Disk Mass Limits and Lifetimes of Externally Irradiated Young Stellar Objects Embedded in the Orion Nebula

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Abstract
We present 1.3 mm wavelength interferometric observations of externally irradiated young stellar objects (proplyds) embedded within the Orion Nebula, including the three largest circumstellar disks seen in silhouette against the background nebular light. One field is centered on the 2'' diameter edge-on disk 114-426. The second field is centered on the large teardrop-shaped object 182-413 (HST 10), which contains a very opaque 04 diameter edge-on disk. This field 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 absence of continuum emission and upper bounds on the gas masses from the lack of CO emission. These limits imply circumstellar disk masses less than 0.015 M⊙ for the observed sources and upper bounds on the column density of 13CO of N(13CO) < 1.4 × 1015 cm-2 averaged over the synthesized beam. Comparison with lower bounds on the dust content derived from the visibility of the circumstellar material in silhouette against the background nebular light and the extinction toward the embedded central star implies that 13CO may be less abundant 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 the Orion Nebula that are still surrounded by circumstellar material have been exposed to external UV radiation for less than 105 yr and possibly for as little as 104 yr.