Where are the Mini Kreutz-family Comets?

Abstract

The Kreutz family of sungrazing comets contains over 2000 known members, many of which are believed to be under ~100 m sizes (mini comets) and have only been studied at small heliocentric distances (r _H) with space-based SOHO/STEREO spacecraft. To understand the brightening process of mini Kreutz comets, we conducted a survey using CFHT/MegaCam at moderate r _H guided by SOHO/STEREO observations. We identify two comets that should be in our search area but are not detected, indicating that the comets have either followed a steeper brightening rate within the previously reported rapid brightening stage (the brightening burst), or the brightening burst starts earlier than expected. We present a composite analysis of the pre-perihelion light curves of five Kreutz comets that cover to ~1 AU. We observe significant diversity in the light curves that can be used to grossly classify them into two types: C/Ikeya-Seki and C/SWAN follow the canonical r_H^-4 while the others follow r_H^-7. In particular, C/SWAN seems to have undergone an outburst (Δm > 5 mag) or a rapid brightening (n >~ 11) between r _H = 1.06 AU and 0.52 AU, and shows hints of structural/compositional differences compared to other bright Kreutz comets. We also find evidence that the Kreutz comets as a population lose their mass less efficiently than the dynamically new comet, C/ISON, and are relatively devoid of species that drive C/ISON's activity at large r _H. Concurrent observations of C/STEREO in different wavelengths also suggest that a blueward species such as CN may be the main driver for brightening bursts, instead of sodium as previously thought.