Near-Infrared Rotational Variability in Comet-Asteroid Transition Object 944 Hidalgo

Document Type

Conference Proceeding

Publication Date


Publication Title

American Astronomical Society, Department of Planetary Sciences


Dynamical arguments indicate that 944 Hidalgo is most likely an extinct or dormant comet. Hidalgo's Tisserand invariant (T = 2.07) suggests strongly that this object came either from the Kuiper belt or the Oort cloud (e.g., Weissman et al. 2002). We obtained low-resolution near-infrared spectra in the 0.8-2.4 micron region on UT Oct 22, 23, Nov 19 and Dec 11, 2004, using the SpeX instrument on NASA's Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii (Oct. and Nov.) and the NICS instrument on the Telescopio Nazionale Galileo (TNG) on La Palma, Spain (Dec.). Our reflectance spectra show a range of slopes. To characterize these slope differences, we normalized each spectrum to 1.0 reflectance at 1.25 microns and measured the reflectance at 2.2 microns. These values are listed in the table for the seven spectra obtained on the two dates when we have temporal coverage, Oct 22 and 23. The uncertainty in each reflectance value is ± 3%. Hidalgo's rotational light curve has a period of 10.06 hours and amplitudes ranging from 0.31 to 0.6 magnitudes in the visible (Harris and Warner 2006, Minor Planet Center). We define the time of our first observation on Oct. 22 as zero rotational phase and give the other six phases in the table. The table shows a systematic temporal variation of the spectral slope consistent with the rotational period. Based on an unpublished visible light curve obtained 10 days earlier (C. Hergenrother personal communication) we determine that one of the small ends of Hidalgo corresponds to our "reddest” spectrum (phase 0.36) while one of the broad sides has the flattest spectrum (phase 0.77).