spectro-polarimetry: a new tool for asteroid characterisation? stefano bagnulo 1, alberto cellino 2,...
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POLARIZATION BY REFLECTION - > 0 du/~whu/intermediate/polari zation/polar1.htmlTRANSCRIPT
SPECTRO-POLARIMETRY: A NEW TOOL
FOR ASTEROID CHARACTERISATION?
Stefano Bagnulo1, Alberto Cellino2, & Michael Sterzik3
1: Armagh Observatory, UK2: INAF – Osservatorio Astrofisico di Torino, Italy
3: European Southern Observatory, Germany
EPSC 2014Cascais, 10 September 2014
SPECTRO-POLARIMETRY: A NEW TOOL
FOR ASTEROID CHARACTERISATION!
Stefano Bagnulo1, Alberto Cellino2, & Michael Sterzik3
1: Armagh Observatory, UK2: INAF – Osservatorio Astrofisico di Torino, Italy
3: European Southern Observatory, Germany
EPSC 2014Cascais, 10 September 2014
POLARIZATION BY REFLECTION
- > 0
http://background.uchicago.edu/~whu/intermediate/polarization/polar1.html
POLARIZATION BY REFLECTION
Incident light (unpolarized)
Reflected light (polarized)
PHASE ANGLE
Q/I
Negative branch
Positive branch
FORS @ VLT
ISIS @ WHT
PRELIMINARY QUESTION:
How polarization spectra change with phase-angle?
Let’s first look at the polarization spectra of identical objects observed at different phase angle…
YES!
YES!
YES!
PRELIMINARY QUESTION:
How polarization spectra change with phase-angle?
DOES THEIR SHAPE CHANGE WITH PHASE-ANGLE?
COMPARING DATA TAKEN AT DIFFERENT PHASE-ANGLES
IF PQ(λ,α) = A(λ) * B(α) THEN pq = pQ(λ)
YES!
To each asteroid we associate two normalized spectra, one representative of the positive branch, and one representative of the negative branch
MORE PRELIMINARY QUESTIONS Do asteroids of similar taxonomic classes
have similar polarization spectra? Do asteroids of different taxonomic classes
have different polarization spectra?
(Note that we prefer a complex rather than a simple answer)
YES! Low albedo:
dp(λ)/dλ >0 in the positive branch
dp(λ)/dλ <0 in the negative branch
OR
dP(λ,α)/dλ >0
YES! Intermediate albedo:
dp(λ)/dλ <0 in the positive branch
dp(λ)/dλ >0 in the negative branch
OR
dP(λ,α)/dλ < 0
YES! High albedo (Nysa)
dp(λ)/dλ > 0
OR
dP(λ,α)/dλ > 0 if P>0
dP(λ,α)/dλ < 0 if P<0
B- and C-type (low albedo):
dP/dλ > 0S-type (intermediate albedo):
dP/dλ < 0
High albedo Xc asteroid Nysa has
dP/dλ < 0 if P < 0 dP/dλ > 0 if P > 0
Results fully consistent with the analysis carried out by Lupisko & Kiselev (1995) and by Belskaya et al. (2009) based on multi-colour broadband polarimetry
MORE PRELIMINARY QUESTIONS Do asteroids of similar taxonomic classes
have similar polarization spectra? Do asteroids of different taxonomic
classes have different polarization spectra?
What is the relationship between polarization spectra and reflectivity spectra?
HOW POLARISED SPECTRA COMPARE TO REFLECTIVITY SPECTRA?
HOW POLARISED SPECTRA COMPARE TO REFLECTIVITY SPECTRA?
HOW POLARISED SPECTRA COMPARE TO REFLECTIVITY SPECTRA?
S-type
HOW POLARISED SPECTRA COMPARE TO RELFECTIVITY SPECTRA?
236 Honoria (L-type)
REFLECTIVITY
POLARISATION in the NEGATIVE BRANCH
In the negative branch, some asteroids violate the Umov law!
CONCLUSIONS Spectro-polarimetry offers a new degree of constraint
for the characterisation of the atmosphere-less bodies of our solar system
It complements spectro-photometry and should be taken into account for asteroids classification
Future attempts of modelling the surface structure of asteroids should take into account both I and Q spectra!