Probing the Final Stages of Protoplanetary Disk Evolution with ALMA [EPA]

The evolution of a circumstellar disk from its gas-rich protoplanetary to gas-poor debris stage is not well understood. It is apparent that disk-clearing progresses from the inside-out on a short time-scale, and photoevaporation models are frequently invoked to explain this process. However, the photoevaporation rates predicted by recent models differ by up to two orders of magnitude, resulting in uncertain time-scales for the final stages of disk clearing. The best candidates for studying this stage are weak line T Tauri stars (WTTS) with significant IR excess. We here aim to provide observational constraints on theories of disk-clearing by measuring the dust masses and CO content of a sample of such WTTS. We use ALMA band-6 to obtain continuum and $^{12}$CO(2-1) line fluxes for a sample of 24 WTTS stars with a known IR-excess. For these systems, we infer the dust mass from the continuum observations, and derive disk luminosities and ages to allow comparison with previously detected systems. We detect continuum emission in only 4 of 24 systems, and no $^{12}$CO(2-1) emission in any. For those systems without a continuum detection, the dust mass and fractional disk luminosity upper-limits suggest they are in the debris disk regime, making them some of the youngest debris disks known. Of those with a continuum detection, three are possible photoevaporating disks but photodissociation has likely reduced the CO abundance to below our detection limit. The low fraction of continuum detections implies that once accretion onto the star stops, the clearing of the majority of dust progresses very rapidly. Most WTTS with IR excess are not in transition but resemble debris disks. The dust in these disks is either primordial and survived the disk clearing, or is of second generation origin. In the latter case, the presence of giant planets within these systems might be the cause.

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A. Hardy, C. Caceres, M. Schreiber, et. al.
Wed, 22 Apr 15

Comments: submitted to A&A