118 research outputs found
Tidal Excitation of Modes in Binary Systems with Applications to Binary Pulsars
We consider the tidal excitation of modes in a binary system of arbitrary
eccentricity. For a circular orbit, the modes generally undergo forced
oscillation with a period equal to the orbital period (). For an eccentric
orbit, the amplitude of each tidally excited mode can be written approximately
as the sum of an oscillatory term that varies sinusoidally with the mode
frequency and a `static' term that follows the time dependence of the tidal
forcing function. The oscillatory term falls off exponentially with increasing
\b (defined as the ratio of the periastron passage time to the mode period),
whereas the `static' term is independent of \b. For small \b modes (\b
\approx 1), the two terms are comparable, and the magnitude of the mode
amplitude is nearly constant over the orbit. For large \b modes (\b \gta a
few), the oscillatory term is very small compared to the `static' term, in
which case the mode amplitude, like the tidal force, varies as the distance
cubed. For main sequence stars, , , and low order -modes generally
have large \b and hence small amplitudes of oscillation. High overtone
-modes, however, have small overlap with the tidal forcing function. Thus,
we expect an intermediate overtone -mode with \b \sim 1 to have the
largest oscillation amplitude. The dependence on mode damping and the stellar
rotation rate is considered, as well as the effects of orbital evolution. We
apply our work to the two binary pulsar system: PSR J0045-7319 and PSR
B1259-63.Comment: 28 pages of uuencoded compressed postscript. 9 postscript figures
available by anonymous ftp from ftp://brmha.mit.edu/ To be published in ApJ
Angular momentum transport by gravity waves and its effect on the rotation of the solar interior
We calculate the excitation of low frequency gravity waves by turbulent
convection in the sun and the effect of the angular momentum carried by these
waves on the rotation profile of the sun's radiative interior. We find that the
gravity waves generated by convection in the sun provide a very efficient means
of coupling the rotation in the radiative interior to that of the convection
zone. In a differentially rotating star, waves of different azimuthal number
have their frequencies in the local rest frame of the star Doppler shifted by
different amounts. This leads to a difference in their local dissipation rate
and hence a redistribution of angular momentum in the star. We find that the
time scale for establishing uniform rotation throughout much of the radiative
interior of the sun is years, which provides a possible explanation
for the helioseismic observations that the solar interior is rotating as a
solid body.Comment: 10 pages, tex, 3 figures. To appear in ApJ lette
Differential rotation enhanced dissipation of tides in the PSR J0045-7319 Binary
Recent observations of PSR J0045-7319, a radio pulsar in a close eccentric
orbit with a massive B-star companion, indicate that the system's orbital
period is decreasing on a timescale of years, which is
much shorter than the timescale of 10^9 years given by the standard
theory of tidal dissipation in radiative stars. Observations also provide
strong evidence that the B-star is rotating rapidly, perhaps at nearly its
break up speed. We show that the dissipation of the dynamical tide in a star
rotating in the same direction as the orbital motion of its companion (prograde
rotation) with a speed greater than the orbital angular speed of the star at
periastron results in an increase in the orbital period of the binary system
with time. Thus, since the observed time derivative of the orbital period is
large and negative, the B-star in the PSR J0045-7319 binary must have
retrograde rotation if tidal effects are to account for the orbital decay. We
also show that the time scale for the synchronization of the B-star's spin with
the orbital angular speed of the star at periastron is comparable to the
orbital evolution time. From the work of Goldreich and Nicholson (1989) we
therefore expect that the B-star should be rotating differentially, with the
outer layers rotating more slowly than the interior. We show that the
dissipation of the dynamical tide in such a differentially rotating B-star is
enhanced by almost three orders of magnitude leading to an orbital evolution
time for the PSR J0045-7319 Binary that is consistent with the observations.Comment: 8 pages, tex. Submitted to Ap
Wind-Fed GRMHD Simulations of Sagittarius A*: Tilt and Alignment of Jets and Accretion Discs, Electron Thermodynamics, and Multi-Scale Modeling of the Rotation Measure
Wind-fed models offer a unique way to form predictive models of the accretion
flow surrounding Sagittarius A*. We present 3D, wind-fed MHD and GRMHD
simulations spanning the entire dynamic range of accretion from parsec scales
to the event horizon. We expand on previous work by including nonzero black
hole spin and dynamically evolved electron thermodynamics. Initial conditions
for these simulations are generated from simulations of the observed Wolf-Rayet
stellar winds in the Galactic Centre. The resulting flow tends to be highly
magnetized () with an density profile
independent of the strength of magnetic fields in the winds. Our simulations
reach the MAD state for some, but not all cases. In tilted flows, SANE jets
tend to align with the angular momentum of the gas at large scales, even if
that direction is perpendicular to the black hole spin axis. Conversely, MAD
jets tend to align with the black hole spin axis. The gas angular momentum
shows similar behavior: SANE flows tend to only partially align while MAD flows
tend to fully align. With a limited number of dynamical free parameters, our
models can produce accretion rates, 230 GHz flux, and unresolved linear
polarization fractions roughly consistent with observations for several choices
of electron heating fraction. Absent another source of large-scale magnetic
field, winds with a higher degree of magnetization (e.g., where the magnetic
pressure is 1/100 of the ram pressure in the winds) may be required to get a
sufficiently large RM with consistent sign.Comment: Accepted by MNRAS. Animations for several figures in the paper are
available at
https://www.youtube.com/playlist?list=PL3pLmTeUPcqSd4jVBnRubYQpa-Dma25i
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