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

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 objects 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

Additional observations and improved ephemeris of VV Puppis

Wetensch. publicatieFaculteit der Wiskunde en Natuurwetenschappe

Fluorescence in Astrophysical Plasmas

Following the initial detection by Bowen in 1934 of the strong O III linesbeing due to accidental resonance with strong He II radiation, many strong spectralemission lines are explained as produced by fluorescence. Many of these areFe II lines pumped by H Lyα, as a consequence of strong radiation from hydrogenand a favorable energy level structure for Fe II. The lines are observed in manytypes of objects with low density plasma components. The Weigelt condensationsin the vicinity of the massive star Eta Carinae is one location where these lines areobserved and can be studied in detail, as well as been used for diagnostics.These gas condensations do not only show a spectrum indicating a nonequilibriumexcitation but also non-equilibrium ionization, where the strong hydrogenradiation plays a key role. Early studies identified certain strong lines beingthe result of Resonance Enhanced Two-Photon Ionization (RETPI). Further investigationssuggest that RETPI can be the responsible mechanism for the ionizationstructure of gas condensation.We will review the resonance processes, with emphasis on the Eta Carinae spectrum.Large spectral, spatial and temporal coverage is available for this fascinatingobject, allowing for detailed analysis

Multiple Stressor Impacts

This repository contains the regression metadata, the 174 paired-stressor response datasets as well as the R-code used in the study: Birk et al. 2020. Impacts of multiple stressors on freshwater biota across spatial scales and ecosystems. Nat. Ecol. Evol. https://doi.org/10.1038/s41559-020-1216-4 The folder "Data" includes the regression metadata ("0_Metadata.csv") as semicolon-separated values and the file "00_Legend_Metadata.md" specifying the column-headers; the 174 paired-stressor response datasets as semicolon-separated values and the file "01_Legend_CaseIDs.md" specifying the column-headers. Note that the variable details are specified in the file "0_Metadata.csv". The folder "R-script" includes the R-code ("cleancode.R") used to analyse the paired-stressor response datasets, and the related R-functions ("functions.R") embedded in the R-code