Alberto Cellino - MP1104 COST WG Meeting Warsaw, 7/05/2012 INAF --Osservatorio Astrofisico di Torino...

Alberto Cellino - MP1104 COST WG Meeting Warsaw, 7/05/2012

INAF --Osservatorio Astrofisico di Torino

The importance of Polarimetry for asteroid studies

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1 CeresAsteroid Polarimetry: What do we measure ?Partial linear polarization and

polarization – phase curves.

Presence of a “Negative polarization branch”

Curve described by a few parameters

Plane of the sky

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Scattering Earth body

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Scattering plane, SP Sun




Is a fundamental relation in asteroid science, where:

D is the equivalent diameter in km.

3.1236 is a wavelength-dependent constant (the indicated value is for V colour)

pv is the geometric albedo in V. It is defined as the ratio between the object brightness at zero phase angle and that of an ideal, flat and perfectly Lambertian disk, having the same projected surface of the object. The albedo is an important physical parameter, being related to surface composition, texture and more in general to the history of the object.

H is the absolute magnitude in V, that is the apparent brightness that would be measured at unit distance from the Sun and the observer, and at zero phase angle.

)log(5.02.01236.3)log( VpHD




log pV = C1 log (h) + C2

log pV = C3 log (Pmin) + C4

The slope –Albedo “law”

Zellner et al. 1977

Polarimetry is in principle an excellent technique to derive asteroid albedoes.

Used in the past in asteroid taxonomy to distinguish between E, M, P classes (the big X complex).

Some advantages over thermal radiometry, whose results are model-dependent, require observations in different IR bands, and generally suffer from poor knowledge of H (and lightcurve effects, when measurements in different thermal IR bands are not simultaneous). Radiometric albedoes for small asteroids observed in one single IR band have uncertainties of the order of 60%.




Some Problems of PolarimetryPolarimetry is not intrinsically efficient, since several observations of the same object at different phase angles are required.

In the case of NEOs this problem is less severe, since the phase angle changes more quickly with respect to main belt asteroids. Moreover, NEOs can be observed at large phase angles, and the (positive) linear polarization of the objects at large phase angles is expected to be fairly different for low- and high-albedo objects. This is interesting for the ESA SSA project.

Polarimetry is intrinsically demanding in terms of telescope size, since polarimeters split the incoming light into beams of radiation linearly polarized along different directions. Thus, a large collecting area is required.




After the first pioneering era in the 70s, not very much was done in the next 20 years. Since mid-90s, there has been a phase of Renaissance of asteroid polarimetry due to the availability of new instruments and actively involved teams, mainly in Europe and Argentina




IRAS albedo distributions

D > 50 km D < 50 km

Problem: Is this true?




Evidence for a slight systematic difference between IRAS and polarimetric albedoes (the latter being generally lower).

Analysis limited to intermediate-albedo objects, due to instrumental limitations at CASLEO.

Need of deriving polarimetric albedoes of small, low-albedo objects




Peculiar properties (inversion angles) of F-type Asteroids

Some F-types exhibit evidence of a cometary nature (Phaeton, Wilson-Harrington).

2008 TC3 was an F-type!!

(Belskaya et al., 2005)




An exciting discovery: the “Barbarians”

Masiero and Cellino (2009)

(dashed and dotted lines show, for a comparison, typical polarization-phase curves for B-type ((24) Themis) and L-type ((12) Victoria) asteroids, respectively)




The search for “Barbarians”: First results

Gil-Hutton et al. (2008)

Known objects:

234 (Ld - class),

172, 236, 387, 980 (L – class)

679 (K - class)

Possible association with some dynamical families




Another result: VLT polarimetric observations of members of the Karin and Koronis families

No statistically relevant difference has been found for the polarimetric phase curves of objects belonging to the Koronis family (> 1 Gyrs old) and to the “children” objects belonging to the Karin family (5.8 Myrs old). This suggests that the space-weathering mechanism acts very quickly in affecting the albedo of S-type asteroid surfaces

Cellino et al., Icarus 209, 556-563 (2010)




Wavelength dependence of linear polarization

Opposite behaviour among moderate-albedo and low-albedo asteroids.

Similar effects observed also for comets.

UBVRI Itokawa observations at CASLEOBelskaya et al., Icarus 199, 97-105 (2009)




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Asteroid polarimetry as a support to space missions:

the examples of Steins and Lutetia




NEA Polarimetry

Even a single measurement of polarization at phase > 40 deg can be sufficient to obtain an overall albedo estimation! Very small (faint) NEAs are observable with the VLT (as shown by the case of Apophis).

C-type S-type

E-type

Presence of a wide branch of positive polarization

with a maximum near 90°

NEO polarimetry is inherently efficient, since the rate of variation of the phase angle is fast for these objects, and a polarimetric slope can be obtained in a short time. Interesting for the purposes of ESA SSA




Linear fits of UBVRI data obtained in five consecutive nights, covering a large interval of phase angles

EXAMPLE: POLARIMETRIC EXAMPLE: POLARIMETRIC OBSERVATIONS OF (25143) OBSERVATIONS OF (25143) ITOKAWA AT LARGE PHASE ITOKAWA AT LARGE PHASE ANGLES in 2004 at CASLEOANGLES in 2004 at CASLEO




From the obtained polarimetric slope, the geometric albedo of Itokawa turns out to be: pv = 0.24 ± 0.03

The size obtained from the polarimetric albedo pv = 0.24 and H, derived from an extensive set of observations by different authors, gives a size which agrees within 7% with the Hayabusa results.

But... How is it possible? Itokawa seems to be very depleted in regolith particles. How can it behave like much larger, and regolith-rich main belt asteroids?




log pV = C1 log (h) + C2

General problem and Recommendation by IAU Comm.15:

Different authors are using different sets of C1 and C2 coefficients: C1 = -1.0 C2 = -1.78 (Bowell & Zellner 1974)

C1 = -0.983 0.082 C2 = -1.731 0.066 (Lupishko & Mohamed, 1996)

C1 = -1.118 0.071 C2 = -1.779 0.062 (Cellino et al., 1999)

Among them, only the latter was derived using thermal IR data reduced using the (H,G) system, which takes into account the opposition brightness surge at zero phase.

It is urgent to converge to a new and unique choice of C1 and C2




Example: the determination of the albedo of the Potentially Hazardous Asteroid (99942) ApophisObservations performed at the UT2 ESO VLT (8-m Kueyen) telescope. (Delbò et al., 2007)

Results: pv = 0.33 ± 0.08

The error is mostly a consequence of the calibration uncertainty

Assuming G=0.25 (Q-type), H = 19.7, and the size of Apophis turns out to be D = 270 30

m




The slope-albedo relation must be recalibrated using only high-quality V-band polarimetry of asteroids having accurately derived albedoes. The best target list at present is the one by Shevchenko and Tedesco (2006), including objects whose albedoes were derived from both occultation and in situ (four objects) size measurements, coupled with accurate estimates of absolute magnitudes (obtained using H,G, so let us keep our fingers crossed! ).




The nominal errors on the two coefficients suggest that more work and additional observations are needed.

Current results of an on-going observing campaign at CASLEO

Observed S&T objects

C1 = -0.97 ± 0.07

C2 = -1.68 ± 0.08




Comparison with S&T albedos

Testing different

forms of the slope – albedo relation

C1 = -0.97 ± 0.07

C2 = -1.68 ± 0.08




ceaP br 1)/(

Looking also for new relations not purely based on the polarimetric slope only, but on more global fits of the whole phase – polarization curve.




A tentative “road map” for the future:

• Primary goal: New accurate polarization

measurements of the Shevchenko-Tedesco

targets, to improve albedo determination.

Theoretical work certainly needed.

Peculiar objects (Barbarians, F-class,...)

Need of continuous collaborations between

European researchers (Torino, Kharkiv, Helsinki,

Paris, Nice, Armagh...).

Alberto Cellino - MP1104 COST WG Meeting Warsaw, 7/05/2012 INAF --Osservatorio Astrofisico di Torino...

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