4 Laboratory spectra and observational results

In order to interpret the obtained spectral behaviours we compared the observational results with the whole sample of mineral and meteorite emissivities available in the literature (Salisbury et al. 1991a,b, ASTER spectral library on http://speclib.jpl.nasa.gov). Moreover, new laboratory spectra of a few selected samples at different grain dimensions, have been obtained with the Capodimonte Observatory Bruker IFS66v interferometer, following the procedures already described in Dotto et al. (2000) and Barucci et al. (2002). Since asteroid spectra seem to be dominated by the effect of fine particles (Le Bertre & Zellner 1980), all the laboratory samples we considered are particulate.

In Figs. 3-7 we plot the "Relative Obs/Mod'' obtained for each observed asteroid, compared with the emissivities of some mineral and/or meteorite analogs. In the considered wavelength range the most diagnostic feature is the Christiansen peak which is associated with the principal molecular vibration band, where the refractive index changes rapidly, and occurs at a wavelength that for silicates is just short of the Si-O stretching vibration bands. This feature, directly related to the mineralogy and the grain size, appears generally as a peak between 7.5 and 9.5 $\mu$m.

 

77 Frigga.   The taxonomic classification of this object is still an open problem. It has been classified as MU-type (Tholen 1989), D2-type (Barucci et al. 1987), and Xe (Bus & Binzel 2002).

Figure 3: Relative PHT-S Obs/Mod of 77 Frigga compared with the winonaite meteorite Winona (continuous line). The spectrum of Winona is vertically offset for clarity.

On the basis of our ISO data we obtained a diameter of $70 \pm 4$ km and an albedo of  $0.146 \pm 0.005$, in close agreement with the previous IRAS determinations. These values have been computed using STM on PHT-S and PHT-P data, eliminating the observation at 25 $\mu$m which gives an overestimation of about 40% in the diameter.

Figure 3 shows the emissivity obtained by dividing the PHT-S data by the STM expected flux. The comparison of this spectrum with the whole sample of available meteorite and mineral emissivities was unsuccessfull. The only possible analogy, even if very weak, seems to be with the emissivity of the Winona meteorite sample belonging to the ASTER database. Winona belongs to the winonaite meteorites, a group of primitive achondrites associated with the IAB/IIICD iron meteorites. Its structure is characterized by silicate inclusions in IAB irons. Although this is just a tentative interpretation of the obtained ISO mid-infrared spectrum of Frigga, the silicated iron nature of the meteorite Winona seems to be in agreement with the debated classification of this asteroid.

 

114 Kassandra and 308 Polyxo.   Asteroids 114 Kassandra and 308 Polyxo have been classified as belonging to the T class by Tholen (1989), as a D2-type object by Barucci et al. (1987), and as Xk and T, respectively, by Bus & Binzel (2002). On the basis of ISO PHT-P and PHT-S data we obtained diameter and albedo values which are in agreement (to within 10%) with the previous determinations obtained on the basis of the IRAS data.

Britt et al. (1992) showed that 114 Kassandra has a 0.4-2.5 $\mu$m spectrum very similar to troilite, while 308 Polyxo shows in the same wavelength range a completely different spectral behavior. In the mid-infrared range, on the contrary, these two objects show a spectral behavior similar to that one of Ornans, a CO3 carbonaceous chondrite meteorite. Figures 4 and 5 show the comparison between the emissivity of Kassandra and Polyxo and the laboratory emissivity of an Ornans sample at grain sizes between 0 and 20 $\mu$m. The behaviour of the spectrum around 9.2 $\mu$m seems to be consistent with the Christiansen peak of the Ornans sample. This result is surprising since Ornans is a type 3 carbonaceous meteorite chondrite (CO3) which seems to show the presence of aqueous alteration processes (Zolensky & McSween 1988). 308 Polyxo has been shown to have a strong feature of water of hydration feature at 3.0 $\mu$m while in the spectrum of 114 Kassandra this feature is completely absent (Jones et al. 1990). These differences in the interpretation of the surface composition can be due to possible variations with different rotational phases. Further observations at near-infrared wavelength of these two asteroids are needed in order to investigate the possibility that these objects have really undergone some aqueous alteration processes.

 

511 Davida.   Asteroid 511 Davida is the fifth biggest asteroid with an IRAS diameter of 326 km. For this object PHT-P obtained multi-filter photometric data only at 10, 25, and 60 $\mu$m. On the basis of our complete ISO data sample we computed a diameter of 303 km and an albedo of 0.064.

Figure 4: Relative PHT-S Obs/Mod of 114 Kassandra compared with the CO3 carbonaceous chondrite meteorite Ornans (continuous line). The spectrum of Ornans is vertically offset for clarity.

Figure 5: Relative PHT-S Obs/Mod of 308 Polyxo compared with the CO3 carbonaceous chondrite meteorite Ornans (continuous line). The spectrum of Ornans is vertically offset for clarity.

Davida as been classified by Tholen (1989), Barucci et al. (1987) and Bus & Binzel (2002) as belonging to the C class. Jones et al. (1990) detected hydrated silicates (via 3-$\mu$m spectrometry) on the surface of this asteroids, while Hiroi et al. (1996) found a good match between the 0.3-3.6 $\mu$m spectrum of Davida and that one of the unusual CI/CM meteorite B-7904. On the basis of the analysis of heated Murchison samples they inferred that B-7904 has probably been heated up to 500-600 $^{\circ}$C.

Figure 6 shows the emissivity of Davida between 5.8 and 11.6 $\mu$m which is consistent with that one of carbonaceous chondrites as reported by Salisbury et al. (1991a). In particular the spectral behaviour between 6.5 and 11.5 $\mu$m suggests the comparison with the emissivity of the CM-type carbonaceous chondrite meteorite Murchison given by the ASTER database. This result seems to confirm that Davida is a large body which suffered aqueous alteration processes.

 

914 Palisana.   Asteroid 914 Palisana is classified as CU-type by Tholen (1989) and as a D3-type by Barucci et al. (1987). On the basis of our ISO data we computed a diameter of 71 km and an albedo of 0.113. Also in this case the PHT-P measurement at 25 $\mu$m gives an overestimation of diameter and albedo of more than 30%.

Figure 6: Relative PHT-S Obs/Mod of 511 Davida compared with the CM-type carbonaceous chondrite meteorite Murchison (continuous line). The spectrum of Murchison is vertically offset for clarity.

Fitzsimmons et al. (1994) showed that Palisana exhibits in the visible range large-scale absorption characteristic of phyllosilicates.

The spectral behavior, shown in Fig. 7, is after 7 $\mu$m flat and featureless. We tried to compare this object with many mineral and meteorite emissivities. In particular we compared the obtained spectrum with the emissivity of phyllosilicates and CI/CM carbonaceous chondrite meteorites which are related to aqueous alteration products. None of the available laboratory spectra of minerals and meteorites matched the observed spectrum. As an example we report in Fig. 7 the emissivities of a sample of the CI-type meteorite Orgueil at grain sizes between 0 and 50 $\mu$m and a sample of powdered kaolinite (phyllosilicate), obtained by laboratory experiments at the Capodimonte Observatory.

Figure 7: Relative PHT-S Obs/Mod of 914 Palisana compared with the CI-type carbonaceous chondrite meteorite Orgueil (dashed line) and a phyllosilicate kaolinite (continuous line). The spectra of Orgueil and kaolinite are vertically offset for clarity.