Limits on the Size and Orbit Distribution of Main Belt Comets

TL;DR

This study conducted the most sensitive search to date for main belt comets using TALCS data, finding no new candidates but setting upper limits on their population and activity levels, and suggesting many asteroids may have low-level activity.

Contribution

It provides the first large-scale, sensitive survey constraining the size, distribution, and activity of main belt comets, and establishes limits on their abundance and activity levels.

Findings

No new MBC candidates detected at current sensitivity.

Faint activity detected statistically in the asteroid population.

Less than 400,000 MBCs brighter than H=21 in the main belt.

Abstract

The seven known main belt comets (MBCs) have orbital characteristics of main belt asteroids yet exhibit dust ejection like comets. In order to constrain their physical and orbital properties we searched the Thousand Asteroid Light Curve Survey (TALCS; Masiero et al. 2009) for additional candidates using two diagnostics: tail and coma detection. This was the most sensitive MBC survey effort to date, extending the search from MBCs with H~18 (D~1 km) to H~21 (D~150 m). We fit each of the 924 TALCS objects to a PSF model incorporating both a coma and nuclear component to measure the fractional contribution of the coma to the total surface brightness. We determined the significance of the coma detection using the same algorithm on a sample of comparable null detections. We did not identify any MBC candidates with this technique to a sensitivity limit on the order of cometary mass loss rate of about 0.1 kg/s. Our tail detection algorithm identified statistically significant flux in a segmented annulus around the candidate object. We show that the technique can detect tail activity throughout the asteroid belt to the level of the currently known MBCs. Although we did not identify any MBC candidates with this technique, we find a statistically significant detection of faint activity in the entire ensemble of TALCS asteroids. This suggests that many main belt asteroids are active at very low levels. We set 90% upper confidence limits on the number distribution of MBCs as a function of absolute magnitude, semimajor axis, eccentricity, and inclination. There are <~ 400000 MBCs in the main belt brighter than H_V=21 (~150 m) and the MBC:MBA ratio is <~ 1:400. We further comment on the ability of observations to meaningfully constrain the snow line's location. Under some reasonable and simple assumptions we claim 85% confidence that the contemporary snow line lies beyond 2.5 AU.