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Spectro-Polarimetry as a potentially powerful tool for A rbser
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asteroid studies
INAF --Osservatorio Astrofisico di Torino
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A. Cellino & S. Bagnulo - Helsinki, 19-21/08/2013
Asteroid Spectroscopy and Spectrophotometry
(From Bus et al. 2002)
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Spectro-Polarimetry as a potentially powerful tool for A rbser
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asteroid studies
INAF --Osservatorio Astrofisico di Torino
A. Cellino & S. Bagnulo - Helsinki, 19-21/08/2013
Asteroid taxonomy has been
traditionally based on
spectrophotometric properties, in
the wavelength range covering
UBVRI colors.
The distribution of
different
taxonomic classes
as a function of
heliocentric
distance is related
to the general
composition
gradient of our
Solar System
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Spectro-Polarimetry as a potentially powerful tool for A rbser
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asteroid studies
INAF --Osservatorio Astrofisico di Torino
A. Cellino & S. Bagnulo - Helsinki, 19-21/08/2013
The mineralogical interpretation of reflectance spectra
E, M and P classes are characterized by very different albedo
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magh
Spectro-Polarimetry as a potentially powerful tool for A rbser
vat
O
asteroid studies
INAF --Osservatorio Astrofisico di Torino
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A. Cellino & S. Bagnulo - Helsinki, 19-21/08/2013
Some fundamental problems in current
asteroid science:
 The determination of asteroid masses and densities
 The measurement of asteroid sizes and shapes
 The determination of the relation between composition
and taxonomic classification
 The physical characterization of potential Earth impactors
In all the above problems a crucial
role is played by the problem of
determining asteroid albedos
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Spectro-Polarimetry as a potentially powerful tool for A rbser
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asteroid studies
INAF --Osservatorio Astrofisico di Torino
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A. Cellino & S. Bagnulo - Helsinki, 19-21/08/2013
log(D)  3.1236 0.2H  0.5 log( pV )
Is a fundamental relation in asteroid science, where:
D is the equivalent diameter in km.
3.1236 is a wavelength-dependent constant (the indicated value is for V colour)
pv is the geometric albedo in V. It is defined as the ratio between the object
brightness at zero phase angle and that of an ideal, flat and perfectly
Lambertian disk, having the same projected surface of the object. The albedo
is an important physical parameter, being related to surface composition,
texture and more in general to the history of the object.
H is the absolute magnitude in V, that is the apparent brightness that would
be measured at unit distance from the Sun and the observer, and at zero
phase angle. It varies at different solar oppositions of the same object.
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Spectro-Polarimetry as a potentially powerful tool for A rbser
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asteroid studies
INAF --Osservatorio Astrofisico di Torino
Thermal
Radiometry
A. Cellino & S. Bagnulo - Helsinki, 19-21/08/2013
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magh
Spectro-Polarimetry as a potentially powerful tool for A rbser
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asteroid studies
INAF --Osservatorio Astrofisico di Torino
A. Cellino & S. Bagnulo - Helsinki, 19-21/08/2013
Thermal radiometry results,
particularly for albedo, are
model-dependent and suffer
from poor knowledge of the
value to assign to the absolute
magnitude H.
Radiometric albedoes for small
asteroids observed in one single
IR
band
have
intrinsic
uncertainties up to 60%, due to
the uncertainty in the thermal
model to be applied in different
cases.
Akari versus Wise albedoes
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Spectro-Polarimetry as a potentially powerful tool for A rbser
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asteroid studies
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A. Cellino & S. Bagnulo - Helsinki, 19-21/08/2013
INAF --Osservatorio Astrofisico di Torino
Asteroid Polarimetry: What
do we measure ?
1 Ceres
( I   I // )
Pr 
( I   I // )
Partial linear polarization and
polarization – phase curves.
(%)
Plane of th e sky
I
Scat t ering
body
I 



Scat t ering plane, SP
.
Eart h

Sun
 Presence of a “Negative polarization branch”
 Curve described by a few parameters
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Spectro-Polarimetry as a potentially powerful tool for A rbser
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asteroid studies
INAF --Osservatorio Astrofisico di Torino
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A. Cellino & S. Bagnulo - Helsinki, 19-21/08/2013
The derivation of the Albedo from polarimetric
properties: the “slope – albedo law”.
log pV = C1 log (h) + C2
Log(pV) = (-1.035 ± 0.090) log(h) – (1.724 ±
0.104)
Improvement of the
calibration
of
the
slope-albedo relation
needed: work curretly
in progress
magh
Spectro-Polarimetry as a potentially powerful tool for A rbser
vat
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asteroid studies
INAF --Osservatorio Astrofisico di Torino
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A. Cellino & S. Bagnulo - Helsinki, 19-21/08/2013
Pros:
Polarimetry is in principle an excellent technique to derive
the albedoes of main belt asteroids and NEOs of all sizes.
No dependence on absolute magnitude!
Used in the past in asteroid taxonomy to distinguish between E, M, P
classes.
Cons:
Time consuming! You need weeks to get a satisfactory
coverage of the phase- polarization curve of a main belt
asteroid! (much less time is needed for near-Earth
asteroids).
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Spectro-Polarimetry as a potentially powerful tool for A rbser
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asteroid studies
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A. Cellino & S. Bagnulo - Helsinki, 19-21/08/2013
INAF --Osservatorio Astrofisico di Torino
Courtesy of I.N. Belskaya
C-type
.
S-type
E-type
NEA Polarimetry
Presence of a wide branch
of positive polarization
with a maximum near
90°polarimetry is inherently
NEO
efficient, since the rate of
variation of the phase angle is
fast for these objects, and a
polarimetric slope can be
obtained in a short time.
Interesting for the purposes of
ESA SSA program
Even a single measurement of polarization at phase > 40 deg can be
sufficient to obtain an overall albedo estimation! Very small (faint) NEAs
are observable with the VLT (as shown by the case of Apophis).
magh
Spectro-Polarimetry as a potentially powerful tool for A rbser
vat
O
asteroid studies
INAF --Osservatorio Astrofisico di Torino
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A. Cellino & S. Bagnulo - Helsinki, 19-21/08/2013
First results about
wavelength dependence of
linear polarization
Opposite behaviour among
moderate-albedo and lowalbedo asteroids.
Similar effects observed also for
comets.
Belskaya et al., Icarus 199, 97-105 (2009)
Full spectro-polarimetry never
attempted so far for asteroids!
UBVRI Itokawa
observations at
CASLEO
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Spectro-Polarimetry as a potentially powerful tool for A rbser
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asteroid studies
INAF --Osservatorio Astrofisico di Torino
EXAMPLE:
POLARIMETRIC
OBSERVATIONS OF (25143)
ITOKAWA AT LARGE PHASE
ANGLES in 2004 at CASLEO
Linear fits
of UBVRI
data
obtained in
five colors in
consecutive
nights,
covering a
large
interval of
phase angles
A. Cellino & S. Bagnulo - Helsinki, 19-21/08/2013
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magh
Spectro-Polarimetry as a potentially powerful tool for A rbser
vat
O
asteroid studies
INAF --Osservatorio Astrofisico di Torino
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A. Cellino & S. Bagnulo - Helsinki, 19-21/08/2013
The preliminary results obtained so far, if confirmed, would
imply that one single spectro-polarization measurement
obtained at different wavelengths either at large phase
angles (> 20o) or at a phase angle around 10o could be
sufficient to distinguish between asteroids belonging to
different albedo and taxonomic classes.
Stay tuned! We have been awarded the first
observing run of asteroid spectro-polarimetry
at VLT in the next observing semester !
magh
Spectro-Polarimetry as a potentially powerful tool for A rbser
vat
O
asteroid studies
INAF --Osservatorio Astrofisico di Torino
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ry
A. Cellino & S. Bagnulo - Helsinki, 19-21/08/2013
Other interesting targets for
observations: the “Barbarians”
future
Known objects:
234 (Ld - class),
172, 236, 387,
980 (L – class)
679 (K - class)
Unusual spectra, presence
of CAI compounds on the
surfaces.
Possible association with
some dynamical families.
Masiero and Cellino (2009)
(dashed and dotted lines show, for
a comparison, typical polarizationphase curves for B-type ((24)
Themis) and L-type ((12) Victoria)
asteroids, respectively)
magh
Spectro-Polarimetry as a potentially powerful tool for A rbser
vat
O
asteroid studies
A. Cellino & S. Bagnulo - Helsinki, 19-21/08/2013
INAF --Osservatorio Astrofisico di Torino
Asteroid polarimetry as a
support to space missions: the
examples of Steins and Lutetia
0.0
1.5
21 Lutetia
2867 Steins
-0.5
1.0
K
0.5
Pmin (%)
Polarization degree (%)
2867E
E AR
-1.0
0.0
F
F
-1.5
-0.5
Sw V
SSw
Sq-SSS
SKS
SSSS SSS
M
M M
M
B
C
-2.0
-1.0
-1.5
-2.5
0
5
10
15
20
Phase angle (deg)
25
30
12
Courtesy of I.N. Belskaya
16
21
Cx
ChCh
Ch
C
Ch
Ch Ch
Cqh
Ch Cgh
Ch
20
inv(deg)
L
24
28
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magh
Spectro-Polarimetry as a potentially powerful tool for A rbser
vat
O
asteroid studies
INAF --Osservatorio Astrofisico di Torino
A. Cellino & S. Bagnulo - Helsinki, 19-21/08/2013
Thank you
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