Recent Advances in Asteroid Polarimetry A.Cellino, E. Ammannito, S. Bagnulo, I.N. Belskaya, R. Gil-Hutton, P. Tanga, E.F. Tedesco

General properties of asteroid linear polarization. A few classical parameters: P min Inversion angle α inv Polarimetric slope h

The phase – polarization curves of asteroids can be adequately fit by an exponential – linear relation: Where α is the phase angle A, B, C are three parameters to be determined by best-fit procedures This allows us to better determine “classical” parameters like P min and h, and also to build new polarimetric parameters

The derivation of the Albedo from polarimetric properties: the classical slope – albedo relation. log p V = C 1 log (h) + C 2 (from Geake and Dollfus, 1986) Currently, Different authors use different calibration coefficients… The slope-albedo relation needs to be recalibrated, and the best way is to use only high-quality V-band polarimetry of asteroids having accurately derived albedos. The best target list at present is the one by Shevchenko and Tedesco (2006), including objects whose albedos were derived from both occultation and in situ (four objects) size measurements, coupled with reliable estimates of absolute magnitudes.

Akari versus Wise: albedosAkari versus Wise: Sizes Poor knowledge of the value to assign to the absolute magnitude H. V magnitude measurements are never done simultaneously with thermal IR measurements. The albedo values found by thermal Radiometry are very uncertain. Can we do better than this?

We have used all the polarimetric measurements for asteroids belonging to the Shevchenko & Tedesco list available in the literature, as well as a number of new data obtained by us at the CASLEO observatory (Argentina) We limit our analysis to polarimetric measurements obtained in the standard V filter. we only use values of linear polarization Pr having nominal errors less than 0.2%. We use only polarimetric measurements obtained at phase angles larger than or equal to 14 degrees of phase, a value well in the region of the negative polarization branch where Pr starts to increase linearly with phase. We require to have at least 5 available measurements, and that the interval of phase angles covered by the data is not less than 3 degrees.

Some results Polarimetric slope

Pmin

Polarimetric slope 0.08 otherwise Use calibration obtained using all objects Do not use P min in general, but: if |P min | 0.08 Use albedo from Ψ for any object How to derive geometric albedo from polarimetric data: A general recipe.

This is what “normal” asteroids do...

... And these are Barbarians, so-called after the prototype, (234) Barbara (Cellino et al., 2006). Strong negative polarization around 20° of phase. Very high values of the inversion angle! Only six objects mentioned in the literature until last year.

Barbarians have anomalous spectral abundances of the spinel mineral, found in CAIs in meteorites. They could be survivors of the oldest and most primitive planetesimals accreted in our Solar System! The Watsonia dynamical family is a reservoir of Barbarians. The Barbarian properties are not simply due to surface properties Spectra from Sunshine et al. (2008) Cellino et al. (2014)

Object IdBus & Binzel TaxonomyDeMeo et al. TaxonomyNotes (234) BarbaraLdLSpinel-rich (172) BaucisL- (236) HonoriaLL (387) AquitaniaLL Spinel-rich Close to Watsonia (402) ChloeKL (599) LuisaKL Spinel-rich Close to Watsonia (679) PaxKL (729) WatsoniaLL(Watsonia family) (980) AnacostiaLL Spinel-rich Close to Watsonia (1372) Haremari--(Watsonia family) (2085) HenanLL Spinel-rich. Family? (5492) ThomaL-(Watsonia family) (42635), (56233), (106059), (106061), (144854), (236408) --(Watsonia family) List of known Barbarians More on this in P. Tanga’s talk!

Peculiar properties (inversion angles) of F-class Asteroids Some F-class asteroids exhibit evidence of a cometary nature (Phaeton, Wilson- Harrington) TC3 was an F-class. Important for the interpretation of the future Gaia data. (Belskaya et al., 2005)

Unique example of rotation-dependent polarization found so far among asteroids. Possibility to link remote-sensing polarization measurements to local surface properties observed in situ by the DAWN probe. This can be done computing the sub-Earth points at the epochs of ground-based observations. The case of (4) Vesta

Blue : Eucritic composition Green: Howarditic composition Yellow: Howarditic + diogenitic Cyan : Eucritic + howarditic

Albedo Map of Vesta

Elevation Map of Vesta

Green: 1978 data, phase = 11.1 o o Red : 1986 data, phase = 13.2 o

Slope –albedo data of Vesta nicely fit the relation for objects having albedo > … but something must be better understood! Violation of the Umov law, possibly related to regolith thickness and/or composition

Spectro-polarimetry: The future of asteroid polarimetric studies? (236) Honoria: another violation of the Umov law (Bagnulo, Cellino & Sterzik, 2015)

Summary: The Renaissance of Asteroid Polarimetry We have now new coefficients to calibrate the classical slope - albedo relation and new polarimetric parameters to be also used to derive more reliable albedo estimates. DAWN observations of Vesta provide a first “ground truth” for the calibration of the polarimetry – albedo relations. Barbarian Polarimetric behaviour very likely linked to spinel abundance, which is diagnostic of extremely ancient bodies. Polarimetry and Spectroscopy nicely complement each other! Polarimetric studies useful for the scientific verification of future Gaia- based asteroid taxonomy. The case of the F class. A lot of empirical evidence! Need of better theoretical models. We also need to understand cases of violation of the Umov “law”.