The Science of Sungrazers, Sunskirters, and Other Near-Sun Comets

Abstract

This review addresses our current understanding of comets that venture close to the Sun, and are hence exposed to much more extreme conditions than comets that are typically studied from Earth. The extreme solar heating and plasma environments that these objects encounter change many aspects of their behaviour, thus yielding valuable information on both the comets themselves that complements other data we have on primitive solar system bodies, as well as on the near-solar environment which they traverse. We propose clear definitions for these comets: We use the term near-Sun comets to encompass all ob- jects that pass sunward of the perihelion distance of planet Mercury (0.307 AU). Sunskirters are defined as objects that pass within 33 solar radii of the Sun's centre, equal to half of Mercury's perihelion distance, and the commonly-used phrase sungrazers to be objects that reach perihelion within 3.45 solar radii, i.e. the fluid Roche limit. Finally, comets with orbits that intersect the solar photosphere are termed sundivers. We summarize past studies of these objects, as well as the instruments and facilities used to study them, including space-based platforms that have led to a recent revolution in the quantity and quality of relevant observations. Relevant comet populations are described, including the Kreutz, Marsden, Kracht, and Meyer groups, near-Sun asteroids, and a brief discussion of their origins. The importance of light curves and the clues they provide on cometary composition are emphasized, together with what information has been gleaned about nucleus parameters, including the sizes and masses of objects and their families, and their tensile strengths. The physical processes occurring at these objects are considered in some detail, including the disruption of nuclei, sublimation, and ionisation, and we consider the mass, momentum, and energy loss of comets in the corona and those that venture to lower altitudes. The different components of comae and tails are described, including dust, neutral and ionised gases, their chemical reactions, and their contributions to the near-Sun environment. Comet-solar wind interactions are discussed, including the use of comets as probes of solar wind and coronal conditions in their vicinities. We address the relevance of work on comets near the Sun to similar objects orbiting other stars, and conclude with a discussion of future directions for the field and the planned ground-and space-based facilities that will allow us to address those science topics.

Supercatastrophic Disruption of Asteroids in the Context of SOHO Comet, Fireball, and Meteor Observations

“…Over 3000 comets, most passing within the G16 limit, have been observed and this sample may include SCDing asteroids. Lamy et al (2013) is the last comprehensive analysis of this sample, while Jones et al (2018) provide a useful overview of near-Sun comets: a brief review of the relevant orbital details follows. Below we will adopt the terminology proposed by Knight & Walsh (2013) where comets passing within the Sun's fluid Roche lobe are labelled 'sungrazers', while those passing further out are 'sunskirters'.…”

Planetesimals around stars with TESS (PAST) – I. Transient dimming of a binary solar analogue at the end of the planet accretion era

“…Low-amplitude ( 1%) quasi-periodic dipping with Keplerian regularity but varying amplitude and shape has been explained by dust clouds emanating from "evaporating" ultra-short period planets which by themselves are too small to produce a detectable transit signal (Rappaport et al 2012;Sanchis-Ojeda et al 2015). Lowamplitude dips with shapes consistent with transiting "exocomets" have been identified in the light curves of several main sequence stars, none of which have detectable infrared excess indicative of disks (Rappaport et al 2018;Ansdell et al 2019;Zieba et al 2019); the transiting objects could be larger analogs to the Sun-grazing comets discovered by the SOHO satellite (Battams & Knight 2017;Jones et al 2018). The anomalous star KIC 8462852 (Boyajian et al 2016) may be the extreme member of a population of such stars (Wyatt et al 2018).…”

SUISEI—A Versatile Global Model of Comets with Applications to Small Solar System Bodies

“…SUISEI has proven to be a unique and valuable model to understand the relevant physical processes and properties of small Solar System bodies, including near-Sun comets [25].…”