The physical study of some asteroid families using mini-

SONG

Xiao-bin WangYunnan Observatory, CAS

2011.9.19

Charleston, South Carolina

Outline Asteroids and asteroid families

Photometric observation for asteroids

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

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.

Most asteroids are in the asteroid belt (main belt).

Main belt

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 asteroidsThe 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 Planetesi

mals (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

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

‘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

Family types

Asteroid family can provide us:

Insights into collisional processes (Formation of the families is an evidence to the collision evolutio

n of asteroids) The interior structures and strengths (Most of large members are aggregates of re-accumulated smalle

r fragments)

The compositions of asteroids (The mineralogical composition of the different bodies, implies th

eir common origin)

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 satelliteDisruption of a 119 km asteroid: form Koronis familyDisruption 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 f

or the simulation.

More samples are needed, especially for the targets with long periods

pravec(2000)

The Lack of the slow rotation samples.

Till August 2011, 285,078 numbered minor planets

The periods of nearly 4000 minor planets are known now

About a hundred of asteroids have the spin orientation measurements

A few of asteroids’ shape are known

More photometric observation for individual asteroid are needed

Photometric observation of asteroids

Determine spin rate

Determine spin orientation

Inverse shape of asteroid

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

2. Conversely, we can estimate the spin parameters and its shape from its light curves 0 60 120 180 240 300 360

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Rotational phase angle

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 tri

axial ellipsoid stably rotating about its shortest axis3. Epoch: can determine the sense of spin , rotatio

n period and the orientation of spin axis4. Shape inversion Model the asteroid as a polyhedron with triangular facets

The shape inversion

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.

The determination of spin orientation More than two apparitions’ observations

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

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Present works Observational experience (Since 2000, photometric data of several tens of C-type asteroids were o

btained with 1m telescope at Yunnan Observatory)

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

Mini-SONG has large field of view It is easy to observe the asteroids in ma

in belt Using this network, the observation ca

n 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

Core members of C-type asteroid families

Members without photometric data

Potential binary asteroids

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

The core of Themis family includes name H period(hour) 24 Themis 7.03 8.37 62 Erato 8.21 9.22 90 Antiope 8.10 16.50 binary 171 Ophelia 8.15 6.66 binary? 268 Adorea 7.95 7.80 316 Goberta 9.89 8.60 379 Huenna 8.63 7.02 triple 383 Janina 9.52 6.40 461 Saskia - 10.31 468 Lina 9.54 16.33 binary 846 Lipperta 10.19 1641.0

Only 66 members have period values

Targets can be observed in this winter. 461 Saskia 10.31, 515 Athalia 10.75 561 Ingwelde 11.38, 637 Chrysothemis 11.50 767 Bondia 10.10, 991 McDonalda 11.12 1229 Tilia 11.18, 1247 Memoria 10.46 1383 Limburgia 12.01, 1445 Konkolya 11.13 1539 Borrelly 10.98, 1624 Rabe 11.31 1686 de Sitter 10.89, 1698 Christophe 11.21 1778 Alfven 11.63, 1788 Kiess 12.00 1851 Lacroute 12.26, 1895 Larink 12.10 1898 Cowell 12.19, 1953 Rupertwildt 11.87 1986 Plaut 12.11, 2016 Heinemann 11.69 2039 Payne-Gaposchkin 12.42, 2142 Landau 12.05 2153 Akiyama 11.84 2163 Korczak 11.53 2164 Lyalya 11.67

Thanks!