Elsevier

Icarus

Volume 188, Issue 2, June 2007, Pages 451-456
Icarus

Ceres lightcurve analysis—Period determination

https://doi.org/10.1016/j.icarus.2006.11.025Get rights and content

Abstract

The sidereal period of Ceres is refined from 9.075 h to 9.074170±0.000002, making use of recent and historical lightcurves spanning almost 50 years. An observed increase in the amplitude of the lightcurve with solar phase angle is consistent with bright, discrete albedo features contributing a greater fraction of light as the defect of illumination increases. Observations near the same phase angle over this time span show no evidence of changes that would indicate active surface processes.

Introduction

The value of the sidereal period of Ceres in the literature has been unchanged for two decades, 9.075 h. The primary purpose of this paper is to improve the poor precision of the Ceres rotation period, which became apparent while attempting to align ground-based lightcurves with recent Hubble Space Telescope (HST) observations. The uncertainty in the period corresponds to ∼40° in planetographic longitude over one Earth year. A higher precision in the period allows us to search for anomalous photometric behavior and check for any indication of changes hinting at potential ongoing surface processes. This is motivated by the possibility of a present-day subsurface ocean within Ceres (McCord and Sotin, 2005), the presence of clay minerals on its surface (King et al., 1992) and the tentative detection of OH (A'Hearn and Feldman, 1992).

Recent HST images of Ceres were used to construct a shape model and refine the solution for the rotation pole to within a few degrees (Thomas et al., 2005). The resultant obliquity of Ceres is low; zero obliquity is within the pole uncertainty. The shape of Ceres was found to be gravitationally relaxed and implied internal differentiation into a rocky core and an ice-rich mantle. The HST images were also used to construct high resolution albedo maps of the surface of Ceres (Li et al., 2006). Variations in albedo were discernible.

The amplitude of the Ceres lightcurve at visible wavelengths is small, ∼0.03 magnitudes. Lightcurves of most asteroids are related to their irregular shape, however Ceres' smooth oblate spheroid shape suggests that its lightcurve variations are from albedo features. Li et al. (2006) use these albedo features to coarsely reproduce the Ceres lightcurve shape.

We have collected lightcurves of Ceres aided by compilations of asteroid lightcurves in the NASA Planetary Data System (Lagerkvist et al., 1995, Harris et al., 2006). Table 1 contains a summary of the lightcurves and associated aspect data. All lightcurves have been observed in the Johnson V band. The aspect data in the table are associated with the zero UT time of the first night of each set of observations. Aspect data are discussed again under the analysis section. During the 1975/1976 opposition of Ceres, there was a campaign to observe its lightcurve from sites around the world to resolve whether the period was 9 or 18 h. These lightcurves were published by Tedesco et al. (1983) and the resultant period value of 9.075±0.001h has been used since then.

Lightcurves in Table 1 span several generations in instrumentation. The earliest lightcurves listed were observed with photoelectric devices that were still state-of-the-art and relatively new to astronomy and planetary science. In contrast, the 2004 lightcurve observed by one of us (GAE) used equipment available to amateur astronomers, see Appendix A for details.

Section snippets

Analysis

The lightcurves used to constrain the period of Ceres were selected based on spread of time, data quality and solar phase angles less than 8°. Ideally, the lightcurves used would be observed with the same phase and aspect in order to avoid the uncertainty associated aligning lightcurves with different shapes. Low phase angle lightcurves were the most common in the data available. The widest baseline between lightcurves used was between the years 1958 and 2004 (aspect details in Table 1). The

Acknowledgements

We would like to thank Carol Neese for assistance with the database from which the lightcurve data from the literature were retrieved, and Jian-Yang Li for discussion pointing to directions for future research. This work was supported by the NASA Planetary Astronomy Program, Grant NNG05GF37G (PI MVS). We also thank two anonymous reviews for comments to improve the manuscript.

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