37th DPS Meeting, 4-9 September 2005
Session 56 TNOs and Centaurs
Poster, Thursday, September 8, 2005, 6:00-7:15pm, Music Recital Room

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[56.14] Nightly Observations in BVI of 25 KBOs and Centaurs Over a Whole Observing Season

B. E. Schaefer (Louisiana S. U.), D. Rabinowitz, S. Tourtellotte (Yale)

Photometry and colors of small icy bodies in the outer Solar System can provide valuable information on the surface properties and how they vary with orbital parameters. Due to the faintness of all Kuiper Belt Objects (KBOs), few have been looked at photometrically over more than a few short observing runs. To solve questions of demographics and correlations, many well-sampled light curves are needed. This is now possible with service observing, where a number of KBOs can be monitored nightly. Here we describe our results for 25 KBOs and Centaurs which we have monitored nightly with BVI photometry for the last three years using the SMARTS 1.3-m on Cerro Tololo. The results are well-sampled phase functions (with an average of 120 magnitudes per object) in three pass bands spanning the observable range of phase angles. We measure reflectance versus phase angle, rotational light curves, and the opposition surge versus wavelength. We are also capable of discovering long rotation periods which cannot be detected in short observing runs. By revisiting the same objects in different years, we find long-term secular variations.

Our sample of objects has surges which vary from zero slope to 0.30 mag/deg in the V-band (with an average slope of 0.13 mag/deg). The colors and surge slopes are apparently correlated, with the trend being for the redder objects to have steeper surges. We find several objects with breaks in the phase function, which is likely to be a dividing line between where the surge is dominated by shadow hiding or coherent backscattering. We find three objects (Quaoar, 29981, and 2002 PN34) with surge slopes that vary greatly with wavelength, for example, for Quaoar, the slopes are 0.303 ±0.036 in the I-band and 0.099 ±0.025 in the B-band as expected for coherent backscattering. Overall, the opposition surges are significantly correlated with the orbital radius (or possibly with size), with Centaurs having small surge slopes while objects with semimajor axis >25AU have higher surge slopes.

In our sample, we find the previously observed color bimodality, with no objects from 1.74 < B-I < 1.96 for zero phase. However, if we look at the colors for these same objects at a phase angle of 2 degrees, the color-color diagram is considerably smeared and the bimodality disappears. This smearing effect might explain some of the variations in the measured bimodality and it might also point to the bimodality as originating from opposition surge effects.

This work is supported by NASA under grants NAG5-13533 and NAG5-13369.

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Bulletin of the American Astronomical Society, 37 #3
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