Chondrule Formation and Protoplanetary Disk Heating by Current Sheets in Nonideal Magnetohydrodynamic Turbulence

Abstract

We study magnetic field steepening due to ambipolar diffusion in protoplanetary disk environments and draw the following conclusions. Current sheets are generated in magnetically active regions of the disk where the ionization fraction is high enough for the magnetorotational instability to operate. In late stages of solar nebula evolution, the surface density is expected to decrease and dust grains are expected to gravitationally settle to the midplane. If the local dust-to-gas mass ratio near the midplane is increased above cosmic abundances by factors k10 3 , current sheets reach high enough temperatures to melt millimeter-sized dust grains and hence may provide the mechanism to form meteoritic chondrules. In addition, these current sheets possibly explain the near-infrared excesses observed in spectral energy distributions (SEDs) of young stellar objects. Direct imaging of protoplanetary disks via a nulling interferometer or, in the future, a multiband, adaptive optics coronagraph can test this hypothesis.

Chondrules: Ubiquitous Chondritic Solids Tracking the Evolution of the Solar Protoplanetary Disk

“…Chondrules are mainly composed of olivine ((Fe,Mg) 2 SiO 4 ) and pyroxene ((Fe,Mg)SiO 3 ) minerals, which crystallised within minutes to days between 1800 and 1300 K (Scott 2007). Several heat sources have been proposed for the thermal processing of chondrule precursors, including shock waves (Boss and Graham 1993;Connolly and Love 1998;Hood 1998;Connolly et al 2006), current sheets (Joung et al 2004), x-winds (Shu et al 1997), magnetised disk wind (Salmeron and Ireland 2012), and colliding planetesimals (Asphaug et al 2011;Sanders and Scott 2012). A longstanding paradigm used to constrain chondrule-formation models is the so-called chemical complementarity that apparently exists between chondrules and matrix in individual chondrite groups (Hezel and Palme 2010;Palme et al 2015;Ebel et al 2016).…”

Chondrites and Their Components

“…envelopes of giant protoplanets (Nelson and Ruffert, 2005), current sheets in magnetically active nebula regions (Joung et al, 2004), and ablation of 1-500 m sized planetesimals by shock (Genge, 2000). Note that the constraints listed in Table 7 apply to what can be called 'archetypal chondrules,' which are most closely represented by type II chondrules.…”

Petrology and origin of amoeboid olivine aggregates in CR chondrites

“…Thus, this final episode of fracturing and recrystallization (reheating) occurred after the condensation of olivine. This CAI may have experienced multiple excursions into hotter regions of the nebula (e.g., Cassen 2001) or experienced pulses of energy like those produced by shock waves (e.g., Desch and Connolly 2002) or current sheets (e.g., Joung et al 2004) in the protoplanetary disk.…”