Astrophysical processes contributing to the formation of meteoritic components

Details Astrophysical processes contributing to the formation of meteoritic components Astrophysical processes contributing to the formation of meteoritic components

Jul 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29q.454c&link_type=abstract

Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 454

Astronomy and Astrophysics

Astrophysics

2

Abundance, Aluminum Isotopes, Chondrule, Inclusions, Interstellar Gas, Magnetic Field Reconnection, Stellar Mass Accretion, Astrophysics, Photodissociation, Solar Corona, Stellar Evolution, Stellar Luminosity

Scientific paper

Ca-Al rich Inclusions (CAIs) commonly have a Al-26/Al-27 ratio of 5 x 10-5, but this ratio is smaller in chondrules; a few CAI chondrule time intervals in the range 2-6 m.y. exist, and there are many more cases in which numbers in this range are lower limits. Circumstellar gaseous disks about young stellar objects progressively disappear at ages of 3-10 m.y. The commonality of the Al-26/Al-27 abundance ratios in CIAs is readily understood; the accretion time for a star like the Sun is about 105 yr, short compared to the Al-26 half-life (7.2 x 105 yr). This is determined from stellar evolution and collapse calculations and comparisons to the luminosities of young stellar objects and also by observations of the rate at which disks are dissipated in FU Orionis stars. A process such as bipolar outflow lifts hot gas upward and radially outward before expelling it from the solar system; CAIs that condense from it will fall into the midrange of the solar nebula. The Balbus-Hawley magnetic instability amplifies magnetic fields and sets up a two-stream radial motion, toward and away from the Sun; this can move hot gas through CAI condensation and out into the nebula during accretion. Note that in both cases the hot gas must have previously contained these refractory materials as solids from which the gaseous products of lower- temperature evaporation had been convected away, or the CAI may be a residue from such an evaporation process; both alternatives may frequently occur in a general accretion scenario. These processes cease with the transition to the minimum solar nebula with negligible further accretion onto the Sun; within the nebula, under benign conditions, the CAIs can be preserved for millions of years. This appears to be an excellent environment for the fusion and melting of the dustballs to form the chondrules primarily by exposure to optical radiation from the flares taking place by magnetic reconnection in the gap; these flares, like those on the Sun, should have durations comparable to chondrule cooling times. In this picture the CAI chondrule time interval should be variable within a range of several million years.

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