UCLA

In order to have a better understanding of the early history of the solar system, it is scientifically important to study comets and active asteroids, which are believed to be leftovers from the formation epoch. This work presents our studies of two of the least understood families of cometary objects -- active asteroids and near-Sun comets, in terms of their non-gravitational effects and physical properties. We also present a summary of our two short-term surveys specifically for Kreutz-group comets, and an examination of a widely adopted cometary nucleus-extraction technique.

We first systematically investigate the non-gravitational effects of the known active asteroids. Two of the members (313P/Gibbs

amp; 324P/La Sagra) exhibit statistically significant non-gravitational effects, while for the remaining members we are only able to place upper limits. The result is broadly consistent with the fact that the mass loss of active asteroids is generally less than that of typical comets. Thus, previous dynamical studies of active asteroids without consideration of the non-gravitational effects are still valid, and the majority were likely formed in situ in the main belt rather than from other cometary sources.

For near-Sun comets, we conducted ground-based surveys from the Canada-France-Hawaii telescope (CFHT) and the VLT Survey Telescope (VST) in 2012 and 2014, respectively, without detection of any dwarf Kreutz-group comets. Our non-detection of two bright members which were later discovered by the {\it Solar and Heliospheric Observatory} ({\it SOHO}) but within our CFHT search region suggests that dwarf Kreutz comets brighten much more rapidly than previously thought, or they undergo outburst at larger heliocentric distances. We also present an unprecedentedly detailed study of near-Sun comet C/2015 D1 ({\it SOHO}), which was the first sunskirting comet observed from the ground over the past half century. This comet disintegrated around perihelion due to excessive thermal stress within its nucleus, or to rotational instability. The enormous mass loss ($\dot{M}_\mathrm{N} \sim 10^5$ kg s$^{-1}$) caused a strong non-gravitational effect. Together with photometric measurements, the nucleus mass and radius are inferred ($M_\mathrm{N} \sim 10^8$-10$^9$ kg, $R_\mathrm{N} \sim 50$-150 m), and we probe the emission of dust grains and dust-size distribution based on the morphology, as well as its composition using {\it SOHO} multiband observations.

Finally, we examine the nucleus-extraction technique, an important tool for revealing cometary nucleus sizes, based on which statistics of the nucleus-size distribution are established. By testing the method on our synthetic comet images, we identify an obvious systematic bias stemming from neglect of the distortion of the coma brightness profile after convolution with point-spread functions (PSF). Thus, we conclude that published nucleus-size determinations using this technique are likely invalid. Our main suggestion is to better apply the technique on high spatial resolution images of weakly active comets when the nucleus signal occupies $\gtrsim$10\% of the total around the central region.