2 Observations and data reduction

The data of the asteroids presented have been obtained during different observing runs in the course of 1997.

The observations were performed at the European Southern Observatory of La Silla (Chile) and at the Asiago Astrophysical Observatory (Italy).

At La Silla we used the 1.5 m telescope equipped with a Boller & Chivens spectrograph and a Loral Lesser CCD as detector (2048$\times$2048 pixels). The grating used was a 225 gr/mm, with a dispersion of 331 Å/mm in the first order. The CCD has a 15 $\mu$m square pixels, giving a dispersion of about 5 Å/pixel in the wavelength direction. The spectral range is about 0.48 $< \lambda <$ 0.92 $\mu$m with a FWHM of about 10 Å.

At the Asiago Observatory we used the 1.82 m telescope equipped with a Boller & Chivens spectrograph and a Thomson CCD (430 $\times$ 600 pixels) as detector. The grating was a 150 gr/mm with a dispersion of 340 Å/mm in the first order. The CCD has 23 $\mu$m square pixels giving a dispersion of about 7.8 Å/pixel in the wavelength direction. The spectral coverage is about 0.5$< \lambda <$ 0.9 $\mu$m with an instrumental FWHM of 15.6 Å.

Each spectrum was recorded through a slit oriented in the East-West direction. The slit was opened to about 8 arcsec in order to reduce effects due to differential refraction and the possibility of losing signal due to guiding errors of the telescopes.

In Table 1 we report the circumstances of the observations (date and site), the visual magnitude of the asteroids, the solar analog stars used for reduction and some physical characteristics of the observed objects: semimajor axis (AU), diameter (km) and albedo derived from IRAS observations, and taxonomic type (Tholen taxonomy).

During each night, we also recorded bias, flat-field, calibration lamp, spectrophotometric standard and solar analog stars spectra at different intervals throughout the night. The stars were observed at airmasses similar to those of the objects.

  

Figure 1: a) reflectance spectrum of the asteroid 51 Nemausa with superimposed its linear continuum, computed with a linear least squares fit to the smoothed spectral data points. b) residual spectra of 51 Nemausa created as a result of the asteroid spectra in A being divided by the linear background. The same procedure was applied to all the other asteroids

Solar analog stars (Hardorp 1978) are fundamental in the final step of the reduction procedure to remove the solar contribution from the spectra of the asteroids and to obtain the asteroidal reflectivities. Eight stars have been used: Hyades64, 16 Cyg B, HD 28022, HD 44594, HD 89010, HD 20630, HD 86728, HD 76151. Their choice is connected with the observational period.

The spectra were reduced using ordinary procedures of data reduction with the software packages Midas and IDL.
These procedures include: subtraction of the bias from the raw data, flattening of the data in order to remove large scale structures, cosmic ray removal, background subtraction, collapsing the two dimensional spectra, wavelength calibration, atmospheric extinction. The reflectivity of the asteroids was then obtained by dividing the spectra of the objects by the respective solar analog spectrum.

All asteroid spectra are normalized at 1 around 5500 Å.

Spectra used in these studies, even with a good signal to noise ratio, have been smoothed with a median filter technique. Moreover, in order to study aqueous absorption features, which may be very weak, we treated each asteroid/solar-analog-spectrum as a continuum with discrete absorption features superimposed on it, as described by Vilas et al. (1993, 1993b). For the spectra, a simple linear continuum is defined by a linear least squares fit to the smoothed spectral data points. We then divided each individual spectrum by the continuum, thus removing a sloped background (Fig. 1). If residual absorption features are present in these processed spectra, these features can then be easily recognized. In Figs. 2 and  3 we report the spectra of the observed asteroids divided by their linear backgrounds.

  

Figure 2: Reflectance spectra of the observed asteroids. The spectra are normalized to 5500 Å and a linear continuum has been removed

  

Figure 3: Reflectance spectra of the observed asteroids. The spectra are normalized to 5500 Å and a linear continuum has been removed