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Spectroscopy of Centaurs Asbolus and Chiron: Observations over a full rotacional period

Authors: J. Romon-Martin, M.A. Barucci, C. de Bergh, N. Peixinho

Ref.: American Astronomical Society, DPS Meeting #33, #08.05; Bulletin of the American Astronomical Society 33, 1034 (2001)

Abstract: Spectroscopic observations of the centaurs Asbolus and Chiron were obtained in June 2001 at the Very Large Telescope (ESO, Chile). For each centaur, several spectra were recorded in order to find possible changes in the spectrum over a rotational period. Such a study is particularly important for Asbolus. Although Barucci et al. (2000 A&A 357) and Brown et al. (2000 AJ 119) did not observe any feature in spectra of Asbolus, Kern et al. (2000 ApJ 542) obtained two HST-NICMOS spectra of Asbolus over a 1.7 h interval, one with a strong absorption feature at 1.6 micron attributed to water ice, and the other without. They suggested that one side of Asbolus would contain a large amount of water ice, whereas the other side would show no water ice absorption feature. Our observations were performed in the near-infrared range (1.1-2.5 microns) using the ISAAC IR spectro-imager. Three near-infrared spectra of Chiron, and six spectra of Asbolus have been obtained, covering a wide part of the rotational period of each Centaur (full period for Asbolus). Asbolus spectra do not show any obvious water ice absorption feature around 1.6 and 2 microns. Since the full rotational period has been covered, this would mean that there is no strong water ice absorption on any side of Asbolus. This is not at all consistent with observations from Kern et al. However, their observations were obtained in 1998, whereas ours come 3 years later. May water ice absorption features have ``disappeared" from Asbolus surface between 1998 and 2001 ? Chiron spectra show some kind of absorption features in the K band, especially a strong decrease beyond 2.3 microns. Preliminary results would indicate that this feature varies from one spectrum to the other. However no clear evidence of water ice absorption can be seen on these spectra.