70 research outputs found
Global Alfven Wave Heating of the Magnetosphere of Young Stars
Excitation of a Global Alfven wave (GAW) is proposed as a viable mechanism to
explain plasma heating in the magnetosphere of young stars. The wave and basic
plasma parameters are compatible with the requirement that the dissipation
length of GAWs be comparable to the distance between the shocked region at the
star's surface and the truncation region in the accretion disk. A two-fluid
magnetohydrodynamic plasma model is used in the analysis. A current carrying
filament along magnetic field lines acts as a waveguide for the GAW. The
current in the filament is driven by plasma waves along the magnetic field
lines and/or by plasma crossing magnetic field lines in the truncated region of
the disk of the accreting plasma. The conversion of a small fraction of the
kinetic energy into GAW energy is sufficient to heat the plasma filament to
observed temperatures.Comment: Submitted to ApJ, aheatf.tex, 2 figure
Modeling the line variations from the wind-wind shock emissions of WR 30a
The study of Wolf-Rayet stars plays an important role in evolutionary
theories of massive stars. Among these objects, ~ 20% are known to be in binary
systems and can therefore be used for the mass determination of these stars.
Most of these systems are not spatially resolved and spectral lines can be used
to constrain the orbital parameters. However, part of the emission may
originate in the interaction zone between the stellar winds, modifying the line
profiles and thus challenging us to use different models to interpret them. In
this work, we analyzed the HeII4686\AA + CIV4658\AA blended lines of WR30a
(WO4+O5) assuming that part of the emission originate in the wind-wind
interaction zone. In fact, this line presents a quiescent base profile,
attributed to the WO wind, and a superposed excess, which varies with the
orbital phase along the 4.6 day period. Under these assumptions, we were able
to fit the excess spectral line profile and central velocity for all phases,
except for the longest wavelengths, where a spectral line with constant
velocity seems to be present. The fit parameters provide the eccentricity and
inclination of the binary orbit, from which it is possible to constrain the
stellar masses.Comment: accepted for publication in the MNRA
The role of damped Alfven waves on magnetospheric accretion models of young stars
We examine the role of Alfven wave damping in heating the plasma in the
magnetic funnels of magnetospheric accretion models of young stars. We study
four different damping mechanisms of the Alfven waves: nonlinear, turbulent,
viscous-resistive and collisional. Two different possible origins for the
Alfven waves are discussed: 1) Alfven waves generated at the surface of the
star by the shock produced by the infalling matter; and 2) Alfven waves
generated locally in the funnel by the Kelvin-Helmholtz instability. We find
that, in general, the damping lengths are smaller than the tube length. Since
thermal conduction in the tube is not efficient, Alfven waves generated only at
the star's surface cannot heat the tube to the temperatures necessary to fit
the observations. Only for very low frequency Alfven waves ~10^{-5} the ion
cyclotron frequency, is the viscous-resistive damping length greater than the
tube length. In this case, the Alfven waves produced at the surface of the star
are able to heat the whole tube. Otherwise, local production of Alfven waves is
required to explain the observations. The turbulence level is calculated for
different frequencies for optically thin and thick media. We find that
turbulent velocities varies greatly for different damping mechanisms, reaching
\~100 km s^{-1} for the collisional damping of small frequency waves.Comment: 29 pages, 12 figures, to appear in The Astrophysical Journa
Alfvenic Heating of Protostellar Accretion Disks
We investigate the effects of heating generated by damping of Alfven waves on
protostellar accretion disks. Two mechanisms of damping are investigated,
nonlinear and turbulent, which were previously studied in stellar winds
(Jatenco-Pereira & Opher 1989a, b). For the nominal values studied, f=delta
v/v_{A}=0.002 and F=varpi/Omega_{i}=0.1, where delta v, v_{A} and varpi are the
amplitude, velocity and average frequency of the Alfven wave, respectively, and
Omega_{i} is the ion cyclotron frequency, we find that viscous heating is more
important than Alfven heating for small radii. When the radius is greater than
0.5 AU, Alfvenic heating is more important than viscous heating. Thus, even for
the relatively small value of f=0.002, Alfvenic heating can be an important
source of energy for ionizing protostellar disks, enabling angular momentum
transport to occur by the Balbus-Hawley instability.Comment: 21 pages, 9 figures. Accepted for publication in Ap
Alfven waves as a driving mechanism in stellar winds
Alfven waves have been invoked as an important mechanism of particle
acceleration in stellar winds of cool stars. After their identification in the
solar wind they started to be studied in winds of stars located in different
regions of the HR diagram. We discuss here some characteristics of these waves
and we present a direct application in the acceleration of late-type stellar
winds.Comment: Accepted for publication in Advances in Space Research. Presented at
the World Space Environment Forum 2007, Egypt. 9 pages, 2 figure
Surface Alfven Wave Damping in a 3D Simulation of the Solar Wind
Here we investigate the contribution of surface Alfven wave damping to the
heating of the solar wind in minima conditions. These waves are present in
regions of strong inhomogeneities in density or magnetic field (e. g., the
border between open and closed magnetic field lines). Using a 3-dimensional
Magnetohydrodynamics (MHD) model, we calculate the surface Alfven wave damping
contribution between 1-4 solar radii, the region of interest for both
acceleration and coronal heating. We consider waves with frequencies lower than
those that are damped in the chromosphere and on the order of those dominating
the heliosphere. In the region between open and closed field lines, within a
few solar radii of the surface, no other major source of damping has been
suggested for the low frequency waves we consider here. This work is the first
to study surface Alfven waves in a 3D environment without assuming a priori a
geometry of field lines or magnetic and density profiles. We determine that
waves with frequencies >2.8x10^-4 Hz are damped between 1-4 solar radii. In
quiet sun regions, surface Alfven waves are damped at further distances
compared to active regions, thus carrying additional wave energy into the
corona. We compare the surface Alfven wave contribution to the heating by a
variable polytropic index and find that it an order of magnitude larger than
needed for quiet sun regions. For active regions the contribution to the
heating is twenty percent. As it has been argued that a variable gamma acts as
turbulence, our results indicate that surface Alfven wave damping is comparable
to turbulence in the lower corona. This damping mechanism should be included
self consistently as an energy driver for the wind in global MHD models.Comment: Accepted to ApJ (scheduled September '09), 22 pages, 8 figure
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