DISK MASS LIMITS AND LIFETIMES OF EXTERNALLY IRRADIATED YOUNG STELLAROBJECTS EMBEDDED IN THE ORION NEBULA

We present 1.3 mm wavelength interferometric observations of externall y irradiated young stellar objects (proplyds) embedded within the Orio n Nebula, including the three largest circumstellar disks seen in silh ouette against the background nebular light. One field is centered on the 2 '' diameter edge-on disk 114-426. The second field is centered o n the large teardrop-shaped object 182-413 (HST 10), which contains a very opaque 0.'' 4 diameter edge-on disk. This held also contains four other proplyds, including the large pure silhouette 183-405 (HST 16). We derive upper bounds on the dust masses of these disks from the abs ence of continuum emission and upper bounds on the gas masses from the lack of CO emission. These limits imply circumstellar disk masses les s than 0.015 M-. for the observed sources and upper bounds on the colu mn density of (CO)-C-13 of N((CO)-C-13) < 1.4 x 10(15) cm(-2) averaged over the synthesized beam. Comparison with lower bounds on the dust c ontent derived from the visibility of the circumstellar material in si lhouette against the background nebular light and the extinction towar d the embedded central star implies that (CO)-C-13 may be less abundan t in these circumstellar environments than in normal molecular clouds. The nondetection statistics are combined with estimates of radiation- induced mass-loss rates to derive an upper bound on the UV irradiation time for these young stellar objects. The young stellar objects in th e Orion Nebula that are still surrounded by circumstellar material hav e been exposed to external UV radiation for less than 10(5) yr and pos sibly for as little as 10(4) yr.