2015ApJ...806..221A

We present results from an 850 µm survey of the ∼5 Myr old λ Orionis star-forming region. We used the SCUBA-2 camera on the James Clerk Maxwell Telescope to survey a ∼0.°5-diameter circular region containing 36 (out of 59) cluster members with infrared excesses indicative of circumstellar disks. We detected only one object at > 3 σ significance, the Herbig Ae star HD 245185, with a flux density of ∼74 mJy/beam corresponding to a dust mass of ∼150 M_⊕. Stacking the individually undetected sources did not produce a significant mean signal but gives an upper limit on the average dust mass for λ Orionis disks of ∼3 M_⊕. Our follow-up observations of HD 245185 with the Submillimeter Array found weak CO 2-1 line emission with an integrated flux of ∼170 mJy km/s but no ¹³CO or C¹⁸O isotopologue emission at 30 mJy km/s sensitivity, suggesting a gas mass of ≲ 1 M_Jup. The implied gas-to-dust ratio is thus ≳ 50 times lower than the canonical interstellar medium value, setting HD 245185 apart from other Herbig Ae disks of similar age, which have been found to be gas rich; as HD 245185 also shows signs of accretion, we may be catching it in the final phases of disk clearing. Our study of the λ Orionis cluster places quantitative constraints on planet formation timescales, indicating that at ∼5 Myr the average disk no longer has sufficient dust and gas to form giant planets and perhaps even super-Earths; the bulk material has been mostly dispersed or is locked in pebbles/planetesimals larger than a few mm in size.