POLARIMETRY OF THE SMALL BODIES OF THE SOLAR SYSTEM Stefano Bagnulo Armagh Observatory Alberto Cellino INAF Torino 3 rd SOLARNET WORKSHOP Granada, 27 May 2015 Ludmilla Kolokolova University of Maryland Gian Paolo Tozzi INAF Firenze Irina Belskaya Kharkiv National University

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

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

= - Negative branch Positive branch ~ albedo

p V ≈ 0.09 p V ≈ 0.20 p V ≈ 0.80

THE ALBEDO-POLARIZATION RELATIONSHIP P v = C 1 Log(h) + C 2 P v = C 3 Log(P min ) + C 4 C 1 ≈ C 2 ≈ C 3 ≈ C 4 ≈ -1

THE ALBEDO-POLARIZATION RELATIONSHIP Cellino et al. (2015), MNRAS (in press)

THE ALBEDO-POLARIZATION RELATIONSHIP

WHAT ELSE POLARIMETRIC CURVES TELL US?

EMPIRICAL FIT TO THE POLARIZATION CURVES  TAXONOMY  0 is the inversion angle b is the amplitude and affects also the slope at the inversion angle ( ~ albedo) c 1 affects the position of the minimum c 2 affects the position of the maximum

THE BARBARIANS

Cellino et al. (2015)

TRANSNEPTUNIAN OBJECTS

TRANSNEPTUNIAN OBJECTS (IXION) Bohnhardt et al. (2002)

TRANSNEPTUNIAN OBJECTS Bagnulo et al. (2008)

CENTAURS

TRANSNEPTUNIAN OBJECTS + CENTAURS Bagnulo et al. (2006), Belskaya et al. (2010)

COMETS

COMETS Stinson et al. (2015, in prep.)

COMETS Nucleus Dust Tail Coma Ion Tail

AGGREGATES T matrix code for cluster of spheres (Mackowki & Mischcenko 2011)

AGGREGATES Radius of the monomers ~ 0.1 µm Number of monomers > 256 Refractive index (based on in-situ data for comet Halley): 1.88-i0.47 at =0.45 µm and 1.98-i0.48 at =0.60 µm Composition: silicate 31.76%, iron 2.56%, carbonaceous materials 65.68% (1/3 organic refractory, 2/3 amorphous carbon)

CONSTRAINTS FROM THE POLARIMETRIC COLOUR

CIRCULAR POLARIMETRY Rosenbush et al. (2007)

CIRCULAR POLARIMETRY Violation of the mirror symmetry in the medium Asymmetry of the medium where light experience multiple scattering Asymmetry of the optical properties of the material (e.g., asymmetry in crystal structure of the material, or in the structure of the molecules)

THE BUILDING BLOCKS OF LIFE (AS WE KNOW) ARE CHIRAL All amino acids of biological origin are left-handed All sugars of biological origin are right-handed

CIRCULAR POLARIMETRY Violation of the mirror symmetry in the medium Asymmetry of the medium where light experience multiple scattering Asymmetry of the optical properties of the material (e.g., asymmetry in crystal structure of the material, or in the structure of the molecules) Asymmetry of the scatterers themselves (alignment of particles due to some mechanism)

AN ALTERNATIVE EXPLANATION

NULCEUS OF COMET 2P/ENCKE  0 ~ 13 o (Boehnhardt et al. 2008)

NUCLEUS OF 133P/ELST-PIZARRO Bagnulo et al. (2010)

OTHER COMET NUCLEI Stinson et al. (2015, in prep.)

ASTEROID SPECTROSCOPY

PRELIMINARY QUESTION How polarization spectra depend on phase-angle? DOES THEIR SHAPE CHANGE WITH PHASE-ANGLE?

NORMALISING POLARIZATION SPECTRA IF P Q (λ,α) = A(λ) * B(α) THEN p q = p Q (λ)

ORIGINAL NORMALIZED Bagnulo et al. (2015)

DO ASTEROIDS WITH SIMILAR REFLECTANCE SPECTRA SHOW SIMILAR POLARIZATION SPECTRA?

NO!

S-type THE UMOV EFFECT: P ~ 1/R REFLECTANCE SPECTRA POLARIZATION SPECTRA

VIOLATION OF THE UMOV EFFECT (P ~ R) 236 Honoria (L-type) REFLECTANC E POLARIZATIO N

YES! Low albedo: dp(λ)/dλ >0 in the positive branch dp(λ)/dλ <0 in the negative branch

YES! Intermediate albedo: dp(λ)/dλ <0 in the positive branch dp(λ)/dλ >0 in the negative branch

Results fully consistent with the analysis carried out by Lupisko & Kiselev (1995) and by Belskaya et al. (2009) based on multi-colour broadband polarimetry

CONCLUSIONS  Spectro-polarimetry offers a new degree of constraint for the characterization of the atmosphere-less bodies of our solar system  Future attempts of modeling the surface structure of asteroids should take into account both I and Q spectra!