Licandro, J.; Campins, H.; Kelley, M.; Hargrove, K.; Pinilla-Alonso, N.; Cruikshank, D.; Rivkin, A. S.; Emery, J.
Referencia bibliográfica
Astronomy and Astrophysics, Volume 525, id.A34
Fecha de publicación:
1
2011
Revista
Número de citas
91
Número de citas referidas
84
Descripción
Context. (65) Cybele is the most representative member of a population
of primitive asteroids in the outer edge of the main belt, the Cybele
asteroids. Recent dynamical models suggest that a significant fraction
of them originated in the primordial transneptunian disk, so the study
of the physical properties of these asteroids is potentially a useful
test of these models. Aims: Our aim is to obtain information on
the surface composition of this asteroid. In particular we want to
obtain information on the composition and properties of the regolith and
the possible presence of ices and organic materials. Methods: We
present 2-4 μm and 5-14 μm spectroscopy of (65) Cybele obtained
with the NASA IRTF telescope and Spitzer Space Telescope respectively.
We compare the results with spectra of Trojan asteroids and asteroid
(24) Themis. We analyze the 2-4 μm spectrum using scattering models
and we apply thermal models to the 5-14 μm data. Results: The
2-4 μm spectrum of (65) Cybele presents an absorption band centered
at ~3.1 μm and more weaker bands in the 3.2-3.6 μm region, very
similar to those observed in (24) Themis. No hydrated silicates are
detected. From the spectrum in the 5-14 μm region an effective
diameter D = 290 ± 5 km, a beaming paramete η = 0.967
± 0.014, and a geometric visible albedo pV = 0.05
± 0.01 are derived using the NEATM thermal model. The emisivity
spectrum in the 5-14 μm range exhibits an emission plateau at about 9
to 12 μm with an spectral contrast of ~5%. This emission is similar
to that of Trojan asteroids and active comets and may be due to small
silicate grains being imbedded in a relatively transparent matrix, or to
a very under-dense (fairy-castle) surface structure. The lower amplitude
of the silicate emission in Cybele's spectrum with respect to that of
Trojan asteroids could be attributed to larger dust particles and/or a
slightly denser structure. Conclusions: The surface of (65)
Cybele is covered by a fine anhydrous silicate grains mantle, with a
small amount of water ice and complex organic solids. This is similar to
comet surface where non-equilibrium phases coexist. The presence of
water-ice and anhydrous silicates is indicative that hydration did not
happened or is incomplete, suggesting that the temperatures were always
sufficiently low.
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