85 research outputs found
Formulation of Non-steady-state Dust Formation Process in Astrophysical Environments
The non-steady-state formation of small clusters and the growth of grains
accompanied by chemical reactions are formulated under the consideration that
the collision of key gas species (key molecule) controls the kinetics of dust
formation process. The formula allows us to evaluate the size distribution and
condensation efficiency of dust formed in astrophysical environments. We apply
the formulation to the formation of C and MgSiO3 grains in the ejecta of
supernovae, as an example, to investigate how the non-steady effect influences
the formation process, condensation efficiency f_{con}, and average radius
a_{ave} of newly formed grains in comparison with the results calculated with
the steady-state nucleation rate. We show that the steady-state nucleation rate
is a good approximation if the collision timescale of key molecule tau_{coll}
is much smaller than the timescale tau_{sat} with which the supersaturation
ratio increases; otherwise the effect of the non-steady state becomes
remarkable, leading to a lower f_{con} and a larger a_{ave}. Examining the
results of calculations, we reveal that the steady-state nucleation rate is
applicable if the cooling gas satisfies Lambda = tau_{sat}/tau_{coll} > 30
during the formation of dust, and find that f_{con} and a_{ave} are uniquely
determined by Lambda_{on} at the onset time t_{on} of dust formation. The
approximation formulae for f_{con} and a_{ave} as a function of Lambda_{on}
could be useful in estimating the mass and typical size of newly formed grains
from observed or model-predicted physical properties not only in supernova
ejecta but also in mass-loss winds from evolved stars.Comment: 44 pages, 10 figures, 1 table, accepted for publication in Ap
Pure iron grains are rare in the universe
The abundant forms in which the major elements in the universe exist have
been determined from numerous astronomical observations and meteoritic
analyses. Iron (Fe) is an exception, in that only depletion of gaseous Fe has
been detected in the interstellar medium, suggesting that Fe is condensed into
a solid, possibly the astronomically invisible metal. To determine the primary
form of Fe, we replicated the formation of Fe grains in gaseous ejecta of
evolved stars by means of microgravity experiments. We found that the sticking
probability for formation of Fe grains is extremely small; only several atoms
will stick per hundred thousand collisions, so that homogeneous nucleation of
metallic Fe grains is highly ineffective, even in the Fe-rich ejecta of Type Ia
supernovae. This implies that most Fe is locked up as grains of Fe compounds or
as impurities accreted onto other grains in the interstellar medium
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