142 research outputs found
On the alignment of PNe and local magnetic field at the galactic centre: MHD numerical simulations
For the past decade observations of the alignement of PNe symmetries with
respect to the galactic disk have led to conflicting results. Recently
observational evidence for alignment between PNe and local interstellar
magnetic fields in the central part of the Galaxy () has been
found. We studied the role of the interstellar magnetic field on the dynamical
evolution of a PN by means of an analytical model and from 3D MHD numerical
simulations. We test under what conditions typical ejecta would have their
dynamics severely modified by an interstellar magnetic field. We found that
uniform fields of G are required in order to be dynamically dominant.
This is found to occur only at later evolutionary stages, therefore being
unable to change the general morphology of the nebula. However, the symmetry
axis of bipolar and elliptical nebulae end up aligned to the external field.
This result can explain why different samples of PNe result in different
conclusions regarding the alignment of PNe. Objects located at high galactic
latitudes, or at large radii, should present no preferential alignment with
respect to the galactic plane. PNe located at the galactic centre and low
latitudes would, on the other hand, be preferentiably aligned to the disk.
Finally, we present synthetic polarization maps of the nebulae to show that the
polarization vectors, as well as the field lines at the expanding shell, are
not uniform even in the strongly magnetized case, indicating that polarization
maps of nebulae are not adequate in probing the orientation, or intensity, of
the dominant external field.Comment: to appear in MNRA
Wind-wind collision in the eta Carinae binary system - III. The HeII 4686 line profile
We modeled the HeII 4686 line profiles observed in the eta Carinae binary
system close to the 2003.5 spectroscopic event, assuming that they were formed
in the shocked gas that flows at both sides of the contact surface formed by
wind-wind collision. We used a constant flow velocity and added turbulence in
the form of a gaussian velocity distribution. We allowed emission from both the
primary and secondary shocks but introduced infinite opacity at the contact
surface, implying that only the side of the contact cone visible to the
observer contributed to the line profile. Using the orbital parameters of the
binary system derived from the 7 mm light curve during the last spectroscopic
event (Paper II) we were able to reproduce the line profiles obtained with the
HST at different epochs, as well as the line mean velocities obtained with
ground based telescopes. A very important feature of our model is that the line
profile depends on the inclination of the orbital plane; we found that to
explain the latitude dependent mean velocity of the line, scattered into the
line of sight by the Homunculus, the orbit inclination should be close to 90
degrees, meaning that it does not lie in the Homunculus equatorial plane, as
usually assumed. This inclination, together with the relative position of the
stars during the spectroscopic events, allowed us to explain most of the
observational features, like the variation of the Purple Haze with the orbital
phase, and to conciliate the X-ray absorption with the postulated shell effect
used to explain the optical and UV light curves.Comment: to appear in the MNRA
Damping of MHD turbulence in partially ionized gas and the observed difference of velocities of neutrals and ions
Theoretical and observational studies on the turbulence of the interstellar
medium developed fast in the past decades. The theory of supersonic magnetized
turbulence, as well as the understanding of projection effects of observed
quantities, are still in progress. In this work we explore the characterization
of the turbulent cascade and its damping from observational spectral line
profiles. We address the difference of ion and neutral velocities by clarifying
the nature of the turbulence damping in the partially ionized. We provide
theoretical arguments in favor of the explanation of the larger Doppler
broadening of lines arising from neutral species compared to ions as arising
from the turbulence damping of ions at larger scales. Also, we compute a number
of MHD numerical simulations for different turbulent regimes and explicit
turbulent damping, and compare both the 3-dimensional distributions of velocity
and the synthetic line profile distributions. From the numerical simulations,
we place constraints on the precision with which one can measure the 3D
dispersion depending on the turbulence sonic Mach number. We show that no
universal correspondence between the 3D velocity dispersions measured in the
turbulent volume and minima of the 2D velocity dispersions available through
observations exist. For instance, for subsonic turbulence the correspondence is
poor at scales much smaller than the turbulence injection scale, while for
supersonic turbulence the correspondence is poor for the scales comparable with
the injection scale. We provide a physical explanation of the existence of such
a 2D-3D correspondence and discuss the uncertainties in evaluating the damping
scale of ions that can be obtained from observations. However, we show that the
statistics of velocity dispersion from observed line profiles can provide the
spectral index and the energy transfer rate of turbulence. Also, comparing two
similar simulations with different viscous coefficients it was possible to
constrain the turbulent cut-off scale. This may especially prove useful since
it is believed that ambipolar diffusion may be one of the dominant dissipative
mechanism in star-forming regions. In this case, the determination of the
ambipolar diffusion scale may be used as a complementary method for the
determination of magnetic field intensity in collapsing cores. We discuss the
implications of our findings in terms of a new approach to magnetic field
measurement proposed by Li & Houde (2008).Comment: ApJ accepte
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
Turbulence in collisionless plasmas : statistical analysis from numerical simulations with pressure anisotropy
In recent years, we have experienced increasing interest in the understanding of the physical properties of collisionless plasmas, mostly because of the large number of astrophysical environments (e. g. the intracluster medium (ICM)) containing magnetic fields that are strong enough to be coupled with the ionized gas and characterized by densities sufficiently low to prevent the pressure isotropization with respect to the magnetic line direction. Under these conditions, a new class of kinetic instabilities arises, such as firehose and mirror instabilities, which have been studied extensively in the literature. Their role in the turbulence evolution and cascade process in the presence of pressure anisotropy, however, is still unclear. In this work, we present the first statistical analysis of turbulence in collisionless plasmas using three-dimensional numerical simulations and solving double-isothermal magnetohydrodynamic equations with the Chew-Goldberger-Low laws closure (CGL-MHD). We study models with different initial conditions to account for the firehose and mirror instabilities and to obtain different turbulent regimes. We found that the CGL-MHD subsonic and supersonic turbulences show small differences compared to the MHD models in most cases. However, in the regimes of strong kinetic instabilities, the statistics, i.e. the probability distribution functions (PDFs) of density and velocity, are very different. In subsonic models, the instabilities cause an increase in the dispersion of density, while the dispersion of velocity is increased by a large factor in some cases. Moreover, the spectra of density and velocity show increased power at small scales explained by the high growth rate of the instabilities. Finally, we calculated the structure functions of velocity and density fluctuations in the local reference frame defined by the direction of magnetic lines. The results indicate that in some cases the instabilities significantly increase the anisotropy of fluctuations. These results, even though preliminary and restricted to very specific conditions, show that the physical properties of turbulence in collisionless plasmas, as those found in the ICM, may be very different from what has been largely believed. Implications can range from interchange of energies to cosmic ray acceleration.Publisher PDFPeer reviewe
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