Subarcsecond Millimeter and Submillimeter Observations of Circumstellar Disks

o o o oo o o oo o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o Millimeter and submillimeter bservati ns pr vide a valuable t l f r pr bing the c l, dusty material in the disks ar und y ung stars. With ng ing instrumental upgrades, the technique f interfer metry n w btains subarcsec nd res luti n at these wavelengths, capable f spatially res lving the size scales ass ciated with planet f rmati n in nearby star-f rming dark cl uds. We describe in detail the subarcsec nd millimeter and submillimeter bservati ns f the well-kn wn l w-mass y ung stellar bjects HL Tau and L1551 IRS5 t illustrate what may be learned fr m such bservati ns. High-res luti n multifrequency c ntinuum bservati ns yield estimates f surface mass density distributi ns and ptical depths, pr viding imp rtant c nstraints n accreti n mechanisms as well as starting p ints f r m deling the ev luti n f the primitive s lar nebula. Drawing n bservati ns f several l w-mass pre-main-sequence systems, we examine p wer law disk m dels, departures fr m simple p wer law structure, and the influence f c mpani n b dies. We als discuss bservati ns f maser emissi n as subarcsec nd pr bes f gas kinematics in s me s urces.