Kruegel E. The physics of interstellar dust (IOP, 2003)(T)(584s).djvu: eKnigu

Kruegel E. The physics of interstellar dust (IOP, 2003)(T)(584s).djvu

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Date Jul 3, 2004

After N steps, one has arrived at the
position Rn = Rn-i +Ln- If the vectors L,- are of constant length L but in an
arbitrary direction, the mean square of the distance, (R2^), grows like
(R%) = NL2...
For instance, a 10 MQ star has
acr ~ 2.4 mm, so all particles with sizes of interstellar grains (~0.1 /xm) are
blown away by radiation pressure...
(9.39)
Because F = /xV = M dV/dt ~ MV/t&s, the friction coefficient can be
expressed through the damping time ?dis,
ix = —...
The Poynting-Robertson effect also
works when the photons are not absorbed but isotropically scattered...
The observational evidence comes from clouds of
high velocity (v > 100 km s) where more than half of Si and Fe is in the
gas phase, whereas in normal clouds only a fraction of order 1 % of these atoms
is in the gas phase...
The examples refer to water because its properties are
experimentally well established but the physics apply, with some modifications,
also to interstellar grains...
Although the entropy of a macroscopic system can only increase, on a
microscopic level all processes are reversible and S fluctuates...
(9.60)
This is a very general equation, all the physics is contained in the coefficients for
evaporation and accretion, an and f}n...
Let us write the time derivative of the concentration of
rc-mers as
-rold
Cn =
where c°ld is the old value of the concentration a small time step x ago; we want
to find the new one, cn...
At time t = 0, we start with a configuration where only very small clusters exist
and put cn = 0 for n > g...
(9.76)
Jo
If the cooling is due to an adiabatic expansion, K = T/c\~l is a constant,
where k = Cp / Cv is the ratio of specific heats...
Similarity relations
The number of variables that enters the nucleation calculations is quite large
(vo, m,c\,%,T, (v), s, po, To) but one can drastically reduce the parameter space
to only two variables: /x from (9.70) and A defined below [Yam77]...
However, there is no information
in the spectra about grain size or the dependence of growth on the physical
parameters of the medium...
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...
When
the cylinder is tilted further, gy and Q±_ converge and for extremely oblique
incidence they tend to zero...
The refractive index of the cylinder material in
figure 10.4 is m = 1.7 + z'0.03, representative of silicate at optical and near IR
wavelengths...
We will argue in the discussion of figure 10.10
that, for cylinders, the maximum wavelength Amax is proportional to grain size...
The ratio C \\/C±_ at optical and infrared wavelengths
In the optical region, polarization is well described by the Serkowski curve which
can be quantitatively reproduced by applying Mie theory to infinite cylinders...
Figure 10.12 depicts how the optical depth x{X) and the polarization p{X)
produced by small silicate cigars change with wavelength...
Polarized emission versus polarized extinction
Sometimes towards a dust cloud one detects polarized submillimeter radiation
and simultaneously polarized light at much shorter wavelengths from embedded
or background stars...
A0.14)
2n
When the wave has crossed a distance /, the components Ey and Ej_ of the electric
vector E are out of phase by an angle
Ad, = l(K&* - Kfa) = lNAC*>ha...
As viewed from
the rotating body, L, therefore, changes like
—) =Lxft) + T A1.5)
dt /body
Grain rotation 349
or in components
XX = IXQ)X - (l)y(DZ{Iy - IZ)
Xy = lyCOy - COZ<DX(IZ - Ix) A1.6)
TZ = IZCDZ ~ @x@y(Ix ~ Iy)...