Singularities in the gravitational capture of dark matter through long-range interactions

We re-examine the gravitational capture of dark matter (DM) through long-range interactions. We demonstrate that neglecting the thermal motion of target particles, which is often a good approximation for short-range capture, results in parametrically inaccurate results for long-range capture. When the particle mediating the scattering process has a mass that is small in comparison to the momentum transfer in scattering events, correctly incorporating the thermal motion of target particles results in a quadratic, rather than logarithmic, sensitivity to the mediator mass, which substantially enhances the capture rate. We quantitatively assess the impact of this finite temperature effect on the captured DM population in the Sun as a function of mediator mass. We find that capture of DM through light dark photons, as in e.g. mirror DM, can be powerfully enhanced, with self-capture attaining a geometric limit over much of parameter space. For visibly-decaying dark photons, thermal corrections are not large in the Sun, but may be important in understanding long-range DM capture in more massive bodies such as Population III stars. We additionally provide the first calculation of the long-range DM self-evaporation rate.

Gravity assists gravitational scattering and the perturbation rings in the Solar system

One of the types of gravitational scattering in the Solar system within the framework of the model of the restricted three-body problem (R3BP) is gravity assist maneuvers of the “particles of insignificant mass” [1] (spacecraft, asteroids, comets, etc.). For their description, a physical analogy with the beam scattering of charged α particles in a Coulomb field is useful. However, unlike the scattering of charged particles, there are external restrictions for the possibility of gravity assists executing related from the restricted size of planet’s sphere of influence. At the same time, internal restrictions for the gravity assists performance estimated by the effective radii of planets are known from the literature on R3BP [2] (gravitational capture by the planet, falling into it). They depend from the particle asymptotic velocity relative the planet. For obvious reasons, their influence cuts off the possibility of effective gravity assists performance [3]. In this work the generalized estimates of the sizes of the near-planetary regions (“perturbation rings”), falling into which is a necessary condition for the implementation of gravity assists, are presented. The detailed analysis shows that Neptune and Saturn have the characteristic “perturbation rings” of the largest sizes in the Solar system, and Jupiter occupies only the fourth place in this checklist.

The low-velocity encounter of asteroid 2006 RH120 with the Earth

Исследовано низкоскоростное сближение астероида 2006 RH120 с Землей в 2006 г. Это первый наблюдаемый астероид с длительным временным гравитационным захватом в сферу Хилла Земли. Выполнено моделирование движения объектов из области низкоскоростных сближений малых тел с Землей. Обнаружены объекты с временными спутниковыми захватами в смысле Эверхарта. Найден объект, подобный астероиду 2006 RH120. The low-velocity encounter of the asteroid 2006 RH120 with the Earth in 2006 has been investigated. This is the first observed asteroid with a long-term gravitational capture into the Hill sphere of the Earth. The modeling of the motion of objects from the area of low-velocity approaches of small bodies with the Earth is carried out. Objects with temporary satellite captures in the sense of Everhart have been detected. An object similar to the asteroid 2006 RH120 has been found.

Gravitational Capture Cross-Section of Particles by Schwarzschild-Tangherlini Black Holes

We study the capture cross-section of massless (photon) and massive test particles by the Schwarzschild–Tangherlini black hole, which is a solution of pure general relativity in higher dimensional spacetime with R×SD−2 topology. It is shown that an extra dimension weakens the gravitational attraction of a black hole, and consequently, radii of all the characteristic circular orbits, such as the radius of a photonsphere decrease in the higher dimensions. Furthermore, it is shown that in higher dimensions, there are no stable and bounded circular orbits. The critical impact parameters and capture cross-sections of photons and massive particles are calculated for several higher dimensions and it is shown that they also decrease with increasing dimension. Moreover, we calculate the capture cross-section of relativistic and non-relativistic test particles in the higher dimensions..

MIKE High Resolution Spectroscopy of Raman-scattered O VI and C II Lines in the Symbiotic Nova RR Telescopii

We present a high-resolution optical spectrum of the symbiotic nova RR Tel obtained with MIKE at Magellan-Clay telescope. RR Tel is a wide binary system of a hot white dwarf and a Mira with an orbital period of a few decades, where the white dwarf is accreting through gravitational capture of some fraction of material shed by the Mira. We find broad emission features at 6825, 7082, 7023, and 7053 Å, which are formed through Raman scattering of far-UV O VI ⋋⋋ 1032 and 1038 Å, C II ⋋⋋ 1036 and 1037 Å with atomic hydrogen. Raman O VI features exhibit clear double-peak profiles indicative of an accretion flow with a characteristic speed ∼ 30 km s−1, whereas the Raman C II features have a single Gaussian profile. We perform a profile analysis of the Raman O VI by assuming that O VI emission traces the accretion flow around the white dwarf with a fiducial scale of 1 AU. A comparison of the restored fluxes of C II ⋋⋋ 1036 and 1037 from Raman C II features with the observed C II ⋋ 1335 multiplet is consistent with the distance of RR Tel ∼ 2.6 kpc based on interstellar extinction of C II.

Orbits

Astronomy and recording of the motion of celestial objects are ancient practices, but what is now called astrophysics arguably began with Newtonian gravity and the concept of orbits as dynamical phenomena. This chapter provides a modern perspective on the early problems of gravitational dynamics, from Kepler’s laws up to the virial theorem for an arbitrary number of gravitating bodies. Even a simple gravitating system with only two bodies turns out to have many interesting features, such as the gravitational capture of material by protoplanets. The study of orbital motions also includes applications to observing extrasolar planets and stars near the black hole at the centre of the Milky Way.