A SENSITIVE 10 MU-M SEARCH FOR EMISSION ARISING FROM CIRCUMSTELLAR DUST ASSOCIATED WITH SOLAR-TYPE PRE-MAIN-SEQUENCE STARS

Excess infrared radiation above photospheric levels is used to diagnose the presence of circumstellar disks surrounding solar-type pre-main-sequence (PMS) stars, and to determine the evolutionary time-scales for these disks. Our analysis combines published optical photometry, near-infrared fluxes, and IRAS fluxes for a sample of 83 solar-type PMS stars in Taurus-Auriga, with new, highly sensitive 10 μm measurements made with the NASA Infrared Telescope Facility (IRTF). Of sample stars with ages t<3 Myr, ∼1/2 show excess 2.2 and 10 μm emission consistent with that expected from optically thick disks extending inward to the stellar surface. By an age t∼10 Myr, fewer than 10% of our sample show evidence of dust emission from optically thick disks, and hence must have accreted or destroyed their disks, or have begun to assemble distributed gas and dust into larger bodies. Hence, the timescale over which disks survive as infrared-luminous, optically thick structures is t<10 Myr. Out of a sample which contains 33 solar-type PMS stars surrounded by optically thick disks (typical age, t∼3 Myr), three show evidence of inner holes. These holes are signified by small near-infrared (λ≤25 μm) excesses arising in optically thin regions located at r<0.2 AU, and large far-infrared excesses produced in regions r>1 AU. Disks with inner holes may be "transition structures" which have begun to assemble material into larger bodies in the terrestrial planet region. The frequency of occurrence of transition structures in our sample suggests that a given disk evolves from an optically thick disk to an optically thin structure on a timescale t∼0.3 Myr.