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!