433 research outputs found
Evaluation of the microseismic motion at the KAGRA site based on the ocean wave data
The microseismic motion, which is the ambient ground vibration caused by
ocean waves, affects ground-based gravitational detectors. In this study, we
characterized the properties of the microseismic motion at the KAGRA site and
the ocean waves at 13 coasts of Japan, such as the seasonal variation and the
correlation between them. As a result, it almost succeeded to explain the
microseismic motion at the KAGRA site by the principal components of the ocean
wave data. One possible application of this study is the microseismic forecast
and its example is also shown
Accretion-powered Stellar Winds as a Solution to the Stellar Angular Momentum Problem
We compare the angular momentum extracted by a wind from a pre-main-sequence
star to the torques arising from the interaction between the star and its
Keplerian accretion disk. We find that the wind alone can counteract the
spin-up torque from mass accretion, solving the mystery of why accreting
pre-main-sequence stars are observed to spin at less than 10% of break-up
speed, provided that the mass outflow rate in the stellar winds is ~10% of the
accretion rate. We suggest that such massive winds will be driven by some
fraction of the accretion power. For observationally constrained
typical parameters of classical T-Tauri stars, needs to be between a
few and a few tens of percent. In this scenario, efficient braking of the star
will terminate simultaneously with accretion, as is usually assumed to explain
the rotation velocities of stars in young clusters.Comment: Accepted by ApJ Letter
Accretion-Powered Stellar Winds II: Numerical Solutions for Stellar Wind Torques
[Abridged] In order to explain the slow rotation observed in a large fraction
of accreting pre-main-sequence stars (CTTSs), we explore the role of stellar
winds in torquing down the stars. For this mechanism to be effective, the
stellar winds need to have relatively high outflow rates, and thus would likely
be powered by the accretion process itself. Here, we use numerical
magnetohydrodynamical simulations to compute detailed 2-dimensional
(axisymmetric) stellar wind solutions, in order to determine the spin down
torque on the star. We explore a range of parameters relevant for CTTSs,
including variations in the stellar mass, radius, spin rate, surface magnetic
field strength, the mass loss rate, and wind acceleration rate. We also
consider both dipole and quadrupole magnetic field geometries.
Our simulations indicate that the stellar wind torque is of sufficient
magnitude to be important for spinning down a ``typical'' CTTS, for a mass loss
rate of yr. The winds are wide-angle,
self-collimated flows, as expected of magnetic rotator winds with moderately
fast rotation. The cases with quadrupolar field produce a much weaker torque
than for a dipole with the same surface field strength, demonstrating that
magnetic geometry plays a fundamental role in determining the torque. Cases
with varying wind acceleration rate show much smaller variations in the torque
suggesting that the details of the wind driving are less important. We use our
computed results to fit a semi-analytic formula for the effective Alfv\'en
radius in the wind, as well as the torque. This allows for considerable
predictive power, and is an improvement over existing approximations.Comment: Accepted for publication in Ap
Probing the Edge of the Solar System: Formation of an Unstable Jet-Sheet
The Voyager spacecraft is now approaching the edge of the solar system. Near
the boundary between the solar system and the interstellar medium we find that
an unstable ``jet-sheet'' forms. The jet-sheet oscillates up and down due to a
velocity shear instability. This result is due to a novel application of a
state-of-art 3D Magnetohydrodynamic (MHD) code with a highly refined grid. We
assume as a first approximation that the solar magnetic and rotation axes are
aligned. The effect of a tilt of the magnetic axis with respect to the rotation
axis remains to be seen. We include in the model self-consistently magnetic
field effects in the interaction between the solar and interstellar winds.
Previous studies of this interaction had poorer spatial resolution and did not
include the solar magnetic field. This instability can affect the entry of
energetic particles into the solar system and the intermixing of solar and
interstellar material. The same effect found here is predicted for the
interaction of rotating magnetized stars possessing supersonic winds and moving
with respect to the interstellar medium, such as O stars.Comment: 9 pages, 4 figures, accepted for publication in ApJ
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