Transcript Powerpoint of lecture #8

Lecture L7 - AST3020
Understanding dust
1. Clearing stage: do planets clear dust?
2. Comets
3. Asteroids
4. Planetoids
5. Zodiacal light
6. IDPs (Interplanetary dust particles)
Clearing the junk left at the
construction site:
Oort cloud formation
Kuiper belt
10th planet(s)
Two-body interaction: a small planetesimal
is scattered by a large one, nearly missing it
and thus gaining an additional velocity of up
to ~vesc (from the big body with mass Mp)
Gravitational
slingshot
The total kinetic energy after encounter, assuming that
initially both bodies were on nearly-circular orbits is
vK  vesc
EK 
2
2
2
(we neglect the random part depending on the angle between the two
components of final velocity).
If the total energy of the small body after encounter,
E=Ek + Epot,
is positive, then the planetesimal will escape from the planetary
system.
v esc 
2
2GM p
Rp
, vK 
2
GM
r
GM
2
E pot  
  vK ,
r
2
2
v K  v esc
Ek 
,
2
2
2
v esc  v K
E  E pot  E k 
.
2
E  0  escape
Condition of escpe :
2
2M p r
v esc

2
M Rp
vK
1
2
Planet
v esc
2
vK
Conclusions:
Earth
Mars
0.14
0.04
Terrestrial planets in the solar system
cannot eject planetesimals
Jupiter’s core
Jupiter
Saturn
Uranus
Neptune
5
21
14
10
19
Giant planets (even cores) can eject
planetesimals out of the solar system
Any cleared region may be seen as a gap
in SED.
So far no firm detection of exoplanets
this way… but the process definitely
happened in the solar system, leaving
behind the Oort Cloud.
GM
E 
2a
Typical perturbation by
planets ~ 0.01 (1/AU)
E=0
Jan Oort
(1902-1992)
found that
a~ (2-7)*1e4 AU for
most new comets.
Oort cloud of comets: the source of the so-called new comets
size ~ Hill radius of the Sun in the Galaxy ~ 260,000 AU
  1011 , rL  r 3  / 3 ~ 8500 pc
3
3 104 ~ 1.3 pc
Q: Porb = ?
inner part flattened, outer elliptical
Out of
152 new comets
~50 perturbed
recently by 2 stars
(one slow, one fast
passage)
excess of retrograde
orbits,
aphelia clustered on
the sky
Fomation of Oort cloud
Kuiper belt, a theoretical entity since 1949 when
Edgeworth first mentioned it and Kuiper independently
proposed it in 1951, was discovered by D. Jewitt and J. Lu
in 1993 (1st object), who later estimated that 30000
asteroid-sized (typically 100 km across) super-comets
reside there.
Gerard Kuiper (1905-1973)
Interestingly, we now observe
that Kuiper belt apparently ends
at r ~ 50 AU,
so the original drawings were
incorrect!
Smith & Terrile (1984)
The Kuiper belt is home to quite
a Zoo of planetoids or plutinos,
some of which are larger than
the recently demoted (former)
planet Pluto.
10th planet(s):
super-Pluto’s:
Sedna, “Xena”
also starring:
Plutinos!
Don’t worry…
it’s hard to see!
Better image on
the next slide.
The 10th planet (temp. name “Xena” or UB313) first seen in 2003.
It has a moon (announced in Sept. 2005)
See the home page of the discoverer of planetoids, Michael Brown
http://www.gps.caltech.edu/~mbrown/
Images of the four largest
Kuiper belt objects
from the Keck Observatory
Laser Guide Star Adaptive
Optics system.
Satellites are seen
around all except for
2005FY9; in 75% of cases!
In comparison, only 1 out of 9
Kuiper belt objects, also
known as TNOs (TransNeptunian Objects) have
satellites.
On October 31 2005, 2 new moons of Pluto have been
found by the Hubble Space Telescope/ACS
Charon
Pluto
IDPs: Interplanetary Dust Particles
Comet Hale-Bopp (1997)
10 -100 km
10 m
European (ESA) Giotto mission
saw comet Halley’s nucleus in 1986, confirming the basic
concept of comet nucleus
as a few-km sized chunk
of ice and rocks stuck
together (here, in the form
of a potato, suggesting
2 collided “cometesimals”)
The bright jets are from the
craters or vents through
which water vapor and the
dust/stones dragged by it
escape, to eventually
spread and form head
and tail of the comet.
Borrely-1 imaged by NASA in 2001
Why study comets?
Comet Wild-2 is a good example:
this 3km-planetesimal was thrown out in the giant
impacts era from Saturn-Neptune region into the
Oort cloud, then wandered closer to Uranus/Jupiter
& has recently been perturbed by Jupiter
(5 orbits ago) to become a short-period comet
(P~5 yr)
Gas tail
Comet Hale-Bopp
Dust tail
Comet Temple1,
on the other hand,
is a short-period comet that survived >100 passages so we are eager to study differences between the more
and the less pristine bodies.
Stardust NASA mission - reached comet Wild-2 in 2004
Storeoscopic view of comet Wild-2 captured by Stardust
http://stardust.jpl.nasa.gov/index.html and in particular:
http://stardust.jpl.nasa.gov/mission/index.html
http://stardust.jpl.nasa.gov/science/details.html
Stardust NASA mission - reached comet Wild-2 in 2004
The probe also carried aerogel - a ghostly material that NASA
engineered (like a transparent, super-tough styrofoam, 2 g of
it can hold a 2.5 kg brick - see the r.h.s. picture).
Aerogel was used to capture cometary particles (l.h.s. picture)
which came back and landed on Earth in Jan. 2006.
Tracks in aerogel, Stardust sample of
dust from comet Wild 2. That comet was
residing in the outer solar system until a
close encounter with Jupiter in 1974.
OLIVINES, Mg-Fe silicate solid state solutions (also found by Stardust) are
the dominant building material of both our and other planetary systems.
Forsterite, Mg2SiO4
Fayalite, Fe2SiO4
"I would say these materials came from the inner, warmest
parts of the solar system or from hot regions around other
stars,"
"The issue of the origin of these crystalline silicates still
must be resolved. With our advanced tools, we can examine
the crystal structure, the trace element composition and the
isotope composition, so I expect we will determine the origin
and history of these materials that we recovered from Wild 2."
D. Brownlee (2006)
Deep Impact NASA probe - impacted comet Tempel1 on July 4,
2005 (v =10.2 km/s) - see the movie frames of the actual impact
of the probe taken by the main spacecraft, taken 0.83s apart.
The study showed that Temple1 is porous: the impactor dug a
deep tunnel before exploding.
Comet
Temple 1
nucleus
~10m
resolution
Here is the Deep Impact description
http://deepimpact.jpl.nasa.gov/home/index.html
See http://stardust.jpl.nasa.gov/science/feature001.html
about the differences between comets Wild-2 and Temple 1.
Other missions are ongoing….
http://rosetta.esa.int
Rosetta mission by ESA (European Space
Agency)
will first fly by astroids Steins and Lutetia near
Mars
after the arrival at the comet
Churyumov-Gerasimenko in 2014, the
spacecraft will enter an orbit around the
comet and continue the journey together.
A lander will descend onto the surface.
These particles have been delivered to Earth for $free$
IDP (cometary origin?)
Brownlee particles
collected in the
stratosphere
Chonditic meteorite
Donald Brownlee, UW
Brownlee
particle
Brownlee particle
A few out of a thousand subgrains
shows isotopic anomalies, e.g., a
O(17) to O(16) isotope ratio 3-5
times higher than all the rest - a
sign of pre-solar nature.
Glass with Embeded Metals and Sulfides - found in IDPs
Nano-rocks composed of a mixture
of materials, some pre-solar
Out of this
world
(pre-solar
isotopes,
composition
of GEMS)
Figure 1. Transmission electron micrographs of GEMS within thin sections of chondritic IDPs.
(A) Bright-field image of GEMS embedded in amorphous carbonaceous material (C).
Inclusions are FeNi metal (kamacite) and Fe sulfides. (B) Dark-field image. Bright inclusions
are metal and sulfides; uniform gray matrix is Mg-rich silicate glass. (C and D) Dark-field
images of GEMS with "relict" Fe sulfide and forsterite inclusions.
Dust modeler’s toolkit
Definitions of Qsca, Qabs, Qext
Simplified case of no diffraction
Mie theory
Mie theory program online at
http://omlc.ogi.edu/calc/mie_calc.html
Temperature calculation with Mie theory
Scattering patterns
Polarization
Radiation coefficients
How Mie theory helped understand beta Pictoris
+ other systems
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 (e.g., Mirza’s
1501 project!)
tau = optical thickness perpendicular
to the disk (vertical optical thicknass)
Or, as a bare minimum, an empirical model of dust
(e.g., stolen from comets)
Mie theory of scattering
(+absorption, polarization,Qrad)
C. F. Bohren and D. R. Huffman (Editors),
Absorption and Scattering of Light by Small Particles
(Wiley-Interscience, New York, 1983).
Gustav Mie
(1869-1957)
``Resonant’’ scattering from Mie theory
Wavelength = 0.55 um
Ocean water in air, Qsca
m=1.343 + 0i
Carbon in air, Qsca
m=1.95 - 079i
Air bubble in seawater, Qsca
Carbon in air, Qabs
m=1.95 - 079i
Scattering of red light (0.65 um) on water droplets of radius r
How Mie theory works in terms of reflection and/or surface
electromagnetic waves.
GLORY
RAINBOW
Laboratory-measured optical constants
These peaks are
caused
by Si=O bond
vibration
Wavelength (um)
s=1 um
s=9 um
s=3 um
s=20 um
silicates
s=1 um
s=3 um
s=9 um
s=20 um
H2O
Radiation pressure
coefficient depends on
composition, as well
as porosity
Radiation pressure on mixture may be stronger than
on pure components
Radiation pressure on
ISM dust in three
prototype debris disks.
Notice the logarithmic
scale! ISM particles are
absorbent, which
enhances the effect.
Radiative Rutherford
scattering off a star
(the same Coulomb
+1/r potential applies!)
A good candidate
material can be found
for the beta Pictoris disk
SED and broadband
photometry modeling
Choosing the plausible material
and
Calculating the temperature of solids
EQUILIBRIUM COOLING SEQUENCE
Chemical unity
of nature… and it’s
thanks to
stellar nucleosynthesis!
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.
Silicates
silicates
ices
However, this may backfire.
T vs. r
beta Pictoris
Temperature-distance
relationship and
Ice boundary
location in beta Pic
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 r^2) = 4A Qabs(IR) sigma T^4
L = stellar luminosity, r = distance to star, L/4pi r^2 = flux of energy,
T = equilibrium temperature of the whole particle, e.g., dust grain,
sigma = Stefan-Boltzmann constant (see physical constants table)
sigma T^4 = 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) - emissivity=absorptivity (Kirchhoffs law!) in the infared,
where thermal radiation is emitted
Equilibrium temperature of solid bodies falls with the square-root of r
T^4 = [Qabs(vis)/ Qabs(IR)] L/(16 pi r^2 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)
Theoretical surface temperature T of planets if Qabs(IR)=1, and the actual
surface temperature Tp. Differences are 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
Optical thickness:
  (r ) 
 eq ( r ) 
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)
What is the optical thickness
  (r ) ?
It is the fraction of the disk surface covered by dust:
here I this example it’s 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 r=100 AU opt.thickness~3e-3
disk around Vega
opt.thickness~1e-4
zodiacal light disk (IDPs) solar system ~1e-7
STIS/Hubble imaging
(Heap et al 2000)
Modeling
(Artymowicz,unpubl.):
parametric, axisymmetric disk
cometary dust phase function
Vertical optical
thickness 
Radius r [AU]
Vertical
profile of
dust density
Height z [AU]
Mirza Ahmic’s (2006) best fit to HST/STIS data (b Pic)
Model of dust distribution uses empirical ZL
scattering phase function and two overlapping disks,
inclined by a few degrees
model
Fitting method: multiparametric fit (~18 par.) using simplex algorithm
Mirza Ahmic’s best fit to HST/ACS data (b Pic)
Why the differences??
Chemistry/mineralogy/crystallinity of dust
All we see so far are 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)
Are all planetary systems made of the same material?
Microstructure of circumstellar
disks: identical with IDPs
(interplanetary dust particles)
mostly Fe+Mg silicates
(Mg,Fe)SiO3
(Mg,Fe)2SiO4
HD142527
cold outer
disk
warm disk
The disk particles
are made of the
Earth-type minerals!
(olivine, pyroxene,
FeO, PAH= Polycyclic
Aromatic Hydrocarbons)
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,
observations).
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.
~90% amorphous
Beta
compar
e
~60% amorphous
~45%
amorphous
Pic,
~95% crystalline
We do not at present see in our statistics of Vega-type
stars any simple time-evolution of dustiness or
crystallinity of solids in circumstellar disks.
?
Why
Annealing could be thermal (in proximity to stars),
while transport done by outflows.
Does migration of dust explain observations??