Global Simulations of Protoplanetary Disks With Ohmic Resistivity and Ambipolar Diffusion

Abstract

Protoplanetary disks are believed to accrete onto their central T Tauri star because of magnetic stresses. Recently published shearing box simulations indicate that Ohmic resistivity, ambipolar diffusion and the Hall effect all play important roles in disk evolution. In the presence of a vertical magnetic field, the disk remains laminar between 1-5 au, and a magnetocentrifugal disk wind forms that provides an important mechanism for removing angular momentum. Questions remain, however, about the establishment of a true physical wind solution in the shearing box simulations because of the symmetries inherent in the local approximation. We present global MHD simulations of protoplanetary disks that include Ohmic resistivity and ambipolar diffusion, where the time-dependent gas-phase electron and ion fractions are computed under FUV and X-ray ionization with a simplified recombination chemistry. Our results show that the disk remains laminar, and that a physical wind solution arises naturally in global disk models. The wind is sufficiently efficient to explain the observed accretion rates. Furthermore, the ionization fraction at intermediate disk heights is large enough for magnetorotational channel modes to grow and subsequently develop into belts of horizontal field. Depending on the ionization fraction, these can remain quasi-global, or break-up into discrete islands of coherent field polarity. The disk models we present here show a dramatic departure from our earlier models including Ohmic resistivity only. It will be important to examine how the Hall effect modifies the evolution, and to explore the influence this has on the observational appearance of such systems, and on planet formation and migration.

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

“…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.…”

Effects of the Planetary Temperature on the Circumplanetary Disk and on the Gap

“…Nelson et al 2013;Barker & Latter 2015;Richard et al 2016) and disk wind (e.g. Gressel et al 2015;Suzuki & Inutsuka 2014;Suzuki et al 2010). For the circumplanetary disk, the spiral wake also thought to promote accretion Zhu et al 2016).…”

On the Grain-Modified Magnetic Diffusivities in Protoplanetary Disks

“…The calculations in Bai (2011a), Mohanty et al (2013), and Dzyurkevich et al (2013) included the full dependence of diffusivities on field strength, though again they did not explicitly discuss the dependence, and they did not focus on the Hall effect. The full dependence of diffusivities on field strength was included in recent simulations by Gressel et al (2015), though the Hall effect was not considered. Also, the grain abundance considered there is not sufficiently large for the effect discussed in this paper to be significant (see our empirical criterion in Section 3.2).…”

“…We focus on the disk interior (rather than the surface), where grains are likely the dominant charge carrier and are mostly singly charged, and systematically explore how magnetic diffusivities depend on field strength. We hope to provide researchers with useful guidance for modeling nonideal MHD effects from ionization chemistry, which has been fruitful in recent years (e.g., Turner et al 2007;Bai 2013Bai , 2014bBai , 2015Bai & Stone 2013;Lesur et al 2014;Gressel et al 2015;Simon et al 2015).…”