43 research outputs found
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
Search for long-lived particles decaying to e ± μ ∓ ν
Abstract: Long-lived particles decaying to e±μ∓ν, with masses between 7 and 50GeV/c2 and lifetimes between 2 and 50ps, are searched for by looking at displaced vertices containing electrons and muons of opposite charges. The search is performed using 5.4fb-1 of pp collisions collected with the LHCb detector at a centre-of-mass energy of s=13TeV. Three mechanisms of production of long-lived particles are considered: the direct pair production from quark interactions, the pair production from the decay of a Standard-Model-like Higgs boson with a mass of 125GeV/c2, and the charged current production from an on-shell W boson with an additional lepton. No evidence of these long-lived states is obtained and upper limits on the production cross-section times branching fraction are set on the different production modes