13 research outputs found
Warps, bending and density waves excited by rotating magnetized stars: results of global 3D MHD simulations
We report results of the first global three-dimensional magnetohydrodynamic
simulations of the waves excited in an accretion disc by a rotating star with a
dipole magnetic field misaligned from the star's rotation axis (which is
aligned with the disc axis). The main results are the following: (1) If the
magnetosphere of the star corotates approximately with the inner disc, then we
observe a strong one-armed bending wave (a warp). This warp corotates with the
star and has a maximum amplitude between corotation radius and the radius of
the vertical resonance. The disc's center of mass can deviate from the
equatorial plane up to the distance of z_w\approx 0.1 r. However, the effective
height of the warp can be larger, h_w \approx 0.3 r due to the finite thickness
of the disc. Stars with a range of misalignment angles excite warps. However,
the amplitude of the warps is larger for misalignment angles between 15 and 60
degrees. (2) If the magnetosphere rotates slower, than the inner disc, then a
bending wave is excited at the disc-magnetosphere boundary, but does not form a
large-scale warp. Instead, high-frequency oscillations become strong at the
inner region of the disc. These are (a) trapped density waves which form inside
the radius where the disc angular velocity has a maximum, and (b) inner bending
waves which appear in the case of accretion through magnetic Raleigh-Taylor
instability. These two types of waves are connected with the inner disc and
their frequencies will vary with accretion rate. Bending oscillations at lower
frequencies are also excited including global oscillations of the disc. In
cases where the simulation region is small, slowly-precessing warp forms.
Simulations are applicable to young stars, cataclysmic variables, and accreting
millisecond pulsars.Comment: 26 pages, 25 figure
Accretion, Outflows, and Winds of Magnetized Stars
Many types of stars have strong magnetic fields that can dynamically
influence the flow of circumstellar matter. In stars with accretion disks, the
stellar magnetic field can truncate the inner disk and determine the paths that
matter can take to flow onto the star. These paths are different in stars with
different magnetospheres and periods of rotation. External field lines of the
magnetosphere may inflate and produce favorable conditions for outflows from
the disk-magnetosphere boundary. Outflows can be particularly strong in the
propeller regime, wherein a star rotates more rapidly than the inner disk.
Outflows may also form at the disk-magnetosphere boundary of slowly rotating
stars, if the magnetosphere is compressed by the accreting matter. In isolated,
strongly magnetized stars, the magnetic field can influence formation and/or
propagation of stellar wind outflows. Winds from low-mass, solar-type stars may
be either thermally or magnetically driven, while winds from massive, luminous
O and B type stars are radiatively driven. In all of these cases, the magnetic
field influences matter flow from the stars and determines many observational
properties. In this chapter we review recent studies of accretion, outflows,
and winds of magnetized stars with a focus on three main topics: (1) accretion
onto magnetized stars; (2) outflows from the disk-magnetosphere boundary; and
(3) winds from isolated massive magnetized stars. We show results obtained from
global magnetohydrodynamic simulations and, in a number of cases compare global
simulations with observations.Comment: 60 pages, 44 figure
MRI-driven Accretion on to Magnetized stars: Global 3D MHD Simulations of Magnetospheric and Boundary Layer Regimes
We discuss results of global 3D MHD simulations of accretion on to a rotating
magnetized star with a tilted dipole magnetic field, where the accretion is
driven by the magneto-rotational instability (MRI). The simulations show that
MRI-driven turbulence develops in the disc, and angular momentum is transported
outwards due primarily to the magnetic stress. The turbulent flow is strongly
inhomogeneous and the densest matter is in azimuthally-stretched turbulent
cells. We investigate two regimes of accretion: a magnetospheric regime and a
boundary layer (BL) regime. In the magnetospheric regime, the accretion disc is
truncated by the star's magnetic field within a few stellar radii from the
star, and matter flows to the star in funnel streams. The funnel streams
flowing towards the south and north magnetic poles but are not equal due to the
inhomogeneity of the flow. In the BL regime, matter accretes to the surface of
the star through the boundary layer. The magnetic field in the inner disc is
strongly amplified by the shear of the accretion flow, and the matter and
magnetic stresses become comparable. Accreting matter forms a belt-shaped
region on the surface of the star. The belt has inhomogeneous density
distribution which varies in time due to variable accretion rate. Results of
simulations can be applied to classical T Tauri stars, accreting brown dwarfs,
millisecond pulsars, dwarf novae cataclysmic variables, and other stars with
magnetospheres smaller than several stellar radii.Comment: 15 pages, 13 figures, accepted by MNRA
An Earth-mass planet orbiting alpha Centauri B
International audienceExoplanets down to the size of Earth have been found, but not in the habitable zone--that is, at a distance from the parent star at which water, if present, would be liquid. There are planets in the habitable zone of stars cooler than our Sun, but for reasons such as tidal locking and strong stellar activity, they are unlikely to harbour water-carbon life as we know it. The detection of a habitable Earth-mass planet orbiting a star similar to our Sun is extremely difficult, because such a signal is overwhelmed by stellar perturbations. Here we report the detection of an Earth-mass planet orbiting our neighbour star α Centauri B, a member of the closest stellar system to the Sun. The planet has an orbital period of 3.236 days and is about 0.04 astronomical units from the star (one astronomical unit is the Earth-Sun distance)