[Paper Review] Linking Outer Disk Pebble Dynamics and Gaps to Inner Disk Water Enrichment

Anusha Kalyaan, Paola Pinilla+3 more|arXiv (Cornell University)|Sep 6, 2021

Astrophysics and Star Formation StudiesPhysics and Astronomy144 references44 citations

TL;DR

This paper investigates how gaps in the outer protoplanetary disk block the inward drift of icy pebbles, thereby regulating water vapor enrichment in the inner disk. Using a volatile-inclusive disk evolution model, it finds that gaps at 7–15 au are most effective at blocking 1–10 mm pebbles—responsible for most ice mass—minimizing inner-disk water enrichment and potentially inhibiting Earth and super-Earth formation.

ABSTRACT

Millimeter continuum imaging of protoplanetary disks reveals the distribution of solid particles and the presence of substructures (gaps and rings) beyond 5-10 au, while infrared (IR) spectra provide access to abundances of gaseous species at smaller disk radii. Building on recent observational findings of an anti-correlation between the inner disk water luminosity and outer dust disk radius, we aim here at investigating the dynamics of icy solids that drift from the outer disk and sublimate their ice inside the snow line, enriching the water vapor that is observed in the IR. We use a volatile-inclusive disk evolution model to explore a range of conditions (gap location, particle size, disk mass, and alpha-viscosity) under which gaps in the outer disk efficiently block the inward drift of icy solids. We find that inner-disk vapor enrichment is highly sensitive to the location of a disk gap, yielding for each particle size a radial "sweet spot" that reduces the inner-disk vapor enrichment to a minimum. For pebbles of 1-10 mm in size, which carry the most mass, this sweet spot is at 7-15 au, suggesting that inner gaps may have a key role in reducing ice delivery to the inner disk and may not allow the formation of Earths and super-Earths. This highlights the importance of observationally determining the presence and properties of inner gaps in disks. Finally, we argue that the inner water vapor abundance can be used as a proxy for estimating the pebble drift efficiency and mass-flux entering the inner disk.

Motivation & Objective

• To understand how outer disk gaps influence the delivery of icy solids to the inner disk, where they sublimate and enrich the gas with water vapor.
• To investigate the role of gap location, particle size, disk mass, and viscosity in controlling inner-disk water vapor abundance.
• To determine whether inner-disk water vapor abundance can serve as a proxy for pebble mass flux into the inner disk.
• To assess the implications of gap efficiency for planet formation, particularly the formation of Earths and super-Earths.
• To reconcile observational anti-correlations between inner water luminosity and outer dust disk size by modeling pebble drift dynamics.

Proposed method

• Developed a volatile-inclusive disk evolution model tracking water in both ice and vapor phases across the disk.
• Incorporated a gap in the outer disk as a pressure bump that traps solids, using a parameterized gap profile to simulate different locations and strengths.
• Simulated radial drift of icy pebbles (1–10 mm) under varying disk masses (5% and 10% M⊙), α-viscosity (10⁻³ to 10⁻²), and gap positions (3–30 au).
• Tracked time-evolving water vapor abundance in the inner disk (r < 5 au), accounting for sublimation within the snow line and accretion onto the star.
• Used linear regression to compare model outputs with observed anti-correlation between water luminosity and outer dust disk radius from Banzatti et al. (2020).
• Evaluated the efficiency of gaps in blocking ice mass delivery by comparing inner-disk vapor enrichment across different gap locations and particle sizes.

Experimental results

Research questions

• RQ1 How does the location of a gap in the outer disk affect the delivery of icy pebbles to the inner disk and subsequent water vapor enrichment?
• RQ2 What particle size range is most efficiently blocked by gaps at different locations, and how does this affect total ice mass delivery?
• RQ3 Can inner-disk water vapor abundance serve as a proxy for pebble mass flux into the inner disk?
• RQ4 How do disk mass and α-viscosity modulate the effectiveness of gaps in reducing inner-disk water enrichment?
• RQ5 What are the implications of gap efficiency for the formation of Earths and super-Earths via pebble accretion?

Key findings

• For each particle size, there exists a radial 'sweet spot' in gap location that minimizes inner-disk water vapor enrichment, with the most effective blocking occurring at 7–15 au for 1–10 mm pebbles.
• Gaps at 7–15 au are most efficient at blocking 1–10 mm pebbles, which carry the highest ice mass and drift fastest, reducing inner-disk water enrichment by up to 90% compared to no-gap scenarios.
• Gaps at 30 au are less effective at blocking larger pebbles but can block smaller particles; however, these deliver less ice mass and thus have a smaller impact on inner-disk enrichment.
• Disks with efficient gaps (e.g., at 7–15 au) may be unable to form Earths or super-Earths via pebble accretion, as the supply of icy solids is drastically reduced in the inner disk.
• Lower α-viscosity increases inner-disk water vapor persistence by reducing vapor diffusion, leading to higher and longer-lasting vapor abundances even with similar pebble flux.
• Inner-disk water vapor luminosity correlates strongly with pebble mass flux, supporting its use as a proxy for estimating the rate of icy solid delivery into the inner disk.