SPECTRO-POLARIMETRY: A NEW TOOL FOR ASTEROID CHARACTERISATION? Stefano Bagnulo 1, Alberto Cellino 2, & Michael Sterzik 3 1: Armagh Observatory, UK 2: INAF – Osservatorio Astrofisico di Torino, Italy 3: European Southern Observatory, Germany EPSC 2014 Cascais, 10 September 2014

SPECTRO-POLARIMETRY: A NEW TOOL FOR ASTEROID CHARACTERISATION! Stefano Bagnulo 1, Alberto Cellino 2, & Michael Sterzik 3 1: Armagh Observatory, UK 2: INAF – Osservatorio Astrofisico di Torino, Italy 3: European Southern Observatory, Germany EPSC 2014 Cascais, 10 September 2014

POLARIZATION BY REFLECTION - > 0 du/~whu/intermediate/polari zation/polar1.html

POLARIZATION BY REFLECTION Incident light (unpolarized) Reflected light (polarized) PHASE ANGLE

Q/I Negative branch Positive branch

VLT

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 P Q (λ,α) = A(λ) * B(α) THEN p q = p Q (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 > 0 S-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?

S-type

HOW POLARISED SPECTRA COMPARE TO RELFECTIVITY SPECTRA? 236 Honoria (L-type) REFLECTIVITY POLARISATIO N 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!