiter, as well as the cores of close in gas giants are thus thought to form in this region of the disk.…
The complexity of composite spectra of close binary star system makes study of the spectra of their component stars extremely difficult. For this reason there exists very little information on the photospheric chemical composition of stars…
Almost all massive stars are part of a binary system. Given the wide range of separations at which these companions are found, several observational techniques have been adopted to characterize them, but contrasts greater than 4 in the…
We present the results of a high-resolution imaging survey of 12 brown dwarfs and very low mass stars in the closest (~145 pc) young (~5 Myr) OB association, Upper Scorpius. We obtained images with the Advanced Camera for Surveys/High…
We present new near-IR observations of 76 Class I/flat-spectrum objects in the nearby (d < 320 pc) Perseus, Taurus, Chamaeleon I and II, rho Ophiuchi, and Serpens dark clouds. These observations are part of a larger systematic infrared…
The origin of planetary mass objects (PMOs) wandering in young star clusters remains enigmatic, especially when they come in pairs. They could represent the lowest-mass object formed via molecular cloud collapse or high-mass planets ejected…
The widespread prevalence of close-in, nearly coplanar super-Earth- and sub-Neptune-sized planets in multiple-planet systems was one of the most surprising results from the Kepler mission. By studying a uniform sample of Kepler "multis"…
Within the SONYC - Substellar Objects in Nearby Young Clusters - survey, we investigate the frequency of free-floating planetary-mass objects (planemos) in the young cluster NGC1333. Building upon our extensive previous work, we present…
We report the present day mass functions (PDMFs) of 3 young open clusters over a mass range from 30 Jupiter masses to 10~\msunn. The PDMFs of the 3 clusters are remarkably similar, suggesting little impact of specific conditions (stellar…