Absorption bands on the spectra of 21 Lutetia and comparison with Rosetta data M. Lazzarin, S. Marchi Astronomy Dept. Padova Venezia, 19th September 2007

Lutetia observations Lutetia was previously classified as an M-type asteroid because of the high IRAS albedo (0.221+/-0.200) (Tholen 1989, Barucci 1987). --Howell et al . (1994), Burbine and Binzel (2000) found an infrared spectrum unusually flat compared to other M asteroids. --Rivkin et al. (2000) find the 3 micron abs. band typical of the presence of aqueous altered material (they find that more than 75% of large (D>65km) M-types are hydrated). --Bus and Binzel (2002) from SMASSII data classify Lutetia as a Xk type, more primitive. --Birlan et al. (2004) obtained an almost flat featureless infrared spectrum similar to carbonaceous chondrite spectra, so a more primitive composition. --Busarev et al. (2004) found the presence of features around 0.43 micron probably associated to hydrated silicates. --Lazzarin et al. (2004) found a feature around 0.43 mic. and around 0.51 mic: hydrated silicates and/or carbon rich compounds ? --Mueller et al. (2005) with thermal infrared obs. find an albedo =0.208+/-0.025. --Barucci et al. (2005) find a good match between the vis(new)+nir (Birlan et al. 2004) spectrum of Lutetia with the CV3 carbonaceous chondrite Vigarano. --Prokofeva et al. (2005) confirm the band aroud 0.43 micron. --Birlan et al. (2006), with NIR obs., confirmed a similarity with the CV-CO meteorites. --Barucci et al. (2006), with NIR obs., find again a composition compatible with a primitive body

Spettri di maggio first obs Spectra of Lutetia obtained at NTT-ESO on May 6th 2003: they cover the rotational period. The behaviour is similar to C-type. We found two abs. bands at about 0.43 and 0.51 mic. tentatively attributed respect. to a ferric iron spin-forbidden absorption present in phyllosilicates, result of aqueous alteration activity and to chrystalline iron oxides or porphyrins, carbon rich compounds. Lazzarin, Marchi, Magrin, Barbieri, A&A 425,25, 2004.

Spectra of Lutetia obtained on Dec. 2004 at NTT-ESO with Gr#1(0. 4-0 Spectra of Lutetia obtained on Dec. 2004 at NTT-ESO with Gr#1(0.4-0.95 micron) and Gr#5(0.4-0.7 micron, higher resolution) + one spectrum of May 2003. We could not investigate the region shortward of 0.45 mic. because the solar analogues presented several differences at those wavelengths, so we preferred to cut off the short-wavelength range of the spectra. In this case we cannot confirm the band around 0.43 mic. reported previously

Spectrum of Lutetia rectified From the analysis of these new spectra we can confirm a spectral behaviour of the average C-complex asteroids of Bus Taxonomy (Bus, 1999). However it should be noted that this classification does not include albedo, and some spectral slope variations may result not only from compositional variations, but also from roughness variations and space weathering effects. The two new spectra confirm the presence of the absorption feature around 0.51 micron. Precisely, we detected a narrow feature around 0.47 mic. and one around 0.52 mic. included in a wider depression from 0.45 to about 0.55 micron. M. Lazzarin, S. Marchi, L. Moroz, S. Magrin, A&A submitted

Interpretation of the bands 0.45-0.55 micron feature: The broad inflection between 0.45 and 0.55 mic. may be explained by superposition of many absorption bands due to spin-allowed and spin-forbidden crystal field transitions and charge-transfer transitions. Such inflections are observed in spectra of some pyroxenes and pyroxene-rich mineral assemblages (e.g. some meteorites). Also spectra of some olivines show similar features, though the major absorption is centered at 0.45 mic. In general, pyroxenes show extreme spectral diversity, sometimes lack two typical near infrared bands at 0.9 and 2 microns, and may exhibit a variety of different absorption features in the visible due to the presence of trace elements. Other possible candidates could be some Fe-sulfides like troilite, pyrrhotite and intermediate phases that show similar inflections as a function of composition, heating and oxidation degree (Cloutis and Burbine, 1999).

The broad feature around 0. 45-0. 55 mic The broad feature around 0.45-0.55 mic. is present in reflectance spectra of some meteorites, including some ordinary chondrites and CH metal-rich carbonaceous chondrites (e.g. CH-chondrite PCA91467) Nearly all spectra of ureilites (Cloutis and Hudon 2004, Gaffey 1976) show such feature. Reflectance spectra of some ureilites also show a weak feature at 0.52 mic. superimposed onto a broader depression between 0.45 and 0.55 mic. Ureilites are carbon-bearing achondrites, containing olivine and several types of pyroxenes - pigeonite, augite and sometimes low-Ca pyroxene.

0.52 micron feature: might be due to traces of Fe3+ in the tetrahedral sites of clinopyroxenes. Spin-forbidden transitions in tetrahedral Fe3+ produce absorption at ~0.52 mic. Note that some shifts of band positions in the visible range as a function of temperature are also possible, and it is difficult to predict the exact behavior. Another possible contribution to this band at its short-wavelength side is 0.505-0.510 mic. band typical of pyroxenes, being due to spin-forbidden crystal field transitions in Fe2+ (e.g. Hazen et al. 1978). It moves to longer wavelengths with increasing Fe2+ and Ca2+ content in pyroxenes (Hazen et al. 1978).

0.47 micron feature: it may be due to spin-allowed crystal field transitions in Ti3+ in pyroxene (usually at 0.455-0.475 mic.), although 0.475 mic. band is also prominent in the spectra of Fe-rich orthopyroxenes, where it is due to Fe2+. Hazen et al. (1978) suggested that the band could be due to superposition of Ti3+ and Fe2+ effects. Cloutis (2001) reports a 0.46 mic. feature in reflectance spectra of some components in carbonaceous chondrites and tentatively assigns it to spin-allowed transitions in Fe2+. Cloutis et al. (2004) also report that a band at 0.46 microns is present in reflectance spectra of Fe-bearing spinels, where its position correlates with Fe2+ content.

CONCLUSIONS We conclude that Lutetia is a rather unusual asteroid: bright (high albedo) and lacks deep near-infrared absorption bands, but shows UV-vis bands very likely due to silicates The broad ''inflection'' observed around 0.5 mic. may be present in the spectra of both primitive and differentiated meteorites, though is more common for spectra of achondrites compared to those of chondrites. To simulate the spectral-albedo behaviour of Lutetia we would probably need the presence of coarse metal or coarse sulfides which would increase reflectance without producing additional bands, and without affecting significantly the bands present in the spectrum. Such an assemblage is for example found in some carbonaceous meteorites - CH and CB-chondrites- that are rich in metal.

NAC data superimposed on Lutetia spectra

Lutetia Vis+Nir (Nir from Birlan et al. 2004) Our results agree with those of Birlan et al. 2004. On the basis of ephemeris and pole determination, their data refer to almost the same region we observed.