Polarimetry: a primary tool for the physical characterization of the - - PowerPoint PPT Presentation

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 . 2 . 1236 . 3 ) log(

V

p H D − − =

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

α

α

C e A P

B r

+ − =

−

) 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

• nly high-quality V-band polarimetry of

asteroids having accurately derived albedos. The best target list at present is the

• ne by Shevchenko and Tedesco

(2006), including objects whose albedos were derived from both

• ccultation 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
• btained 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

• ur Solar System!

For more information, see the Barbarian Poster!

The Watsonia dynamical family is a reservoir

• f Barbarians.

The Barbarian properties are not simply due to surface properties

Peculiar properties (inversion angles)

• f 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!