Astron. Astrophys. 351, 759-765 (1999)

1. Introduction

Chondrules are millimetre sized, spherical to irregular shaped objects that constitute the major component of most chondrite meteorites that originate in the region between Mars and Jupiter and which fall to the Earth. They appear to have crystallised rapidly from molten or partially molten drops and were described (Sorby, 1877) as "molten drops in a fiery rain". The properties of the chondrules and chondrites have been exquisitely deduced from an extensive series of experiments and two conferences have been devoted completely to chondrules (King, 1983; Hewins et al., 1996). The mineralogy of chondrules is dominated by olivine (SiO₄) and pyroxene ((FeMg)SiO₃) and there is a wide range of compositions for all elements. This diversity is consistent with the melting of heterogeneous precursor solids or dust balls. The age of chondrules indicate they formed very early in the solar system. The calcium-, aluminium-rich inclusions (CAIs) are refractory inclusions in carbonaceous and ordinary chondrites that predate the chondrules by several million years and are the oldest known solid materials produced in the nebula (Swindle et al., 1996). The first 10⁷ years in the complicated development of the solar system has been comprehensively covered in an attempt to understand the CAI to chondrule time interval of several million years (Cameron, 1995).

The presence of volatile elements in the chondrules indicate that the high temperature melting period lasted for a matter of seconds to minutes. Experiments based on chemical and textural compositions of chondrules suggest cooling rates that were much slower than radiative cooling of isolated chondrules and imply they were made in some large quantity in relatively opaque nebular domains (Yu & Hewins, 1998; Brearley & Jones, 1998). Volatile elements such as alkalis and sulphur occur in chondrule interiors as primary constituents and indicate that some chondrule precursor materials must have reacted with cool nebula gases at ambient temperatures less than 650 K.

The heat source that melted the chondrules remains uncertain and a critical summary of the heating mechanisms was given by Boss (1996). These methods include giant lightning flashes (Horanyi et al., 1995) and shock wave heating of the precursor materials (Wood, 1988). All heat sources proposed to form the chondrules are local to the solar nebula. We propose that the chondrules were flash heated to melting point by a nearby GRB when the precursor materials efficiently absorbed x-rays and low energy -rays. The distance to the source was about 300 light years (or 100 pc) for a GRB output of 10⁵³ ergs and was estimated using the minimum value of erg g^-1 required to heat and melt the precursor grains (Grossman et al., 1988; Wasson, 1993). The role of nearby supernovae that preceded the formation of the solar system have been considered (Cameron et al., 1995) along with the serious consequences for life on Earth of nearby supernovae (Ruderman, 1974; Clark et al., 1977) and GRBs (Thorsett, 1995). The consequences of a nearby GRB on the early solar nebula have not been considered elsewhere.

The properties of GRBs relevant to chondrule formation are presented in Sect. 2. The absorption of x-rays and -rays by gas and dust in the solar nebula is considered in Sect. 3. The effects of sudden chondrule production on the formation of the planets are presented in Sect. 4. The probability of a GRB producing chondrules is considered in Sect. 5.

© European Southern Observatory (ESO) 1999

Online publication: November 3, 1999
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