FORMATION OF GIANT PLANETS BY DISK INSTABILITY ON WIDE ORBITS AROUND PROTOSTARS WITH VARIED MASSES

Doppler surveys have shown that more massive stars have significantly higher frequencies of giant planets inside similar to 3 AU than lower mass stars, consistent with giant planet formation by core accretion. Direct imaging searches have begun to discover significant numbers of giant planet candidates around stars with masses of similar to 1 M circle dot to similar to 2 M circle dot at orbital distances of similar to 20 AU to similar to 120 AU. Given the inability of core accretion to form giant planets at such large distances, gravitational instabilities of the gas disk leading to clump formation have been suggested as the more likely formation mechanism. Here, we present five new models of the evolution of disks with inner radii of 20 AU and outer radii of 60 AU, for central protostars with masses of 0.1, 0.5, 1.0, 1.5, and 2.0 M circle dot, in order to assess the likelihood of planet formation on wide orbits around stars with varied masses. The disk masses range from 0.028 M circle dot to 0.21 M circle dot, with initial Toomre Q stability values ranging from 1.1 in the inner disks to similar to 1.6 in the outer disks. These five models show that disk instability is capable of forming clumps on timescales of similar to 10(3) yr that, if they survive for longer times, could form giant planets initially on orbits with semimajor axes of similar to 30 AU to similar to 70 AU and eccentricities of similar to 0 to similar to 0.35, with initial masses of similar to 1 M-Jup to similar to 5 M-Jup, around solar-type stars, with more protoplanets forming as the mass of the protostar (and protoplanetary disk) is increased. In particular, disk instability appears to be a likely formation mechanism for the HR 8799 gas giant planetary system.