Other
Scientific paper
Dec 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994icar..112..430r&link_type=abstract
Icarus (ISSN 0019-1035), vol. 112, no. 2, p. 430-447
Other
36
Adiabatic Conditions, Chemical Composition, Chondrule, Protostars, Shock Fronts, Temperature Effects, Cooling, Kinetic Energy, Melting, Velocity Distribution
Scientific paper
Previous models of solid grain melting in solar nebula shocks have neglected gas cooling behind the shock front, i.e., they considered adiabatic shocks. The effect of gas cooling in the postshock region is studied. It was found that shocked gas is cooled efficiently by dipole molecules and small dust particles, and this results in a sharp increase in gas density in the postshock region. Submillimeter and larger grains cross the region of cooling before being decelerated, and are heated by the drag more strongly than in an adiabatic shock. This effect opens the possibility of melting of millimeter-size dust aggregates (chondrule precursors) in the cooler region, even when the matter is transparent for the thermal radiation of the aggregates. The possibility of formation of chondrule precursors in the collapsing presolar cloud and of melting of the precursors in the radiative accretional shock during formation of the solar nebula is speculated. In such a shock, grains could be melted at helicentric radii up to the inner part of asteroidal belt if the density of infalling gas were only few times larger than the average density of infalling gas corresponding to an accretion rate of 10-5 solar mass/yr, the limiting radius for melting increases with desity. Enhancement of drag heating due to fast cooling of postshock gas is also significant for shocks of other origin in the solar nebula. At the central plane of a minimum-mass solar nebula, chondrule precursors could be melted in shocks with velocity greater than or equal to 10 km/sec. The duration of the molten state and the cooling rate of a grain are determined by the grain's kinetic energy and the rate of its decay (not the time scale of radiational cooling for the grain] . Millimeter-size copact grains have temperatures greater than or equal to 1600 K for less than 105 sec, and greater than or equal to 700 K for approximately 104 sec, times that are appropriate for chrondrules.
Huen Ip Wing
Ruzmaikina T. V.
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