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

This topic: Main > Ltypes
Topic revision: 08 Feb 2018, PaoloTanga
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