Probing the final stages of protoplanetary disk evolution with ALMA

Abstract

Context. The evolution of a circumstellar disk from its gas-rich protoplanetary to gas-poor debris stage is not well understood. It is
\napparent that disk clearing progresses from the inside-out on a short time-scale, and photoevaporation models are frequently invoked
\nto explain this process. However, the photoevaporation rates predicted by recent models differ by up to two orders of magnitude,
\nresulting in uncertain time-scales for the final stages of disk clearing.
\nAims. Photoevaporation theories predict that the final stages of disk clearing progress in objects which have ceased accretion but
\nstill posses considerable material at radii far from the star. Weak-line T Tauri stars (WTTS) with infrared emission in excess of that
\nexpected from the stellar photosphere are likely in this configuration. We here aim to provide observational constraints on theories of
\ndisk-clearing by measuring the dust masses and CO content of a sample of young (1.8-26.3 Myr) WTTS.
\nMethods. We use ALMA band-6 to obtain continuum and [SUPERSCRIPT 12]CO(2-1) line fluxes for a sample of 24 WTTS stars with known infrared
\nexcess. For these WTTS, we infer the dust mass from the continuum observations, and derive disk luminosities and ages to allow
\ncomparison with previously detected WTTS.
\nResults. We detect continuum emission in only 4 of 24 WTTS, and no [SUPERSCRIPT 12]CO(2-1) emission in any. For those WTTS without a
\ncontinuum detection, their ages and derived upper-limits suggest they are debris disks, making them some of the youngest known. Of
\nthose with a continuum detection, 3 are possible photoevaporating disks, although the lack of CO detection is suggestive of a severely
\nreduced gas-to-dust ratio.
\nConclusions. The low fraction of continuum detections implies that once accretion onto the star stops, the clearing of the majority
\nof dust progresses very rapidly. Most WTTS with infrared excess are likely not in transition but are instead young debris disks, with
\ntheir dust being either primordial having survived the disk clearing, or of second generation origin. In the latter case, the presence of
\ngiant planets within these WTTS might be the cause.