GeminiFocus 2015 Year in Review | Page 15

grains, which coagulate over time, grow in size, and settle towards the disk midplane. Present theories hold that giant planet formation in disks likely occurs via one of two mechanisms: core accretion or gravitational instability. In the core accretion scenario, ice-covered dust particles collide and stick together, growing into ever-larger rocky bodies; planetesimals form from this buildup of rocky material, and their collisions eventually build super-Earth-mass planetary “cores.” Such massive cores can then rapidly collect gas from the disk to form giant planets. In contrast, the gravitational instability theory holds that planets form when a perturbation in the disk causes a large amount of disk material to collapse and form a planet essentially all at once. Hence, this rapid process is similar to that by which a young central star forms from its birth cloud. Under either scenario, once a massive planet forms, co-orbiting material either accretes onto the planet or is accelerated radially in the disk via spiral density waves, which cause material approaching them to speed up — until they reach the perturbed regions, where they slow down and linger. These mechanisms result in ring-like or spiral structures in the disk characterized by sharp radial gradients in both surface density and particle size. The predicted spiral and ring structures have indeed been observed in disks around young stars in nearby star-forming clouds with telescopes such as the Sub-Millimeter Array (SMA) and Atacama Large Millimeter Array (ALMA), and with near-infrared cameras on 8-meter-class telescopes, such as Subaru and Gemini. However, these observations are typically probing planet formation around young stars at orbital separations many times that of the gas giants in our Solar System. formation around young nearby stars within ~ 30 AU, where we can search for evidence for gas giant planet formation on scales similar to that of our Solar System. We do so by focusing on a handful of solar mass stars within ~ 300 light years (ly) of Earth that are surrounded by, and actively accreting material from, gas-rich circumstellar disks. Target: V4046 Sagittarii Our team has been closely scrutinizing one such star-disk system: V4046 Sagittarii (Sgr). Lying at a distance of just ~ 240 ly, V4046 Sgr is an extraordinary binary star system surrounded by a massive disk of gas and dust roughly 0.1 solar mass. With an age of only ~ 20 million years, this system provides us with an excellent opportunity to search for evidence of recent or ongoing planet formation. The V4046 Sgr system also offers an intriguing twist: Any planets spawned in its disk would have to orbit twin stars (both only slightly less massive than our Sun) separated by just ~ 9 solar radii. Interestingly, the V4046 Sgr disk exhibits a central clearing out to ~ 30 AU (i.e., ~ 1/2 arcsecond) at submillimeter wavelengths. The presence of this inner “hole” suggests that one or more planets may be forming close to the central stars, carving out an opening within the disk (Figure 1 shows an artist’s rendering of the V4046 Sgr disk based on Figure 1. Artist’s impression of the the binary stardisk system V4046 Sgr. Image credit: NASA/ JPL-Caltech/T. Pyle (SSC). Our team is interested in studying planet January 2016 2015 Year in Review GeminiFocus 13