arXiv subjects

Keiya Murashima

Publications and source records attributed to Keiya Murashima.

3 recordsLinked to original sources

arXiv2026

Dependence on the Equation of State in SPH Simulations of Proto-Uranian Disk Formation from a Giant Impact

The $98^\circ$ obliquity of Uranus is widely attributed to a giant impact that ejected material and formed a debris disk, which subsequently coalesced into its regular satellites. Previous Smoothed Particle Hydrodynamics (SPH) studies have yielded inconsistent disk compositions, a discrepancy often linked to the variety of numerical and physical modeling assumptions. We address this by presenting SPH simulations that systematically test three distinct EOS models alongside two SPH schemes (standard SPH, and the enhanced density-independent SPH). We utilized a $3M_{\oplus}$ impactor and explored a range of impact parameters which are capable of reproducing Uranus's current spin state. Our primary finding is that for impacts capable of reproducing Uranus's current rotation, the choice of EOS or SPH scheme barely affects macroscopic features such as the post-impact rotation period, disk mass, or disk size; these properties are primarily controlled by the impact's angular momentum. In contrast, the disk's rock fraction is highly EOS-dependent. Our results clarify that while disk mass and size are robust outcomes, the final disk composition is highly model-dependent. Therefore, accurate EOS modeling, integrated with detailed disk evolution studies, is essential to definitively validate the giant impact scenario for Uranus.

astro-ph.EP↗

arXiv2024

Modifications of SPH towards three-dimensional simulations of an icy moon with internal ocean

There are some traces of the existence of internal ocean in some icy moons, such as the vapor plumes of Europa and Enceladus. This implies a region of liquid water beneath the surface ice shell. Since liquid water would be essential for the origin of life, it is important to understand the development of these internal oceans, particularly their temperature distribution and evolution. The balance between tidal heating and radiative cooling is believed to sustain liquid water beneath an icy moon's surface. We aim to simulate the tidal heating of an internal ocean in an icy moon using 3-dimensional numerical fluid calculations with the Smoothed Particle Hydrodynamics (SPH) method. We incorporated viscosity and thermal conduction terms into the governing equations of SPH. However, we encountered two issues while calculating rigid body rotation using SPH with a viscous term: (1) conventional viscosity formulations generated unphysical forces that hindered rotation, and (2) there was artificial internal energy partitioning within the layered structure, which was due to the standard SPH formulations. To address the first issue, we modified the viscosity formulation.For the second, we adopted Density Independent SPH (DISPH) developed in previous studies to improve behavior at discontinuous surfaces. Additionally, we implemented radiative cooling using an algorithm to define fluid surfaces via the particle method. We also introduced an equation of state accounting for phase transitions. With these modifications, we have refined the SPH method to encompass all necessary physical processes for simulating the evolution of icy moons with internal oceans.

astro-ph.EP↗

arXiv2019

Impact of Stellar Superflares on Planetary Habitability

High-energy radiation caused by exoplanetary space weather events from planet-hosting stars can play a crucial role in conditions promoting or destroying habitability in addition to the conventional factors. In this paper, we present the first quantitative impact evaluation system of stellar flares on the habitability factors with an emphasis on the impact of Stellar Proton Events. We derive the maximum flare energy from stellar starspot sizes and examine the impacts of flare associated ionizing radiation on CO$_2$, H$_2$, N$_2$+O$_2$ --rich atmospheres of a number of well-characterized terrestrial type exoplanets. Our simulations based on the Particle and Heavy Ion Transport code System [PHITS] suggest that the estimated ground level dose for each planet in the case of terrestrial-level atmospheric pressure (1 bar) for each exoplanet does not exceed the critical dose for complex (multi-cellular) life to persist, even for the planetary surface of Proxima Centauri b, Ross-128 b and TRAPPIST-1 e. However, when we take into account the effects of the possible maximum flares from those host stars, the estimated dose reaches fatal levels at the terrestrial lowest atmospheric depth on TRAPPIST-1 e and Ross-128 b. Large fluxes of coronal XUV radiation from active stars induces high atmospheric escape rates from close-in exoplanets suggesting that the atmospheric depth can be substantially smaller than that on the Earth. In a scenario with the atmospheric thickness of 1/10 of Earth's, the radiation dose from close-in planets including Proxima Centauri b and TRAPPIST-1 e reach near fatal dose levels with annual frequency of flare occurrence from their hoststars.

astro-ph.SR↗