Polarimetry: a primary tool for the physical characterization of the

Asteroids

A. Cellino (INAF Torino Obs.) & S. Bagnulo (Armagh Obs.)

Some fundamental problems in current asteroid science:

The measurement of asteroid sizes and composition

The interpretation in terms of composition of taxonomic

classification based on reflectance spectra

The physical characterization of potential Earth impactors

In all the above problems a crucial role is played by the problem of determining asteroid albedos

)log(5.02.01236.3)log( VpHD −−=

where: D = size, H = absolute magnitude, pV = geometric albedo

The Thermal Radiometry technique. Good for sizes, less good for albedo (errors > 30% due to thermal model limitations and rough estimates of the brightness in the Visible).

General properties of asteroid linear polarization.

A few classical parameters: • Pmin • Inversion angle αinv • Polarimetric slope h

αα CeAP Br +−= − )1( /

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 Pmin and h, and also to build new polarimetric parameters

The derivation of the Albedo from polarimetric properties: the classical slope – albedo relation.

log pV = C1 log (h) + C2

(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.

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.

A possible reassessment of the classical slope – albedo relation

The new Ψ – albedo relation, where Ψ = Pr(30°) – Pr(10°)

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 a few months ago.

Barbarians have anomalous spectral abundances of the spinel mineral, found in CAIs in meteorites.

Barbarians could be remnants of the oldest

and most primitive planetesimals accreted in

our Solar System!

For more information, see the Barbarian Poster!

The Watsonia dynamical family is a reservoir of Barbarians. The Barbarian properties are not simply due to surface properties

Peculiar properties (inversion angles) of F-class Asteroids Some F-class

asteroids exhibit evidence of a cometary nature (Phaeton, Wilson-Harrington). 2008 TC3 was an F-class. Important for the interpretation of the future Gaia data. (Belskaya et al., 2005)

Dollfus et al. (1989, Asteroids II book)

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

• We have now new coefficients to calibrate the classical slope - albedo relation. New polarimetric parameters can also be used to derive more reliable albedo estimates.

• DAWN observations of Vesta can 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.

• The Barbarian properties are not limited to the surface of the objects. The parent body of Watsonia was “homogeneously Barbarian”. (A really wild asteroid!)

• A lot of empirical evidence! Need of better theoretical models.

• Polarimetry and Spectroscopy nicely complement each other!

• See Bagnulo’s presentation to better understand the advantages of a marriage of Spectroscopy and Polarimetry!