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Project Progress

In 2006, Hsieh & Jewitt (Science, Volume 312, pp. 561-563) observed three asteroids with comet-like activity inside the main asteroid belt. These Main Belt Comets (MBCs) represent the third known reservoir of comets in the solar system, after the Oort cloud and Kuiper belt, and are a potential source of water for terrestrial planets.

We have employed two different search methods to find cometary activity.

1. We have constructed a non-parametric search algorithm that examines angular segments around an asteroid, and finds the brightest segment as a fraction of the source brightness. This figure of merit is then compared to stars from the same image, which are subject to the same systematic biases. The rank of the asteroid relative to its calibration stars is compared over many images using a Kolmogorov-Smirnov (KS) test to find the likelihood that an object shows excess light (a tail). Existing MBCs are readily detected over a series of 20 images (Figure 1)

2. We have constructed a similar approach to find comae instead of tails. Images of an asteroid are stacked over an observing run, in parallel with carefully chosen neighboring calibration stars. The final stacked asteroid image is then modeled as a sum of the stacked stellar image plus a coma model. A χ² statistic is then used to ascertain whether a significant coma signature is present.

The first approach, tail detection, did not yield a significant tail detection (Figure 2). However, the overall ensemble of KS probability values disagrees with the null flat case, suggesting there may be an overall sub detection threshold signature of cometary activity among asteroids, an issue we will study further (Figure 3). The second approach also did not detect MBCs, but the presence of comae even in the four existing MBCs is a matter of debate.

This work is currently in the last stages of preparation for publication.