Dispersal of protoplanetary disks by central wind stripping

We present a model for the dispersal of protoplanetary disks by winds from either the central star or the inner disk. These winds obliquely strike the flaring disk surface and strip away disk material by entraining it in an outward radial-moving flow at the wind-disk interface which lies several disk scale heights above the mid-plane. The disk dispersal time depends on the entrainment velocity at which disk material flows into this turbulent shear layer interface. If the entrainment efficiency is ~10% of the local sound speed, a likely upper limit, the dispersal time at 1 AU is ~6 Myr for a disk with a surface density of 10^3 g cm^{-2}, a solar mass central star, and a wind with an outflow rate 10^{-8} Msun/yr and terminal velocity 200 km/s. When compared to photoevaporation and viscous evolution, wind stripping can be a dominant mechanism only for the combination of low accretion rates (< 10^{-8} Msun/yr) and wind outflow rates approaching these accretion rates. This case is unusual since generally outflow rates are < 0.1 of of accretion rates.

💡 Research Summary

The paper introduces a quantitative framework for a disk‑dispersal mechanism that has received comparatively little attention: stripping of protoplanetary material by a fast wind launched from the central star or the innermost regions of the disk. The authors picture the wind as a radially expanding, high‑velocity flow (typical mass‑loss rates of 10⁻⁸ M☉ yr⁻¹ and terminal speeds of ~200 km s⁻¹) that strikes the flared surface of the disk at an oblique angle. This impact creates a turbulent shear layer located a few scale heights above the mid‑plane. Within this layer, disk gas is entrained into the wind and carried outward. The key parameter governing the efficiency of this process is the entrainment velocity vₑₙₜ, which the authors bound by a fraction ε of the local sound speed cₛ. They argue that ε≈0.1 (i.e., vₑₙₜ≈0.1 cₛ) is a plausible upper limit because stronger turbulence would disrupt the shear interface.

Using a simple analytic treatment, the mass‑loss rate from a ring of radius r is expressed as Ṁ_strip(r)=2πr Σ(r) vₑₙₜ, where Σ(r) is the surface density. Substituting the wind density ρ_w≈Ṁ_w/(4πr²v_w) yields a dispersal timescale τ_disp≈Σ/(ε cₛ ρ_w). For a fiducial disk with Σ(1 AU)=10³ g cm⁻², a solar‑mass star, and the wind parameters mentioned above, the authors obtain τ_disp≈6 Myr at 1 AU. This timescale is comparable to, but generally longer than, the typical lifetimes inferred from photoevaporation models (1–3 Myr) and viscous evolution (10⁶–10⁷ yr depending on the α‑parameter).

A systematic comparison shows that wind stripping can dominate only under a narrow set of conditions: the accretion rate onto the star must be low (Ṁ_acc ≲ 10⁻⁸ M☉ yr⁻¹) and the wind mass‑loss rate must be a substantial fraction of the accretion rate (Ṁ_w/Ṁ_acc ≈ 0.1–1). Observational surveys of T Tauri stars typically find Ṁ_w/Ṁ_acc ≈ 0.01–0.1, implying that for most systems wind stripping is a secondary effect. Nevertheless, in the late stages of disk evolution, when accretion has dwindled, the mechanism could become comparable to or even exceed photoevaporative loss, especially at radii where the wind density remains relatively high.

The authors acknowledge several simplifications. The shear layer is treated as a one‑dimensional interface with a prescribed entrainment efficiency; the detailed turbulence spectrum, magnetic field geometry, and possible ionization of the disk surface are not modeled. Moreover, the back‑reaction of the wind on the disk’s angular momentum budget is ignored. These omissions point to the need for high‑resolution three‑dimensional magnetohydrodynamic simulations that can capture the full interaction between wind, magnetic fields, and the disk’s stratified structure.

In conclusion, the study provides the first analytic description of wind‑driven stripping as a viable disk‑dispersal channel. While the parameter space where it overtakes photoevaporation or viscous spreading is limited, the mechanism offers a natural explanation for rapid clearing in systems with unusually strong outflows or very low accretion rates. Future work should focus on (1) detailed numerical simulations to validate the entrainment prescription, (2) observational diagnostics—such as high‑velocity molecular line wings or spatially resolved wind‑disk interfaces with ALMA—and (3) incorporating the wind’s torque into global disk evolution models. By doing so, the community can assess whether wind stripping is merely a niche process or an essential piece of the broader puzzle of protoplanetary disk lifetimes.