Astron. Astrophys. 321, 652-659 (1997)

4. Observations

According to modern understanding of the process of collisional evolution of the asteroid belt, it is practically sure that during its lifetime an asteroid like (7) Iris should have suffered energetic collisional events. Even in the absence of a dynamical family associated with this asteroid, some observational indication of the occurrence of such events could come from the optical properties of Iris' surface. In particular, as quoted in Sect. 2, some evidence for albedo and/or surface texture heterogeneity has been noticed by some authors. This might suggest the presence of some large impact crater(s) on the surface of Iris. The existence of a large crater could witness an energetic event occurred in the past, which could be associated with some episode of massive injection of fragments into the 3/1 resonance. Moreover, the presence of a large crater could produce observable spectroscopic variegation of the surface, at least in the case of deeply excavated zones in a differentiated body. In the case that such evidence of differentiation could be achieved by spectroscopic means, this could be interesting from the point of view of the kind of meteorites that could originate from Iris. In particular, it is known that OCs could hardly be reconciled with a fully differentiated parent body

In order to test the mineralogical properties of (7) Iris we obtained several reflectance spectra in visible wavelengths at different rotational phases. At the time of our measurements the angle between the rotation axis and the direction of the Earth (aspect angle) was about (or ) according to Magnusson's (1989) pole determination. We are aware that near-infrared data would be more sensitive to features more strongly diagnostic of particular silicate assemblages. However even optical data are very useful for deriving useful mineralogic information (Vilas & McFadden 1992, Vilas et al. 1994).

Spectra were taken at Asiago-Ekar observatory using a 1.82 m Cassegrain telescope equipped with a Boller & Chivens Spectrograph and a CCD THOMSON TH7882 Thick UV-Coated pixels, each pixel having dimensions of m m. The grating had 150 gr/mm with a dispersion of 339 Å/mm in the first order. In addition to the spectra we have taken several biases, flat fields, calibration lamps and several spectra of HD 191854 and 64 Hyades solar analogues (Hardorp 1978). They are claimed to be indistinguishable from solar spectrum; in particular the latter is one of the best known solar analogues. The reduction technique was standard, using the IRAF package. In Table 2. the circumstances of the observations are shown; the last column represents the rotational phase ( based on a period of .139). Fig. 3 shows the averaged spectrum of (7) Iris compared to that of (6) Hebe (Migliorini et al. 1996). Their similarity in the visible region of wavelength is striking. The spectral range is not exactly the same since the spectra were obtained using two different telescopes; however, the spectral trends are closely similar. In order to confirm the accuracy of our observations, data from the ECAS survey (Zellner et al., 1985) are also superimposed to the average spectrum of Iris shown in Fig. 3, and the agreement is excellent in the wavelength range covered by the spectrum. In addition, we have also compared our spectrum with the one obtained in the framework of the SMASS survey (Xu et al. 1995). Also in this case, no appreciable differences were found. In Fig. 4 we can see Iris' spectra corresponding to different rotational phases. In order to emphasise variations among the spectra, taken at different rotational phases, we plotted in Fig. 5 the ratio between each spectrum of Fig. 4 with the averaged one of Fig. 3. No strong differences are visible, apart from fluctuations of the red part due to the low sensibility of CCD and also to telluric absorption band centered at about 9200 Å. On the whole, our observations do not indicate the presence of spectral variations related to surface heterogeneity.

Table 2. Observational Circumstances

Fig. 3. Averaged reflectance spectrum of (7) Iris compared with (6) Hebe (Migliorini et al. 1996). On the spectrum of Iris is superimposed the 8-color survey (Tholen 1984).

Fig. 4. Rotationally resolved spectra of (7) Iris. Data are vertically offset for clarity.

Fig. 5. Ratio between the averaged spectrum of (7) Iris (Fig. 3) and the spectra taken at different rotational phases (Fig. 4). All the spectra are flat to within few percent; some difference are found in the red part ( Å) due to telluric water absorptions and the relatively low-efficiency of CCD.

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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