SPECTROSCOPIC AND POLARIMETRIC SURVEY OF L-TYPE ASTEROIDS

Introduction:
Spectroscopic and polarimetric observations are
of fundamental importance in order to derive
information about the surface properties of asteroids.
In 2006, the asteroid (234) Barbara was
found to possess a pecular polarimetric response
characterized by an abnormaly large
value of the inversion angle. Later on, several
other similar asteroids were discovered. They were called Barbarian asteroids in
reference to (234) Barbara.
Spectroscopically, all the Barbarian asteroids
belong to the L, Ld and in a few cases K types
in the Bus and Binzel visible taxonomy.
When a classification extended to near-infrared
is considered, all those known to date group
into the L class.
The most commonly accepted explanation for
the polarimetric anomaly is the high abundance
of Spinel-bearing, fluffy (type A) CAIs (Calcium-Aluminium
rich Inclusion) whose optical
properties could reproduce the observed
polarimetry. If this hypothesis is confirmed,
Barbarian asteroids could contain an
anomalous amount of the first minerals condensed
after the formation of the Solar system.
Based on these motivations, we organized a
systematic campaign of spectroscopy and polarimetry
of Barbarians/L-type asteroids.
Observations:
35 asteroids were observed using spectroscopy
or polarimetry, or both. These asteroids include
members of Watsonia, Henan and Klumpkea
families which are known to be made of Barbarian
and/or L type asteroids.
The polarimetric data were obtained with the
ToPol instrument at the Calern station of the
Observatoire de la Côte d’Azur. The spectroscopic
data were obtained at the IRTF NASA
facility.
Data analysis:
The phase-polarization curve of the polarimetric
data was analysed to derive the inversion angle
of the asteroids. The spectroscopic data were
analysed using a Hapke spectral mixing model.
This method combines end-member spectra in
order to fit the asteroid spectrum and derive its
composition. The end-members used are the
Allende matrix from which CAIs were removed, MgO rich olivine, and fluffy type A CAI.
A Hapke space-weathering model was used
in order to simulate the space-weathering effect
on the spectra of asteroids.
Results:
The polarimetric data allowed us to discover 3
new Barbarian asteroids and to remove 4 asteroids
from the Barbarian candidate list.
We found that the inversion angle of Barbarian
asteroids ranges from 26° to 30°.
High CAI abundances ranging from 5 to 50%
were found in L type asteroid spectra. We find
for the first time a link between the abundance
of CAIs and the polarimetric response. Spaceweathering
also appears to play a role in polarimetry,
showing an aging effect on the granularity
of the surface regolith.