The poorly known L-type asteroids and the intriguing Barbarians

We investigate the poorly known L-type asteroids as some of them (known as "Barbarians") are known to show peculiar polarimetric properties, and near-infrared spectra hinting to a high aboundance of the most ancient materials present in the Solar System.

What is a Barbarian?

Definition: an asteroids exhibiting – in the phase vs. polarization curve – an exceptionally high inversion angle (> 25°) and a deep minimum. Prototype: 234 Barbara, 1st to be discovered (A. Cellino, 2006)

Tentative explaination of the polarimetric anomaly

• “A very speculative possibility”: very large impact craters/concavities on the surface (Cellino et al. 2006). An anomalous variety of incidence/scattering angles
• CO3/CV3 spinel-bearing meteorites have high negative polarization and inversion angles (Zellner 1977).
• All Barbarians asteroid spectra shows the same absorbtion feature near
  

Spinel = MgAl2O4 often associated to metals (Fe, Cr), resulting in a reddening of the spectrum + wide absorption feature at 2 um

The "speculative possibility" of the presence of large impact craters/concavities on the surface was later on supported by the non-convex light curve inversion of the asteroid 234 Barbara.This non-convex inversion was made feasible thanks to the observations of two well constrained stellar occultation of Barbara in 2009. The first one occurred on the 5th of October (left figure) and was observed in Europe by members of IOTA/ES. The second occurred on the 21st of November and was observed in North America by members of IOTA. The plots have been generated with the Occult software package.

In 2009 (Delbo et al.) realises the first VLTI-MIDI direct determination of asteroid sizes. One of the targets of this study was (234) Barbara.Thank to these interforetric measurement, Delbo et al., not only determine the size of Barbara, but also gives constrain about its shape. They found that Barbara should either be bi-lobated or be a double asteroid.

These two informations together with the well sampled light curves observations of Barbara allows to determine a non-convex shape using the KAOLA inversion technique.

The result of the non-convex light curve inversion shows a very irregular object with deep large scale concavities.

Which are the known Barbarians?

		Bus (2002)	De Meo (2009)	Diameter (km)
172	Baucis	L	-	65
234	Barbara	Ld	L	45.7
236	Honoria	L	L	90
387	Aquitania	L	L	99
402	Chloe	K	L	55
458	Hercynia	Ld	-	42
599	Luisa	K	L	68
679	Pax	K	L	66
729	Watsonia	L	L	52
980	Anacostia	L	-	85
5492	Thoma (Watsonia fam.)	L	-	13.7
	6 Watsonia family members	-	-	< 5

Note that all barbarians belong to the L-class as defined by De Meo (2009), i.e. when the Near-IR spectrum is included in the classification.

Which asteroids we are studying?

We target known and candidate barbarians, such as:

12	Victoria	8.66035
42	Isis	13.58364
122	Gerda	10.685
172	Baucis	27.40
234	Barbara	26.47
236	Honoria	12.34
269	Justicia	16.50
387	Aquitania	24.15
402	Chloe	10.66
458	Hercynia	21.8136
478	Tergeste	16.103354
599	Luisa	9.566
606	Brangane	12.29
611	Valeria	6.98
642	Clara	8.23080
679	Pax	8.452
729	Watsonia	25.19
753	Tiflis	9.825965
824	Anastasia	252.0
908	Buda	15.57
980	Anacostia	20.115
1284	Latvia	25
1332	Marconia	32.121
1372	Haremari	15.25
1400	Tirela	13.356
1406	Komppa	5.50
1702	Kalahari	24
1858	Lobachevskij	7
2085	Henan	110.68
2354	Lavrov
2448	Sholokhov	10.060
2733	Hamina	17
2850	Mozaiskij	19
3287	Olmstead	4.800
3349	Manas	17
3734	Waland	53
3844	Lujiaxi	13.33
4426	Roerich	21
4456	Mawson	19
4607	Seilandfarm	3.97
4619	Polyakhova	13
4726	Federer	82
4737	Kiladze	45
4917	Yurilvovia
5067	Occidental	23
5492	Thoma	20
5840	Raybrown
6730	Ikeda
7062	Meslier
7245	1991 RN10	40
7304	Namiki	8.87
7411	1990 QQ1
7562	Kagiroino_oka	30
7763	Crabeels	49
8250	Cornell	17
9222	Chubey
10165	1995 BL2
12443	Paulsydney
12532	1998 KW54
13150	Paolotesi
15552	Sandashounkan	32.4
16498	Passau
19369	1997 YO
26219	1997 WO21
67255	2000 ET109	3.70

Our international network for photometry

Observatory	Aperture (in m)	Longitude	Latitude
AOdM	0.8	000◦44′13” E	42◦01′28” N
C2PU	01/04/2015	006◦56′00” E	43◦45′00” N
Borowiec	0.4	016◦52′48” E	52◦24′00” N
ZADKO	1	115◦42′49” E	31◦21′24” S
RBT	0.7	111◦58′45” W	31◦42′00” N
SARA	0.6	070◦48′23” W	30◦10′10” S
IAA	01/05/2015	003◦23′09” W	37◦03′47” N

References

-- PaoloTanga - 09 Jul 2015