Astron. Astrophys. 357, L53-L56 (2000)

2. Observations and data reduction

The observations were carried out with the Infrared-Cooled grating spectrometer ISAAC (Moorwood et al. 1999) at the first Unit Telescope (Antu) of the ESO-VLT, on 23-24 May 1999. The 1 to 2.5 microns wavelength range camera is equipped with a Rockwell Hawaii 1024 x 1024 pixel Hg:Cd:Te array. The pixel scale is 0.146 arcsec.

The object was observed when it was at an heliocentric distance of 8.94 AU, a geocentric distance of 7.99 AU, and at a phase angle of 2.35 degrees. The estimated apparent visual magnitude at the time of our observations is 18.5. Using the color indexes measured by Davies et al. (1998) the corresponding estimated J, H and K magnitudes are, respectively, 16.8, 16.5 and 16.4.

ISAAC was used in its Low Resolution spectroscopic mode, using the low resolution grating at different orders with different order sorting filters. The details of the observations are given in Table 2.

Table 2. Journal of the observations on 24 May 1999 (UT time at mid exposure).

A slit 1 arc-second wide was used, which corresponds to a spectral resolution of about 500. The observations were done by nodding the object along the slit by 10 arcsec between 2 positions A and B. The 2 averaged A and B images in each spectral range (J, H and K) were subtracted from each other. The A-B and B-A images were flat-fielded, corrected for spatial and spectral distortion and finally combined with a 10 arcsec offset. This procedure allows to sum the signal up while providing a double subtraction of the OH sky lines which perfectly subtract out.

The spectra were extracted from the resulting combined images, and wavelength calibrated from a sky spectrum obtained from the A images corrected for distortion. Because of the blending of the OH lines at the spectral resolution used, the wavelength calibration is typically accurate to within one pixel.

The same procedure was used for the calibrators, which were flat fielded with the same flat fields as the ones used for 8405 Asbolus. As calibrators, we used both a solar-type star (HD 144585; Hardorp 1978), and two C-type asteroids 566 Stereoskopia and 128 Nemesis. Although there are no published near-infrared spectra of these two asteroids, C-type asteroids are known to have, in the near-infrared, a featureless and, in general, flat spectrum, which makes them good solar analogs. In fact, by dividing the two asteroid spectra between themselves, a perfectly flat continuum is obtained in the three spectral ranges, outside regions of strong telluric absorptions. The Asbolus spectra presented in Fig. 1 have been divided by the calibrator ones corresponding to the best fit in air-masses: 566 Stereoskopia in J and H, and HD 144585 in K. The edges of each spectral region have been cut to avoid too low S/N spectral regions. To improve the S/N ratio, the resulting spectra were then smoothed by gaussian filtering of = 15 pixels (FWHM = 2.35 ), providing a final spectral resolution of 100 in J and H and 90 in K. The average S/N ratio after smoothing is between 10 and 20, depending on the wavelength.

Fig. 1. Spectral reflectance of 8405 Asbolus in the J, H and K ranges. The spectra in the three different bands have been adjusted using the average J, H and K colors of Davies et al. (1998). The spectral reflectances have been arbitrarily normalized to 1 at 0.55 micron. The upper curves correspond to the same spectra as in the lower curve, but smoothed to a spectral resolution of 100 in J and H and 90 in K. The zero level of the spectra in the upper curves has been shifted by 1.5 for clarity.

In the lack of absolute calibration, the spectra in the different spectral bands have been adjusted using the colors of Asbolus as measured by Davies et al. (1998) over three different nights in March and April 1997, and assuming solar colors V-J=1.07, V-H=1.36 and V-K=1.42 (Degewij et al. 1980) (Fig. 1).

© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000
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