Great Migrations

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Lecture L22 ASTB21
1. Discovery and study of dusty disks in
Vega-type systems 2. Evidence of planetesimals
and planets in the Beta Pictoris system 3.
Replenished dust disks collisions and nature of
dust
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Discovery and study of dusty disks Scattered
light tells us how the scattering area is
distributed around the star and how reflective
particles are Thermal radiation measurements
and images (at wavelengths of 10 microns and
larger) tell us how the absorbing and emitting
area of particles is distributed around the star
and how hot particles are. Neither the optical
nor the mid-infrared images/data alone allow us
to separate the contributions of the area and the
emissivity (scattering/emission coefficient).
Albedo (percentage of light scattered) can only
be found by comparing observations done in the
visible and mid-infrared (or far-IR) spectral
domains.
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Infrared excess stars (Vega phenomenon)
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Beta Pictoris thermal radiation imaging (10
um) Lagage Pantin (1993)
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1984
1993
Beta Pictoris, visible scattered
starlight comparison with IR data yields a high
albedo, A0.4-0.5 (like Saturns rings but very
much unlike the black particles of cometary crust
or Uranus rings).
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A new edge- on disk! NICMOS/ HST (Schneider et
al 2005) near-IR band (scattered light)
This is how disks look when just discovered
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This is how disks look a decade later - much
better quality data, fewer artifacts, disks
appear smoother.
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Disk of Alpha Pisces Austrini (a PsA)
Fomalhaut a bright southern star type A
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HD 141569A is a Herbig emission star 2 x solar
mass, 10 x solar luminosity, hydrogen emission
lines H are double, because they come from a
rotating inner gas disk. CO gas has also been
found at r 90 AU. Observations by Hubble Space
Telescope (NICMOS near-IR camera).
Age 5 Myr, a transitional disk
Gap-opening PLANET ? So far out??
R_gap 350AU dR 0.1 R_gap
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HD 14169A disk gap confirmed by new
observations (HST/ACS)
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5 Myr
20 Myr
200 Myr
4567 Myr
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Evidence of planetesimals and planets in the
vicinity of beta Pictoris 1. Lack of dust near
the star (r Falling
Evaporating Bodies 3. Something large (a planet)
needed to perturb FEBs so they approach the
star gradually. 4. The disk is warped somewhat,
like a rim of cowboy hat, and that requires
the gravitational pull of a planet on an orbit
inclined by a few degrees to the plane of the
disk. 5. Large reservoir of parent (unseen)
bodies of dust needed, of order 100 Earth
masses of rock/ice. Otherwise the dust would
disappear quickly, on collisional time scale
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B Pic b(?) sky?
Beta Pictoris
Evidence of large bodies (planetesimals, comets?)
11 micron image analysis converting observed
flux to dust area (Lagage Pantin 1994)
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FEB Falling Evaporating Bodies hypothesis in
Beta Pictoris
FEB
star

absorption line(s) that move on the time scale
of days as the FEBs cross the line of sight
H K calcium absorption lines are located in the
center of a stellar rotation-broadened line
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1. Temperature of solid particles around a
star 2. Finding out the dust distribution
(optical thickness) 3. Radiation pressure -
size distribution of particles - elliptic
orbits of stable particles 4. Collisional
lifetime orbital period / optical
thickness 5. Composition and crystallinity of
particles
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Calculating the temperature of dust larger
bodies
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The physics of dust and radiation is very
simple In the past the amount of dust
hidden by coronograph mask had to be
reconstructed using MEM maximum entropy
method or other models. Today scattered light
data often suffice. tau optical thickness
perpendicular to the disk (vertical optical
thicknass)
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Equilibrium temperature of solid particles (from
dust to atmosphereless planets) A Qsca
albedo (percentage of light scattered) Qabs
absorption coefficient, percentage of light
absorbed Qabs Qsca 1 (this assumes the
size of the body wavelength of starlight,
otherwise the sum, called extinction
coefficient Qext, might be different) total
absorbing area A, total emitting area 4 A
(spherical particle) Absorbed energy/unit time
Emitted energy /unit time A Qabs(vis) L/(4 pi
r2) 4A Qabs(IR) sigma T4 L stellar
luminosity, r distance to star, L/4pi r2
flux of energy, T equilibrium temperature of
the whole particle, e.g., dust grain, sigma
Stefan-Boltzmann constant (see physical constanys
table) sigma T4 energy emitted from unit area
of a black body in unit time Qabs(vis) - in the
visible/UV range where starlight is
emitted/absorbed Qabs(IR) - emissivityabsorptivi
ty (Kirchhoffs law!) in the infared, where
thermal radiation is emitted
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Equilibrium temperature of solid bodies falls
with the square-root of r T4
Qabs(vis)/ Qabs(IR) L/(16 pi r2 sigma) which
can be re-written using Qabs(vis) 1-A as T
280 K (1-A)/Qabs(IR) (L/Lsun)(1/4)
(r/AU)(-1/2) Table of theoretical surface
temperature T of planets if Qabs(IR)1, and the
actual surface temperature Tp. Differences
between the two mostly due to greenhouse effect
Body Albedo A T(K) Tp(K)
comments _________________________________________
________________ Mercury
0.15 433 433 Venus 0.72 240 540 huge
greenhouse Earth 0.45 235 280
greenhouse Moon 0.15 270 270 Mars 0.25 210
220 weak greenhouse asteroid (typical)
0.15 160 160 Ganimede 0.3 112 112 Titan 0.2
86 90(?) Pluto 0.5 38 38
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Optical thickness
perpendicular to the disk in
the equatorial plane (percentage of starlight
scattered and absorbed, as seen by the outside
observer looking at the disk edge-on,
aproximately like we look through the beta
Pictoris disk)
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What is the optical thickness ? It
is the fraction of the disk surface covered by
dust here I this example its about 2e-1 (20) -
the disk is optically thin ( transparent,
since it blocks only 20 of light) picture
of a small portion of the disk seen from
above Examples beta Pic disk at r100 AU
opt.thickness3e-3 disk
around Vega opt.thickness1e-4
zodiacal light disk (IDPs) solar system
1e-7
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STIS/Hubble imaging (Heap et al 2000)
Modeling (Artymowicz,unpubl.) parametric,
axisymmetric disk cometary dust phase function
Vertical optical thickness ?
Vertical profile of dust density
Radius r AU
Height z AU
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Dust processing collisions 1. Collisional time
formula 2. The analogy with the early solar
system (in the region of todays TNOs
trans-Neptunian objects, or in other
words, Kuiper belt objects, KBOs these are
asteroid-sized bodies up to several hundred km
radius)
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Time between collisions (collisional lifetime) of
a typical alpha meteoroid. Obviously, inversely
proportional to the optical thickness (doubling
the optical depth results in 2-times shorter
particle lifetime, because the rate of
collision doubles).
P orbital period, depends on radius as in
Keplers III law. This formula is written with a
numerical coefficient of 1/8, to reflect the
fact that a disk made of equal-sized particles
needs to have the optical thickness of about 1/4
to make every particle traversing it vertically
collide with some disk particle, on average. The
vertical piercing of the disk is done every
one-half period, because particles are on
inclined orbits and do indeed cross the disk
nearly vertically, if on circular orbits. If the
orbits are elliptic, a better approximate
formula has a coefficient of 12 replacing 8 in
the above equation.
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How does the Vega-phenomenon relate to our Solar
System (Kuiper belt, or TNOs - transneptunian
objects)
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Chemistry/mineralogy/crystallinity of dust all
we see so far is silicate particles similar to
the IDPs (interplanetary dust particles from
our system) ice particles are not seen, at
least not in the dust size range (that is also
true of the IDPs)
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Microstructure of circumstellar disks identical
with IDPs (interplanetary dust particles) mostly
FeMg silicates (Mg,Fe)SiO3 (Mg,Fe)2SiO4
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Small dust is observed due to its large total
area Parent bodies like these (asteroids,
comets) are the ultimate sources of the dust, but
remain invisible in images due to their small
combined area
Comet
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A rock is a rock is a rock which one is from
the Earth? Mars? Beta Pic? Its hard to tell
from just spectroscopy or even looking at it!
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EQUILIBRIUM COOLING SEQUENCE
Chemical unity of nature and its thanks to
stellar nucleosynthesis!
Silicates
silicates
T(K)
What minerals will precipitate from
a solar-composition, cooling gas? Mainly
Mg/Fe-rich silicates and water ice. Planets are
made of precisely these things.
ices
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The disk particles are made of the Earth-type
minerals! (olivine, pyroxene, FeO, PAH
Polycyclic Aromatic Hydrocarbons)
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Crystallinity of minerals Recently, for the
first time observations showed the difference in
the degree of crystallinity of minerals in the
inner vs. the outer disk parts. This was done by
comparing IR spectra obtained with single
dish telescopes with those obtained while
combining several such telescopes into an
interferometric array (this technique, long
practiced by radio astronomers, allows us to
achieve very good, low-angular resolution, observa
tions). In the following 2 slides, you will see
some inner and outer disk spectra - notice
the differences, telling us about the
different structure of materials amorphous
silicates typical dust grains precipitating
from gas, for instance in the interstellar
medium, no regular crystal structure crystalline
grains same chemical composition, but forming a
regular crystal structure, thought to be
derived from amorphous grains by some heating
(annealing) effect at temperatures up to 1000 K.
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90 amorphous
60 amorphous
45 amorphous
compare
Beta Pic,
95 crystalline
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THE END of L22