44 research outputs found
Binary YORP and Evolution of Binary Asteroids
The rotation states of kilometer sized near earth asteroids are known to be
affected by the YORP effect. In a related effect, Binary YORP (BYORP) the
orbital properties of a binary asteroid evolves under a radiation effect mostly
acting on a tidally locked secondary. The BYORP effect can alter the orbital
elements in years for a primary with a
secondary at . It can either separate the binary
components or cause them to collide. In this paper we devise a simple approach
to calculate the YORP effect on asteroids and BYORP effect on binaries
including effects due to primary oblateness and the sun. We apply this to
asteroids with known shapes as well as a set of randomly generated bodies with
various degrees of smoothness. We find a strong correlation between the
strengths of an asteroid's YORP and BYORP effects. Therefore, a statistical
knowledge on one, could be used to estimate the effect of the other. We show
that the action of BYORP preferentially shrinks rather than expands the binary
orbit and that YORP preferentially slows down asteroids. This conclusion holds
for the two extremes of thermal conductivities studied in this work and
assuming the asteroid reaches a stable point, but may break down for moderate
thermal conductivity. The YORP and BYORP effects are shown to be smaller than
what could be naively expected due to near cancellation of the effects on small
scales. Taking this near cancellation into account, a simple order of magnitude
estimate of the YORP and BYORP effects as function of the sizes and smoothness
of the bodies is calculated. Finally, we provide a simple proof showing that
there is no secular effect due to absorption of radiation in BYORP.Comment: Accepted to Astronomical Journa
Rich: Open Source Hydrodynamic Simulation on a Moving Voronoi Mesh
We present here RICH, a state of the art 2D hydrodynamic code based on
Godunov's method, on an unstructured moving mesh (the acronym stands for Racah
Institute Computational Hydrodynamics). This code is largely based on the code
AREPO. It differs from AREPO in the interpolation and time advancement scheme
as well as a novel parallelization scheme based on Voronoi tessellation. Using
our code we study the pros and cons of a moving mesh (in comparison to a static
mesh). We also compare its accuracy to other codes. Specifically, we show that
our implementation of external sources and time advancement scheme is more
accurate and robust than AREPO's, when the mesh is allowed to move. We
performed a parameter study of the cell rounding mechanism (Llyod iterations)
and it effects. We find that in most cases a moving mesh gives better results
than a static mesh, but it is not universally true. In the case where matter
moves in one way, and a sound wave is traveling in the other way (such that
relative to the grid the wave is not moving) a static mesh gives better results
than a moving mesh. Moreover, we show that Voronoi based moving mesh schemes
suffer from an error, that is resolution independent, due to inconsistencies
between the flux calculation and change in the area of a cell. Our code is
publicly available as open source and designed in an object oriented, user
friendly way that facilitates incorporation of new algorithms and physical
processes
Instability of Supersonic Cold Streams Feeding Galaxies II. Nonlinear Evolution of Surface and Body Modes of Kelvin-Helmholtz Instability
As part of our long-term campaign to understand how cold streams feed massive
galaxies at high redshift, we study the Kelvin-Helmholtz instability (KHI) of a
supersonic, cold, dense gas stream as it penetrates through a hot, dilute
circumgalactic medium (CGM). A linear analysis (Paper I) showed that, for
realistic conditions, KHI may produce nonlinear perturbations to the stream
during infall. Therefore, we proceed here to study the nonlinear stage of KHI,
still limited to a two-dimensional slab with no radiative cooling or gravity.
Using analytic models and numerical simulations, we examine stream breakup,
deceleration and heating via surface modes and body modes. The relevant
parameters are the density contrast between stream and CGM (), the Mach
number of the stream velocity with respect to the CGM () and the
stream radius relative to the halo virial radius (). We
find that sufficiently thin streams disintegrate prior to reaching the central
galaxy. The condition for breakup ranges from for
to for
. However, due to the large stream
inertia, KHI has only a small effect on the stream inflow rate and a small
contribution to heating and subsequent Lyman- cooling emission.Comment: The main astrophysical results are Figure 22 and Figure 23. Final 7
pages are appendices. Accepted to MNRA