On the origin of the Kreutz family of sungrazing comets

ABSTRACT We evaluate numerically three different models for the parent comet of the Kreutz family of sungrazers: (i) A Centaur on a highly inclined or retrograde orbit that diffuse to the inner planetary region where it became a sungrazer (Model 1). (ii) A parent comet injected from the Oort cloud straight into a near-parabolic, sungrazing orbit. Near perihelion the comet was disrupted by tidal forces from the Sun giving rise to a myriad of fragments that created the Kreutz family (Model 2). (iii) A two-step process by which an Oort cloud comet is first injected in a non-sungrazing, Earth-crossing orbit where its semimajor axis decreases from typical Oort cloud values (a ∼ 104 au) to around 102 au, and then it evolves to a sungrazing orbit by the Lidov–Kozai mechanism (Model 3). Model 1 fails to produce sungrazers of the Kreutz type. Model 2 produces some Kreutz sungrazers and has the appeal of being the most straightforward. Yet the impulses received by the fragments originated in the catastrophic disruption of the parent comet will tend to acquire a wide range of orbital energies or periods (from short-period to long-period orbits) that is in contradiction with the observations. Model 3 seems to be the most promising one since it leads to the generation of some sungrazers of the Kreutz type and, particularly, it reproduces the clustering of the argument of perihelion ω of the observed Kreutz family members around 60°–90°, as a natural consequence of the action of the Lidov–Kozai mechanism.

Probing the solar corona magnetic field with sungrazing comets

<p>Space telescopes of the SoHO, STEREO and SDO missions have occasionally acquired observations of comets, providing an interesting opportunity to investigate the structure and dynamics of the heliospheric plasma.  Cometary plasma tails exhibit a wave-like motion, which is believed to be a response to the physical conditions of the local interplanetary medium. Furthermore, sungrazing comets diving in the solar atmosphere provide us with an unprecedented way to diagnose the coronal plasma at distances which are unaccessible from the current spacecraft. Here, we present observations of Comet Lovejoy C/2011 W3 from SDO/AIA, which was seen to cross the EUV solar corona in December 2011. The cometary ions produced by the sublimation of the comet nucleus were channelled along the magnetic field lines forming some filamented structures. Such structures appear to show small amplitude kink oscillations, which are used to determine the magnitude of the coronal magnetic field by coronal seismology.</p>

Relativistic Gravitation Based on Symmetry

We present a Relativistic Newtonian Dynamics ( R N D ) for motion of objects in a gravitational field generated by a moving source. As in General Relativity ( G R ), we assume that objects move by a geodesic with respect to some metric, which is defined by the field. This metric is defined on flat lab spacetime and is derived using only symmetry, the fact that the field propagates with the speed of light, and the Newtonian limit. For a field of a single source, the influenced direction of the field at spacetime point x is defined as the direction from x to the to the position of the source at the retarded time. The metric depends only on this direction and the strength of the field at x. We show that for a static source, the R N D metric is of the same form as the Whitehead metric, and the Schwarzschild metric in Eddington–Finkelstein coordinates. Motion predicted under this model passes all classical tests of G R . Moreover, in this model, the total time for a round trip of light is as predicted by G R , but velocities of light and object and time dilation differ from the G R predictions. For example, light rays propagating toward the massive object do not slow down. The new time dilation prediction could be observed by measuring the relativistic redshift for stars near a black hole and for sungrazing comets. Terrestrial experiments to test speed of light predictions and the relativistic redshift are proposed. The R N D model is similar to Whitehead’s gravitation model for a static field, but its proposed extension to the non-static case is different. This extension uses a complex four-potential description of fields propagating with the speed of light.

Comets: extremal states and their observational manifestations

Current status and prospects of further investigations are considered for: 1.Anomaly distribution of Na-atoms emission in the cometary heads; 2. Mechanisms of X-ray generation in comets; 3. Evolution of sungrazing comets near the Sun.