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Kruegel E. The physics of interstellar dust (IOP, 2003)(T)(584s).djvu |
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Size 3.9Mb Date Jul 3, 2004 |
For an ideal surface, tunneling to a site n grid spacings away is not a random walk,
but the well is reached after a time tD ~ n ¦ ?tun- A real grain has defects on its
surface, i.e...
From Mie theory, one can compute
the internal field of the grain (the relevant formula is B.45)) and thus the Poynting
vector...
In such circumstances, C^ is much
smaller than the geometrical cross section and V accordingly smaller, too...
The computations pertaining to figure 9.6 also yield, of course, the time
evolution of the mean square deviation r2 = (x2 + y2)...
With the appropriate numbers for air (kinematic viscosity v ~ 0.15 cm2 s,
density p ~ 1.2 x 10~3 g cm, r) = pv ~ 1.8 x 10~4 g cm s), the turbulent
and laminar deceleration become equal, 6nrjaV = na2pV2, when the drop has
a radius a ~ 0.1 mm and sinks at a velocity V ~ lms...
The threshhold impact energy for liberating an atom from
the solid phase is a few times larger than the binding energy of an atom
(~5 eV)...
A supernova rate
Grain formation 301
tsn = 0.03 yr then implies that all interstellar material of the Milky Way is
processed in a time
M
'snr = „ gaS ~ 2 x 108 yr...
L is Loschmidt's number, and the gas constant R, the Boltzmann
constant k and L are related through
(9.51)
The Clausius-Clapeyron equation is, with obvious modifications, also valid
with respect to sublimation, which is the phase transition between the solid and
gas phases, and thus applies to the evaporation of interstellar grains and their
mantles...
When 1 mol has become gaseous,
B) Isothermal expansion from pressure pr to poo of the 1 mol of gas that has
been vaporized in step A),
W2 = RT\n(pt/Poo)...
One finds typical values of the critical saturation parameter for water at room
temperature around 10 which implies in view of (9.56) a critical seed radius rcr of
a few angstroms...
where An, Bn,Cn, Dn are known coefficients (so for the moment, An is not the
surface area of an rc-mer)...
Let N(t) be the number of monomers in clusters that were formed at time tcr
and and let R(t) be their radius, so 4nR3/3 = Nvo...
The first, /x, is
fixed by the ratio of surface tension over temperature and thus incorporates grain
properties...
Many
reddened stars appear to be polarized, up to 10% or so, although the emission
from the stellar photosphere is certainly not...
What counts is the difference
Acext = qxt - cf\
When there are many such cylinders with their axes all parallel, one speaks
of picket fence alignment...
Suppose the cylinder circles in the (x, y)-plane of a Cartesian coordinate
system, presented by the page of this book, and its momentary location is
characterized by the angle <f> (see figure 10.3)...
This has consequences for the temperature of dust, its
emission and the degree to which the emission is polarized; extinction is usually
irrelevant at these wavelengths...
For this purpose, we go back to figure 10.4
which displays the difference A<2ext = <2y — <2_i_ in the extinction efficiency
of rotating cylinders as a function of size parameter x = 2na/X, and also for
various inclination angles * of the rotation axis...
For
cylinder radius a — 0.025 /xm, the optical constant m(X) = 1.85 + 1.23i is fixed and
has the value of aC at 0.55 /xm; for the other cylinder radii, m(X) is from figure 7.19
and variable...
We can improve
the infinite cylinder model by treating ellipsoids as a more realistic alternative...
However, there exist significant offsets in the resonance peaks, as can be checked
by scrutinizing figure 10.12, if necessary with the help of a ruler...
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