Abstract
The evolution of a circumstellar disk from its gas-rich protoplanetary stage to its gas-poor debris stage is not understood well. It is apparent that disk clearing progresses from the inside-out on a short time scale and models of photoevaporation are frequently used to explain this. However, the photoevaporation rates predicted by models differ by up to two orders of magnitude, resulting in uncertain time scales for the final stages of disk clearing. Photoevaporation theories predict that the final stages of disk-clearing progress in objects that have ceased accretion but still posses considerable material at radii far from the star. Weak-line T Tauri stars (WTTS) with infrared excess are likely in this configuration. We aim to provide observational constraints on theories of disk-clearing by measuring the dust masses and CO content of a sample of young (1.8-26.3 Myr) WTTS. We used ALMA Band 6 to obtain continuum and 12 CO(2-1) line fluxes for a sample of 24 WTTS stars with known infrared excess. We detect continuum emission in only four of 24 WTTS, and no 12 CO(2-1) emission in any. For those WTTS where no continuum was detected, their ages and derived upper limits suggest they are debris disks, which makes them some of the youngest debris disks known. Of those where continuum was detected, three are possible photoevaporating disks, although the lack of CO detection suggests a severely reduced gas-to-dust ratio. 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 infrared excess are likely not in transition but are instead young debris disks, whose dust is either primordial and has survived disk-clearing, or is of second-generation origin.
Highlights
Circumstellar disks typically fall into two categories – the massive gas-rich protoplanetary disks and the gas-poor debris disks
Andrews & Williams (2005) show that, owing to continuum emission being optically thin at mm wavelengths, the dust mass can be estimated by a simple equation of the form Mdust = Cν × Fν where Cν is a constant for a given frequency Fν
All the above sources in our sample are beyond the stage of active gas accretion. Their spectral energy distribution (SED) are suggestive of a depleted dust mass, and here we confirm this, either with their low 1.3 mm flux or with their non-detection from ALMA
Summary
Circumstellar disks typically fall into two categories – the massive gas-rich protoplanetary disks and the gas-poor debris disks. Protoplanetary disks are a natural consequence of the star formation process, with the majority of their evolution being dominated by viscous accretion onto the star (e.g., Lynden-Bell & Pringle 1974). It has long been suggested that debris disks could be a later stage of evolution from the protoplanetary disks, but the nature of the main physical processes that drive this evolution is ill-understood and remains one of the biggest questions in the field. To understand this phenomenon, the nature of a class of disks known as transition disks (TDs), needs to be investigated
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