Young stars with brown dwarfs

Because most nearby stars are old, studying young planetary systems entails looking beyond the solar neighborhood. Sasha Hinkley of the University of Exeter in the UK and his collaborators are using two of the world’s largest optical telescopes, the Keck II in Hawaii and the Very Large Telescope in Chile, to do just that. Their hunting ground is the Scorpius– Centaurus Association, a region of ongoing star formation 10 million to 20 million years old and 400 light-years away. At that distance, an object 10–100 times the mass of Jupiter (M_J) is still bright enough to be detected directly—provided its orbit is 30 astronomical units (AU) or wider. Closer orbits hit the telescopes’ diffraction limits. To search for those close-in objects, Hinkley and his collaborators use aperture mask interferometry. An opaque mask with a set of 7 or 9 holes is placed in the telescope’s pupil. The collection of holes generates interference patterns, such as the one shown here, that encode a range of angular distances. In their pilot study, the researchers targeted 140 stars whose masses range from 1.5 to 4.5 solar masses. When observed with the mask, six stars turned out to have companions; one has two companions. None of the companions are planets: Four are small red stars, and three are brown dwarfs—substellar objects whose masses are too low to initiate hydrogen fusion. For stars of the targets’ mass and age, no companions with the same combination of mass (20–200 M_J) and orbital separation (10–30 AU) have ever been observed before. (S. Hinkley et al., Astrophys. J. Lett. 806, L9, 2015, doi:10.1088/2041-8205/806/1/L9 .)