Five steps in the evolution from protoplanetary to debris disk

Abstract

The protoplanetary disks seen around Herbig Ae stars eventually dissipate leaving just a tenuous debris disk, comprised of planetesimals and the dust derived from them, as well as possibly gas and planets. This paper uses the properties of the youngest (10-20 Myr) A star debris disks to consider the transition from protoplanetary to debris disk. It is argued that the physical distinction between these two classes should rest on the presence of primordial gas in sufficient quantities to dominate the motion of small dust grains (rather than on the secondary nature of the dust or its level of stirring). This motivates an observational classification based on the dust emission spectrum which is empirically defined so that A star debris disks require fractional excesses < 3 at 12 µm and < 2000 at 70 µm. We also propose that a useful hypothesis to test is that the planet and planetesimal systems seen on the main sequence are already in place during the protoplanetary disk phase, but are obscured or overwhelmed by the rest of the disk. This may be only weakly true if the architecture of the planetary system continues to change until frozen at the epoch of disk dispersal, or completely false if planets and planetesimals form during the relatively short dispersal phase. Five steps in the transition are discussed: (i) the well-known carving of an inner hole to form a transition disk ; (ii) depletion of mm-sized dust in the outer disk, where it is noted that it is of critical importance to ascertain whether this mass ends up in larger planetesimals or is collisionally depleted; (iii) final clearing of inner regions, where it is noted that multiple debris-like mechanisms exist to replenish moderate levels of hot dust atInstitute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK later phases, and that these likely also operate in protoplanetary disks; (iv) disappearence of the gas, noting the recent discoveries of both primordial and secondary gas in debris disks which highlight our ignorance in this area and its impending enlightenment by ALMA; (v) formation of ring-like structure of planetesimals, noting that these are shaped by interactions with planets, and that the location of the planetesimals in protoplanetary disks may be unrelated to that of dust concentrations therein that are set by gas interactions.

Disk Dispersal: Theoretical Understanding and Observational Constraints

“…Ercolano et al 2011, Koepferl et al 2015 for a theoretical perspective). Similarly, Wyatt (2008) argues that the disk mass derived from the sub-mm remains more or less constant (albeit with a wide dispersion, see Carpenter et al 2014) and shows a sharp decline at 10 Myr, perhaps again indicating this transition to the debris disk phase (see also Wyatt et al 2014). Since debris disks are almost always gas-free, the 10 Myr time also serves as an upper limit on the gas disk dispersal time, which is also consistent with gas observations (Zuckerman et al 1995, Dent et al 2013.…”

“…This conversion needs to be rapid and highly efficient, if not, substantial amounts of dust may remain after gas disk removal. Debris disk processes such as PR drag could remove the remnant primordial dust (see chapter by Wyatt et al ), but these mechanisms act on Myr timescales and are not consistent with the rapid transition to the debris disk stage (e.g., Luhman et al 2010, Wyatt et al 2014. The most likely scenario is that planet formation has already removed most of the dust before gas disk dispersal, although this suggests a causal link between these two processes (see discussion in Gorti et al 2015).…”

Resolved Co Gas Interior to the Dust Rings of the Hd 141569 Disk

“…Its young age, gas disk, and active accretion are reminiscent of a protoplanetary disk, although the strength of the infrared excess is much more typical of a debris disk. With constraints on the temperature and density structure of the gas we can compare to typical protoplanetary disks and debris disks to gain insight as to the origin of the gas and dust in the system, as well as their influence on each other (e.g., Wyatt et al 2015).…”

Fingerprints of giant planets in the photospheres of Herbig stars

“…Several sources are in the 10 Myr regime. At such ages, many systems already show low emission levels consistent with debris disks (e.g., Hardy et al 2015;Wyatt et al 2015). This is particularly surprising since A stars are thought to lose their disks faster (Ribas et al 2015) and suggests that the giant planet systems responsible for clearing large portions of the inner disk may contribute to disk longevity by trapping material in the outer disk.…”