A unified framework for producing CAI melting, Wark–Lovering rims and bowl-shaped CAIs

Kurt Liffman,David A. Paterson,Nicolas Cuello

doi:10.1093/mnras/stw1563

Abstract

Calcium Aluminium Inclusions (CAIs) formed in the Solar System, some 4,567 million years ago. CAIs are almost always surrounded by Wark-Lovering Rims (WLRs), which are a sequence of thin, mono/bi-mineralic layers of refractory minerals, with a total thickness in the range of 1 to 100 microns. Recently, some CAIs have been found that have tektite-like bowl-shapes. To form such shapes, the CAI must have travelled through a rarefied gas at hypersonic speeds. We show how CAIs may have been ejected from the inner solar accretion disc via the centrifugal interaction between the solar magnetosphere and the inner disc rim. They subsequently punched through the hot, inner disc rim wall at hypersonic speeds. This re-entry heating partially or completely evaporated the CAIs. Such evaporation could have significantly increased the metal abundances of the inner disc rim. High speed movement through the inner disc produced WLRs. To match the observed thickness of WLRs required metal abundances at the inner disc wall that are of order ten times that of standard solar abundances. The CAIs cooled as they moved away from the protosun, the deduced CAI cooling rates are consistent with the CAI cooling rates obtained from experiment and observation. The speeds and gas densities required to form bowl-shaped CAIs are also consistent with the expected speeds and gas densities for larger, ~ 1 cm, CAIs punching through an inner accretion disc wall.

Highlights

Calcium Aluminium-rich Inclusions (CAIs) are sub-millimetre to centimetre sized, light grey, irregular to spherical-shaped rocks that are found in most primitive meteorites, but are most commonly found in carbonaceous chondrites
The re-entry surface heating and the passage of the CAI through the hot gas at the inner disc wall are the reasons, we suggest, why most CAIs are surrounded by Wark-Lovering Rims (WLRs), which are a sequence of thin, mono/bi-mineralic layers of hibonite ((Ca,Ce)(Al,Ti,Mg)12O19), perovskite (CaTiO3), spinel (MgAl2O4), melilite ((Ca,Na)2(Al,Mg,Fe2+)[(Al,Si)SiO7].), anorthite (CaAl2Si2O8), pyroxene and olivine ((Mg, Fe)2SiO4), where the total thickness of the rim is in the range of 10 to 100 microns (Wark & Lovering 1977)
As we show the production rate of CAIs decreases significantly with time, so even if CAIs were produced over a million year time frame there would be a bias towards CAIs that were formed at earliest times when the mass accretion rates were higher

Summary

Introduction

Calcium Aluminium-rich Inclusions (CAIs) are sub-millimetre to centimetre sized, light grey, irregular to spherical-shaped rocks that are found in most primitive meteorites, but are most commonly found in carbonaceous chondrites. They are amongst the oldest rocks to form in the Solar System with an estimated age of 4567.30 ± 0.16 million years (Connelly et al 2012). As noted by Lorenz, Ivanova & Shuvalov (2012), the shape of these CAIs is consistent with the shapes obtained from molten droplets that are subject to hypersonic interaction with a rarefied gas. Ivanova et al (2014) suggest that such high speed interaction may be due to shock waves generated by X-ray flares

Results

Discussion

Conclusion