The physical study of some asteroid families using mini- SONG Xiao-bin Wang Yunnan Observatory, CAS 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 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

 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 familyKarin Cluster  ‘ Clumps ’ (e.g. the Juno clump) groups have relatively few members but are clearly distinct from the backgroundJuno clump  ‘ Clans ’ (e.g. the Flora family) groups merge very gradually into the background densityFlora family  ‘ 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 evolution of asteroids)  The interior structures and strengths (Most of large members are aggregates of re- accumulated smaller fragments)  The compositions of asteroids (The mineralogical composition of the different bodies, implies their 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, ) 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.

 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

Several methods can be used to estimate these parameters 1. SAM: simultaneous amplitude – magnitude – aspect 2. WAA: weighted amplitude – aspect M odel 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  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

Present works  Observational experience (Since 2000, photometric data of several tens of C-type asteroids were obtained 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 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  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 Erato Antiope binary 171 Ophelia binary? 268 Adorea Goberta Huenna triple 383 Janina Saskia Lina binary 846 Lipperta

Only 66 members have period values Targets can be observed in this winter.  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  2153 Akiyama  2163 Korczak  2164 Lyalya 11.67

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