Sungrazing Comets: Properties of Nuclei and in Situ Detectability of Cometary Ions at 1 AU

Abstract

A one dimensional sublimation model for cometary nuclei is used to derive size limits for the nuclei of sungrazing comets, and to estimate oxygen ion fluxes at 1 AU from their evaporation. Given that none of the ≈ 300 sungrazers detected by the SOlar and Heliospheric Observatory (SOHO) was observed after disappearing behind the sun, and that small nuclei with a radius of ≈ 3.5 m could be observed, it is assumed that all SOHO sungrazers were completely destroyed. For the case that sublimation alone is sufficient for destruction, the model yields an upper size limit as a function of nuclear density ̺, albedo A and perihelion distance q. If the density of the nuclei is that typical of porous ice (600 kg m −3 ), the maximum size is 63 m. These results confirm similar model calculations by Weissman (1983). An analytical expression is derived that approximates the model results well. We discuss possible modifications of our results by different disruption mechanisms. While disruption by thermal stress does not change the upper size limits significantly, they may be somewhat increased by tidal disruption (up to 100 m for a density of 600 kg m −3 ) dependent on the isotropy of the sublimation process and the tensile strength of the comet. Implications for the Kreutz family of sungrazers are discussed.Oxygen ions from the sublimation of sungrazing comets form a tail. Fluxes from this tail are sufficiently high to be measured at 1 AU by particle detectors on spacecraft, but the duration of a tail crossing is only about half an hour. Therefore the probability of a spacecraft actually encountering a tail of an evaporating sungrazer is only of the order of two percent per year.

Are the sungrazing comets the inner source of pickup ions and energetic neutral atoms?

“…Modelling by Iseli et al (2002) returns the most massive element at 0.63 × 10 12 g, which agrees with the observational value to ∼30%. However, observations of several bright sungrazers by Raymond et al (1998) and Uzzo et al (2001) suggest that their diameters are 5-fold smaller than inferred by Sekanina and Iseli, which indicates a substantially smaller mass.…”

“…Pickup ions from solar system bodies were observed at surprisingly large distances of millions of kilometers: Grünwaldt et al (1997) discovered the Venus PUI tail from the Earth's orbit, Gloeckler et al (2000b) detected the PUI tail from Comet C/1996 B2 Hyakutake, and Gloeckler et al (2004) observed an oxygen and carbon PUI signal that shows characteristics resembling the sungrazer signature discussed by Iseli et al (2002).…”

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

“…Most sporadic near-Sun comets are likely to follow the trends observed in the general comet population, if they have a cometary origin. Kreutz comet nucleus sizes, however, have been estimated to be meters to tens of meters in radius by a variety of modeling and observational techniques, e.g., MacQueen and St. Cyr (1991), Raymond et al (1998), Uzzo et al (2001), Iseli et al (2002), Sekanina (2003), Bemporad et al (2005), Knight et al (2010). These sizes are similar to inferred from high resolution imaging with Hubble Space Telescope and/or thermal IR imaging with Spitzer Space Telescope, such as the 25-60 m range inferred for the fragments of split comets D/1999 S4 LINEAR (Weaver et al 2001); the tens of m estimated for 73P/Schwassmann-Wachmann 3-C (Weaver et al 2006;Reach et al 2009), and the inferred 9-30 m range for 332P/IkeyaMurakami (Jewitt et al 2016).…”

Grain opacity and the bulk composition of extrasolar planets

“…This temperature depends on the vapor pressure law and is in the range 1300-2000 K for silicates (Opik 1958) and 180-220 K for ices (Iseli et al 2002).…”