Using Ice and Dust Lines to Constrain the Surface Densities of Protoplanetary Disks

Abstract

We present a novel method for determining the surface density of protoplanetary disks through consideration of disk "dust lines," which indicate the observed disk radial scale at different observational wavelengths. This method relies on the assumption that the processes of particle growth and drift control the radial scale of the disk at late stages of disk evolution such that the lifetime of the disk is equal to both the drift timescale and growth timescale of the maximum particle size at a given dust line. We provide an initial proof of concept of our model through an application to the disk TW Hya and are able to estimate the disk dust-to-gas ratio, CO abundance, and accretion rate in addition to the total disk surface density. We find that our derived surface density profile and dust-to-gas ratio are consistent with the lower limits found through measurements of HD gas. The CO ice line also depends on surface density through grain adsorption rates and drift and we find that our theoretical CO ice line estimates have clear observational analogues. We further apply our model to a large parameter space of theoretical disks and find three observational diagnostics that may be used to test its validity. First, we predict that the dust lines of disks other than TW Hya will be consistent with the normalized CO surface density profile shape for those disks. Second, surface density profiles that we derive from disk ice lines should match those derived from disk dust lines. Finally, we predict that disk dust and ice lines will scale oppositely, as a function of surface density, across a large sample of disks.

New Constraints From Dust Lines on the Surface Densities of Protoplanetary Disks

“…Recent multiwavelength observations of disks appear to show signatures of particle drift as the radial extent of several disks becomes smaller at longer wavelengths (e.g., Isella et al 2010;Tripathi et al 2017;Guilloteau et al 2011;Banzatti et al 2011;Pérez et al 2012Pérez et al , 2015Tazzari et al 2016). The radial extent of a disk at a particular wavelength is known as a disk dust line (Powell et al 2017) as, in the millimeter, we can assume that emission at the observed wavelength is dominated by particles with a size comparable to that wavelength. As these particles are all in the Epstein drag regime, the surface density of a given disk can be readily determined given the maximum radius where particles of a given size are present.…”

CO and Dust Properties in the TW Hya Disk from High-resolution ALMA Observations

“…While several studies have suggested that warm gas in the TW Hya disk is CO-depleted by one to two orders of magnitude relative to the ISM (Favre et al 2013;Kama et al 2016;Nomura et al 2016;Schwarz et al 2016;Powell et al 2017), the new high angular resolution ALMA observations provide additional insight into how the CO distribution varies radially.…”

“…Estimates for the TW Hya disk's midplane CO snowline location range from 11 to 33 AU Nomura et al 2016;Schwarz et al 2016;Powell et al 2017;van't Hoff et al 2017;Zhang et al 2017), so it is natural to consider whether the apparent steep drop in 12 CO column density at a radius of 15 AU is related. Indeed, several of the aforementioned CO snowline estimates are based on observations of a dip in the C 18 O emission profile at a radius (22 AU) close to where we infer a 12 CO column density drop.…”

Dust Density Distribution and Imaging Analysis of Different Ice Lines in Protoplanetary Disks

“…It is, however, expected that dust dynamics (in particular radial drift and vertical settling) can change the gas surface density and the location of ice lines (Piso et al 2015;Cleeves 2016;Krijt et al 2016;Powell et al 2017;Stammler et al 2017). In addition, the disk temperature can vary by different effects, such as disk dispersal, which can also change the ice line locations (Panić & Min 2017).…”