Magneto-Thermal Disk Winds From Protoplanetary Disks

Abstract

Global evolution and dispersal of protoplanetary disks (PPDs) is governed by disk angular momentum transport and mass-loss processes. Recent numerical studies suggest that angular momentum transport in the inner region of PPDs is largely driven by magnetized disk wind, yet the wind mass-loss rate remains unconstrained. On the other hand, disk mass loss has conventionally been attributed to photoevaporation, where external heating on the disk surface drives a thermal wind. We unify the two scenarios by developing a 1D model of magnetized disk winds with a simple treatment of thermodynamics as a proxy for external heating. The wind properties largely depend on 1) the magnetic field strength at the wind base, characterized by the poloidal Alfvén speed v Ap , 2) the sound speed c s near the wind base, and 3) how rapidly poloidal field lines diverge (achieve R −2 scaling). When v Ap ≫ c s , corotation is enforced near the wind base, resulting in centrifugal acceleration. Otherwise, the wind is accelerated mainly by the pressure of the toroidal magnetic field. In both cases, the dominant role played by magnetic forces likely yields wind outflow rates that well exceed purely hydrodynamical mechanisms. For typical PPD accretion-rate and wind-launching conditions, we expect v Ap to be comparable to c s at the wind base. The resulting wind is heavily loaded, with total wind mass loss rate likely reaching a considerable fraction of wind-driven accretion rate. Implications for modeling global disk evolution and planet formation are also discussed.

Spiral Arms in Disks: Planets or Gravitational Instability?

“…Disks with M disk /M å ∼0.1 have multiple arms and are marginally unstable, with disk accretion rates ∼10 −8 -10 −6 M e yr −1 in the outer disk (e.g., Dong et al 2015a;Hall et al 2016 Bai et al 2016;Bai 2017), will become more dominant as the gravitational instability-driven accretion rate declines. The relatively long lifetime of the multiarm phase of gravitational instability is possibly consistent with the 3/11 (or 27%) incidence rate of multiarm spirals among well-studied Herbig IMS and the 3/24 (or 13%) incidence rate within the volume-limited Herbig IMS sample ( Table 4).…”

Planetesimal Formation by the Streaming Instability in a Photoevaporating Disk

“…As such, these winds exert a torque on the disk so they carry a significant portion of the disk's angular momentum and drive much of the accretion. The process was first proposed by Blandford & Payne (1982), but see also the review by Turner et al (2014) and the recent model by Bai et al (2016).…”

Tracing Slow Winds From T Tauri Stars via Low-Velocity Forbidden Line Emission

“…We anticipate that these models will soon be providing more rigorous tests of whether the LVC might arise in thermal winds. Theoretical models for magnetically driven centrifugal disk winds have also grown increasingly sophisticated and recently the relevance of MRI-driven accretion over much of the disk has been challenged (e.g., Turner et al 2014) putting MHD disk winds back in the spotlight for extracting disk angular momentum to enable accretion onto the star (e.g., Bai et al 2016). Global simulations of these winds have been recently presented by Gressel et al (2015) and thermalchemical models have been investigated by Panoglou et al (2012), but predictables that can be directly compared with observations of TTS forbidden lines are still lacking.…”

“…As dust grains grow they settle toward the midplane. The implication is that disk winds mostly deplete the protoplanetary disk of gas, which consequently increases the dust-to-gas ratio with time (Gorti et al 2015;Bai et al 2016). This increase can directly impact the formation of planetesimals, terrestrial planets, and the cores of giant planets.…”