Protoplanetary Disks in the ALMA Era

The σ Orionis cluster is important for studying protoplanetary disk evolution, as its intermediate age (~3–5 Myr) is comparable to the median disk lifetime. Ansdell et al. used ALMA to conduct a high-sensitivity survey of dust and gas in 92 protoplanetary disks around σ Orionis members with M_* ≳ 0.1 M_⊙. These ALMA observations cover the 1.33 mm continuum and several CO J = 2–1 lines: of 92 sources, 37 are detected in the millimeter continuum, 6 in ¹²CO, 3 in ¹³CO, and none in C¹⁸O. Using the continuum emission to estimate dust mass, the authors find only 11 disks with M_dust ≳ 10 M_⊕, indicating that after only a few million years of evolution most disks lack sufficient dust to form giant planet cores. Stacking the individually undetected continuum sources limits their average dust mass to 5X lower than that of the faintest detected disk, supporting theoretical models that indicate rapid dissipation once disk clearing begins. Comparing the protoplanetary disk population in σ Orionis to those of other star-forming regions supports the steady decline in average dust mass and the steepening of the M_dust – M_* relation with age; studying these evolutionary trends can inform the relative importance of different disk processes during key planet formation eras. External photoevaporation from the central O9 star is influencing disk evolution throughout the region: dust masses clearly decline with decreasing separation from the photoionizing source. Collectively, the findings of Ansdell et al. indicate that giant planet formation is inherently rare and/or well underway by a few million years of age.

Illustration caption: Continuum images at 1.33 mm of the 37 detected disks in the σ Orionis ALMA sample, ordered by decreasing flux density. Images are 2 x 2 arcsec; the typical beam size of 0.31 × 0.25 arcsec is shown in the first panel (top row, left end).

Publication: M. Ansdell (University of Hawai‘i at Mānoa) et al., An ALMA Survey of Protoplanetary Disks in the σ Orionis Cluster, 2017, Astronomical Journal, 153, 240.