54 research outputs found
Spectral variability of classical T Tauri stars accreting in an unstable regime
Classical T Tauri stars (CTTSs) are variable in different time-scales. One
type of variability is possibly connected with the accretion of matter through
the Rayleigh-Taylor instability that occurs at the interface between an
accretion disc and a stellar magnetosphere. In this regime, matter accretes in
several temporarily formed accretion streams or `tongues' which appear in
random locations, and produce stochastic photometric and line variability. We
use the results of global three-dimensional magnetohydrodynamic simulations of
matter flows in both stable and unstable accretion regimes to calculate
time-dependent hydrogen line profiles and study their variability behaviours.
In the stable regime, some hydrogen lines (e.g. H-beta, H-gamma, H-delta,
Pa-beta and Br-gamma) show a redshifted absorption component only during a
fraction of a stellar rotation period, and its occurrence is periodic. However,
in the unstable regime, the redshifted absorption component is present rather
persistently during a whole stellar rotation cycle, and its strength varies
non-periodically. In the stable regime, an ordered accretion funnel stream
passes across the line of sight to an observer only once per stellar rotation
period while in the unstable regime, several accreting streams/tongues, which
are formed randomly, pass across the line of sight to an observer. The latter
results in the quasi-stationarity appearance of the redshifted absorption
despite the strongly unstable nature of the accretion. In the unstable regime,
multiple hot spots form on the surface of the star, producing the stochastic
light curve with several peaks per rotation period. This study suggests a CTTS
that exhibits a stochastic light curve and a stochastic line variability, with
a rather persistent redshifted absorption component, may be accreting in the
unstable accretion regime.Comment: 20 pages, 11 figures, 1 table, accepted for publication in MNRA
Jets and Disk-Winds from Pulsar Magnetospheres
We discuss axisymmetric force-free pulsar magnetospheres with magnetically
collimated jets and a disk-wind obtained by numerical solution of the pulsar
equation.
This solution represents an alternative to the quasi-spherical wind solutions
where a major part of the current flow is in a current sheet which is unstable
to magnetic field annihilation.Comment: 6 figures, accepted for publication in the Ap
Possible Quasi-Periodic Oscillations from Unstable Accretion: 3D MHD Simulations
We investigate the photometric variability of magnetized stars, particularly
neutron stars, accreting through a magnetic Rayleigh-Taylor-type instability at
the disk-magnetosphere interface, and compare it with the variability during
stable accretion, with the goal of looking for possible quasi-periodic
oscillations. The lightcurves during stable accretion show periodicity at the
star's frequency and sometimes twice that, due to the presence of two funnel
streams that produce antipodal hotspots near the magnetic poles. On the other
hand, lightcurves during unstable accretion through tongues penetrating the
magnetosphere are more chaotic due to the stochastic behaviour of the tongues,
and produce noisier power spectra. However, the power spectra do show some
signs of quasi-periodic variability. Most importantly, the rotation frequency
of the tongues and the resulting hotspots is close to the inner-disk orbital
frequency, except in the most strongly unstable cases. There is therefore a
high probability of observing QPOs at that frequency in longer simulations. In
addition, the lightcurves in the unstable regime show periodicity at the star's
rotation frequency in many of the cases investigated here, again except in the
most strongly unstable cases which lack funnel flows and the resulting
antipodal hotspots. The noisier power spectra result in the fractional rms
amplitudes of the Fourier peaks being smaller.
We also study in detail the effect of the misalignment angle between the
rotation and magnetic axes of the star on the variability, and find that at
misalignment angles , the star's period always appears in the
lightcurves.Comment: 14 pages, 16 figures, accepted by MNRAS. v2 comments: significant
revision. v3 comments: after referee report. Rewrote QPO section (4.5). v4
comments: final versio
Analytical Hot Spot Shapes and Magnetospheric Radius from 3D Simulations of Magnetospheric Accretion
We present an analytical formula for the position and shape of the spots on
the surface of accreting magnetized stars in cases where a star has a dipole
magnetic field tilted at a small misalignment angle Theta < 30 degrees about
the rotational axis, and the magnetosphere is 2.5-5 times the radius of the
star. We observed that the azimuthal position of the spots varies significantly
when the position of the inner disc varies. In contrast, the polar position of
the spots varies only slightly because of the compression of the magnetosphere.
The azimuthal width of the spots strongly varies with Theta: spots have the
shape of an arc at larger misalignment angles, and resemble a ring at very
small misalignment angles. The polar width of the spots varies only slightly
with changes in parameters. The motion of the spots in the azimuthal direction
can provide phase-shifts in accreting millisecond pulsars, and the "drift" of
the period in Classical T Tauri stars. The position and shape of the spots are
determined by three parameters: misalignment angle Theta; normalized corotation
radius, r_c/R_* and normalized magnetospheric radius, r_m/R_*, where R_* is the
stellar radius.
We also use our data to check the formula for the Alfv\'en radius, where the
main dependencies are r_m \sim (\mu^2/\dot M)^{2/7}, where \mu is the magnetic
moment of the star, and \dot M is the accretion rate. We found that the
dependence is more gradual, r_m \sim (\mu^2/\dot M)^{1/5}, which can be
explained by the compression of the magnetosphere by the disc matter and by the
non-dipole shape of the magnetic field lines of the external magnetosphere.Comment: 13 pages, 12 figures, Submitted to MNRA
Propeller outflows from an MRI disc
We present the results of axisymmetric simulations of MRI-driven accretion
onto a rapidly rotating, magnetized star accreting in the propeller regime. The
stellar magnetosphere corotates with the star, forming a centrifugal barrier at
the disc-magnetosphere boundary which inhibits matter accretion onto the star.
Instead, the disc matter accumulates at the disc-magnetosphere interface and
slowly diffuses into the inner magnetosphere where it picks up angular momentum
and is quickly ejected from the system as an outflow. Due to the interaction of
the matter with the magnetosphere, this wind is discontinuous and is launched
as discrete plasmoids. If the ejection rate is lower than the disc accretion
rate, the matter accumulates at the disc-magnetosphere boundary faster than it
can be ejected. In this case, accretion onto the star proceeds through the
episodic accretion instability in which episodes of matter accumulation are
followed by simultaneous accretion and ejection. During the accretion phase of
this instability in which matter flows onto the star in funnel streams, we
observe a corresponding rise in the outflow rate. Both the accretion and
ejection processes observed in our simulations are highly non-stationary. The
stars undergo strong spin-down due to the coupling of the stellar field with
the disc and corona and we measure the spin-down timescales of around 1 Myr for
a typical CTTS in the propeller regime.Comment: 13 pages, 10 figures, submitted to MNRA
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