Bridging the Gap between Protoplanetary and Debris Disks: Separate Evolution of Millimeter and Micrometer-sized Dust

Abstract

The connection between the nature of a protoplanetary disk and that of a debris disk is not well understood. Dust evolution, planet formation, and disk dissipation likely play a role in the processes involved. We aim to reconcile both manifestations of dusty circumstellar disks through a study of optically thin Class III disks and how they correlate to younger and older disks. In this work, we collect literature and Atacama Large Millimeter/submillimeter Array archival millimeter fluxes for 85 disks (8%) of all Class III disks across nearby star-forming regions. We derive millimeter-dust masses M dust and compare these with Class II and debris disk samples in the context of excess infrared luminosity, accretion rate, and age. The mean M dust of Class III disks is 0.29 ± 0.19 M ⊕. We propose a new evolutionary scenario wherein radial drift is very efficient for nonstructured disks during the Class II phase resulting in a rapid M dust decrease. In addition, we find possible evidence for long infrared protoplanetary disk timescales, ∼8 Myr, consistent with overall slow disk evolution. In structured disks, the presence of dust traps allows for the formation of planetesimal belts at large radii, such as those observed in debris disks. We propose therefore that the planetesimal belts in debris disks are the result of dust traps in structured disks, whereas protoplanetary disks without dust traps decrease in dust mass through radial drift and are therefore undetectable as debris disks after the gas dissipation. These results provide a hypothesis for a novel view of disk evolution.

Demographics of young stars and their protoplanetary disks: lessons learned on disk evolution and its connection to planet formation

“…This is sufficient for drawing inferences about planet formation, but is about an order of magnitude greater than the dust masses of debris disks. This means that there is a significant gap in our knowledge of the late stages of disk dispersal, as testified by contradictory results in recent work (Lovell et al 2021;Michel et al 2021).…”

“…In recent years, surveys with far-IR and millimeter instruments like Herschel, JCMT, ALMA, and WISE have revealed how debris disk mass changes with age across a range of stellar temperatures (Moór et al 2016;Holland et al 2017;Pawellek et al 2021). Recent work by Michel et al (2021) has proposed a separate evolutionary pathway for debris disks: a radial-drift dominated mode leading to rapid dust dissipation in featureless disks (which then do not become observable debris disks), and a slower evolution of structured disks, suggesting that most observed cold debris disks inherit some amount of structure from their protoplanetary predecessors. This proposal is supported by recent theoretical modeling work (Najita et al 2022) and studies of large samples of exoplanet properties and ALMA disk observations (van der Marel and Mulders 2021).…”

“…This conclusion is strengthened by the survey of Ophiuchus protostars across evolutionary stages (ODISEA, Cieza et al 2019), which shows that, while dust mass does decrease with protostellar evolutionary stage, disk dust masses do not decrease monotonically with age (Williams et al 2019), in line with other surveys (Cazzoletti et al 2019). However, with the advent of Gaia, more rigorous membership determination has called the ages of some of these sources into question and demonstrated for example that many of the Class III sources previously identified with Lupus might actually be part of the older surrounding Sco-Cen region, for example Upper Centaurus Lupus with an age of 16 Myr (Pecaut et al 2012;Luhman 2020a;Michel et al 2021).…”

“…While debris disks are usually observed around A and B dwarfs, the lower-mass counterparts are less studied, making comparison with protoplanetary disk populations more uncertain (e.g., Michel et al 2021). Debris disks around M dwarfs are difficult to observe due to the relatively low masses and temperatures in these systems (Luppe et al 2020).…”

“…These two surveys pose questions on the simple interpretation of a decrease in mm-size content in disks with age, and possibly suggest that the evolution of the dust content in disks is subject to replenishment, maybe due to collisions between planetesimals (e.g., Ger- The star-forming histories of most regions are more complex than a single age indicates and new Gaia data on the 3D structure and kinematics are improving our understanding of different subgroups therein (e.g., Krolikowski et al 2021). Similarly, there is an overlap in both sky position and proper motion between the young stars in Lupus with older stars in Sco-Cen that may complicate the interpretation of a rapid disk dispersal from Class II to Class III (Michel et al 2021). Resolving this issue will likely require a thorough analysis of individual stellar properties.…”