486 research outputs found
How to adapt broad-band gravitational-wave searches for r-modes
Up to now there has been no search for gravitational waves from the r-modes
of neutron stars in spite of the theoretical interest in the subject. Several
oddities of r-modes must be addressed to obtain an observational result: The
gravitational radiation field is dominated by the mass current
(gravitomagnetic) quadrupole rather than the usual mass quadrupole, and the
consequent difference in polarization affects detection statistics and
parameter estimation. To astrophysically interpret a detection or upper limit
it is necessary to convert the wave amplitude to an r-mode amplitude. Also, it
is helpful to know indirect limits on gravitational-wave emission to gauge the
interest of various searches. Here I address these issues, thereby providing
the ingredients to adapt broad-band searches for continuous gravitational waves
to obtain r-mode results. I also show that searches of existing data can
already have interesting sensitivities to r-modes.Comment: 8 pages, no figure
Precession of collimated outflows from young stellar objects
We consider several protostellar systems where either a precessing jet or at
least two misaligned jets have been observed. We assume that the precession of
jets is caused by tidal interactions in noncoplanar binary systems. For Cep E,
V1331 Cyg and RNO 15-FIR the inferred orbital separations and disk radii are in
the range 4-160 AU and 1-80 AU, respectively, consistent with those expected
for pre-main sequence stars. Furthermore, we assume or use the fact that the
source of misaligned outflows is a binary, and evaluate the lengthscale over
which the jets should precess as a result of tidal interactions. For T Tau, HH1
VLA 1/2 and HH 24 SVS63, it may be possible to detect a bending of the jets
rather than 'wiggling'. In HH 111 IRS and L1551 IRS5, 'wiggling' may be
detected on the current observed scale. Our results are consistent with the
existence of noncoplanar binary systems in which tidal interactions induce jets
to precess.Comment: 5 pages (including 1 figure), LaTeX, uses emulateapj.sty, to be
published in ApJ Letters, also available at
http://www.ucolick.org/~ct/home.html and
http://www.tls-tautenburg.de/research/research.htm
Corotation Resonance and Diskoseismology Modes of Black Hole Accretion Disks
We demonstrate that the corotation resonance affects only some
non-axisymmetric g-mode oscillations of thin accretion disks, since it is
located within their capture zones. Using a more general (weaker radial WKB
approximation) formulation of the governing equations, such g-modes, treated as
perfect fluid perturbations, are shown to formally diverge at the position of
the corotation resonance. A small amount of viscosity adds a small imaginary
part to the eigenfrequency which has been shown to induce a secular instability
(mode growth) if it acts hydrodynamically. The g-mode corotation resonance
divergence disappears, but the mode magnitude can remain largest at the place
of the corotation resonance. For the known g-modes with moderate values of the
radial mode number and axial mode number (and any vertical mode number), the
corotation resonance lies well outside their trapping region (and inside the
innermost stable circular orbit), so the observationally relevant modes are
unaffected by the resonance. The axisymmetric g-mode has been seen by Reynolds
& Miller in a recent inviscid hydrodynamic accretion disk global numerical
simulation. We also point out that the g-mode eigenfrequencies are
approximately proportional to m for axial mode numbers |m|>0.Comment: 16 pages, no figures. Submitted to The Astrophysical Journa
Discs and Planetary Formation
The formation, structure and evolution of protoplanetary discs is considered.
The formation of giant planets within the environment of these models is also
discussed.Comment: 22 pages, LaTeX (including 6 figures), uses paspconf.sty, epsf.sty
and rotate.sty, to be published in Proceedings of the EC Summer School on
'Astrophysical Discs', eds J. A. Sellwood and J. Goodman, ASP Conf. Serie
On the orbital evolution and growth of protoplanets embedded in a gaseous disc
We present a new computation of the linear tidal interaction of a
protoplanetary core with a thin gaseous disc in which it is fully embedded. For
the first time a discussion of the orbital evolution of cores with eccentricity
(e) significantly larger than the gas-disc scale height to radius ratio (H/r)
is given. We find that the direction of orbital migration reverses for
e>1.1H/r. This occurs as a result of the orbital crossing of resonances in the
disc that do not overlap the orbit when the eccentricity is very small. Simple
expressions giving approximate fits to the eccentricity damping rate and the
orbital migration rate are presented. We go on to calculate the rate of
increase of the mean eccentricity for a system of protoplanetary cores due to
dynamical relaxation. By equating the eccentricity damping time-scale with the
dynamical relaxation time-scale we deduce that an equilibrium between
eccentricity damping and excitation through scattering is attained on a 10^3 to
10^4 yr time-scale, at 1au. The equilibrium thickness of the protoplanet
distribution is such that it is generally well confined within the gas disc. By
use of a three dimensional N-body code we simulate the evolution of a system of
protoplanetary cores, incorporating our eccentricity damping and migration
rates. Assuming that collisions lead to agglomeration, we find that the
vertical confinement of the protoplanet distribution permits cores to build up
from 0.1 to 1 earth mass in only ~10^4 yr, within 1au. The time-scale required
to achieve this is comparable to the migration time-scale. We deduce that it is
not possible to build up a massive enough core to form a gas giant planet
before orbital migration ultimately results in the preferential delivery of all
such bodies to the neighbourhood of the central star. [Abridged]Comment: Latex in MNRAS style, 13 pages with 6 figures, also available from
http://www.maths.qmw.ac.uk/~jdl
Tidally-induced warps in protostellar discs
We review results on the dynamics of warped gaseous discs. We consider tidal
perturbation of a Keplerian disc by a companion star orbiting in a plane
inclined to the disc. The perturbation induces the precession of the disc, and
thus of any jet it could drive. In some conditions the precession rate is
uniform, and as a result the disc settles into a warp mode. The tidal torque
also leads to the truncation of the disc, to the evolution of the inclination
angle (not necessarily towards alignment of the disc and orbital planes) and to
a transport of angular momentum in the disc. We note that the spectral energy
distribution of such a warped disc is different from that of a flat disc. We
conclude by listing observational effects of warps in protostellar discs.Comment: 10 pages, LaTeX (including 1 figure), uses paspconf.sty and epsf.sty,
to be published in Proceedings of the EC Summer School on 'Astrophysical
Discs', eds J. A. Sellwood and J. Goodman, ASP Conf. Serie
The Local Instability of Steady Astrophysical Flows with non Circular Streamlines with Application to Differentially Rotating Disks with Free Eccentricity
We carry out a general study of the stability of astrophysical flows that
appear steady in a uniformly rotating frame. Such a flow might correspond to a
stellar pulsation mode or an accretion disk with a free global distortion
giving it finite eccentricity. We consider perturbations arbitrarily localized
in the neighbourhood of unperturbed fluid streamlines.When conditions do not
vary around them, perturbations take the form of oscillatory inertial or
gravity modes. However, when conditions do vary so that a circulating fluid
element is subject to periodic variations, parametric instability may occur.
For nearly circular streamlines, the dense spectra associated with inertial or
gravity modes ensure that resonance conditions can always be satisfied when
twice the period of circulation round a streamline falls within. We apply our
formalism to a differentially rotating disk for which the streamlines are
Keplerian ellipses, with free eccentricity up to 0.7, which do not precess in
an inertial frame. We show that for small the instability involves
parametric excitation of two modes with azimuthal mode number differing by
unity in magnitude which have a period of twice the period of variation as
viewed from a circulating unperturbed fluid element. Instability persists over
a widening range of wave numbers with increasing growth rates for larger
eccentricities. The nonlinear outcome is studied in a follow up paper which
indicates development of small scale subsonic turbulence.Comment: Accepted for publication in Astronomy and Astrophysic
On the tilting of protostellar disks by resonant tidal effects
We consider the dynamics of a protostellar disk surrounding a star in a
circular-orbit binary system. Our aim is to determine whether, if the disk is
initially tilted with respect to the plane of the binary orbit, the inclination
of the system will increase or decrease with time. The problem is formulated in
the binary frame in which the tidal potential of the companion star is static.
We consider a steady, flat disk that is aligned with the binary plane and
investigate its linear stability with respect to tilting or warping
perturbations. The dynamics is controlled by the competing effects of the m=0
and m=2 azimuthal Fourier components of the tidal potential. In the presence of
dissipation, the m=0 component causes alignment of the system, while the m=2
component has the opposite tendency. We find that disks that are sufficiently
large, in particular those that extend to their tidal truncation radii, are
generally stable and will therefore tend to alignment with the binary plane on
a time-scale comparable to that found in previous studies. However, the effect
of the m=2 component is enhanced in the vicinity of resonances where the outer
radius of the disk is such that the natural frequency of a global bending mode
of the disk is equal to twice the binary orbital frequency. Under such
circumstances, the disk can be unstable to tilting and acquire a warped shape,
even in the absence of dissipation. The outer radius corresponding to the
primary resonance is always smaller than the tidal truncation radius. For disks
smaller than the primary resonance, the m=2 component may be able to cause a
very slow growth of inclination through the effect of a near resonance that
occurs close to the disk center. We discuss these results in the light of
recent observations of protostellar disks in binary systems.Comment: 21 pages, 7 figures, to be published in the Astrophysical Journa
Resonantly-forced Eccentric Ringlets: Relationships between Surface Density, Resonance location, Eccentricity and Eccentricity-gradient
We use a simple model of the dynamics of a narrow-eccentric ring, to put some
constraints on some of the observable properties of the real systems.In this
work we concentrate on the case of the `Titan ringlet of Saturn'.Our approach
is fluid-like, since our description is based on normal modes of oscillation
rather than in individual particle orbits. Thus, the rigid precession of the
ring is described as a global mode, which originates from a standing wave
superposed on an axisymmetric background. An integral balance condition for the
maintenance of the standing-wave can be set up, in which the differential
precession induced by the oblateness of the central planet must cancel the
contributions of self-gravity, the resonant satellite forcing and collisional
effects. We expect that in nearly-circular narrow rings dominated by
self-gravity, the eccentricity varies linearly across the ring. Thus, we take a
first order expansion and we derive two integral relationships from the
rigid-precession condition. These relate the surface density of the ring with
the eccentricity at the center, the eccentricity gradient and the location of
the secular resonance. These relationships are applied to the Titan ringlet of
Saturn, which has a secular resonance with the satellite Titan in which the
ring precession period is close to Titan's orbital period. In this case, we
estimate the mean surface density and the location of the secular resonance.Comment: Accepted for publication in Celestial Mechanics and Dynamical
Astronom
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