Astronomy

17 pages, 325 KiB

Open AccessArticle

by Pravin Kumar Dahal

Astronomy 2022, 1(3), 271-287; https://doi.org/10.3390/astronomy1030016 - 05 Dec 2022

Cited by 1 | Viewed by 1875

We present the geometric optics expansion for circularly polarized gravitational waves on a curved spacetime background, to subleading order. We call spin optics to the subleading order geometric optics expansion, which involves modifying the standard eikonal function by including a specially chosen helicity-dependent correction. We show that the techniques developed for the propagation of electromagnetic waves can also be applied to gravitational waves in the limit of spin optics. However, one needs to account for the difference in the photon and graviton helicity, which we do here. Full article

16 pages, 1366 KiB

Open AccessArticle

Creating a CLOUDY-Compatible Database with CHIANTI Version 10 Data

by Chamani M. Gunasekera, Marios Chatzikos and Gary J. Ferland

Astronomy 2022, 1(3), 255-270; https://doi.org/10.3390/astronomy1030015 - 28 Nov 2022

Viewed by 1552

Abstract

Atomic and molecular data are required to conduct the detailed calculations of microphysical processes performed by cloudy to predict the spectra of a theoretical model. cloudy now utilizes three atomic and molecular databases, one of which is CHIANTI version 7.1. CHIANTI version 10.0.1 is available, but its format has changed. cloudy is incompatible with the newer version. We have developed a script to convert the version 10.0.1 database into its version 7.1 format so that cloudy does not have to change every time there is a new CHIANTI version with an evolved format. This study outlines the steps taken by the script for this version format change. We have also found a modest number of significant changes to spectral line intensities/luminosities calculated by cloudy with the adoption of CHIANTI version 10.0.1. These changes are a result of improvements to collision strength data. Full article

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9 pages, 256 KiB

Open AccessArticle

Modified Gravity and a Space Probe–Venus Mission

by Alexander P. Yefremov

Astronomy 2022, 1(3), 246-254; https://doi.org/10.3390/astronomy1030014 - 18 Nov 2022

Viewed by 1175

Abstract

A comparison of gravitational forces and a space probe’s trajectory parameters is made for two different models of the sun’s field, expressed in Schwarzschild and isotropic coordinates. It is shown that these two representations of a single Schwarzschild solution give, in the tangent space format, different deflections from classical finite trajectories and, hence, from one other; greatly amplified by a planet’s (Venus’) gravity assist, this effect renders it possible to experimentally specify the format of the gravity law that dominates the solar system. Full article

11 pages, 994 KiB

Open AccessArticle

Energetic Neutral Atom (ENA) Imaging Simulation of the Distant Planetary Magnetosphere and ENA Emission Discussion of the Solar Wind

by Li Lu, Qinglong Yu, Shuai Jia and Yuan Chang

Astronomy 2022, 1(3), 235-245; https://doi.org/10.3390/astronomy1030013 - 03 Nov 2022

Viewed by 2107

Abstract

We doubt whether the “Energetic Neutral Atom (ENA) ribbon” signals, especially the peak ones, scanned remotely by IBEX-Hi at the lunar resonance orbit, are really from the heliopause, which involves assessing the scale of solar wind particle energy loss throughout the solar system. The ENA imaging simulation results at the Earth’s orbit show that the scale of the planetary magnetosphere with a telemetry distance of AU magnitude is too small to contribute to the IBEX-Hi ribbon. However, the simulated effective ENA differential fluxes provide a reference for the physical scale evaluation of the huge magnetic structure in the heliopause. The ENA differential flux of the “ENA emission cone” generated by the charge exchange between the solar wind ion flow and local neutral gas near the Earth’s orbit is also comparable to the measured peak of the IBEX-Hi ribbon, which may be the main ENA emission source of the ribbon’s measured peak. The 2D ENA imaging measurements at the Lagrange points proposed here can be used to investigate the ENA ribbon origination by using the energy spectral lag vs the disparity of the ENA images. Full article

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13 pages, 713 KiB

Open AccessArticle

Stability and Damping in the Disks of Massive Galaxies

by John Herbert Marr

Astronomy 2022, 1(3), 222-234; https://doi.org/10.3390/astronomy1030012 - 14 Oct 2022

Viewed by 1805

Abstract

After their initial formation, disk galaxies are observed to be rotationally stable over periods of >6 Gyr, implying that any large velocity disturbances of stars and gas clouds are damped rapidly on the timescale of their rotation. However, it is also known that despite this damping, there must be a degree of random local motion to stabilize the orbits against degenerate collapse. A mechanism for such damping is proposed by a combination of inter-stellar gravitational interactions, and interactions with the Oort clouds and exo-Oort objects associated with each star. These mechanisms may produce rapid damping of large perturbations within a time period that is short on the scale of observational look-back time, but long on the scale of the disk rotational period for stars with small perturbations. This mechanism may also account for the locally observed mean perturbations in the Milky Way of 8–15 km/s for younger stars and 20–30 km/s for older stars. Full article

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96 pages, 971 KiB

Open AccessArticle

K-Essence Lagrangians of Polytropic and Logotropic Unified Dark Matter and Dark Energy Models

by Pierre-Henri Chavanis

Astronomy 2022, 1(3), 126-221; https://doi.org/10.3390/astronomy1030011 - 28 Sep 2022

Cited by 5 | Viewed by 2267

Abstract

We determine the k-essence Lagrangian of a relativistic barotropic fluid. The equation of state of the fluid can be specified in different manners depending on whether the pressure is expressed in terms of the energy density (model I), the rest-mass density (model II), or the pseudo rest-mass density for a complex scalar field in the Thomas-Fermi approximation (model III). In the nonrelativistic limit, these three formulations coincide. In the relativistic regime, they lead to different models that we study exhaustively. We provide general results valid for an arbitrary equation of state and show how the different models are connected to each other. For illustration, we specifically consider polytropic and logotropic dark fluids that have been proposed as unified dark matter and dark energy models. We recover the Born-Infeld action of the Chaplygin gas in models I and III and obtain the explicit expression of the reduced action of the logotropic dark fluid in models II and III. We also derive the two-fluid representation of the Chaplygin and logotropic models. Our general formalism can be applied to many other situations such as Bose-Einstein condensates with a

{| φ |}^{4}

(or more general) self-interaction, dark matter superfluids, and mixed models. Full article

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Astronomy, Volume 1, Issue 3 (December 2022) – 6 articles

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