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

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3. Discussion and conclusion

The spectra in the J, H and K regions show a rather featureless behavior. 8405 Asbolus had already been observed in August 1998 by Brown (2000) who obtained at the Keck I telescope a flat spectrum between 1.5 and 2.4 microns at a resolution of about 100. Our observations, which include in addition a spectrum in the J band, confirm the lack of spectral signature observed by Brown (2000). No obvious feature is detected, in particular no absorption due to water ice (bands at 1.5 and 2 µm). Since we may have been looking at a different region of 8405 Asbolus, and since our spectra have a higher S/N ratio, the observations reported here provide additional support for the lack of detectable water ice on the surface of this object. If water ice had a 2-micron absorption as deep as those found in 1997 CU26 spectra (Brown et al. 1998), or in 2060 Chiron spectra (Luu et al. 2000), which are about 10 [FORMULA] deep, we would have detected it.

On Fig. 2, we show a composite spectrum that includes the spectrum obtained in the visible in March 1998 with the ESO-NTT by Barucci et al. (1999). One should note that, since the visible and near infrared data were obtained at different times, they may not correspond to the same area on the Centaur 8405 Asbolus, and therefore this composite spectrum should be used with some caution. This is also the case for the near-infrared colors that were used for the normalization in the J, H and K windows. On Fig. 2 are also shown data corresponding to average V-R and V-I colors as measured by Davies et al. (1998) in May 5-8, 1997, and assuming solar colors of V-R=0.36 and V-I=0.71 as in Davies et al., that indicate a surface slightly "bluer" than observed by Barucci et al. (1999).

[FIGURE] Fig. 2. A composite spectrum of 8405 Asbolus that includes visible data from 1998 by Barucci et al. (1999), and the near-infrared data reported in this paper. As on Fig. 1, the spectra in the J, H and K bands have been adjusted using the average V-J, V-H and V-K colors of Davies et al. (1998). The R and I mean relative reflectances (normalized to 1 at V) of Davies et al. are also shown, as well as the error bars on all Davies et al. (1998) reflectances. Two different models with spatial mixtures are presented. The albedo at 0.55 µm is 0.025 for the carbon+kerogen model, and 0.055 for the carbon+tholin+water ice model. Grain sizes are: 0.1 µm for amorphous carbon, 0.1 µm for tholins and 10 µm for water ice.

The spectrum of 8405 Asbolus, with a slope of 17%/103Å in the visible range (up to 7500 Å) as measured by Barucci et al. (1999), has a general shape very similar to that of the reddest D-type asteroids (Lazzarin et al. 1995).

As a first check of the possible materials contributing to the spectrum of 8405 Asbolus, we ran a radiative transfer model (Douté & Schmitt 1998) considering a simple geographical distribution of Titan tholins (Khare et al. 1984) - these complex organic solids have been included in models of the surface of 5145 Pholus to account for the steep red slope of the spectrum from 0.4 to 1 micron (see, e.g., Cruikshank et al. 1998) - and amorphous carbon (Zubko et al. 1996).

The spectrum of 8405 Asbolus was scaled to a geometric albedo at 0.55 micron as close as possible to the value of 0.04 (5145 Pholus albedo: 0.04[FORMULA]0.03 and 1997 CU26 albedo: 0.045[FORMULA]0.01; see Davies 2000 and Jewitt & Kalas 1998). The best overal fit was obtained with most of the surface covered by sub-micron amorphous carbon grains and the remaining by tholins, and an albedo of 0.04 at 0.55 µm. Tholins allow to reproduce the visible red slope, and amorphous carbon (a dark spectrally featureless material often included in models of dark solar system objects) provides the low albedo and the slight infrared slope.

Kerogen-like organic compounds are plausible constituents of the surface of D-type asteroids (see e.g. Gradie & Veverka 1980). We therefore also considered this kind of material. Some type of kerogens (see Fig. 8 from Cruikshank 1987) can possibly replace the Titan tholins in our models as their spectra display similar red slopes in the visible. Furthermore, they have no absorptions in the near infrared. Due to the much smaller albedo of the kerogens, less amorphous carbon will be needed. Using the laboratory data from Clark published in Cruikshank (1987), we found that, although a combination of 50 [FORMULA] of this type of kerogen and 50 [FORMULA] of amorphous carbon can reproduce the overall shape of the spectrum (Fig. 2), the fit with the spectrum of 8405 Asbolus is not as good below 0.5 µm, and the albedo is only 0.025 at 0.55 µm.

To investigate the limits on the water ice abundance that can be derived from the spectrum, we added a small area of pure water ice at 130 K (Grundy & Schmitt 1998), the expected surface temperature around 9 AU. We found that, in the case of our tholins plus amorphous carbon areal mixture, no more than a few percents of the surface (depending on the grain size) can be covered with pure water ice in order to keep the strong 2-micron ice band within the noise level in the K region (Fig. 2). In the case of the kerogen plus amorphous carbon mixture, no good fit could be obtained when water ice was included.

Different surface representations, in particular intimate mixtures, need to be investigated to better evaluate the materials present, their mode of coexistence and their relative abundances.

8405 Asbolus has orbital characteristics in common with those of the Centaurs 2060 Chiron (perih.=8.4, aph.=18.8), 1997 CU26 (perih.=13.1, aph.= 18.4) and 5145 Pholus (perih.=8.7, aph.=31.8). Pholus is however much redder than 8405 Asbolus, which suggests different surface properties. 8405 Asbolus is only slightly redder than 2060 Chiron (Barucci et al. 1999, Davies et al. 1998), but it does not show any sign of activity. The colors of 8405 Asbolus are, in fact, more comparable to those of the Centaur 1997 CU26. However, H2O ice is clearly detected on 1997 CU26, 2060 Chiron and 5145 Pholus, but not on 8405 Asbolus. The presence or absence of detectable H2O ice at the surface of Centaurs (as well as the colors of the objects) therefore does not seem to be merely related to the orbital characteristics of the objects.

Luu et al. (2000) have suggested that water ice is common on the surface of Centaurs and that water ice detection is directly connected to cometary-like activity (with water ice detected only when there is no activity). Indeed, water ice was not seen on 2060 Chiron in 1993 (Luu et al. 1994) at a time when its activity was very high. It has been seen on this body only since 1996 (Luu et al. 2000, and Foster et al. 2000), during a period of low activity. However, our observations of 8405 Asbolus do not support this, as the absence of detectable water ice on this body would imply that it is currently active, which has not been demonstrated so far.

Higher signal-to-noise observations, and observations made a few hours apart so that different areas on 8405 Asbolus can be studied, would be very useful to confirm or reject the lack of water ice signatures.

8405 Asbolus will pass through perihelion in 2002, and we are planning to observe it intensively with different techniques to detect any possible activity from that time on.

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© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000