Dust Migration and Morphology in Optically Thin Circumstellar Gas Disks

Abstract

We analyze the dynamics of gas-dust coupling in the presence of stellar radiation pressure in circumstellar disks, which are in a transitional stage between the gas-dominated, optically thick, primordial nebulae, and the dust-dominated, optically thin Vega-type disks. Dust grains undergo radial migration, either leaving the disk owing to a strong radiation pressure or seeking a stable equilibrium orbit in corotation with gas. In our models of A-type stars surrounded by a total gas mass from a fraction to dozens of Earth masses, the outward migration speed of dust is comparable with the gas sound speed. Equilibrium orbits are circular, with exception of those signiÐcantly a †ected by radiation pressure, which can be strongly elliptic with apocenters extending beyond the bulk of the gas disk. The migration of dust gives rise to radial fractionation of dust and creates a variety of possible observed disk morphologies, which we compute by considering the equilibrium between the dust production and the dust-dust collisions removing particles from their equilibrium orbits. Large grains (typically km) are distributed Z200 throughout most of the gas disk. Smaller grains (in the range of 10È200 km) concentrate in a prominent ring structure in the outer region of the gas disk (presumably at radius D100 AU), where gas density is rapidly declining with radius. The width and density, as well as density contrast of the dust ring with respect to the inner dust disk, depend on the distribution of gas and the mechanical strength of the particles. Our results open the prospect for deducing the distribution of gas in circumstellar disks by observing their dust. We have qualitatively compared our models with two observed transitional disks around HR 4796A and HD 141569A. Dust migration can result in observation of a ring or a bimodal radial dust distribution, possibly very similar to the ones produced by gap-opening planets embedded in the disk, or shepherding it from inside or outside. We conclude that a convincing planet detection via dust imaging should include speciÐc nonaxisymmetric structure (spiral waves, streamers, resonant arcs) following from the dynamical simulations of perturbed disks.

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

“…Takeuchi & Artymowicz (2001) predict a single ring, while two distinct rings are observed (Weinberger et al 1999;Clampin et al 2003;Biller et al 2015). A Saturn-mass planet embedded in the outer disk could carve out a gap and stir up the grains enough to create two rings (Wyatt 2005), and Biller et al (2015) find tentative evidence of a stellocentric offset that is consistent with perturbations from a planet.…”

“…These grains will move outward until they reach the outer edge of the gas disk. For HD 141569 this can occur for smallish grains (tens of microns), while larger grains (∼hundreds of microns) settle into stable orbits within the gaseuous disk where radiation pressure and gas drag balance (Takeuchi & Artymowicz 2001). Poynting-Robertson drag will drive grains with sizes of tens to hundreds of microns inward toward the star (Li & Lunine 2003).…”

“…Most observations focus on this dust emission (Wyatt 2008;Krivov 2010;Matthews et al 2014, and references therein) but the presence of gas can substantially influence the observed dust structures (Takeuchi & Artymowicz 2001;Lyra & Kuchner 2013). While surveys have found that most debris disks have a negligible gas mass (Pascucci et al 2006), some systems possess significant amounts of gas in addition to their debris-like dust distributions.…”

“…The presence of a gas reservoir has implications for the origin and evolution of the dust within this system. Cospatial gas and dust can lead to sharp rings without the need for shepherding planets (Lyra & Kuchner 2013), while collisions with gas as dust is pushed outward by radiation pressure can lead to a ring at the outer edge of the gaseous disk (Takeuchi & Artymowicz 2001).…”

The Depletion of Water During Dispersal of Planet-Forming Disk Regions

“…Observations of a wavelength-dependent decrease in infrared excesses (e.g., Skrutskie et al 1990;Haisch et al 2001;Espaillat et al 2012;Ribas et al 2015), or spatially resolved cavities in continuum dust emission (e.g., Andrews et al 2011;van der Marel et al 2016), indicate that dust grains in the inner disk regions are depleted before those at larger radii. Disks that show these properties have been proposed to be in a transition phase ("transitional disks") toward their dispersal (e.g., Strom et al 1989;Skrutskie et al 1990;Takeuchi & Artymowicz 2001). The concept of disks in "transition" has been developing ever since its early definition, under new evidence provided by larger samples, more detailed observations, and improved models (e.g., Espaillat et al 2014;Owen 2016).…”