Transcript 幻灯片 1 - James Neff

The physical study of some
asteroid families using miniSONG
Xiao-bin Wang
Yunnan Observatory, CAS
2011.9.19
Charleston, South Carolina
Outline
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Asteroids and asteroid families
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Photometric observation for asteroids
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The methods for determining the spin
parameters and shape of asteroids
Research experiences and present works
A program for photometric observation of
asteroids using mini-SONG
Asteroids
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Asteroids are small rocky
fragment left over from
the formation of our solar
system about 4.6 billion
years.
They orbit the Sun
between the orbits of the
Mars and Jupiter.
Main belt
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Most asteroids are in the
asteroid belt (main belt).
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Asteroids’ size in diameter from 952km,
to less than 1 km
The total mass of all asteroids is less
than that of the Earth's Moon
Formation of asteroids
The first theory
(1)Asteroids are the remains of
fragments of a planet that was
destroyed in a massive collision
long time ago
Most scientists accepted theory
(2)Asteroids are the remains of
Planetesimals (the gravitational
perturbation of Jupiter prevented
the formation of a planet in the
‘asteroid belt’, then those small
bodies suffered massive collisions
with each other)
Which theory is more reasonable, that
needs to be tested.
Asteroid families
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Family is thought as the
result of the collisional
disruption of a larger
body (so, families are
direct proofs of collision
evolution of asteroid belt)
137 significant clusters
of asteroids in proper
elements space
Family types
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‘Cluster’ (such as the Karin Cluster ,90 members) is used to
describe a small asteroid family
‘Clumps’ (e.g. the Juno clump) groups have relatively few
members but are clearly distinct from the background
‘Clans’ (e.g. the Flora family) groups merge very gradually
into the background density
‘Tribes’ groups are less certain to be statistically significant
against the background either because of small density or
large uncertainty in the orbital parameters of the members
Asteroid family can provide us:
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Insights into collisional processes
(Formation of the families is an evidence to the collision
evolution of asteroids)
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The interior structures and strengths
(Most of large members are aggregates of reaccumulated smaller fragments)
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The compositions of asteroids
(The mineralogical composition of the different bodies,
implies their common origin)
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The age of family
(Old families are thought to contain few small members
due to the YORP effect)
.
Collisional processes to form some families
with the disruption of a bigger asteroid are
simulated by Michel et al. (2003, Nature, Vol.
461, 608-611)
For example:
Disruption of a 100 km asteroid: forming a large fragment
(contain 50% of the mass of the parent body) and a big satellite
Disruption of a 119 km asteroid: form Koronis family
Disruption of a 164 km asteroid: form Flora family
Disruption of a 284 km asteroid: form Eunomia family
The shapes, sizes and spin-rate distributions in
the asteroid family are the important input
data for the simulation.
More samples are needed, especially for the
targets with long periods
pravec(2000)
The Lack of the
slow rotation
samples.
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Till August 2011, 285,078 numbered minor planets
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The periods of nearly 4000 minor planets are known
now
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About a hundred of asteroids have the spin
orientation measurements
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A few of asteroids’ shape are known
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More photometric observation for individual asteroid
are needed
Photometric observation of asteroids
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Determine spin rate
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Determine spin orientation
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Inverse shape of asteroid
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Determine density for binary asteroid
The determination of the spin
parameters and shape
1.
The shape of light
curves of asteroids is
related to the shape
of asteroid, spin rate
and spin orientation
25
Conversely, we can
estimate the spin
parameters and its
shape from its light
curves
¦È=5
23
=10
22
=15
0
0
0
=30
21
ares of intersection
2.
0
24
0
=45
20
0
=60
19
0
=90
18
17
16
15
14
13
12
11
0
60
120
180
240
Rotational phase angle
300
360
Several methods can be used to estimate
these parameters
1. SAM: simultaneous amplitude–magnitude–
aspect
2. WAA: weighted amplitude–aspect
Model an asteroid as a uniformly bright, featureless,
smooth triaxial ellipsoid stably rotating about its
shortest axis
3. Epoch: can determine the sense of spin ,
rotation period and the orientation of spin
axis
4. Shape inversion
Model the asteroid as a polyhedron with triangular facets
The shape inversion
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This method can yield a model shape
closely related to the convex hull of
the body, as well as the sidereal
rotation period, the sense of spin,
and the orientation of the rotation
pole.
Requirements for photometric data

The determination of spin period
The observation in one apparition can be used to
determine the spin period. Long last observation is
needed for long spin period of asteroid.
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The determination of spin orientation
More than two apparitions’ observations
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The determination of shape
The tri-axes ellipsoid shape
More than two apparitions’ observations
The convex hull shape
The more apparitions’ observations, the best the shape is
determined
cos   sin  sin  p  cos cos p cos(  p )
( ,  ) : observer position,
  0 ~ 360,   23.5
( p ,  p ) : orientation  of  spin
Present works
 Observational experience
(Since 2000, photometric data of several tens of C-type asteroids
were obtained with 1m telescope at Yunnan Observatory)
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Determine spin parameters for part of
targets with SAM and Epoch methods
Determine shape of (360) and (171)
(In collaboration with Karri Muinonen and Alberto Cellino)
The shape of (360)Carlova
Light curves of (360)Carlova
The shape of (171)Ophelia
Binary structure
A program for photometric observation
of asteroids using mini-SONG
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Mini-SONG has large field of view
It is easy to observe the asteroids
in main belt
Using this network, the observation
can cover a quite long time
Good datasets can be obtained so
as to inverse the spin parameters
and shape of asteroids (we can get light
curves in 4 different apparitions during the 5 years
runing of mini-SONG)
The interested families
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Core members of C-type asteroid
families
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Members without photometric data
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Potential binary asteroids
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Eugenia
C and X type
Chloris
C type
Lydia
C and X type
Liberatrix C type
Watsonia
L type
Thisbe
B type
Pallas
B or C type
Phaeo
X type
Astrid
C type
Hoffmeister C or F type
Dora
C type
Eos
K type
Themis
C type
Hygiea
C (10 Hygiea)
Veritas
C, P, and D type
Themis family

A well-defined asteroid family with 550 members. Most
family members are C-class asteroids with low albedo
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The core of Themis family includes
name
24 Themis
62 Erato
90 Antiope
171 Ophelia
268 Adorea
316 Goberta
379 Huenna
383 Janina
461 Saskia
468 Lina
846 Lipperta
-
H
period(hour)
7.03 8.37
8.21 9.22
8.10 16.50
binary
8.15 6.66
binary?
7.95 7.80
9.89 8.60
8.63 7.02
triple
9.52 6.40
10.31
9.54 16.33
binary
10.19 1641.0
Only 66 members have period values
Targets can be observed in this winter.
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461 Saskia
10.31, 515 Athalia
561 Ingwelde
11.38, 637 Chrysothemis
767 Bondia
10.10, 991 McDonalda
1229 Tilia
11.18, 1247 Memoria
1383 Limburgia
12.01, 1445 Konkolya
1539 Borrelly
10.98, 1624 Rabe
1686 de Sitter
10.89, 1698 Christophe
1778 Alfven
11.63, 1788 Kiess
1851 Lacroute
12.26, 1895 Larink
1898 Cowell
12.19, 1953 Rupertwildt
1986 Plaut
12.11, 2016 Heinemann
2039 Payne-Gaposchkin 12.42,
2142 Landau
12.05
2153 Akiyama
11.84
2163 Korczak
11.53
2164 Lyalya
11.67
10.75
11.50
11.12
10.46
11.13
11.31
11.21
12.00
12.10
11.87
11.69
Thanks!