Thanatology in protoplanetary discs

Abstract

Protoplanetary discs are poorly ionised due to their low temperatures and high column densities and are therefore subject to three "non-ideal" magnetohydrodynamic (MHD) effects: Ohmic dissipation, ambipolar diffusion, and the Hall effect. The existence of magnetically driven turbulence in these discs has been a central question since the discovery of the magnetorotational instability (MRI). Early models considered Ohmic diffusion only and led to a scenario of layered accretion, in which a magnetically "dead" zone in the disc midplane is embedded within magnetically "active" surface layers at distances of about 1-10 au from the central protostellar object. Recent work has suggested that a combination of Ohmic dissipation and ambipolar diffusion can render both the midplane and surface layers of the disc inactive and that torques due to magnetically driven outflows are required to explain the observed accretion rates. We reassess this picture by performing three-dimensional numerical simulations that include all three non-ideal MHD effects for the first time. We find that the Hall effect can generically "revive" dead zones by producing a dominant azimuthal magnetic field and a large-scale Maxwell stress throughout the midplane, provided that the angular velocity and magnetic field satisfy Ω · B > 0. The attendant large magnetic pressure modifies the vertical density profile and substantially increases the disc scale height beyond its hydrostatic value. Outflows are produced but are not necessary to explain accretion rates 10 −7 M yr −1 . The flow in the disc midplane is essentially laminar, suggesting that dust sedimentation may be efficient. These results demonstrate that if the MRI is relevant for driving mass accretion in protoplanetary discs, one must include the Hall effect to obtain even qualitatively correct results.

On the water delivery to terrestrial embryos by ice pebble accretion

“…The formation of terrestrial embryos as dry as the present Earth was possible if A15, page 15 of 19 moderately strong turbulence (α > ∼ 10 −3 ) was present at 1 AU. However, the latest magnetohydrodynamical disk models (e.g., Bai & Stone 2013;Lesur et al 2014) suggest that turbulence is considerably weaker than this requirement in inner regions of protoplanetary disks 4 For r out = 100 AU and α = 10 −4 , embryos as dry as the present Earth would not have formed, but embryos containing < ∼ 1 wt% water would have formed if the snow line migrated on a timescale of > ∼ 2 Myr. This is one plausible scenario that can explain the origin of our dry Earth because the snow line in the nebula could indeed have migrated on a similar timescale (Sect.…”

Gravito-Turbulent Disks in Three Dimensions: Turbulent Velocities Versus Depth

“…Turbulence driven by the magnetorotational instability (MRI; Balbus & Hawley 1991, 1998) is perhaps the most intensively studied mechanism. Recent studies explore a host of non-ideal magnetohydrodynamic effects that strongly affect the character of transport in poorly ionized disks (e.g., PerezBecker & Chiang 2011a, 2011bBai 2011;Wardle & Salmeron 2012;Bai & Stone 2013;Bai 2013;Simon et al 2013aSimon et al , 2013bKunz & Lesur 2013;Lesur et al 2014). Disk self-gravity is another option for sufficiently massive disks (e.g., Paczynski 1978;Gammie 2001;Forgan et al 2012).…”

EXor OUTBURSTS FROM DISK AMPLIFICATION OF STELLAR MAGNETIC CYCLES

“…Second, the naive estimate for r crit above lies close to the inner edge of the dead zone (nominally 0.1 AU in the original model of Gammie 1996). A dynamically significant β z that extends into the dead zone opens up a stroner path to instability, because in the Hall-dominated inner disk not just the strength but also the polarity of the net field has a decisive impact on accretion (Lesur et al 2014;Bai 2014;Simon et al 2015).…”

“…In the presence of the Hall effect, though, simulations suggest that the value of α given the favorable aligned orientation of the net field is not small, even in the nominal dead zone. Assuming frequent reversals of the net field's polarity, the appropriate α to use in the above estimate is the average of the aligned and anti-aligned Hall values, which is likely to be comparable to 10 −2 (Lesur et al 2014). Adopting as the criterion for Hall effect physics to impact accretion at the inner edge of the dead zone that β z 10 5 , equation (10) suggests that it is possible (but not guaranteed) for residual stellar fields diffusing through the disk to play a role.…”

“…Slightly further out, in the dead zone, the Hall effect (Wardle 1999;Balbus & Terquem 2001) is the most important non-ideal MHD process (for reviews see Armitage 2011;Turner et al 2014). Simulations show that the efficiency of angular momentum transport in the Hall-dominated regime differs dramatically depending upon whether the net field is aligned or anti-aligned to the rotation axis (Lesur et al 2014;Bai 2014;Simon et al 2015). We therefore suggest that EXor outbursts occur due to the MHD response of the inner disk to the changing strength and polarity of a stellar-derived net magnetic field.…”