ALMA Survey of Lupus Class III Stars: Early Planetesimal Belt Formation and Rapid Disk Dispersal

Abstract

Class III stars are those in star forming regions without large non-photospheric infrared emission, suggesting recent dispersal of their protoplanetary disks. We observed 30 class III stars in the 1-3 Myr Lupus region with ALMA at $\sim 856\mu$m, resulting in 4 detections that we attribute to circumstellar dust. Inferred dust masses are $0.036-0.093M_{\oplus }$, $\sim 1$ order of magnitude lower than any previous measurements; one disk is resolved with radius $\sim 80$ au. Two class II sources in the field of view were also detected, and 11 other sources, consistent with sub-mm galaxy number counts. Stacking non-detections yields a marginal detection with mean dust mass $\sim 0.0048M_{\oplus }$. We searched for gas emission from the CO J=3-2 line, and present its detection to NO Lup inferring a gas mass ($4.9\pm 1.1$) $\times 10^{-5}{M}_{\oplus }$ and gas-to-dust ratio $1.0\pm 0.4$. Combining our survey with class II sources shows a gap in the disk mass distribution from $0.09-2M_{\oplus }$ for $>0.7M_{\odot }$ Lupus stars, evidence of rapid dispersal of mm-sized dust from protoplanetary disks. The class III disk mass distribution is consistent with a population model of planetesimal belts that go on to replenish the debris disks seen around main sequence stars. This suggests that planetesimal belt formation does not require long-lived protoplanetary disks, i.e., planetesimals form within $\sim$2 Myr. While all 4 class III disks are consistent with collisional replenishment, for two the gas and/or mid-IR emission could indicate primordial circumstellar material in the final stages of protoplanetary disk dispersal. Two class III stars without sub-mm detections exhibit hot emission that could arise from ongoing planet formation processes inside $\sim 1$ au.

30 pages, 20 figures. This is a pre-copyedited, author-produced PDF of an article accepted for publication in MNRAS following peer review

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