Properties of Circum(sub)stellar Accretion Disks

We have completed a systematic survey for disks around young brown dwarfs and very low mass stars. By choosing a well-defined sample and by obtaining sensitive thermal IR observations, we can make an unbiased measurement of the disk fraction around such low mass objects. We find that ≈75% of our sample show intrinsic IR excesses, indicative of circum(sub)stellar disks. We discuss the physical properties of these disks and their relation to the much better studied disks around solar-mass stars. The high incidence of disks around substellar objects also raises the possibility of planetary systems around brown dwarfs. 1. A Survey for Brown Dwarf Disks The existence of circumstellar disks and their role in planet formation are well established for young solar-type stars. However, little is known about disks around young substellar objects. Such circum(sub)stellar disks might provide a laboratory for studying the physical processes of disks over a wide range of (central object and disk) mass. In addition, the presence of disks around young substellar objects may be an important clue to the origin of brown dwarfs. We have recently completed a large thermal IR (L-band; 3.8 μm) survey to study the frequency and properties of disks around young brown dwarfs and very low mass stars (Liu et al. 2002; see also Liu 2002). Our sample comprises young (∼1–3 Myr) objects in nearby star-forming regions which have been spectroscopically classified to be very cool, corresponding to masses of ∼15 to ∼100 MJup based on current models. As described in our paper, the objects constitute a well-defined sample and are largely free of selection biases. A priori, brown dwarf disks are expected to be harder to detect than disks around stars because of lower contrast. Substellar objects are less luminous and have shallower gravitational potentials. Hence, their disks should be relatively cool and may have negligible excesses in the commonly used JHK (1.1−2.4 μm) bands. In fact, we find that thermal IR data are required to detect most disks 1 Beatrice Watson Parrent Fellow. Email: [email protected]