A&A 399, 297-313 (2003)
DOI: 10.1051/0004-6361:20021734
P. Woitke1 - Ch. Helling1,2
1 - Zentrum für Astronomie und Astrophysik, Technische
Universität Berlin, Hardenbergstraße 36, 10623 Berlin, Germany
2 -
Konrad-Zuse-Zenrum für Informationstechnik Berlin,
Takustraße 7, 14195 Berlin, Germany
Received 9 August 2002 / Accepted 19 November 2002
Abstract
In this paper, we quantify and discuss the physical and
surface chemical processes leading to the formation, temporal
evolution and sedimentation of dust grains in brown dwarf and giant gas
planet atmospheres: nucleation, growth, evaporation and gravitational
settling. Considering dust particles of arbitrary sizes in the
different hydrodynamical regimes (free molecular flow, laminar flow,
turbulent flow), we evaluate the equilibrium drift velocities (final
fall speeds) and the growth rates of the particles due to accretion
of molecules. We show that a depth-dependent maximum size of the
order of
exists, which depends on
the condensate and the stellar parameters, beyond which
gravitational settling is faster than growth. Larger particles can
probably not be formed and sustained in brown dwarf atmospheres. We
furthermore argue that the acceleration towards equilibrium drift is
always very fast and that the temperature increase of the grains due
to the release of latent heat during the growth process is
negligible. Based on these findings, we formulate the problem of
dust formation coupled to the local element depletion/enrichment of
the gas in brown dwarf atmospheres by means of a system of partial
differential equations. These equations state an extension of
the moment method developed by Gail
Sedlmayr (1988) with an
additional advective term to account for the effect of
size-dependent drift velocities of the grains. A dimensionless
analysis of the new equations reveals a hierarchy of nucleation
growth
drift
evaporation, which characterises the
life cycle of dust grains in brown dwarf atmospheres. The developed
moment equations can be included into hydrodynamics or classical
stellar atmosphere models. Applications of this description will be
presented in a forthcoming paper of this series.
Key words: stars: atmospheres - stars: low-mass, brown dwarfs - dust, extinction - molecular processes - methods: numerical
Brown dwarfs are the only stars known so far that are cool enough to
host small solid particles or fluid droplets (henceforth called dust or dust grains) in their atmospheres. The dust has a
strong influence on the opacity and hence on the structure of the
atmosphere as well as on the spectral appearance of brown dwarfs
(Allard
2001; Marley
2002; Tsuji 2002; Cooper
2002), e.g.by
smoothing out molecular bands and thermalising the radiation, by
increasing the temperature below optically thick cloud layers, and in
particular, by affecting the element composition of the gas
depth-dependently.
Furthermore, the dust component seems to be responsible for a wealth
of variability phenomena recently observed (Bailer-Jones
Mundt 2001a,b; Bailer-Jones 2002; Martín
2001;
Eislöffel
Scholz 2001). The observed light variations are partly
non-periodic and, thus, cannot be explained solely by rotation and
magnetic spots.
In order to study the atmospheres of these ultra-cool stars and giant gas planets, a consistent physical description of the formation, temporal evolution and gravitational settling of dust grains is required, which states a new fundamental problem to the classical theory of stellar atmospheres. A better physical description of the dust component is likely to provide the key not only to understand the variability of brown dwarfs, but also the structure of their atmospheres and the observations of brown dwarfs in general.
In comparison to other astronomical sites of effective dust formation
(Sedlmayr 1994), the atmospheres of brown dwarfs provide special
conditions for the dust formation process. The convection replenishes
the gas in the upper layers with fresh uncondensed gas from the deep
interior, probably in a non-continuous and spatially inhomogeneous
way. The convection energises turbulence which creates strongly
varying thermodynamical conditions on small scales, causing an
inhomogeneous and time-dependent distribution of the dust
(Helling
2001, henceforth called Paper I).
Three further points are to be mentioned:
(i) The stellar gravity (
)
is about a hundred
times larger than in the earth's atmosphere and roughly 104 to
105 times larger than in the circumstellar envelopes of red giants.
This strong gravity puts severe physical constraints on the dynamical
behaviour of the forming dust component. Once formed from the gas
phase, dust grains are immediately forced to sink downwards. The
atmosphere can be expected to clean up from dust grains via
gravitational settling on time-scales ranging in minutes to months,
depending on the dust grain size. This is just the opposite as encountered
in the circumstellar envelopes of red giants, where the forming dust grains
are accelerated outwards due to radiation pressure.
(ii) The dust forming gas in brown dwarf atmospheres is very dense,
.
On the one hand
side, these high densities simplify a physical description of the dust
formation process, since chemical equilibrium in the gas phase can be
assumed. On the other hand side, the large densities lead to a quite
different molecular composition of the gas (for example a simultaneous
occurrence of CH4, H2O and CO2, i.e.no CO-blocking), like in
planetary atmospheres (e.g.Lodders
Fegley 1994).
Consequently, different chemical preconditions for the dust formation
process are present as e.g.in M-type giants (Gail
Sedlmayr 1998). Other nucleation species and other
surface chemical reactions can be important for the dust growth
process. Other solid compounds and even fluid phases can be stable in
a brown dwarf atmosphere.
(iii) The high densities lead furthermore to a qualitatively different
dynamical behaviour of the gas flow around a dust grain. The mean free
path lengths of the gas particles can be smaller than a typical
diameter of a grain, leading to small Knudsen numbers
.
This affects the range of applicability of certain
physical descriptions at hand, concerning for example the drag force
or the growth of a dust grain due to accretion of molecules from the
gas phase, which may be limited by the diffusion of the molecules
toward the grain's surface.
One of the key processes to understand the structure of planetary and
brown dwarf atmospheres - including element depletion and
weather-like features - is the dust sedimentation, which means a
non-zero relative motion between the dust particles and the
surrounding gas, known as the drift problem. Various
approaches have been carried out to simulate the dynamics of
dust/gas mixtures, e.g.in the circumstellar envelopes of late-type stars
(Gilman 1972; Berruyer
Frisch 1983; MacGregor
Stencel 1992; Krüger
1994, 1997). Mostly, two-fluid
approaches have been applied, assuming a constant dust grain size,
where nucleation is disregarded or assumed to be followed by an
instantaneous growth to the mean particle size. Simis
(2001) and Sandin
Höfner (2003) have relaxed this approach by
allowing for a varying mean grain size, according to the results of a
time-dependent treatment of the dust nucleation and growth according
to (Gail
1984; Gail
Sedlmayr 1988).
However, an unique velocity of the dust component is assumed. Assuming
stationarity, Krüger
(1995) have developed a bin
method for the 1D drift problem in stellar winds including a full
time-dependent description of the dust component, which explicitly
allows for a size-dependent drift velocity. This powerful approach
has inspired Lüttke (2002) to develop an
adaptive bin tracking algorithm where the evolution of each bin is
followed in time and space, using the multi-grid method of
Nowak (1993).
In the business of fitting the spectra of brown dwarfs and extra-solar
gas planets, much simpler approaches have been adopted so far in order
to study the effects of element depletion and dust sedimentation by
gravitational settling. In the frame of static model atmospheres with
frequency-dependent radiative transfer, the usual procedure is to
remove heavy elements like Ti, Fe, Mg,
from the object's
atmosphere, assuming that these elements have been consumed by dust
formation guided by stability arguments (Burrows
1997;
Burrows
Sharp 1999; Saegers
Sasselow 2000). Depending on the purpose of the model, dust
formation is either simply disregarded, the dust is assumed to be
fully present or to have rained out completely, leaving behind a
saturated gas. An extensive time-scale study of dust formation and
sedimentation for the atmospheres of Jupiter, Venus, and Mars has been
presented by Rossow (1978). Ackermann
Marley (2001) have extended these time-scale
considerations to the turbulent regime for large dust Reynolds numbers
by adopting various data fits. Cooper
(2002) have presented
further time-scale arguments in consideration of an atmosphere
with prescribed supersaturation to arrive at a maximum size of dust
particles as function of depth, emphasising the influence of
particles sizes on the resulting spectra. However, usually much
simpler ad-hoc assumptions about the grain size distribution are
made, e.g.relying on the size distribution function known from the
interstellar medium (e.g.Allard
2001). Very
recently, Tsuji (2002) has published photospheric models based on the
assumption that the dust particles remain very small (smaller than the
critical cluster size) such that the particles are continuously
evaporating and re-forming. In this case, the problem of the
gravitational settling does not occur. All these simple approaches
allow for an easy use of up-to-date solid opacity data in the
simulations, but a consistent theoretical description of the dust
component is still not at hand.
In this paper, we aim at a solution of this new problem in stellar
atmospheres. We formulate a physical description of the formation, the
temporal evolution and the gravitational settling of dust grains in brown
dwarf atmospheres, consistently coupled to the element consumption from the
gas phase, by modifying and extending the moment method developed by
Gail
Sedlmayr (1988). This description is based on partial differential
equations for the moments of the dust grain size distribution function in
conservation form, which avoids an elaborate and time-consuming binning of the
size distribution function, thus making a straightforward inclusion into
hydrodynamics and classical stellar atmosphere calculations possible.
After the outline of the forces in the equation of motion, the concept
of equilibrium drift is discussed in Sect. 2.
Section 3 contains a physical description of the dust
growth by accretion of molecules in the free molecular flow (
)
and in the viscous case (
).
Section 4 investigates the influence of the latent
heat of condensation and the frictional heating due to particle
drift on the growth process. In Sect. 5, our new
description of the dust component by means of moment equations is
developed. The character of these equations is discussed by analysing the
corresponding dimensionless equations and characteristic numbers
in Sect. 6. Section 7 comprises our
conclusions and future aims.
The trajectory
of a spherical dust particle of radius a and
mass
,
floating in a gaseous
environment like a stellar atmosphere, is determined by Newton's law
The gravitational force on the grain is given by
The radiative force is given by the momentum transfer from the ambient
radiation field to the grain due to absorption and scattering of photons
However, as the following rough estimation will demonstrate, the radiative
force is small compared to the other forces in brown dwarf atmospheres and can
be neglected. We simplify the integral in Eq. (3) by pulling out
the flux mean extinction efficiency
.
The
wavelength integrated Eddington flux is given by
,
where
is the Stefan-Boltzmann constant and
is the effective
temperature of the star. Considering furthermore the small particle limit
(SPL) of Mie theory
(Rayleigh limit), the extinction
efficiency is proportional to the grain size
,
and Eq. (3) results in
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Red Giant: | ||
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||
101 |
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104 |
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Brown Dwarf: | ||
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||
101 |
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104 |
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Table 1 demonstrates that the radiative force on dust
grains in brown dwarf atmospheres is always much smaller than the
gravity, even in case of light but opaque grains. Consequently, the radiative force can be
neglected, and the dust grain's equation of motion
(Eq. (1)) simplifies to
An unique description of the frictional force (drag force) is difficult to obtain for brown dwarf atmospheres. These difficulties arise from the fact that the behaviour of the gas flow around the moving dust grain changes qualitatively with changing grain size, changing drift velocity and/or changing thermodynamic state of the gas. There are transitions from freely impinging gas particles to a viscous flow, from a subsonic to a supersonic behaviour, and from a laminar flow to turbulence. The physical conditions in brown dwarf atmospheres are such that all transitions may possibly occur. Reliable physical descriptions of the drag force are only available in certain limited regimes and an unique description must be compiled from these special cases.
In order to quantify the behaviour of the streaming gas flow, the
following characteristic numbers are introduced: the Knudsen number Kn and
the dust Reynolds number
.
The Knudsen number Kn is
defined by the ratio of the mean free path length of the gas particles
to a typical dimension
of the gas flow
under consideration, here given by the diameter of the grain
In contrast, if
,
inter-molecular collisions are
frequent and the stream of gas particles colliding with the dust grain
becomes viscous. In this case, the drag force cannot be obtained via
reduction to elementary collisions. This is the regime of continuum
theory for which well-tested empirical formulae are available, e.g.
from engineering science (viscous case or slip flow), depending
on whether the gas flow around the dust grain is laminar or turbulent.
In order to characterise the transition from laminar (Stokes) friction
to turbulent (Newtonian) friction, the dust grain's Reynolds number
is introduced
In order to evaluate the kinematic viscosity of the gas
we follow the considerations of Jeans
(1967) for a mixture of ideal gases
![]() |
(8) |
Considering the mechanics of rarefied gases, Schaaf (1963)
derives a formula for the drag force by freely impinging gas particles due to
elastic collisions, equally applicable in the subsonic as well as in the
supersonic range,
In this regime, continuum theory is valid for which well-tested
empirical formulae exist. Lain
(1999), carrying out
experimental studies on bubbly flows, arrive at the following
empirical expression for the drag force
Using Eq. (7), the drag force in the viscous case according to
Eq. (14) is found to have the following
asymptotic behaviour
For flows with an intermediate Knudsen number (
), so-called transitions flows, reliable expressions
for the drag force are difficult to obtain. We therefore define a
critical Knudsen number
where
equals
.
Considering the limiting cases of subsonic drift velocities and small
in Eqs. (13) and (16),
respectively, the result is exactly
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(17) |
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Figure 1:
Contour plot of the equilibrium drift velocity
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Considering a dust particle of constant radius a floating in a gas at
constant thermodynamical conditions (,
T) and a constant
velocity
,
the particle will be accelerated until a
force equilibrium is reached, where the gravitational acceleration is
balanced by frictional deceleration
Figure 1 shows the resulting values of
in a brown
dwarf's atmosphere with
.
The equilibrium drift velocities
roughly range in
[10-4,10+6] cm s-1 and are generally smaller
for small particles and large densities. The small bendings of the contour
lines around
are no numerical artifacts but result
from the measured re-increase of the drag coefficient of spherical particles
between
and
(Eq. (15)), associated with the transition from laminar to
turbulent friction.
Small dust particles can sustain longer in the respective atmospheric
layers, whereas large grains will "rain out'' sooner. Only dust
particles 100
m at gas densities
10
can hereby reach a drift velocity beyond the local
velocity of sound
.
However, such particles will remove themselves so
quickly from the respective atmospheric layers (
s) that this case seems very unlikely to be relevant
for any part of the atmosphere, unless there exists a physical process
(convective streams or atmospheric winds) which is capable to produce
supersonic upwinds. The time-scale for gravitational
settling is hereby defined as
Figure 1 demonstrates furthermore that even the
smallest dust particles cannot sustain forever but will slowly sink
into deeper layers. Assuming that the dust particles do not grow
along their way down the atmosphere (which would increase their drift
velocity), a m-particle starting in an atmospheric layer
with
g cm-3 needs about
s (8 months) to pass one
scale height. A dust particle with
m needs only
1/4 hour
.
The destiny of those particles drifting inward is to finally reach an
atmospheric layer where the temperature is high enough to evaporate
them thermally. This sets free the elements the dust grains are
composed off and thereby enriches the surrounding gas in this
layer. Hence, the rain-out will tend to saturate the gas below the
cloud base, where the "cloud base'' is identified with the level in
the atmosphere where the dust grains are just thermodynamically stable
(,
see Sect. 3).
The actual relative velocity of the dust particle with respect to the
gas,
,
can of course deviate from its equilibrium value
defined in Sect. 2.4. It will only asymptotically reach
for
,
if the parameters a,
,
and T are constant. However, considering a dust particle
created in a brown dwarf atmosphere, the particle may grow by
accretion of molecules (
,
see Sect. 3)
and the physical state of the surrounding gas may change with time
(e.g.
)
as the particle sinks into deeper layers of
the atmosphere. Turbulence may furthermore create a time-dependent
velocity field (
), which provides an additional
cause for temporal deviations between
and
.
Thus, an important question for the discussion of the dynamical
behaviour of the dust component in brown dwarf atmosphere is, whether
or not
can be replaced by
,
at least
approximately.
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Figure 2:
Contour plot of the acceleration time-scale towards equilibrium
drift
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In order to discuss this question, we consider the dust particle
acceleration time-scale
towards equilibrium drift. Expressing
the dust particle's equation of motion (Eq. (5)) in terms
of the first-order differential equation
with
and
,
and
assuming small deviations
from the stability point
(where
), the temporal change of y is
.
Accordingly, the acceleration time-scale is given by
,
or
For a powerlaw dependence
,
Eqs. (20) and (21) result in
,
i.e.we find
for
.
Since
Sect. 3 will demonstrate that also the growth of the
dust particles is slow in comparison to
,
we may conclude that
the concept of equilibrium drift provides a good approximation for the
description of the size-dependent relative velocities between dust and
gas in brown dwarf atmospheres.
A fundamental process for the consideration of the time-dependent
behaviour of the dust component in brown dwarf atmospheres is the
growth of the dust particles by accretion of molecules. The
respective reverse process (thermal evaporation, in view of more
complex surface reactions also sometimes denoted by chemical
sputtering) is important at high temperatures. Considering the thermodynamical conditions
in brown dwarf atmospheres, we are again faced with the problem of
qualitative changes of the dynamical behaviour of the gas component
due to different Knudsen numbers.
For large Knudsen numbers (lKn), gas molecules of all kinds are freely
impinging onto the surface of the grain. Some of these dust-molecule
collisions (sometimes a certain sequence of them) will initiate a chemical
surface reaction which causes a growth step (or an evaporation step) of the
dust particle. This case has been extensively studied in the circumstellar
envelopes of AGB stars (Gail
Sedlmayr 1988; Gauger
1990;
Dominik
1993).
The accretion rate, expressed in terms of the increase of the
particle's volume
due to chemical surface
reactions for large Knudsen numbers is given by
The last term on the r.h.s. of Eq. (22) takes into
account the reverse chemical processes, namely the thermal evaporation
rates. It determines the sign of
and hence decides whether
the dust particle grows or shrinks.
is
a generalised supersaturation ratio of the surface reaction "r''
(Dominik
1993) where
is the particle density of the key
species in phase-equilibrium over the condensed dust material.
is not known a priori. In the case of simple surface
reactions, which transform
units of the solid material
from the gaseous into the condensed phase (and vice versa), e.g.
(
,
)
or
(
,
), the generalised
supersaturation ratio
is related to the usual supersaturation ratio
of the dust grain
material
(Gauger
1990) by
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(23) |
For small Knudsen numbers (sKn), the transport of gas molecules to the
surface of the grain (or the transport of evaporating molecules away
from the grain's surface) is not a simple free flight with thermal
velocity as assumed in Eq. (22), but is hindered by
inter-molecular collisions. Consequently, the grain growth and
evaporation is limited by the diffusion of molecules towards or away
from the grain's surface, considering growth or evaporation in the
laminar case, respectively (we disregard here convection as transport
process for the molecules, expected to occur in the turbulent case).
We consider the following particle conservation equation with a
diffusive transport term (see Landau & Lifschitz 1987, Eq. (38.2)
ff)
Considering the static case (
)
and assuming stationary
(
)
and spherical symmetry,
Eq. (24) results in
The solution of Eq. (28) with the boundary
conditions (29) and (30) is
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Figure 3:
Contour plot of the growth time-scale
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For arbitrary Knudsen numbers we apply the same interpolation scheme
as outlined in Sect. 2.3. We define a critical Knudsen
number
by equating Eqs. (22) with (32)
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(33) |
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(34) |
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(35) |
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(37) |
The particle growth is found to be typically 3 orders of magnitude
slower than the acceleration which allows us to assume instantaneous
acceleration (equilibrium drift). For large Knudsen numbers, we find
whereas for small Knudsen numbers
the growth time-scale increases faster for larger grains and becomes
density-independent,
.
Note that the
influence of the drift velocities on the particle growth has not been
considered in Eqs. (22) and (32) such
that for supersonic drift velocities or large dust Reynolds numbers, the
presented physical description is not valid.
An additional dashed line is depicted in Fig. 3, where
.
This line defines a maximum dust
grain size
in a brown dwarf atmosphere. For larger
particles (
above the dashed line), the growth
time-scale exceeds the time-scale for gravitational settling
(
)
which means that such particles
are already removed from the atmosphere before they can be formed.
Consequently, such particles cannot exist. The maximum grain size
varies between
1
m in the thin, outer
atmospheric regions (
)
and
100
m in the dense, inner regions
(
). These values depend on the stellar
parameters and the considered dust material density.
Note, that an absolute minimum of
has been
considered in Fig. 3 since extreme supersaturation
(
)
and solar abundance of silicon in the gas phase
have been assumed. In the case of an Si-depleted or nearly saturated
gas (
),
becomes larger and the maximum
particle radius
becomes smaller. Furthermore, the
values for
are relatively independent of temperature,
but will shift as
in the free molecular flow
case and
in the laminar viscous case,
remembering that
in both cases and that
(see also Eq. (81)).
Figure 3 demonstrates furthermore that dust particles
moving with supersonic drift velocities cannot be expected in brown
dwarf atmospheres. Similarly, the turbulent flow regime with dust
Reynolds numbers
is barely reached at
very large densities. Therefore, we can conclude that for dust grains in
brown dwarf atmospheres the subsonic free molecular flow and the
laminar viscous flow are the important cases to be investigated.
The surface chemical reactions responsible for the growth of a dust
particle liberate the latent heat of condensation
[erg/g] which
causes a heating of the grain as
![]() |
(39) |
A further heating process of the dust particle is given by the
friction caused by the motion relative to the gas,
In order to determine the dust temperature increase, we balance these heating
processes with the net energy losses due to radiation and due to inelastic
collisions. The net radiative cooling rate of a single dust grain is given by
The cooling due to inelastic collisions with gas particles, in particular with
H2, depends again on the Knudsen number. For large Knudsen numbers (
)
the collisional cooling rate is given by
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(46) |
The energy balance of a single dust grain is finally given by
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(52) |
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Figure 4:
Contour plot of the temperature increase
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Figure 4 shows an example for the resulting temperature
increase
of quartz grains. We assume
and again consider the explicit
growth reaction (Eq. (38)) with a release of latent
heat of
eV per reaction at 1000 K, 5.61 eV at
1500 K and 5.49 eV at 2000 K (data reduced from the
enthalpies of formation of the involved molecules and the solid,
source: JANAF-tables, electronic version, Chase
1985).
For the sake of simplicity, we furthermore assume
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(53) |
![]() |
(54) |
Despite these simplifications, Fig. 4 clearly indicates
that the warming of the dust grains due to the release of latent heat
is negligible, being less than 3.5 K all over the relevant parts of
the size-density-plane, where
(compare Fig. 3). Here, we find that this heating is
balanced by collisional cooling
.
Since both heating/cooling rates scale as
for large
Knudsen numbers and as
for small Knudsen numbers, a
constant value for
tunes in for both cases,
K for large Kn, and
K for
small Kn. Note that the calculated temperature differences are always
an upper estimate. The actual temperature differences may be much
smaller because we have assumed solar, undepleted abundances of Si in
the gas phase and
for the calculation of
.
For larger particles (roughly at
as defined in
Sect. 3.1), the character of the energy balance of the dust
particles changes. Here, the frictional heating due to the rapid
relative motion and the radiative cooling dominate, i.e.
.
Much larger temperature deviations up
to 10 000 K result in this case. However, as argued before, such
large grains cannot be formed in brown dwarf atmospheres.
Thus, the resulting increase of the dust temperature
is by far
too small to reach the sublimation temperature
,
unless
gas temperatures very close to
are considered and,
therefore, Eqs. (22) and (32) remain
valid.
The physical and chemical processes discussed so far (nucleation,
growth, evaporation, gravitational settling and element
depletion/enrichment) occur simultaneously in the atmosphere and may
be strongly coupled. Therefore, our aim in this section is to derive a
consistent time-dependent description of the dust component in
brown dwarf or giant gas planet atmospheres. We will
derive a system of partial differential equations which describes the
evolution of the dust component by means of the moments of its size
distribution function. This idea was originally developed by
Gail
Sedlmayr (1988) and extended by Dominik
(1993) to core-mantle and dirty grains. The
resulting differential equations are supposed to be simple and
includable into hydrodynamics or classical stellar atmosphere
calculations. In contrast, we want to avoid an elaborate and
time-consuming multi-component treatment of the dust component, e.g.by
using discrete bins for the dust size distribution function with
individual drift velocities in this paper.
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Figure 5:
Two surface chemical processes (![]() ![]() ![]() |
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The master equation for dust particles
,
,
where
is the distribution function of
dust particles in volume space, is given by
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(56) |
The maximum grain sizes to be expected in brown dwarf atmospheres (Fig. 3) allow us to concentrate on two major cases in the following, namely the subsonic free molecular flow and the laminar viscous flow.
For large Knudsen numbers, the chemical rates depicted in
Fig. 5 can be expressed according to Eq. (22):
Solving Eq. (19) with the frictional force according
to Eq. (13, subsonic case) the equilibrium drift
velocity is
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= | ![]() |
(64) |
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= | ![]() |
(65) |
The derivation of the moment equations for the case of small Knudsen numbers
is analogous to the previous subsection. We express the surface chemical
rates according to Eq. (32) by
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= | ![]() |
(68) |
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= | ![]() |
(69) |
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= | ![]() |
(70) |
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= | ![]() |
(71) |
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= | ![]() |
(73) |
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= | ![]() |
(74) |
For the sake of analysis and discussion, we transform the dust moment
equations derived in Sects. 5.1 and 5.2
into their dimensionless form by introducing reference values as
,
,
,
,
,
,
,
,
,
following the procedure described in Paper I. The reference values are to be
chosen according to the expected order of magnitude of the respective
quantities and the length and time-scales under investigation. After this
substitution, all quantities are dimensionless and can be compared by number.
This allows us to identify the leading terms in the equations, e.g.in the inner and outer regions of a brown dwarf atmosphere. The remaining constants (products of the reference values) can be summarised into characteristic numbers which provide an efficient way to describe the qualitative behaviour of the dust component.
The dimensionless dust moment equations for nucleation, growth,
evaporation, and equilibrium drift write for a subsonic free
molecular flow (
)
The following discussion is based on a typical structure of a brown
dwarf atmosphere with solar abundances in the gas phase, i.e.
neglecting the possible depletion due to dust formation (see
Table 2). As underlying ()-structure we refer to a
brown dwarf model atmosphere with
K and
which has been kindly provided by
T. Tsuji (2002)
. As exemplary dust species we consider solid SiO2(amorphous quartz), growing by the accretion of SiO and H2O
(Eq. (38)). Since the nucleation of SiO2 seems
dubious (the monomer is rather unstable as a free molecule and hence
not very abundant in the gas phase) we consider nucleation of TiO2instead
.
Name | Characteristic | Value | ||||||
Number | inside | outside | ||||||
Mach number |
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||||||
Froude number |
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0.842 |
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0.495 |
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Strouhal number |
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hydrodyn. Knudsen number |
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Knudsen number (Eq. (6)) |
![]() |
![]() |
... |
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Drift number |
![]() |
![]() |
... |
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combined drift number (
![]() |
![]() |
![]() |
... |
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||||
combined drift number (
![]() |
![]() |
![]() |
... |
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|||||||
Sedlmaÿr number
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![]() ![]() |
![]() ![]() |
|||||
![]() ![]() |
![]() ![]() |
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![]() ![]() |
![]() ![]() |
|||||||
Damköhler no. of nucleation |
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0 | 0 |
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Damköhler no. of growth (
![]() |
![]() |
3.95 |
![]() |
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Damköhler no. of growth (
![]() |
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Name | Physical Quantity | Reference Value | ||||||
inside | outside | |||||||
temperature |
![]() |
[K] | 1700 | ... | 1000 | |||
density |
![]() |
[g/cm3] |
![]() |
... |
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|||
thermal pressure |
![]() |
[dyn/cm2] |
![]() |
... |
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|||
velocity of sound |
![]() |
[cm/s] |
![]() |
... |
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|||
velocity |
![]() |
[cm/s] |
![]() |
|||||
length |
![]() |
[cm] | 10+4 | 10+6 | 10+4 | 10+6 | ||
hydrodyn. time |
![]() |
[s] |
![]() |
![]() |
![]() |
![]() |
||
gravitational acceleration |
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[cm/s2] | 10+5 | |||||
mean particle radius |
![]() |
[cm] | 10-3 | ... | 10-6 |
![]() |
||
0th dust moment (
![]() |
![]() |
[1/g] |
![]() |
... |
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![]() |
||
nucleation rate |
![]() |
[1/s] |
![]() |
... |
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|||
growth velocity (
![]() |
![]() |
[cm/s] |
![]() |
... |
![]() |
![]() |
||
growth velocity (
![]() |
![]() |
[cm2/s] |
![]() |
... |
![]() |
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||
diffusion constant (Eq. (26)) |
![]() |
[cm2/s] |
![]() |
... | 2.43 |
![]() |
||
mean free path (Eq. (10)) |
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[cm] |
![]() |
... |
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|||
total hydrogen number density |
![]() |
[1/cm3] |
![]() |
... |
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|||
molecular number density |
![]() |
[1/cm3] |
![]() |
... |
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An analysis of the characteristic numbers in front of the source terms
in Eqs. (77) and (78) (see
Table 2) reveals a hierarchy of nucleation
growth
drift:
Name | Value | ||
dust material density |
![]() |
[g/cm3] | 2.65 |
monomer volume |
![]() |
[cm3] |
![]() |
lower dust grain radius |
![]() |
[cm] |
![]() |
molecular radius |
![]() |
[cm] |
![]() |
physical process |
![]() |
![]() |
time-derivative |
![]() |
|
advective term | 1 | |
nucleation term
![]() |
![]() |
|
growth term
![]() |
![]() |
![]() |
drift term |
![]() |
![]() |
![]() associated mean dust quantity: ![]() ![]() ![]() ![]() |
Table 4 shows some dependencies of the combined characteristic numbers (the squared brackets in Eqs. (77), (78)), which provides scaling laws for the importance of the different processes in the different regimes:
In Table 2, we have assumed
,
i.e.we have
considered time-scales
of the order of
,
appropriate for disordered (e.g.turbulent)
velocity fields, where the l.h.s. terms of Eqs. (77) and
(78) are of comparable importance. However, if
large-scale systematic motions are stable for a long time (e.g.a
circulating thunderstorm or a stable convection roll), the system may
reach a quasi-stationary situation where
becomes much
larger and hence
.
In this case, the first
term on the l.h.s. of Eqs. (77) and (78)
vanishes (see Table 4) whereas all other terms remain
unaffected.
An interesting special case occurs if additionally
,
i.e.when the dust-forming system reaches the static
case. In this case, also the advective terms in
Eqs. (77) and (78) vanish and the source
terms must balance each other. In the
case, this means that
the gain of dust by nucleation and growth must be balanced by the loss
of dust due to rain-out, which means that the gas will be depleted. In
the
case, just the opposite is true, i.e.the loss by
evaporation must be balanced by the gain of dust particles raining in
from above. Consequently, the gas in such undersaturated layers will
be enriched by the condensable elements liberated by the evaporating
grains.
However, both control mechanisms (in the static limit) result in an
efficient transport of condensable elements from the cool upper layers
into the warm inner layers, which cannot last forever. We may conclude
that if the brown dwarf's atmosphere is truly static for a long time,
there is no other than the trivial solution for
Eqs. (77) and (78) where the gas is
saturated (
)
and dust-free (
). This situation
changes, however, if the brown dwarf's atmosphere is turbulent or, in
particular, when it is convective. In that case, the replenishment of
the atmosphere with fresh uncondensed gas from the deep interior will
counteract the downward transport of condensable elements by the
formation and gravitationally settling of dust grains. Simulations of
this quasi-static balance will be the subject of the forthcoming paper
in this series.
A dynamical modelling of the dust component in brown dwarf atmospheres
by means of a moment method as proposed in this paper - consistently
coupled to hydrodynamics, radiative transfer and element depletion in
the scope of hydrodynamical or classical stellar atmosphere
calculations - seems straightforward as soon as two major problems can
be solved:
One idea to construct such a closure condition has been developed by
Deufelhard
Wulkow (1989) and Wulkow (1992),
studying the kinetics of polyreaction systems. The size distribution
function f(V) is here approximated by a weight function
,
which describes the basic shape of f(V), and
modified by a sum of orthogonal polynomials
(k=0,1,2, ... ,n) as
In this paper, we have investigated the basic physical and chemical processes which are responsible for the formation, the temporal evolution and the precipitation of dust grains in brown dwarf atmospheres.
In contrast to other astronomical sites of effective dust formation, the dust particles are embedded in such a dense gas that the Knudsen numbers may fall short of unity. This requires a careful fall differentiation for the different hydrodynamical regimes: free molecular flow (subsonic and supersonic) and slip flow (laminar and turbulent case).
Compiling a general formula for the drag force from the different
special cases, we have shown that the large gravity in brown dwarf
atmospheres forces the dust particles to move with a considerably high
downward drift velocity relative to the gas. The acceleration of the
dust particles (on a time-scale
)
towards the
equilibrium drift velocity (final fall speed) results to be always
much faster than any other considered process (nucleation, growth,
hydrodynamical acceleration and sedimentation) such that an instantaneous acceleration of the particles to equilibrium
drift can be assumed. In contrast, the outward acceleration
of dust grains due to radiation pressure is completely negligible
in brown dwarf atmospheres.
The large drift velocities are found to limit the residence time of the
forming dust grains and hence their maximum size
as
For small Knudsen numbers, the growth of the particles by accretion of
molecules is limited by the diffusion of the molecules towards the
grain surface, and the energy exchange with the surrounding
gas is limited by heat conduction. The latter process co-works
with the radiative gains and losses of the hot grains.
According to our results, the release of latent heat during the growth
does only lead to a small increase of the grain temperature
(K) and has no particular influence on the growth rates.
Based on these findings, we have formulated a system of partial differential equations for the consistent physical description of the dust component in brown dwarf and giant gas planet atmospheres. These moment equations represent an unique tool to model the nucleation, growth and size-dependent equilibrium drift of the dust particles, and the element depletion/enrichment of the gas. We consider such a description as essential, because these processes occur simultaneously and are strongly coupled. The description allows for an inclusion into hydrodynamics or classical stellar atmosphere calculations, although a few unsolved questions still remain, e.g.a reliable closure condition and a clean Knudsen number fall differentiation.
A dimensionless analysis of the moment equations reveals the existence of the following three regimes associated with the formation of a cloud layer:
The life cycle of dust grains in brown dwarf atmospheres is finally completed by convective streams which mix up gas from the deep interior into the upper layers. On a large scale, we expect an intricate balance of this upward mixing of condensable elements by convection with the downward gravitational settling of the condensing dust grains, which will determine the large-scale structure of the element abundances in the atmosphere related to the observation of the various molecular features.
This work will be continued in the next paper of this series by solving the dust moment equations for the special case of a static atmosphere.
Acknowledgements
We thank T. Tsuji for contributing the hydrostatic reference model atmosphere and Akemi Tamanai for providing us with an electronic version of the optical constants of amorphous quartz. The anonymous referee is thanked for helpful comments, in particular concerning the fricional heating. We thank R. Klein for pointing out the work of DeufelhardWulkow. This work has been supported by the DFG (Sonderforschungsbereich 555, Teilprojekt B8, and grand Se 420/19-1 and 19-2). Most of the literature search has been performed with the ADS system.