ALMA Observations of the Largest Proto-Planetary Disk in the Orion Nebula, 114-426: A CO Silhouette

We present ALMA observations of the largest protoplanetary disk in the Orion Nebula, 114–426. Detectable 345 GHz (856 μm) dust continuum is produced only in the 350 AU central region of the ~1000 AU diameter silhouette seen against the bright Hα background in Hubble Space Telescope images. Assuming optically thin dust emission at 345 GHz, a gas-to-dust ratio of 100, and a grain temperature of 20 K, the disk gas-mass is estimated to be 3.1±0.6 Jupiter masses. If most solids and ices have been incorporated into large grains, however, this value is a lower limit. The disk is not detected in dense-gas tracers such as HCO^+ J = 4–3, HCN J = 4–3, or CS = 7–6. These results may indicate that the 114–426 disk is evolved and depleted in some light organic compounds found in molecular clouds. The CO J = 3–2 line is seen in absorption against the bright 50–80 K background of the Orion A molecular cloud over the full spatial extent and a little beyond the dust continuum emission. The CO absorption reaches a depth of 27 K below the background CO emission at V_LSR ≈ 6.7 km s^-1 ~ 0".52 (210 AU) northeast and 12 K below the background CO emission at V_LSR ≈ 9.7 km s^-1 ~ 0".34 (140 AU) southwest of the suspected location of the central star, implying that the embedded star has a mass less than 1 M_⊙