Grain destruction in shocks in the interstellar medium
A. P. Jones, A. G. G. M. Tielens, D. J. Hollenbach, C. F. McKee;
ApJ, 1994, 433, 797
ABSTRACT:Destruction of interstellar dust occurs predominantly in supernova shock waves in the warm neutral/ionized medium (density
approximately = 0.25/cu cm, temperature approximately = 10(exp 4) K).
Recent
theoretical developments and laboratory data for sputtering processes and
grain-grain collisional vaporization allows us to better evaluate the grain
destruction rate in interstellar shocks in the warm medium.
We find that,
independent of composition, grain denstruction in supernova blast waves is
dominated by nonthermal sputtering for shock velocities greater than 50 km/s and
less than or equal to 150 km/s and thermal sputtering at higher shock
velocities.
We use a detailed scheme for the vaporization of grains colliding at high
velocities (v(sub s) greater than or equal to 20 km/s) and show that the grain-grain
collision destruction process is only dominant for shock velocities of less than
or equal to 50-80 km/s and is less important than previously
assumed.
Nevertheless, the grain-grain destruction rates are of order 30%-90% of the
sputtering rates at v(sub s) greater than 100 km/s and less than 200 km/s and are
important in vaporizing the cores of grains.
Detailed results for grain
destruction as a function of grain size and composition are presented.
We also
present results for silicon carbide, iron, ice, and porous test
particles.
For carbonaceous grains we find that the fractional destruction is less
than or equal to 0.29, and for silicate it is less than or equal to 0.45, for
v(sub s) less than or equal to 200 km/s.
We have calculated grain lifetimes,
using the three-phase model of the interstellar medium, and find lifetimes
of 4 x 10(exp 8) yr for carbonaceous grains and 2.2 x 10(exp 8) yr for
silicate grains.
Given that the typical stardust injection timescale of 2.5 x
10(exp 9) yr, we conclude that efficient mechanisms for grain growth in the
interstellar medium must exist in order that a significant fraction of the
refractory elements be incorporated in dust, as observed.
Therefore, although
our improved model has less vaporization of dust due to grain-grain
collisions, sputtering still destroys dust efficiently and grain mantle growth in
the interstellar medium is required, a conclusion reached in previous
models of grain destruction in the interstellar medium.
Carbonaceous
mantles on silicate grains can protect the silicate cores from sputtering
destruction in interstellar shock waves, provided that the protective mantles can
efficiently reform in the interstellar medium.
Also, if the grains are porous the
postshock grain velocities are lower than for solid particles, and grain
destruction is reduced.
Porosity and mantling may increase the grain lifetime by
factors of approximately 3 and approximately 3-4, respectively.
The
fraction of interstellar silicon in silicate stardust is therefore less than or
equal to 0.25, but more silicon might be depleted in the form of a grain
mantle.
KEYWORDS: astronomical models, cosmic dust, particle collisions, shock waves, sputtering, supernovae, vaporizing, carbonaceous materials, ice, iron, numerical analysis, porous materials, silicates, silicon carbides
PERSOKEY:dust, size distribution, ,
CODE: jones94