18 research outputs found
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Comparison of theoretical and observed sodium D2 line profiles for two late-type dwarfs
For many years the Ca II H and K lines and the Mg II h and k lines have been used as chromospheric diagnostics to compute semi-empirical model chromospheres for late-type stars. However, since Johnson (1964) showed that the Na I D1 and D2 lines are collision-controlled in the atmospheres of late-type stars, only a few researchers have studied the detailed NLTE formation of these lines. Here I summarize such a study for the two stars, EQ Vir (dK5e) and Gliese 616.2 (dM1.5e). A detailed discussion of this research can be found in Boice (1985). High resolution observations of the D lines for these two stars have been obtained at the KPNO 4-meter Mayall telescope using the echelle spectrometer with a resolution of 0.158A at D2. 11 refs
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Existence of undiscovered Uranian satellites
Structure in the Uranian ring system as observed in recent occultations may contain indirect evidence for the existence of undiscovered satellites. Using the Alfven and Arrhenius (1975, 1976) scenario for the formation of planetary systems, the orbital radii of up to nine hypothetical satellites interior to Miranda are computed. These calculations should provide interesting comparisons when the results from the Voyager 2 encounter with Uranus are made public. 15 refs., 1 fig., 1 tab
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
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First polymer in Comet Halley: Polyoxymethylene
The RPA2-Positive Ion Cluster Composition Analyser (PICCA) on the Giotto spacecraft detected five mass peaks with regular spacing of about 15 amu up to about 120 amu. Starting at about 45 amu, the peaks decrease in intensity with increasing mass. Within their half-width they are in good agreement with dissociation products of formaldehyde polymer (POM). We suggest a production sequence in which cosmic radiation formed POM from water and carbon monoxide on grains that were aggregated into cometisimals. Other polymers, possibly containing CN, may also exist. Observations suggest that at least some of the H-, C-, and O-containing dust particles detected by the Particle Impact Analyser (PIA) on board of the Giotto spacecraft and its equivalent (PUMA) on the Vega 1 and 2 spacecraft contain POM. The properties of POM are consistent with many of the unexpected observations in the coma. 22 refs., 4 figs., 1 tab
Neutral sodium tails in comets
In the last few years, after a new type of tail composed only of neutral sodium atoms had been discovered in Comet Hale-Bopp, much attention has been given to the release and distribution of sodium in comets. It was not the first time that a sodium tail had been observed in a comet, but the amount af data collected from Comet Hale-Bopp in general and on the sodium emissions in particular is much larger than for any other comet. Several authors tried to identify the sources of the sodium and explain its distribution. However, no clear solution how neutram sodium atoms can be correlated with gas and dust particles has resulted. The presende of a sodium tail superimposed on the dust tail suggests a strong relation of sodium with the dust emitted by the nucleus. Furthermore, analysis of the dust data in the coma and tail provides important insights in the chemical and physical processes relevant to the dust particles. In order to investigate whether release of sodium atoms from a Na-bearing molecule may form an distributed source, we have calculated photodissociation rate coefficients (inverse lifetimes) and excess energies for several Na-bearing molecules. The calculations are based on measured cross sections near the dissociation threshold combined with cross sections at short wavelengths from the separated atom approximation for several Na-bearing molecules in the solar radiation field. Sodium can be used as a tracer of mechanisms working on other elements that are more difficult to observe. Wide interest in the sodium emissions stimulated reanalysis of wide-field images of Comet Hyakutake. It was found that this comet also had a nuetral sodium atom tail, showing that Na tails might be more common than previously thought. Our preliminary conclusion is that the sodium in the fast, narrow tail may also originate from fragmenting dust in the inner coma
The morphology and surface processes of Comet 19/P Borrelly
The flyby of the nucleus of the Comet 19P/Borrelly by the Deep Space 1 spacecraft produced the best views to date of the surface of these interesting objects. It transformed Borrelly from an astronomical object shrouded in coma of gas and dust into a geological object with complex surface processes and a rich history of erosion and landform evolution. Based on analysis of the highest resolution images, stereo images, photometry, and albedo we have mapped four major morphological units and four terrain features. The morphological units are named dark spots, mottled terrain, mesas, and smooth terrain. The features are named ridges, troughs, pits, and hills. In strong contrast to asteroids, unambiguous impact craters were not observed on Borrelly's surface. Because of the relatively short period of this comet, surface erosion by volatile sublimation is, in geologic terms, a very active process. The formation and the morphologies of units and features appear to be driven by differential rates of sublimation erosion. Erosional rates across the comet are probably controlled by solar energy input and the location of the subsolar point during perihelion. Differences in energy input may produce different varieties of sublimation erosional landforms. The terrains on Borrelly suggest that solar energy input could map directly into erosional processes and landforms
Imaging Borrelly
The nucleus, coma, and dust jets of short-period Comet 19P/Borrelly were imaged from the Deep Space 1 spacecraft during its close flyby in September 2001. A prominent jet dominated the near-nucleus coma and emanated roughly normal to the long axis of nucleus from a broad central cavity. We show it to have remained fixed in position for more than 34 hr, much longer than the 26-hr rotation period. This confirms earlier suggestions that it is co-aligned with the rotation axis. From a combination of fitting the nucleus light curve from approach images and the nucleus' orientation from stereo images at encounter, we conclude that the sense of rotation is right-handed around the main jet vector. The inferred rotation pole is approximately perpendicular to the long axis of the nucleus, consistent with a simple rotational state. Lacking an existing IAU comet-specific convention but applying a convention provisionally adopted for asteroids, we label this the north pole. This places the sub-solar latitude at ~60° N at the time of the perihelion with the north pole in constant sunlight and thus receiving maximum average insolation
Observations of comet 19P/Borrelly from the Miniature Integrated Camera and Spectrometer (MICAS) aboard Deep Space 1
Images from the DS1 MICAS CCD camera reveal in three dimensions, the complex characteristics of Borrelly's nucleus, coma, and jets. The images acquired during the last 2 hours of the approach, as the nucleus became resolved and grew to roughly 150 pixels in length, provide stereo coverage of both the nucleus and inner coma over a wide range of phase angle and exposure time. The principal structure in the coma is a sunward-pointed collimated jet that is also visible in ground-based images. This jet is canted about 30 degrees off the sun line and appears to be roughly aligned with the local vertical at the surface from where it originates. Long-exposure images reveal details of the structure of the inner coma. They show the jet, visible at long range, to be composed of at least three discrete components whose locations evidently correspond to specific surface features. The elongated nucleus exhibits topographically distinct terrains and strong albedo variegations (of at least a factor of 2). The jets emanate from within the brighter smoother rolling plains. A consistent model is that the main jets are co-aligned with the rotation axis of the nucleus and issue from regions on the plains that are currently in constant sunlight. The other major terrain is a rough unit that is darker than the average, includes even darker isolated spots, and appears as a jumbled topography. Other surface features include parallel ridges, crater-like depressions, numerous narrow dark fracture-like features, and areas of mottled albedo. However no small fresh impact craters are evident attesting to a geologically young, actively evolving surface
9969 Braille: Deep Space 1 infrared spectroscopy, geometric albedo, and classification
Spectra of Asteroid 9969 Braille in the 1.25–2.6 µm region returned by the Deep Space 1 (DS1) Mission show a ~10% absorption band centered at 2 µm, and a reflectance peak at 1.6 µm. Analysis of these features suggest that the composition of Braille is roughly equal parts pyroxene and olivine. Its spectrum between 0.4 and 2.5 µm suggests that it is most closely related to the Q taxonomic type of asteroid. The spectrum also closely matches that of the ordinary chondrites, the most common type of terrestrial meteorite. The geometric albedo of Braille is unusually high (pv=0.34), which is also consistent with its placement within the rarer classes of stony asteroids, and which suggests it has a relatively fresh, unweathered surface, perhaps due to a recent collision