A SCUBA-2 850-$\mu$m survey of circumstellar disks in the $\lambda$ Orionis cluster

TL;DR

This study used submillimeter observations to investigate circumstellar disks in the 5-million-year-old $\\lambda$ Orionis cluster, finding limited dust and gas, which constrains planet formation timescales.

Contribution

First detailed submillimeter survey of $\\lambda$ Orionis disks, providing new constraints on disk mass and gas content at this age.

Findings

Only one disk detected with significant dust mass.

Upper limit on average dust mass for undetected disks is ~3 M$_{\\oplus}$.

HD 245185 shows low gas-to-dust ratio, indicating disk dispersal.

Abstract

We present results from an 850-$\mu$m survey of the $\sim$ 5 Myr old $\lambda$ Orionis star-forming region. We used the SCUBA-2 camera on the James Clerk Maxwell Telescope to survey a $\sim$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 $>3\sigma$ significance, the Herbig Ae star HD 245185, with a flux density of $\sim74$ mJy beam$^{-1}$ corresponding to a dust mass of $\sim150$ M$_{\oplus}$. Stacking the individually undetected sources did not produce a significant mean signal but gives an upper limit on the average dust mass for $\lambda$ Orionis disks of $\sim3$ M$_{\oplus}$. Our follow-up observations of HD 245185 with the Submillimeter Array found weak CO 2--1 line emission with an integrated flux of $\sim170$ mJy km s$^{-1}$ but no $^{13}$CO or C$^{18}$O isotopologue emission at 30 mJy km s$^{-1}$ sensitivity, suggesting a gas mass of $\lesssim1$ M$_{\rm Jup}$. The implied gas-to-dust ratio is thus $\gtrsim50$ 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 $\lambda$ Orionis cluster places quantitative constraints on planet formation timescales, indicating that at $\sim5$ 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.