Characterizing circumstellar disks at 5 myr: a multiwavelength survey for gas and dust in Upper Scorpius

Abstract

At an age of 1 million years, massive dust and gas rich circumstellar disks are ubiquitous, while by 10 million years most disks have become undetectable. I have focused on the characterization of circumstellar disks in the 5 million year old Upper Scorpius group, a group of stars that probe the middle of this age range, in order to gain insight to the speed at which the disk dissipation process occurs and the time available for giant planet formation. I have carried out the most sensitive survey to-date for millimeter continuum emission from large dust grains in intermediate aged disks. With a dust mass sensitivity of 4x10-3 MJup and under the assumption of the primordial 100-to-1 gas-to-dust ratio, I have found that less than 2% of stars in this group could have su cient mass to continue forming giant planets. Further work with the Sub-Millimeter Array has allowed me to image the dust and gas in the most massive of these disks, revealing a 70 AU inner hole containing sparse dust, a sign-post of the potential presence of giant planets. With Herschel Science Demonstration Phase observations of four stars at ages from 6 to 12 Myr, I showed the utility of the far-infrared oxygen line in tracing the presence of gas in disks. I have built on this work by surveying a subset of the Upper Scorpius disks with the Herschel Space Observatory, combining observations of the [OI] 63 m line with ground based observations of millimeter wavelength CO lines to independently assess the gas content of these disks. Comparison to models of gas-line emission suggest that the gas-to-dust ratio in the most massive disk in this region is 20. If this applies to all Upper Scorpius disks, then only one disk { 0.5% of the population { could potentially form further giant planets. In the eld, 20% of stars host giant planets; this suggests that giant planet formation is complete by the age of Upper Scorpius.